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Dissertations, Theses, and Masters Projects Theses, Dissertations, & Master Projects
1995
Spawning and ecology of early life stages of black drum, Pogonias cormis, in lower Chesapeake Bay
Louis Broaddus Daniel III College of William and Mary - Virginia Institute of Marine Science
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Recommended Citation Daniel, Louis Broaddus III, "Spawning and ecology of early life stages of black drum, Pogonias cormis, in lower Chesapeake Bay" (1995). Dissertations, Theses, and Masters Projects. Paper 1539616624. https://dx.doi.org/doi:10.25773/v5-zmmn-ak77
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SPAWNING AND ECOLOGY OF EARLY LIFE STAGES OF BLACK DRUM, Pogonias cromls, IN LOWER CHESAPEAKE BAY
A Dissertation presented to The Faculty of the School of Marine Science The College of William and Mary in Virginia
In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy
By
Louis Broaddus Daniel, III
1995 UMI Number: 9523179
UMI Microfora 9523179 Copyright 1995, by UMI Coepany. All rights reserved.
This alcrofora edition la protected against unauthorised copying under Title 17, United States Code.
UMI 300 North Zeeb Road Ann Arbor, MI 40103 This dissertation is submitted in partial fulfillment of the requirements for the degree of
Doctor of Philosophy
^ z > a .______^Louis BroSddus Daniel, III
Approved i s i 5
f: John E . Olney, M.A. /'} Committee Chairman/Advisor/
L, in A. Husicficfc, Ph.D. Co-Chairman/Advisor
I 'C . )hn E, Graves, Ph.D.
< k o \ L J J J Peter A. Van Veld, Ph.D,
James H. Cowan, ^Ph■D, niversity of South Alabama
ii TABLE OP CONTENTS PAGE
ACKNOWLEDGEMENTS...... iv
LIST OF TABLES...... vi LIST OF FIGURES...... ix
ABSTRACT...... xii
INTRODUCTION...... 2
MORPHOMETRIC AND GENETIC IDENTIFICATION OF EGGS OF SPRING SPAWNING SCIAENIDS IN LOWER CHESAPEAKE BAY
INTRODUCTION...... 12 METHODS...... 16 RESULTS...... 18 DISCUSSION...... 22
REPRODUCTIVE ECOLOGY AND ASPECTS OF THE EARLY LIFE HISTORY OF BLACK DRUM, Fogonias cromis, IN LOWER CHESAPEAKE BAY
INTRODUCTION...... 29 METHODS...... 33 RESULTS...... 41 DISCUSSION...... 7 7
SPATIO-TEMPORAL ABUNDANCE AND DISTRIBUTION OF GELATINOUS ZOOPLANKTON PREDATORS AND THEIR POTENTIAL IMPACT ON SCIAENID EGG PREY DURING SPRING IN LOWER CHESAPEAKE BAY
INTRODUCTION...... 85 METHODS...... , 90 RESULTS...... , 98 DISCUSSION...... 144 SUMMARY...... 155
REFERENCES...... 157
VITA...... 167
iii ACKNOWL EDGEHENTS
I ant grateful to the members of my committee for their help
and support throughout the course of this study. I am
particularly indebted to my major professor John E, Olney, for allowing me to join the plankton group and guiding me through the
proposal nightmare that eventually led to the present study. His
tireless help, understanding and insight into studies of the early life history of fishes has made the present study what it
is and made me a better scientist. John A. Musick served as co
chair and was instrumental in the maintenance of my sanity with periodic trips to Burgh Westra with Kelly, Pip and Kersey, John
E. Graves mentored all of the work performed in the genetics lab
and provided critical review of this manuscript. Peter A. Van Veld was the outside department member of my committee and though he does not consider himself a chemist, he really is, Pete felt
as though his participation as one of my advisors was minimal, however, nothing could be further from the truth. Finally, James
H. Cowan conducted extensive research in the area off Cape
Charles on black drum while the present study was conducted and we share a common interest in the ecology of these fish.
Many individuals volunteered numerous hours both in the
laboratory and the field. Patricia A. Crewe spent numerous hours aboard vessels, in the laboratory and in the mud, collecting processing and sorting samples. John C. McGovern was my savior
in the field during the first two years of this project and helped in every way. Martin R. Cavaluzzi, John D. Field, Carole iv C. Baldwin and Kathryn C. Kavanaugh all had stints as my office mate, helped in the field and listened to my ramblings about ctenophores and the administration. Jan McDowell provided much needed help in the genetics lab and was very patient. Alice Lee
Tillage, an adopted member of the plankton group played sounding board. Tim Shannon, Suzanne Alexander, William Mathews and John
Poseneau developed, built and serviced the plankton camera, sometimes under very difficult circumstances but always kept it working. The captains of the R/V Bay Eagle, Durand Ward, and R/V
Langley, Charles Wachen, did incredible jobs with their vessels handling and deployment of a difficult piece of sampling gear.
Finally, I thank my Mom and Dad and my wife Ruth for their tremendous support. We are all done now.
v LIST or TABLES Table PACE
MORPHOMETRIC AND GENETIC IDENTIFICATION OF EGGS OF SPRING SPAWNING SCIAENIDS IN LOWER CHESAPEAKE BAY
1.1 Minimum and maximum reported outside egg diameter (OED) and study location for spring spawning sciaenids.■14
1.2 Fragment sizes produced by restriction endonuclease (Hindlll) digestion of mtdna purified from ovarian tissue of spring spawning sciaenids...... ,.,23
REPRODUCTIVE ECOLOGY AND ASPECTS OF THE EARLY LIFE HISTORY OF BLACK DRUM, Pogonlas crontls, IN LOWER CHESAPEAKE BAY
2.1 Criteria for staging field collected, preserved eggs, modified from Moser and Ah 1 strom (1965)...... 34
2.2 Collection, time, egg densities (eggs/l00mJ) , percent dead and physical data for collections of black drum eggs during the 1990 and 1991 time-series experiments... 43 2.3 Numbers, percent non-viable and totals of black drum eggs in each developmental stage during the 1990 Lagrangian time-series experiment...... 47
2.4 Numbers, percent non-viable and totals of black drum eggs in each developmental stage during the 1991 Lagrangian time-series experiment...... 52
2.5 Densities of black drum eggs (eggs/m1) over back-calculated time of spawning on 19 May 1990 and followed throughout the 24 hour experiment...... 53
2.6 Densities of black drum eggs (eggs/mJ) over back-calculated time of spawning on 20 May 1991 and followed throughout the 2 4 hour experiment...... 57
2.7 Results of hatching experiments conducted on 20 May 1991 on eggs of black drum to determine rates hatching and viability...... 61
2.B Stratum-specific mean abundance, range of abundance with production values, 95% confidence intervals and strata-specific percentages of total cruise production estimates for black drum in 1990...... 64 vi 2.9 Summary of cruise-specific, weekly egg production and variance estimates, survey area hydrography and parameters used to calculate seasonal production estimates during 1990...... 65
2.10 Stratum-specific mean abundance, range of abundance with production values, 95% confidence intervals and strata-specific percentages of total cruise production estimates for black drum in 1991...... 71
2.11 Summary of cruise-specific, weekly egg production and variance estimates, survey area hydrography and parameters used to calculate seasonal production estimates during 1991...... *...... ,..72
2.12 Data summary for 1990 and 1991 egg production and population biomass estimates of black drum in lover Chesapeake Bay...... 73
SPATIO-TEMPORAL ABUNDANCE AND DISTRIBUTION OF GELATINOUS ZOOPLANKTON PREDATORS AND THEIR POTENTIAL IMPACT ON SCIAENID EGG PREY DURING SPRING IN LOWER CHESAPEAKE BAY 3.1 Stratum-specific mean and range of density of small N . bachei collected in 1990 and 1991...... 99
1.2 Stratum-specific mean and range of density of large N. bachei collected in 1990 and 1991...... ,,.,102
3.3 Stratum-specific mean and range of density of cydippid stage ctenophores collected in 1990 and 1991,..,,,...... 104
3.4 Stratum-specific mean and range of density of lobate M . leldyi collected in 1990 and 1991...... 109
3.5 Ranges of hydrographic conditions and sample depths at stations occupied in lower Chesapeake Bay during spring 1990 and 1991...... 115
3.6 Stratum-specific center of mass {ZJ for predatory gelatinous zooplankton during spring 1990, ...... 116
3.7 Comparisons of the depth distribution of N. bachei, M. leldyi and cydippids at 5 m intervals in all samples from cruises 6,B and 9 (1990) where taxa co-occurred.... 118
3,0 Stratum-specific center of mass for predatory gelatinous zooplankton during spring 1991...... 119
3.9 Site-specific sciaenid egg prey and gelatinous predator densities (#/m3) during 1990...... 120 vii 3.10 Centers of mass (ZM) of sciaenid eggs and gelatinous predators at each station where eggs were identified in films during 1990 , ,...... 122
3,11 Site-specific sciaenid egg prey and gelatinous predator densities (//m3) during 1991...... -123
3.12 Centers of mass (ZB} of sciaenid eggs and gelatinous predators at each station where eggs were identified in films during 1991...... 124
3.13 Densities of predators and sciaenid eggs at each time interval during the 1990 time-series experiment with tidal stage and halocline depth in meters (HJ - - ...... 126
3.14 Densities of predators and sciaenid eggs at each time interval during the 1991 time-series experiment with tidal stage and halocline depth in meters fHd)...... 131
3-15 Stratum-specific distances (m) between predators and prey during 1990...... 139
3.16 Stratura-Specific estimates of egg production obtained from paired, preserved collections and predator abundance, proportion of the water column cleared, potential number of eggs eaten and percentage of egg production estimates eaten in 1990...... 142
3.17 Stratum-specific estimates of egg production obtained from paired, preserved collections and predator abundance, proportion of the water column cleared, potential number of eggs eaten and percentage of egg production estimates eaten in 1991...... 146
viii LIST or FIGURES
FIGURE PAGE
MORPHOMETRIC AND GENETIC IDENTIFICATION OF EGGS OF SPRING SPAWNING SCIAENIDS IN LOWER CHESAPEAKE BAY
1.1 Composite, weekly frequency distributions of eggs of sciaenids collected during 1990 ...... 20 1.2 Composite, weekly frequency distributions of eggs of sciaenids collected during 1991 ...... 21 1.3 Size distribution of all eggs morphologically typed as sciaenids and identified using genetic techniques...... 24
REPRODUCTIVE ECOLOGY AND ASPECTS OF THE EARLY LIFE HISTORY OF BLACK DRUM, Pogonias cromis, IN LOWER CHESAPEAKE BAY
2.1 Total commercial landings vs. spring landings (April-June) for black drum during the period 1901-1992...... JO
2.2 Ichthyoplankton survey grid...... 3 6 2.3 Dye patch and/or drogue positions mapped during the 1990 Lagrangian time-series experiment...... 44
2.4 Drogue positions mapped during the 1991 Lagrangian time-series experiment ...... 45
2.5 Temperature-salinity plot of all stations sampled during the 1990 Lagrangian time-series experiment...... 48
2.6 Temperature-salinity plot of all stations sampled during the 1991 Lagrangian time-series experiment...... 49
2.7 Temperature-salinity plot of stations sampled from Time o-Time 15 and from Time 18-Time 24 during the 1991 Lagrangian time-series experiment...... 50
2.8 Back-calculated times of spawning for all sciaenid eggs collected during the 1990 Lagrangian time-series experiment...... 54
2.9 Back-calculated times of spawning for all sciaenid eggs collected during the 1991 Lagrangian time-series experiment...... 55 2.10 Densities of black dram eggs in the 0400 and 0700 cohort followed during the 1990 Lagrangian time-series experiment...... 58 2.11 Densities of black drum eggs in the 0100 and 0400 cohort followed during the 1991 Lagrangian time-series experiment...... 59
2.12 Ichthyoplankton survey grid. Triangles represent stations occupied during eight surveys, April-May 1990..62 2.13 Frequency distribution of egg catches made during the 1990 ichthyoplankton survey...... 66 2.14 stations sampled during 1990 cruises that contained eggs of black drum ...... 67
2.15 Cruise-specific estimates of egg production versus temperature (°CJ during 1990 cruises...... 68
2.16 Ichthyoplankton survey grid. Triangles represent stations occupied during eight surveys, Apri1-May 1991..69
2.17 Frequency distribution of egg catches made during the 1991 ichthyoplankton survey...... 74 2 .lfl stations sampled during 1991 cruises that contained eggs of black drum...... 75
2.19 Cruise-specific estimates of egg production versus temperature {°C) during 1991 cruises...... 76 SPATIO-TEMPORAL ABUNDANCE AND DISTRIBUTION OF GELATINOUS ZOOPLANKTON PREDATORS AND THEIR POTENTIAL IMPACT ON SCIAENID EGG PREY DURING SPRING IN LOWER CHESAPEAKE BAY
3.1 Diagram of the paired-net towing frame...... 91
3.2 Abundance estimates of small and large N bachel collected during cruises in 1990 and 1991...... 106
3.3 Abundance estimates of lobate and cydippid stage ctenophores collected during cruises in 1990 and 1991...... 107
3.4 Stratum-specific abundance of large hydromedusae and lobate ctenophores during cruises in 1990...... 112
3.5 Stratum-specific abundance of large hydromedusae and lobate ctenophores during cruises in 1990...... 113
x 3.6 Stratum-specific preserved, sciaenid egg density versus camera derived predator density during 1990.....127
3.7 Stratum-specific preserved, sciaenid egg density versus camera derived predator density during 1991.....128 3.8 Comparisons of center of mass {Z„) for gelatinous predators and sciaenid eggs during the 1990 Lagrangian time-series experiment...... 129
3.9 Comparisons of center of mass (Z,J for gelatinous predators and sciaenid eggs with regards to predicted tidal stage during the 1990 Lagrangian time-series experiment...... 132
3.10 Densities of sciaenid eggs and gelatinous predators during the 1990 Lagrangian time-series experiment...... 133 3.11 Densities of sciaenid eggs and gelatinous predators with differences in center of mass (Zm) during the 1990 Lagrangian time-series experiment...... 134
3.12 Comparisons of center of mass (Z„) for gelatinous predators and sciaenid eggs during the 1991 Lagrangian time-series experiment...... 135
3.13 Comparisons of center of mass (Z„> for gelatinous predators and sciaenid eggs with regards to predicted tidal stage during the 1991 Lagrangian time-series experiment...... 136 3.14 Densities of sciaenid eggs and gelatinous predators during the 1991 Lagrangian time-series experiment..... 139
3.15 Densities of sciaenid eggs and gelatinous predators with differences in center of mass {Z„} during the 1991 Lagrangian time-series.experiment...... 140
3.16 Influence of tidal stage on center of mass (ZB} and distance separated {m} of gelatinous predators and sciaenid eggs during the 1990 Lagrangian time-series experiment...... 143
3.17 Influence of tidal stage on center of mass (Zn) and distance separated (m) of gelatinous predators and sciaenid eggs during the 1991 Lagrangian time-series experiment...... 144
xi ABSTRACT During spring 1990 and 1991, ichthyoplankton surveys were conducted in lower Chesapeake Bay to estimate seasonal egg production, population biomass and the impact of predation by gelatinous zooplankton on early life stages of black drum,
Pogonias crojnis. Rearing experiments indicated that at least three species of sciaenid (silver perch, Balrdieila chrysoura; weakfish, Cynos clan regalis and P, cromis] were spawning in the survey area during both years. Specific identification of eggs based on previously published ranges of outside egg diameter
(OED) were not reliable due to considerable overlap in diameter distributions. Analysis of weekly OED frequency revealed the presence of three modes which differed in temporal occurrence, suggesting the products of three species. Genetic typing of eggs using RFLP analysis of mtDNA confirmed the presence of three species, but demonstrated that eggs of a certain size classes represented two species. Results illustrate that reliance on previously published ranges of egg diameter for specific identification of spring spawning sciaenids may overestimate the spawning biomass of black drum by as much as 50t owing to the misidentification of weakfish eggs as those of black drum.
Black drum enter Chesapeake Bay in early spring and spawn throughout the day over a discrete time period from late April through May in a spatially limited area off the city of Cape
Charles, Virginia. Seasonal egg production is low compared to other sciaenid stocks and estimates of spawner biomass indicate
xii that the population is comprised of relatively few individuals. During 1990 and 1991, peak egg production of black drum occurred in the lower portion of Chesapeake Bay where potential encounter with gelatinous zooplankton predators was least likely. These observations wore consistent between years with vastly different gelatinous zooplankton communities. Analysis of small- scale patterns of spatial coincidence between eggs and predators reveal that chances of physical contact are least in areas of peak production.
xiii SPANNING AND ECOLOGY OF EARLY LIFE HISTORY STAGES OF
BLACK DRUM, Pogonias croais, IN LOWER CHESAPEAKE BAY 2
INTRODUCTION
Black drum, Fogohlas cromis, la the largest of the western Atlantic sciaenids and occurs from Argentina to the Gulf of Maine {Hildebrand and Schroeder 1928, Bleakney 1963, Silverman 1979}. Beckman et al, (1990) reported a maximum age of 43 in the Gulf of Mexico, and preliminary data suggest greater longevity in Chesapeake Bay (C. Jones, old Dominion University; M. Chittenden, Virginia Institute of Marine Science; pers. comm.). The center of abundance of black drum is located in the Gulf of Mexico (Nieland and Parker 1993) where commercial landings increased from 1.9 x 106 kg in 19B2 to nearly s x 104 kg in 1987 (Fitzhugh et al. 1993). Black drum are not specifically targeted throughout most of the South and mid-Atlantic coast but may comprise large portions of commercial catches at some times of the year. In Chesapeake Bay, however, a directed gill net and trotline fishery targets black drum during spring, where annual
landings of 4-5 x 10* kg are comprised of large (15-45 kg) adults
(D. Boyd, Virginia Marine Resources Commission; pers. comm.)
While the overall economic importance of the black drum fishery varies throughout their range, it can be of substantial importance to small, local communities. While reliable data are available for much of the adult life history of black drum in the Gulf of Mexico, data are sparse concerning their early life history, especially egg and larval stages. Black drum spawn from November to June at sea near the 3 mouths of inlets and in large bays from Delaware to Texas
(Simmons and Breuer 1962, Joseph et al. 1964, Holt et al. 1935,
Murphy and Taylor 1989, Peters and McMichael 1990, Fitzhugh et al, 1993, Nieland and Wilson 1993}. Thomas and Smith (1971) reported that spawning occurred from early April to June and peaked around mid-May in Delaware Bay, concurrent histological studies in the northern Gulf of Mexico (Nieland and Wilson 1993;
Fitzhugh et al. 1993) suggest that the spawning season is more protracted farther south, lasting from November/December through
April. Peters and McMichael (1990) collected larvae from January to mid-May in Tampa Bay, Florida, and larvae were collected from January to April in the northwestern Gulf of Mexico (Cowan and
Shaw 1991) . A possible secondary spawning period was reported to occur from late July to November among younger age classes off
Texas (Pearson 192 9); however, recent studies dismiss this hypothesis (Nieland and Wilson 1993; Fitzhugh et al- 1993).
Black drum are batch or serial spawners with a spawning frequency of once every 3-4 days. Average total fecundity estimates calculated for fish in the Gulf of Mexico (4-10 kg) range from 3-3.5 x 10T eggs/female (Nieland and Wilson 1993;
Fitzhugh et al. 1993). Fecundity estimates are unavailable for northern stocks. Female black drum in the Gulf of Mexico mature at age 4 or 6 when they are 628-690 mm fork length (FL). Males mature at a younger age (3-4) and size (552-640 mm FL) (Fitzhugh et al. 1993; Nieland and Wilson 1993).
Black drum eggs are pelagic, range in size from 0,8-1.0 mm, 4 and possess multiple oil globules in early stages that coalesce to a single globule just prior to hatching {Joseph et al, 1964,
Lippson and Horan 1974) . This morphological description is derived from the single published record of black drum eggs collected in the lower Chesapeake Day near Cape Charles, V a . (Joseph et al. 1964). In a recent study (Cowan et al. 1991), black drum eggs were also collected off Cape Charles City, Va.
Extensive ichthyoplankton surveys in the Bay have not detected black drum eggs or larvae (Pearson 1941, Olney 1983, Olney and
Boehlert 1907). Pearson (1941) did not identify pelagic eggs.
Olney (1983) and Olney and Boehlert (1907) could not distinguish between species of sciaenid eggs in their samples since several sciaenids (ie,-Bairdiella chrysoura, cynosclon nebulosus, C. regal is and at least two species of Menticirrhus) in the mid-
Atlantic are spring spawners, and many of their egg diameter and oil globule numbers overlap (Johnson 1978). This identification problem is not intractable, however, since field caught eggs can be raised to hatching for a positive identification (Joseph et al. 1964, Holt et al. 1985). More recently, Graves et al. (1989) have compared restriction fragment lengths of egg mitochondrial
DNA with those of known adults of Paralabrax sp. for a positive identification. This method may prove useful in resolving the identification problems in sciaenids.
Larval black drum have been described (Ditty 1990) and collected in Delaware Bay (Thomas and Smith 1971) and from South
Carolina (McGovern and Wenner 1990) to the Gulf of Mexico 5 {Pearson 1929, Jannke 1971, Price and Schleuter 1905, Peters and
McMichael 1990, Cowan and Shaw 1991). Thomas and Smith {1973)
collected only two specimens (<10 mm TL) in a low salinity (< 4
p.p.t.) tributary of Delaware Bay. Other collections were made
in bays and passes with plankton gear and in shallow, quiet tidal
creeks, secondary bays and lagoons with seines and trawls. King (1971) collected larvae at both surface and mid-water plankton
stations, finding no diel variation in abundance. Peters and
McMichael (1990) collected the majority of their specimens in
mid-water at night in lower Tampa Bay, The nursery habitats of
late larval black drum are shallow tidal creeks and sloughs with
nutrient rich, muddy bottoms {Peters and McMichael 1990). The
abundance of juvenile black drum in Chesapeake Bay is low
compared with more southern waters (Frisbie 1961, Simmons and
Breuer 1962, Joseph et al, 1964, Peters and McMichael 1990). Frisbie (1961) noted that Chesapeake Bay lies near the northern
geographic limit of the reproductive range of black drum and
suggested that environmental conditions, particularly low
temperatures, may severely limit larval and juvenile survival.
Frisbie's hypothesis (1961) and the failure of intensive lower
Chesapeake Bay sampling (Olney and Daniel 1991) to locate larvae
or juvenile nursery grounds, suggest that reproductive success of
this northern stock is consistently low.
Variability in year class strength is believed to be determined during three largely independent transitions (female
biomass to eggs; eggs to larvae and larvae to recruits), each 6 having unique controlling mechanisms {Rothschild 1996). Elements of control within these stages include adult reproductive fitness; predation and starvation of early life history stages; and hydrographic factors that may transport eggs and larvae away from favorable conditions (Blaxter 1971, Houde 1977, shepherd and
Cushing 1960, Billard et al. 1961, Blaxter 19B8). Yearly deviations from "average11 conditions among any of these factors, could result in exceptionally good or poor year classes.
Furthermore, the persistence of anomalous conditions over time, may lead to long term trends of increasing or declining stock size (Houde 1987}.
The number of eggs produced by a fish stock is an important fisheries parameter, but one that usually undergoes high interannual variability (Hislop 1988), Estimates of seasonal egg production are of utility in determining spawning stock size over time (Houde 1977, Lo 1984, Kendall and Picquelle 1990, Olney et al, 1991). Yearly variability in egg production may be influenced by many factors including fluctuations in stock size, population age structure, dietary constraints, temperature and the bioaccumulation of pollutants in gonad tissue that affects reproductive fitness (Rosenthal and Alderdice 1976, Zastrow et al. 1989),
Fertilization success and hatchability determine progression from the egg to larval stage. Fertilization rates of pelagic spawners are highly variable (range = 0 to loot) and may be an important mortality factor in some species (see Markle and 7 Waiwood 1965, for review). Egg size and egg quality affects hatchability and has been shown to be positively correlated with fish age (see Bagenal 1973 and Zastrow et al. 1989, for review). Because black drum spawners in Chesapeake Bay are comprised of the largest fish of the Atlantic stock (Silverman 1979), egg quality may be high. However, studies of adult reproductive fitness are lacking for these older fishes. Field estimates of black drum egg viability based on wild caught, preserved material are presently lacking. Mortality during the egg and larval stages of fishes is highly variable and may significantly affect recruitment success (Rothschild 1987, Cowan et al. 1991, Baily and Houde 1989; Houde 1987, 1989), Recent studies involving drifting, in situ enclosures (mesocosms) suggest that ambient food conditions are suitable for growth and survival of larvae, and that starvation may not be as limiting as previous laboratory studies indicate {ie,-Laurence et al. 1979, de Lafontaine and Leggett 1989, Bailey and Houde 1989, Cowan et al. 1992). Furthermore, starvation would not be applicable to the pre-feeding egg and yolk-sac stages of fishes (Purcell 1985).
Predation is probably the leading cause of mortality during the earliest life stages of fishes (Brewer et al. 1984, Purcell 1985, Bailey and Houde 1989, Govoni and Olney 1990) , however, direct estimates of the impact of zooplanktivores are difficult.
Govoni and Olney (1990) commented that the accurate assessment of the impact of predation on fish eggs and larvae has been impeded a by three problems. Gut content analysis of predators is difficult due to the short residence time of fish eggs and larvae in the guts of predators. Since predators and prey are concentrated in collecting devices, high rates of "net-feeding** would tend to inflate estimates of predation. Finally, inferring predation by the "reciprocal oscillations'1 of predator and prey abundance could actually be revealing spatio-temporal segregation rather than predation (Frank and Leggett 1985).
The potential impact of gelatinous zooplankton as predators on the egg and larval stages of marine fishes is well established in the literature (ie.-Burrell and Van Engel 1976, Moller 1900,
Purcell 1985, de Lafontaine and Leggett 19B8, Gamble and Hay
1989, Purcell 1990, Purcell and Grover 1990, Govoni and Olney 1990, Cowan et al. 1991, Matsakis and Conover 1991, Purcell
1992). Holler (1980) estimated that predation rates on Atlantic herring larvae by the scyphomedusae Aurelia aurita were from 2.6-
4.5 d 1. Predation by three species of hydromedusae on herring larvae in Kulleet Bay, British Columbia during a three year study, ranged from 0.7-92.9%. Annual differences between years were attributed to orders of magnitude increases in predator densities (Purcell and Grover 1990). Enclosure studies of predation by jellyfish on larval fish by de Lafontaine and
Leggett (1988) suggest that larval mortality is strongly dependent upon predator size, larval size and environmental conditions.
Gelatinous zooplanktivores feed via water currents generated 9 by swimming movements that bring prey into contact with tentacles! oral arms or lobes, effectively trapping eggs and yolk sac larvae that are unable to avoid the feeding currents {Purcell
1985}. Most pelagic, gelatinous zooplankton are large in comparison to fish eggs and larvae, have high ingestion rates and do not reach satiation quickly (Purcell 1985, de Lafontaine and
Leggett 1988, Monteleone and Duquay 1988, Govoni and olney 1991}.
Purcell {1990}, however, suggested that satiation may occur at high prey densities and limit predation at hatching. Studies of the ctenophore, Mneroiopsis Jeidyi indicate that they have extremely high metabolic rates, require a constant food supply and may double their population size in 40-4 5 days (Reeve et al.
1978, Monteleone and Duquay 19B5). Starved M. leldyi were found to feed at two times the rate of those that were well fed, an observation attributed to their ability to adjust the feeding apparatus to increase the feeding surface area of the lobes under starved or low prey conditions {Reeve et al. 1978). The capability of ctenophores to self-fertilize, have high fecundity and rapid growth, led Reeve et al. (1978) to conclude that ctenophores could overwhelm other predator taxa at high food levels.
Subtle changes in mortality rates due to variability in development and growth of early life history stages may drastically affect recruitment success (shepherd and Cushing
1980, Houde 1987}. Houde (1987) examined hypothetical differences in mortality rates of larvae and calculated that a 10 25% Increase in instantaneous mortality (Z) was sufficient to decrease the resulting number of recruits threefold. Since slow growth increases the vulnerability of early life stages to predation mortality (Houde 1987, Blaxter 1988, Houde 1989,
Miranda et al. 1990), Shepherd and Cushing (I960] hypothesized that regulation should be most intense in the larval stage when growth was plastic and dependent upon food abundance. They suggested that high densities of larvae may encourage competition, thereby inhibiting growth. Such density-dependent
regulation maybe unlikely since larval fishes are relatively rare when compared to their microzooplankton food (Houde 19&7). Houde (1987, 1989a) argued that growth was largely mediated by temperature and\or less than optimal food levels that increase
stage durations by lowering metabolism and feeding rates, especially at high latitudes where growth rates are already low.
However, variations in growth rates may be so slight that detection from field estimates is difficult. Whether survival is density-dependent or density-independent, slight declines in growth rates that prolong the vulnerability of early life stages to various mortality factors appear to be an important regulator of recruitment in the egg and larval stages of fishes (Shepherd and Cushing 1980, Houde 1987, 1989).
Although the inter-relationship of a number of physical and biological factors influences reproductive potential and recruitment success, their role in the apparent recruitment
failure of black drum is unknown. The absence of collections of 11 wild larvae In Chesapeake Bay and the rarity of juveniles pose formidable questions of factors resulting in failed year-classes.
Without larval specimens, information concerning distribution, growth and mortality rates is unavailable. Furthermore, the infrequent collection of juveniles precludes any index of abundance or identification of critical nursery habitat. 12
MOKFHOMETKIC AND GENETIC IDENTIFICATION OF BOOB 07 SPRING
SPAWNING flCIABNIDB IN LOWER CHESAPEAKE BAY
INTRODUCTION
At least five species of the family Sciaenidae {silver perch, Bairdiella chrysoura; spotted seatrout, Cynoscion nebujosus; weakfish, C, regalis; northern kingfish, Menticirrhus Bax&tilis; and black drum, Pogonias cromis) are purported to spawn during the spring in lower Chesapeake Bay (Joseph et al. 1964; Lippson and Koran 1974; Johnson 1978; Brown 1981; Olney
1983; Cowan et al. 1992), The eggs of spring-spawning sciaenids in lower Chesapeake Bay are morphologically similar, ranging in outside egg diameter (OED) from 0.66 to 1.18 mm, with single or multiple oil globules of varying sizes (Johnson 197B; Olney
19B3). As a result, specific identification of eggs based on morphological criteria is problematic. Holt et al. (1988) suggested that it may not be possible to determine the specific identity of sciaenid eggs using morphological criteria, while
Joseph et al. (1964) reported that positive identification could only be achieved with supplemental hatching studies.
Hatching studies have traditionally been used to positively identify morphologically similar eggs, including those of sciaenids. Joseph et al. (1964) cultured eggs of several sciaenids collected at a single station in southern Chesapeake 13 Boy {16 Hay 1962) and raised larvae to an identifiable size (5-7 mm). The smallest eggs (0.630-0,777 mm) were found to be B. chrysoura while the larger eggs (0.B14-1.110 mm) developed into P. cromis. Culture of eggs {0.777-0.950 mm) collected during early June produced no P. cromis but did result in larvae of fl. chrysoura and C. regalia. In contrast, Olney (1983) suggested that eggs of P. cromls, C. regal is, b . chrysoura and Menticirrhus spp. were included in a size frequency distribution of morphologically similar eggs collected from Hay through August in lower Chesapeake Bay but that identifications based on diameter was ambiguous due to the high degree of overlap in diameter distributions (Table 1) . Confounding this problem is the observation that egg size may change with varying salinity or as the spawning season progresses (Johnson 1978). Because many species of Sciaenidae in lower Chesapeake Bay spawn concurrently and have morphologically similar eggs, most studies have relied either on previously published egg size distributions or rearing for identification {Holt et al. 1985,
1988; Comyns et al. 1991; Saucier and Baltz 1992; saucier et al* 1992). However, the misidentifications that can result from overlapping egg diameter distributions and the time-consuming nature of culture experiments make methods based on other characters desirable*
The application of biochemical genetics has provided an alternative to culture for the positive identification of morphologically similar eggs. Electrophoresis of water soluble Table 1.1. Minimum and maximum reported outside egg diameter (OED) and study location for spring spawning sciaenids. 'H -H xt - a) d *-3 s (N 04 o (No o 04 s A ^1—, +J + ui O < a- a 0 0 0 0 A u a u 0 o c Q j "H ■ £
larvae and juveniles of morphologically similar species of marine
fishes (eg. Morgan 197 5; Smith and Benson 1980; Graves et al.
1988) . Similarly, restriction fragment length polymorphism
(RFLP) analysis of mitochondrial DNA (mtDNA) has been employed to
discriminate between the eggs of three congeneric serranids that
could not be unambiguously identified with a single allozyme
locus {Graves et al. 1989). More recently, direct sequencing and
RFLF analysis of DNA amplified by the polymerase chain reaction
{PCR) has been used to identify morphologically similar larvae of
invertebrates (Olson et al. 1991; Silberman and Walsh 1992).
In this chapter I report that identification of eggs of sciaenids in lower Chesapeake Bay during spring based on
published morphological criteria, rearing experiments, and
genetic analysis are inconsistent. These results indicate that
it is not possible to accurately identify sciaenid eggs using
diameter as the sole criteria. In addition, results of weekly
plots of egg size-frequency distributions and a RFLP analysis of mtDNA used to determine the specific composition of eggs of sciaenids that may be present in lower Chesapeake Bay during
spring are presented,
METHODS
Weekly zooplankton surveys of the lower Chesapeake Bay were
conducted during April and May 1990 and 1991 to determine the
distribution and abundance of eggs of black drum for an estimate ic of their seasonal egg production. Samples of eggs were obtained with an In situ silhouette photography system consisting of paired 60 cm diameter, 335 micron nets fitted to a rigid frame (see Olney and Houde 1993 for a detailed gear description) . All deployments were 5-rain, stepped-oblique tows and yielded a standard plankton sample and a replicate film record. Plankton samples were preserved in 5-8% buffered Formalin and sciaenid eggs were identified using the criteria of Lippson and Horan
(1974) and measured to the nearest 0.025 mm (Zeiss stemi sr
Stereomicroscope}• Ten sub-samples of eggs (n =* 75-100) sorted from preserved plankton samples were re-measured to assess measurement error.
During several cruises in Hay 1990 and 1991, eggs were collected in an area off the city of Cape Charles, Virginia using a 0.5 m Hansen net fitted with 202 micron mesh to seed l liter
Invhoff settling cones for hatching experiments. Eggs were originally separated as Type I (<0.80 mm) and Type n (>0.85 mm) based on the morphological criteria of Joseph et al. (1964),
Rearing chambers were returned to the laboratory and held for 3 to 14 days in which larvae were periodically sacrificed and preserved in 5-8% buffered formalin. Identification of preserved sciaenid larvae using pigment characters was based on Ditty (1989).
Sciaenid eggs collected in the same area during spring 1991,
1992, and 1993 were sorted from fresh plankton samples, measured, placed in scintillation vials with 30-26 p.p.t. seawater, and 17 frozen et -7o°C for genetic analysis. Eggs of most spring- spawning sciaenids generally posses three or fewer oil globules
(usually 1-2) (Johnson 197B), Eggs of Menticfrrhus saxatllis, which could be collected in lower Chesapeake Gay in spring, however, may contain from 1-16 oil globules (Johnson 197B).
Consequently, eggs with > 3 oil globules were omitted from the samples to avoid contamination by the cynoglossid, Symphurus plagiusa and/or the soleid, Trinectes maculatus. After thawing, individual eggs were re-measured prior to homogenization to assess shrinkage.
Sciaenid eggs were genetically typed by comparing mtDNA restriction fragment patterns of individual eggs with those of known adults. To obtain patterns of known adults, mature female sciaenids (B.chrysoura, C t nebulosus, C. regalis, H. saxatilis and P. cromis) were collected by pound net, trawls, and hook and line in April and May 1990 and 1991, Ovarian tissue was excised and frozen at -70flC. MtDNA was purified from ovarian tissue by
CsCl equilibrium density gradient ultracentrifugation following the protocols of Lansman et al. (1981). To determine a restriction enzyme that unambiguously identified the different sciaenid species, aliquots of mtDNA were individually digested with the following restriction enzymes: Apal, Aval, Banl, Banll,
HindiII used according to manufacturer's instructions. The resulting fragments were separated electrophoretically on l.ot agarose mini-gels run at 5 V/cm for 4 hours and visualized with ethidium bromide. la Individual eggs were identified by comparison of RFLP patterns with those of adults. MtDNA-enriched genomic DNA was isolated from individual eggs following the protocols of Graves et al. (1989). Entire DNA samples were digested with a single discriminating restriction endonuclease, separated electrophoretically, and transferred to a nylon filter (Southern transfer) following standard protocols (Sambrook et al. 1990).
Filters were hybridized with highly purified black drum mtDNA nick-translated with biotin-7-dATp, washed, blocked and visualized following the methods of Graves et al. (19B9).
RESULTS
A total of 10,803 sciaenid eggs was sorted from samples collected in 1990 and 1991, Outside egg diameter of all specimens ranged from 0.650 to 1.12 mm. Successive blind readings of samples of 7 5 to 100 eggs were used to assess measurement error. No differences were found in the size frequency distributions indicating good agreement within the
0.025 mm size classes (2-sampled t-test, P < 0.05, n *■ 79).
Qualitative analysis of culture experiments using the two egg types of Joseph et al. (1964) revealed the presence of three species. Cultures containing eggs designated Type I (<0.80 mm) resulted in larvae of B. chrysoura while cultures of eggs designated Type II ( > 0.85 mm) resulted in larvae of C, regal is and P. cromls*
Analysis of preserved ichthyoplankton samples from 1990 and 19 1991 revealed the presence of larvae of B. chrysoura, C, regal is
and P* cromis. No other early life history stages of any other
sciaenids were identified, however, yolk-sac larvae could not be
identified to species. Because rearing studies and analysis of
field caught plankton samples revealed the presence of more than
two species, I could not rely on the criteria of Joseph et al.
(1964) for specific identification. I therefore examined weekly frequency of occurrence of all sciaenid eggs during 1990 (Figure
1) and 1991 (Figure 2). Based on temporal occurrence and size
frequency we identified three modes. The largest eggs (> 0.975 mm), Type C, were most abundant during the period 2 3 April through 9 May. Type C eggs declined in abundance throughout May
in both years. Mid-sized eggs (0.850-0.950 mm), designated Type B, generally appeared later than Types A and C. Type B eggs did
not exceed 5% of the total frequency of sciaenid eggs until 15
May 1990 and 9 May 1991. Type B eggs increased in abundance from mid-May until the end of sampling. The smallest eggs {< 0,050 mm), designated Type A, eggs co-occurred with Type C eggs; however, they did not exceed 5% of the total sciaenid eggs until 8 May 1990 and 9 May 1991. In 1990, Type A eggs peaked in abundance on 15 May and gradually declined throughout the sampling period. In 1991, occurrence of Type A eggs was greatest during the last sample on 28 May.
To test the hypothesis that eggs designated Types A, B and C were separate species assemblages, the mtDNA restriction fragment patterns of known adult sciaenids were compared with those of Figure 1.1.-Composite, weekly frequency distributions eggs of sciaenids collected during 1990. PERCENT FREQUENCY 40' 60 i 60 0 ■ n n l ftl T i r n' i i'I'' l i I i 5 5 5 .5 0 1.15 05 1 0.95 85 0 75 0 65 0 6 0 5 .G .6 .5 1.15 1.05 0.96 0.6G 75 0 65 0 5 .5 5 0 10 1.15 1.05 05 0 65 0 0.75 65 0 - > f * . r . r q ■ J U Type B ■ I| Type Type C 23 April 23 1990 1990 y a 8 M a
1990 y 1 a M USD EGDAEE (mm) DIAMETER EGG OUTSIDE 16 12 8 4 □ 6 075 08 09 10 1.15 1.05 0.95 85 0 5 7 0 65 0 5 7 06 09 1 5 1.15 105 95 0 0.65 75 0 65 0 5 9 10 1.15 1.05 95 0 5 6 0 75 0 5 6 0 4May1990 y a 24 M 5May 0 9 9 1 y a M 15 1 M 31 y 1990 Ma 21
Figure 1.2,-Composite, weekly frequency distributions of eggs of sciaenids collected during 1991. PERCENT FREQUENCY 20 15 10 20 to m i Pi P i i m 0 PiiTTiTill'I'IITi i 0 p r i i l G 07 08 09 t05 1.15 5 t.0 0.95 0.85 0.75 0G5 5 5 .5 0 I 115 S ID 05 0 0.85 75 0 65 0 6 0 5 8 09 1 5 1.15 1 05 0.95 85 0 75 0 65 0 B n Type A E3 E3 T ype ype T ye E3 Type 9APR1991 R P 29 A 3AR 1991 APR 23 c 1991 y a 9 M i ii r i i'i i OUTSIDE EGG DIAMETER (mm) DIAMETER EGG OUTSIDE 20 5 5 .5 95 1 5 15 1 1 05 5 09 0.85 75 0 65 0 5 5 5 5 0 1.15 1 05 95 0 85 0 75 0 65 0 5 .5 5 5 0 1.15 05 1 95 0 85 0 0.75 65 0 8MAY1991 Y A M 28 2MAY1991 Y A M 22 19 MAY19 1991 i-i-i~i~i~p lln 11 m t t m 22 fresh egg samples separated into Types A, B and C. Restriction fragment length polymorphism analysis of mtdna purified from adult B, chryeoura, C. nebulosus, c. regal is, Menticirrhus spp, and P. cromls revealed species-specific restriction fragment patterns for each of the five enzymes. Of the five enzymes, tfindlll showed the greatest differences between species, facilitating visualization using the Southern blotting procedure
(Table 2). A total of 62 eggs, representing all sciaenid egg size classes collected in lower Chesapeake Bay, was identified using diagnostic Jfindlll restriction fragment patterns. Bairdiella chrysoura, c, regalis and p. cromis were the only species of sciaenids identified; no other restriction fragment patterns were observed. Genetic identification of eggs designated Type A (< 0.850 mm, n - 12) resulted in 11 individuals of fl. chrysoura and one specimen (0.625 mm OED size class) of C. regalis (Figure 3).
Cynoscion regal is comprised the majority of type B eggs (0.850-
0.975 mm, n = IB} analyzed, but seven of the 10 largest type B eggs (0.975 mm OED size class) were identified as black drum. Type C eggs, those 1.00 mm and larger (n - 32), all possessed the restriction fragment pattern diagnostic for p. cromis.
DISCUSSIOK
Identifications of eggs of sciaenids are often based on published diameter distributions and/or hatching experiments. Results of hatching experiments and genetic analysis in this 23
Table 1.2.Common fragment sizes produced by restriction endonuclease (J/indlll) digestion of mtDNA purified from ovarian tissue of spring spawning sciaenids.
Species Fragment Sizes (Kb) Bairdlella chryaoura 5.0, 3.9, 2. B, 1.9, 1.7, 1.7, 1.3
Cynoscion nebuloeus 8.5, 4.5, 3.81
Cynoscion regal Is 5.6, 4.3, 4.1, 2.9 Menticirrhus saxatilis 5.4, 3.2, 2.4, 2.0, 1.9, 1.8 Pogonias cromis 3.3, 2.9, 2.7, 2.5, 2.1, 1.3, 1.0
1 J . Gold, Texas A and M, College Station, Tx., personal communication. 34
Figure 1.3.-Size distributions of ail eggs morphologically typed as sciaenids and identified using genetic techniques. 0.7 0 725 0 75 0.775 0 B 0 025 0.850.075 0 9 0.925 0 05 0 975 I ! 025 1 05 1 075 1 1 1 125 Outside Egg Diameter (mm)
M l B chrysoura C regal is P cromis 25
study indicate that samples of eggs of a single site class may represent the products of two or more species. For example, eggs
designated Type I (< O.BO mm) and identified as silver perch by
Joseph et al. (1964} were shown with genetic analysis to contain
eggs of b o t h weakflsh and silver perch. Similarly, eggs designated Type II (> 0.85 mm) and identified as black drum by
Joseph et al. (1964) were shown with rearing and genetic analysis
to contain eggs of both weakfish and black drum.
During the present study, neither hatching experiments nor
genetic analysis identified eggs as black drum that were smaller
than 0.975 mm OED. While temporally limited, the results of this study suggest that the range in size for eggs of black drum
(0.97 5-1.12 5 mm) in lower Chesapeake Bay may be more restricted than those previously reported.
Overlap in egg diameter was also found to occur between
silver perch and weakflsh. Eggs genetically identified as silver
perch ranged in size from 0.650-0.825 mm, in agreement with
previously reported size ranges for silver perch in the
northwestern Gulf of Mexico (0,59 - 0.82 mm, Holt et al. 1988)
and Chesapeake Bay (0.625-0,775 mm, Joseph et al. 1964),
Although Holt et al. (1988) identified eggs of silver perch as
small as 0.590 mm, no sciaenid eggs smaller than 0.650 mm OED
were collected in the present study. Sizes of eggs genetically
identified as weakfish were found to range from 0.82 5-0.975 mm in
diameter. These values are comparable with those reported by
Wisner (1965, 0,84-0,96 mm) but are narrower than the range 2« (0.68-1. IB nun) given by Merriman and Sclar (1952) for Block Island Soundr New York, While the range in sizes for silver perch and weakfish reported in this study agree with past research, overlaps in these ranges preclude the sole use of egg size for identification. Neither Joseph et al. (1964), Olney (1983), nor the present study identified eggs of C. nettulosus or M. saxatilis in samples collected in lower Chesapeake Bay. Fable et al* (1978) described laboratory spawned eggs of C. nebulosus from a single female and reported a mean diameter of 0.77 mm (range 0.70-0.05 mm).
Although based upon a limited sample size, Fable's data indicate that eggs of C, nebulosus could be confused with eggs of B . chrysouraj however, no eggs in our limited sample of this size range (n = 12} were identified as C . nebulosus using mtDNA. A possible explanation for the lack of eggs of C, nebulosus in the present study may be the tendency for adults to spawn in or around vegetated areas (Brown 1981). The absence of eggs of Menticirrhus spp, in this genetic analysis may be explained by our exclusion of eggs with greater than three oil globules.
Additionally, Menticirrhus saxatills reportedly spawns off front beaches and possibly offshore (deSylva 1962), consequently, circulation in the bay may prevent eggs of this species from entering the survey area or they may be transported to areas that were not sampled in our study.
The identification of species-specific restriction fragment patterns for spring-spawning sciaenids is based on the assumption 27 that there is limited intraspecific variation of the diagnostic RPLPs. Recent studies of the population genetics of spotted seatrout, black drum and weakfish (Graves et al. 1992; Gold et al. 1993) indicate that these species exhibit low intraspecific mtdna variability. Furthermore, no variation was found in the RFLP in a survey of mtDNA isolated from 25 adult fl. chrysoura (L. Daniel, unpublished data). Consequently, the common restriction
fragment patterns used to distinguish species in this study were deemed suitable for use in identifications. Variability in egg size distributions with changing salinity and over the spawning season were not examined in this study. Consequently, exact size groupings may only be applicable to the particular salinity regime (19-25 p.p.t.) that 1 sampled.
However, samples were taken throughout peak spawning for black drum and silver perch and may encompass the ranges that may occur for these species in lower Chesapeake Bay. Results of my genetic analysis suggest that identifications of eggs of spring-spawning Sciaenidae in lower Chesapeake Bay based on OED are subject to error. These findings are particularly timely in light of the increased use of fishery- independent assessments of stock size that require precise estimates of egg abundance (egg production method). Because eggs of black drum and weakfish are spatio-temporally coincident and
OEDs overlap, estimates of egg production by black drum in lower Chesapeake Bay may be over-estimated by 50t or greater if identification criteria are based solely on egg size. Likewise, 26 measures of spawning stock biomass will be similarly over estimated, results that could significantly impact management decisions. Comparable biases in estimates of egg production and spawning stock biomass of weakfish could result from egg rois- identifications. However, the more protracted spawning season and greater area of spawning for weakfish in Chesapeake Bay (Olney 198 3) would make these impacts much less severe.
Biochemical techniques are an important tool for the further study of eggs of sciaenids. Genetic analysis has the potential to produce reliable results and permit the storage of samples for
later analysis. Additional studies are needed to survey genetic identifications over the entire spawning season and area to determine if egg sizes change over time or are influenced by seasonal changes in hydrography or age structure of the spawning stock. Finally, the use of genetic techniques, coupled with an extensive examination of morphology could lead to the delineation of other characters that may be useful in separating the eggs of these species. 29
EGO PRODUCTION AMD SPAWNING BIOMASS OF
BLACK DRUM, Pogoniafi cj-omis, IM LOVER CHESAPEAKE BAY
INTRODUCTION
Black drum, pogonias cromis, seasonally migrate along the seaside coast of Virginia and into the Chesapeake Bay. Fish that enter the Bay in the early spring are large {15 - 40 kg) and reproductively active. Richards (1973) examined ovaries of black drum collected in Chesapeake Bay and suggested that spawning began in April when Bay water temperatures reached 15“C, and proceeded until June when water temperatures exceeded 21°C. From
1981 through 1992, an average of 941 (range = 72-99%) of the total, yearly commercial (pound nets and gill nets) landings in
Virginia of black drum were taken during the months April-June
(Figure 2.1), indicating that the spring fishery in Virginia is directed towards a spawning stock. Recent efforts by the
Virginia Marine Resources Commission to protect the black drum fishery include the establishment of a recreational limit of one fish (> 16 inches) per person per day and a total annual commercial quota not to exceed 120,000 pounds.
Few indirect observations of spawning activity of black drum in lower Chesapeake Bay exist in the form of plankton collections of pelagic eggs (Joseph et al. 1964, Cowan et al. 1992). Joseph et al* (1964) collected eggs and reared larvae during cruises in late-May 1962 at stations in lower Chesapeake Bay south of 30
Figure 2■l Total commercial landings versus spring landings (April-June) for black drum during the period 19B1-1992 100 O)(DS(Dlfl^C0Wr oooooooooo ! ( 6 j susol sGuipue-! spuEsnoMl) jo y 31 Kiptopeke. More recently, Cowan et al. (1992) collected eggs of black drum at a limited number of stations off the city of Cape
Charles, Virginia to seed enclosures (mesocosms) for mortality experiments. These investigators provide the only documented occurrence of eggs of black drum in lower Chesapeake Bay.
Larvae of black drum have not been collected in the pelagic environment of Chesapeake Bay (Joseph et al. 1964, Olney 1981,
Olney and Boehlert 1987, Cowan et al. 1992). Numerous faunal surveys in the Bay (Richards and Castagna 1973, Orth and Heck
1900, Cowan and Birdsong 1985} , in which a variety of potential nursery habitats were sampled, have also failed to collect late- larval or juvenile stages of black drum. Collections made during a more recent study (Olney and Daniel 1991) aimed at locating black drum nursery habitat in the summer and fall 1999-1991 in lower Chesapeake Bay and on the seaside of Virginia's eastern shore also was unproductive.
Juvenile black drum are uncommon in Chesapeake Bay
(Hildebrand and Schroeder 1928; Frisbie 1961; Richards and
Castagna 1973). The few fish (n - 350; size range = 22-200 mm in length) collected during the 63-year period between 1928 and 1991 were found in a variety of habitats including shallow, muddy, nutrient rich tidal creeks and high salinity sloughs and embayments. Thirteen large juveniles (140-200 mm SL) were collected by Olney and Daniel (1991) in Nahdua Creek inside
Chesapeake Bay and in Assowoman Cree k on the seaside of the eastern shore. Catch reports by local fisherman of juvenile 32 black drum In pound nets in late-Fall do occur but are sporadic. Frisbie (1961} noted that Chesapeake Bay1 lies near the northern geographic limit of the reproductive range of black drum and suggested that environmental conditions, particularly low temperatures, may severely limit hatching and larval and juvenile survival. Alternatively, seasonal abundance of gelatinous zooplankton predators reportedly peak during the contracted black drum spawning season (Joseph et al. 1964) and may control their recruitment levels in Chesapeake Bay (Cowan et al. 1992}, In this paper, results of a systematic ichthyoplankton survey conducted in lower Chesapeake Bay during April-May 1990 and 1991 are discussed to describe aspects of the spawning ecology of black drum. Specific objectives of the study are to 1) describe the spatio-temporal limits of spawning of black drum in lower Chesapeake Bay and estimate annual egg production; 2} assess egg mortality and diel periodicity of spawning over two 24-h Lagrangian time-series experiments.
KBTHOD0 Eaa Mortality gtudv Field methods- Two Lagrangian time-series studies of mortality of eggs of black drum were conducted in lower Chesapeake Bay in 1990 and 1991 in the area historically considered the black drum spawning grounds. The 1990 experiment was performed approximately three miles northwest of the city of Cape Charles, Virginia (37° 15'N; 76° 04' W). After locating a 33 concentration of fish eggs on Hay 18, the water parcel was marked with 270 L of Rhodamin WT dye deposited at the surface. The dye was tracked visually and with the aid of a fluorometer calibrated to detect dye concentrations. Dye concentrations were flourorcetrlcally measured at 200 p.p.b. at 1120 on 19 Hay, however, by 1645 h, concentrations had fallen to < 1.6 p.p.b and were undetectable after B h, A modified window-shade drogue then was rigged, deployed and followed for the duration of the 2 4-h experiment. Paired plankton samples were obtained with a camera- net system that employed in situ silhouette photography (Olney and Houde 1993). All deployments were 5-min, stepped oblique tows and were conducted within the dye patch or adjacent to the drogue at three hour intervals over a complete tidal cycle. Each camera-net cast yielded a film record and a replicate preserved sample. Only the traditional, preserved sample was used in the analysis. During each camera deployment (n=16), temperature, salinity and depth were recorded at 1 s intervals by a STD (Olney and Houde 1993).
The general location and procedures of the 1991 time-series experiment (20-21 Hay) were identical to those in 1990 except that dye was not used to track the water parcel. Instead, three modified window shade drogues were set at varying depths (1, 3 and 5 m) and followed throughout the 24 hour period. Laboratory methods and data analysis- Eggs of black drum sorted from each preserved sample were staged (Table 2.1) to back-calculate time of spawning and to examine changes in cohort- 34 Table 1.- Criteria for staging field collected, preserved eggs, modified from Moser and Ahlstrom (1985).
Stage Description
1 Newly fertilized egg, cell division has not begun. 2 Gastrula stage, early cell development. 3 Early embryo visible as a streak, notochord becomes visible and there is differentiation of the optic vesicles from the brain.
4 Tip of the tail becomes free from yolk and is broadly rounded.
5 Free tail length is less than one-half the yolk-sac length.
6 Free tail length exceeds one-half yolk-sac length. 35 specific abundance over tine. Time of collection of newly fertilized (Stage I) eggs was recorded for each preserved sample to estimate diel periodicity of spawning. Additionally, time of spawning of all later staged eggs (II-VI) was back-calculated
based on a mean stage duration of 6 h (Johnson 1978; Present study-Table 2.7). Assessment of non-viability vas based upon eggs that had opaque chorions, minimal or no perivitelline space, ruptured yolk or deformed embryos (Olney et al. 1991). Samples of live unidentified sciaenid eggs were sorted from ancillary tows of a 0.5 m Hansen net, and incubated in 1 1 imhoff settling cones. Thirty eggs were p l a c e d in each of six cones, held at ambient sea-surface temperatures (18-19 nc) and aerated using a Whisper 1000 aquarium pump. Cones were emptied, and the contents preserved in 5-8% Formalin after 24, 26, 27, 30, 32 and 3 6 hours to estimate viability and rates of hatching.
Bq q Distribution. Abundance and Seasonal Production Survey methods- Weekly ichthyoplankton cruises (n - 8, 1990; n = 9, 1991) were conducted during the period 9 April to l June 1990 and 1 April to 30 May in 1991. stations (n - 16) were randomly chosen before each cruise from a 3 64-station grid that was divided into 4 areal strata (4 stations each). The strata encompassed the region from 12.8-2 0.9 km from shore and 58 km from South to North, extending from Fisherman Island at the northern base of the Chesapeake Bay Bridge Tunnel to just below
Tangier Island (94K km1, Figure 2,2). During 1990, ancillary 36
Figure 2.2 Ichthyoplankton survey grid 76°10‘ 76°00'
a * 1
. 9 \ SAND SHOAL INLET
SHIP SHOAL IS LA NO
V
05' 37 stations at Ship Shoal Inlet, Sand Shoal Inlet and the eastern side of Fisherman Island, located on the seaside of Virginia's eastern shore, were occupied when time and weather permitted. Samples were obtained with the in situ silhouette photography system, and all deployments were 5 min, stepped oblique tows.
Catches were preserved in buffered formalin (5-81) for later laboratory analysis.
Laboratory Methods and Data Analysis-Preserved plankton samples were whole sorted and examined for occurrence and abundance of sciaenid eggs and larvae. All eggs were measured to the nearest 0.05 mm at 12X using a dissecting microscope. Identifications of eggs were based on outside egg diameter and genetic analysis (Chapter I). Larvae were identified based on melanophore patterns (Ditty 1989). Estimates of egg density were used to estimate strata-specific egg production, determine areas of peak spawning, as well as to estimate seasonal egg production and spawner biomass. Seasonal egg production estimates were determined using a combination of methods from Houde (1977), Kendall and Picquelle (1990) and Olney et al. (1991). Raw counts of eggs of black drum
from preserved samples were standardized to individuals per mJ
for each deployment. Equation 1 describes the method of
calculating the number of eggs per m1 for preserved samples: 38
(1)
where N ■- number of eggs in 4 preserved sample, V - volume filtered (mJ) , and
Z = maximum gear depth. The number of eggs per m1 of sea surface at each of the four stations (including zero values) in a strata with at least one positive station was used to calculate the average density for each stratum. Zero values observed prior to the onset of spawning and in strata where spawning did not occur (ie.- D strata) were not included in production estimates. Values of average egg density then were multiplied by the total number of stations in a strata (A * 96, B = 84, C = 86, D - 99) and the area of a single station (2.59 x 1G6 m2) (Equation 2):
P = f>(At) (A) (2 )
where D = average egg density in a strata, k = number of stations in a strata, and A = area of a single station (2.59 x 10* m2) .
The sum of all strata-specific production values yield an estimate of total production (Pt) for a single cruise. The number of days (D) represented by each cruise was defined as the days of the cruise plus half the days since the last cruise and half the days until the next (Bette and Ahlstrom 1948). The 39 estimate of number of eggs spawned over a cruise period (P,) was derived from equation 3:
where Pt - individual cruise estimate of egg
production, D - number of days represented by each cruise, and
d = egg stage duration (36 hours for
black drum}, The sum of all cruise estimates provided an estimate of total, seasonal egg production for black drum. Estimates of female biomass required to produce the observed number of eggs were obtained by dividing the seasonal egg production by fecundity (eggs/kg). During 1990 and 1991, black drum exhibited a 1:1 sex ratio (Jones and Chittenden, unpublished}; therefore, estimates of population biomass were determined by doubling the female biomass estimate without regards to differences in size between males and females. Variance estimates for each cruise and the total seasonal egg production estimate were calculated using the method of Taft
(I960) as cited in Houde (1977). Since all stations sampled were within the black drum spawning grounds, all samples (including zero values) were included in the estimate of variance. The estimate of the variance in egg abundance under a square meter of sea surface (s1;) was calculated using the method of Cushing 40 (1957) as cited in Houde (1977)♦ The equation for cruise specific variance was:
* A 2 *i? (4)
where SJp = cruise variance estimate,
s,* = the variance estimate for the
number of eggs present under 1 m1 of sea surface for cruise ir
k - the number of stations in cruise i
(1 6 ) , D is defined in Equation (2) , and
A, d are defined in Equation (3), The calculation of atrata-epecific variance utilizes equation 4 with two modifications: first, variance estimates for eggs under 1 a7 of sea surface for cruise i is substituted for the estimate of variance in individual strata; second, the number of stations (k) is equal to 4. Cruise-specific variance estimates are used to calculate variance for total seasonal egg production by:
V = J C S*' (6)
where Sp.1 = variance estimate on the number
of eggs spawned annually, 41 r - the number of cruises upon which the
estimate of annual spawning is based,
and
Sp? “ defined in equation 5, with
confidence intervals {95% C.I.) for the
number of eqgs produced during the spawning season defined by:
Cl (95%) = ± 1,96^3^ (6 }
Sampling was random throughout each stratum during both years. Catches of eggs, however, were not normally, log-normally or contagiously distributed. Nonetheless, Saville (1964) and
Houde (1977) suggested that this method provides a plausible variance estimate. Additionally, these investigators pointed out that this estimate of variance is measured over a cruise period and does not account for variability that may occur on a smaller time scale (ie.-daily changes in spawning intensity or changes in spawning due to hydrographic conditions). Consequently, the error associated with these estimates could be large.
RESULTS
Time of Spawning and Eq q Mortality The 1990 time-series experiment commenced on 19 May at 1000, just prior to the predicted time of slack before ebb, and was terminated Zl min after slack before ebb on the following day. 42 Sunrise occurred at 0549 end 0550 h and sunset occurred at 2005
end 2005 on 19 and 20 May, respectively. Sampling times, depths
sampled, hydrographic data and observed tidal stage are summarized in Table 2.2. Tracking of the dye patch, and subsequently the drogues during the 2 4-h period resulted in a cruise track that covered approximately 15.B km (Figure 3.3).
The maximum distance between any two stations was 5.4 km, and occurred from Time 0 to Time 3 during the predicted maximum ebb current (1,8 km/hr). There was no sample between Time 15 and Time 21,
In 1991, sampling began at 0715 h on 24 Hay (approximately
1-h prior to the predicted time of slack before ebb) and terminated at 0820 h 21 Hay, just over 3.5 h after slack before ebb. Times of sunrise and sunset were 0530 and 1958, respectively, on 20 Hay 1991. Sampling times, depths sampled, hydrographic data and observed tidal stage are summarized in
Table 2.2. Tracking of the drogues over the 24-h period resulted
in transport paths similar to those observed in 1990 (Figure
2.4). Predicted maximum current velocities were greater in 1991, however, and may have accounted for the greater distance travelled during the 1991 experiment (28.4 km). The maximum distance between any two stations was 6,7 km, and occurred during the predicted maximum flood current that occurred between Time 6 and Time 9 (2.2 km/hr).
Average temperature was similar during the two experiments
(1990 - 19.5 C; 1991 = 19.0 C) , however, salinities differed 43 Table 2.2.- Collection, time, egg densities {eggs/lOOm*) , percent dead and physical data for collections of black drum eggs during the 1990 and 1991 time-series experiments.
Collection Time Density Dead Temp (C) Sal (Voo)
13-14 Kay 199 0
Time 0 1000 295 66 19.4 15.5
Time 3 1313 15 12 20. 0 14 . 9
Time 6 1615 139 10 19.4 18. 2
Time 9 1925 252 8 19. 4 18.8
Time 12 2226 523 10 19.4 17.7
Time 15 0054 174 8 19.8 17 . 0
Time 21 0739 280 9 19.7 16.1 Time 24 1111 511 30 18.3 17. 1
23 -2 4 Kay 1991
Time 0 0715 66 8 18.0 25.2
Time 3 1030 78 12 18.3 25.0
Time 6 1350 56 13 18.4 26. 4
Time 9 1645 19 9 19.0 24. 6
Time 12 2000 7 0 19.4 23.9
Time 15 2300 8 0 19.4 22 .2
Time 18 0200 11 0 19.9 18.7
Time 21 0500 31 0 19.3 21.5
Time 24 0820 43 4 19.3 21. 1 44
Figure 2.3 Dye patch and/or drogue positions mapped during the 1990 Lagrangian tirae-series experiments. Diamonds - samples taken during ebb tide; Circles - samples taken during flood tide< $ 1 20 45
Figure 2.4 Drogue positions mapped during the 1991 Lagrangian time-series experiments. Diamonds = samples taken during ebb tide; Circles = samples taken during flood tide.
46 markedly (Table 2.3). Temperature-salinity (T-S) plots representing all samples made during the 1990 experiment (Figure
2.5} indicate that the water column was thoroughly mixed. Water temperatures ranged from 1S.2-22.1°C with the upper 2 meters being warmest. Salinities ranged from 16.1-16.9 °/w at the surface and increased to 19.1-2 2.3 °/aa at the bottom. Depth range of a distinct halocline occurred from 5-7 meters at all stations but was generally shallower during flood tide.
Examination of a T-S plot for samples taken during 1991
(Figure 2.6} also indicated that the water column was thoroughly mixed. Water temperatures ranged from 16.0-20.2 °C at the surface and from IB-19,6"c at the bottom. Bottom salinities were similar during all samples (range * 23.5-29,6 "/«,), however, surface salinities differed between the first six samples and the last three (Figure 2.7), Surface salinities from Time 0 to Time
15 ranged from 19.3-24,3 V,*,, while surface salinities during the remainder of the experiment ranged from 16.3-17.0 7«i suggesting either an influx of fresher, upper Bay water into the sample area or an inability to effectively sample the same water parcel. A consistent halocline depth range could not be identified in 1991, possibly a result of the greater tidal velocity during the 1991 experiment.
Newly fertilized and early developing, stage I and Stage li, eggs occurred at all times of day during the 1990 time-eeries experiment with the exception of Time 12 (1925 h, Table 2.3).
Back-calculated times of spawning also were determined from older 47 Table 2.3.-Numbers, percent non-viable and totals of black drum eggs in each developmental stage during the 1990 time- aeries experiment, 19-20 Hay.
Time I II III IV V VI Non-viable Total
1000 0 47 0 1 1 2 66 117
1313 2 0 1 0 1 3 12 19 1615 0 1 54 2 3 0 10 70
1925 0 0 155 2 1 4 8 170
2226 0 37 7 237 0 3 10 294
0054 38 0 0 92 0 0 8 138
0739 0 73 0 0 56 15 9 153
1111 0 90 0 0 13 83 29 215
Total 40 248 217 334 75 110 152 1176 4a
Figure 2.5 Temperature-salinity plot of all stations sampled during the 1990 Lagrangian time-series experiment. CM CM h-
CM CM CM h- * O CM CM ■ O
CM oi
00 Salinity
CM CO
CO
CM CD
O O ID O CM CM
ajruBjaduuai 49
Figure 2.6 Temperature-salinity plot of all stations sampled during the 1990 Lagrangian time-series experiment. ejniejaduuai so
Figure 2*7 Temperature-salinity plot of stations sampled from Time 0-Time 15 (squares) and from Time 18-Time 24 (crosses) during the 1991 Lagrangian time-series experiment* f SALINITY
LO CM
aanivaadwai 51 eggs. Percent frequency of back-calculated spawning dates
(Figure 2,8} indicate that spawning occurred throughout the day, but was most intense from 0100 h to 0700 h. The observation of protracted daily spawning precluded the identification of a single, daily cohort based on egg staging criteria. The oldest egg collected during the 1990 experiment was estimated to have been spawned at 0400 on 18 Hay while the youngest egg was spawned at 0500 on 20 May. Based on back-calculated spawning times, 18 separate cohorts corresponding to the hour spawned could be identified.
Newly-spawned (Stage 1 and II] eggs also were found to occur throughout the day during the 1991 time-series experiment with the exception of Time 12 and 15 at 1645 and 2000 h (Table 2.4).
The oldest egg collected during the 1991 experiment was estimated to have been spawned at 0700 on 19 May and the youngest egg was spawned at 0800 on 21 May. While back-calculated times of spawning and frequencies throughout the day were similar to 1990, the number of cohorts based on hourly peaks was greater (n = 20]
(Figure 2.9), Time of greatest intensity of spawning was also similar between years (0100-0800 h) .
Black drum e g g s (n * 1,17 6) occurred in all samples (n = 8) taken during the 1990 experiment with densities ranging from 0,2 to 3.9 m J. Because cohort identification was not possible using staging criteria, densities of eggs spawned only on 19 Hay and followed throughout the experiment were used to provide an estimate of mortality based on the 1990 data (Table 2.5). While 52
Table 2.4 Numbers, percent non-viable and totals of black drum eggs in each developmental stage during the 1991 Lagrangian time-series experiment, 20-21 May.
Time 1 II III IV V VI Nonviable Total
0715 9 11 0 1 16 0 7 44
1030 0 23 0 0 6 0 12 41
1350 0 2 25 1 0 5 13 46
1645 0 0 4 4 2 0 9 19
2 0 0 0 0 0 0 4 0 2 0 6
2300 0 1 0 3 1 0 0 5
0200 0 1 0 2 4 0 0 7
0500 13 1 0 5 2 0 0 21
0820 1 13 0 0 7 1 4 26
Total 23 52 29 20 38 e 45 215 S3
Table 2.5 Densities of eggs (eggs/m1) over back-calculated time of spawning on 19 May and followed throughout the 24 hour experiment in 1990.
Time I II III IV V VI Total
1000 0 Q,9Z 0 0 0 0 0*92 0400 1313 Q ,04 0 0,02 0 0 O 0. 05 1313 0113 1615 0 9,02 1,11 Q,P4 0 0 1. 2 1015 0415 0015 1925 0 0 3,39 0,02 0 0 2 . 25 0725 0125 2226 0 O ld 9.12 0 0 4 * 05 1626 1026 0426 0054 0 0 0 1,14 0 0 1. 13 0654 0739 0 0 O 0 9,??. 0.27 1. 26 0739 0139 1111 0 0 0 0 Q.Z5 1,61 1.87 1111 0511 54
Figure 2 .8 Back-calculated times of spawning fur all black drum eggs collected during the 1990 Lagrangian time-series experiment. Times are from 0400, IB May to 0500, ao May. h 0050 I- OOfrO - OOEO - 0030 - 00 LQ - 00*3 - OOE3 - 0033 - 0013 - 0003 -0061. - 000 L - 0011 - 0091 - QOS I - OO H - OC O - 0031 - 00 u - 0001 - 0060
- 0000 - oozo -0090 -0050 -00*0
-OOEO Time -0030 - OOLO — 00*3 — OOE3 — 0033 -0013 -0003 - 0061. -OOBl - o o a - 009 L -OOSl -OOH -ooei -0031 - o o n - ooo t -0060 - 0090 -oozo - 0000 - 0050 -00*0 in cm in in o oi ^ o
(elu) pauM eds S663 \o s a w s u e Q 55
Figure 2.9 Back-calculated times of spawning for all black drum eggs collected during the 1991 Lagrangian time-series experiment. Times are from 0700, 19 May to 0800, 21 Kay. - 0090 - 00Z0 - 0090 -ooso - OGfrO -OOEO - OOSO - 0010 00fr3 DOES DOES OOtS 0003 - 0061. - 009 L OQZl 0091 OOSL OOH OOGL 003 L 00 U 0001 0060 0090 00/0
0090 Tim e 0090 00*0 0000 0030 OOtO 00*3 00£3 0033 001^3 0003 0061 0091 00/1 0091 0091 o o n ooei 003 L oou 0001 0060 0090 00/0
^J-OOCOCJ CMt- t-O o o o o o o d
(e l u ) pauMeds sBBg jo sawsuea 56 members of several cohorts are represented in Table 2.5, eggs
spawned at 0400 and 0700 could be followed for at least 4 sample periods (12 h). Mortality estimates based on temporal changes in densities of eggs in the 0400 cohort was impractical because densities of eggs increased during the first three samples
(Figure 2.10), Density of the 0700 cohort, however, declined
from 1925 on 19 May {2.2 m*) to 0739 on 20 May (0.99 m J) corresponding to an instantaneous rate of mortality of 0.03© h 1.
Black drum eggs occurred in all samples in 1991, however,
total numbers {n = 215) and densities were lower (range ~ 0.10 to 0.54 m 1) than 1990. Examination of eggs spawned on 20 May revealed two cohorts that could be followed throughout most or all of the 24 h experiment (Table 2.6). Eggs spawned between 0115 and 0220 (0100 cohort) could be followed from 0715 on 20 May to 0820 on 21 May while eggs spawned from 0430-0500 (0400 cohort) could be followed from 1030 on 20 Kay to 0500 on 21 May.
Examination of decline in density over time {Figure 2.11) for the
0100 cohort showed an increase in egg density from 0715 (0.2 tnJ) to 1350 (0.4 m J) followed by a steady decrease until the end of the experiment at 0820 (0.02 raJ) {z = 0.16 h 1). The cohort
identified from eggs spawned during 0400 h showed a relatively steady decline in abundance from 1030 (0.54 m 1) to 0500 (0.03 m s)
(z - 0.15 h ') .
Twenty percent of all black drum eggs collected during the
1990 experiment were classified as nonviable (range 0 to 66 1}
(Table 2,4). Nonviable eggs comprised only 8% (range 0 to 13%) 57 Table 2.6 Densities of eggs (eggs/100m1) over back-calculated time of spawning on 20 May and followed throughout the 24 hour experiment in 1991.
Time I II III IV V VI Total 0715 9-15 0-1? 0 0 0 0 0. 34 0715 0115 1010 0 0-54 0 0 0 0 0. 54 0430 1350 0 0,03 0,3$ 0 0 0 0 . 40 0750 0150 1645 0 0 0, 07 0 0 0 0. 07 0445 2000 0 0 0 9,00 0 0 0.06 0200 2300 0 0 0 0,0$ 0-02 0 0 .08 0500 2300 0200 0 0 0 0-03 Q t Q6 0 0.09 2 000 0200 0500 0 0 0 0,97 9 - 0 3 0 0. 10 1100 0500 | 0820 0 0 0 0 0 - 0 1 0,02 0. 03 0820 0220 58
Figure !. 10 Densities of black drum eggs in the 04 00 and 0700 cohort followed during the 1990 Lagrangian time- series experiment. EGG DENSITY (m3) 0.5 2.5 1.5 0 I 3 I 1 1000 1925 • ■ v- S- • : Cs • i-' .V V . x ■%’. 1V v . -v I -:1 ^ ' 1615 0400 t . . 70 COHORT 0700 P M I T COHORT R O H O C 54 f.w ; 2226 739 1111 59
Figure S. 11 Densities of black, drum eggs in the 0100 and 0400 cohort followed during the 1991 Lagrangian time* series experiment. EGG DENSITY (m3) 0.1 0.3 0.4 . I 0.2 0.6 0.5 0.2 0.1 0.3 0.4 0 ! |
\ I i i i j I ■ 'NV- :'.NSV ■■ 715 1030 1350 0100 COHORT 0400 1645 C OHORT R O H O C TIME 2000 2300 200 500 820 CO of the total in the 1991 experiment (Table 3,5). Hatching experiments conducted throughout the 1991 spawning season revealed similar findings, however, percentages of nonviable eggs were never greater than 151 (Table 2.7).
Laboratory experiments-Eggs and larvae held in Imhoff settling cones to determine hatching rates and egg viability, were sorted by size and examined in the laboratory. Two of eight cones were seeded with one species, p, cromis, while the other six contained mixed eggs of the sciaenids (P, cromis, Qairdi&lla chryaoura and Cynosclon regal Is). The percentage of recovery of eggs from the cones averaged 94% (range ■= 77 - 100%). While the rate of hatching during the first 24 to 27 hours was low (0 to
37%), 50 to 100% had hatched by 36 h. One cone seeded with 30 black drum eggs had loot hatch after 36 hours at IS.5-19.0 "C.
Examination of all unhatched eggs indicated that most were viable and probably would have hatched with time. The average percentage of eggs that were viable during the experiment was 93%
(range = 85 to 100%) (Table 2.7).
Eq q and Larval Distributions and Seasonal Eva Production 1990 Egg Distribution and Cruise-Specific Production-Newly spawned eggs of black drum were observed on 1 May 1990 (Cruise 4) and were still present during the last cruise (9} on 31 May.
During the eight cruises in April and May 1990, 128 stations representing 107 locations were sampled (some stations were sampled more than once, Figure 2.12). Average water temperature during the spawning season ranged from 16.1°C-18 . 9°c. From Cruise 61
Table 2.7 Results of hatching experiments conducted! on 2 0 Hay 1991 on eggs of black drum to determine rates of hatching and viability.
Time Held Total Number Percent Hatched Percent Viable 24h 29 12 100
26h 25 20 85 Z7h 10 20 90
30h 36 B5 100
32h 6 05 100
36h 30 100 100 63
Figure 1,12 Ichthyoplankton survey grid. Triangles represent stations occupied during eight surveys, April-May 1990. Soma stations were occupied more than once. 76° 10' 76" 00' 37"
OX CR
MATT A WOMAN CR
* n SAND SHOAL INLET
_ ( ) SHIP SHOAL ISLAND • r ^ J . 63
4 through Cruise 9, 74% (n * 59) of ell samples (n = BO) contained eggs of black drum (Figure 2.13). station specific estimates of abundance ranged from 0.01-37.9 eggs/m7 with the maximum value observed in strata A on 1 Kay (Table 2.8). Eggs of black drum were collected throughout the survey area in 1990
(Figure 2.14). From 1 May to 15 May 1990, however, greater than 80% of all eggs were collected in the area off Cape Charles City,
Virginia in stratum A and B (Table 2.B), Alternatively, few eggs were collected in the lower half of the A strata or the upper D strata. During cruises fl and 9, egg abundance was less localized with production in strata C and D greater than or equal to production in strata A and B. Although egg production had markedly declined by the last cruise, I was unable to document the time of cessation of spawning.
Cruise-specific estimates of egg production calculated for the 1990 spawning season (Table 2.9) ranged from 0,20-2,01 x 10l{).
Peaks in egg production occurred on 1 and 15 May (Figure 2.15).
Examination of the lunar cycle indicated that egg abundance was highest during cruises that occurred during ha If-moon phases.
1991 Egg Distribution and Cruise~specific Product!on-During nine cruises in 1991, 144 stations representing 123 locations were sampled (some stations were sampled more than once. Figure
2.16). Black drum eggs first occurred in samples taken during
Cruise 5 on 29 April and occurred in the last samples during
Cruise 10 on 29 May. Average water temperature during the spawning season ranged from 14.4-23.2°C, Throughout the spawning Table 2.8.-Stratum-specific mean abundance, ranges of abundance (eggs\m2) , with production values (x 10*), 95* confidence intervals {x 10*) and strata-specific percentages
of the total cruise productioni estimate for black drum in 1990. 4# M '— ft ft ft U) a u u S (H o S3 B H o z w u a ft m w Q M ul E 1 u h m in ID < < m u o m 04 01 O 'r~O' v r i c 1COO D I O' O C 01 m o c £ >* □ a rt t ■■ ■ » ...... ■ > ■ . * m H r H r Ht n o m <0 n a o O 4 0 1 . n i Q O o o o o o 4 0 o o o o O O H r a H r H r 1 1 • ■ ■ » * n 1 0 o a 4 0 o O' 01 o CO o H r I . . . ■ * * > * IN o ' f Q oi n O □ o Noi fN o o H a ID fN rH rH 01 DO 1 1 * * f * o n 01 f i D I a CO 4 0 cq n a o 43 rH ■■ . . . ■ * 4 ■ « u ' O n o o o 04 n o H r f" o o o o * » ■ o Q n o o O C q o o o o 404 04 o rH H r o rH o 04 o i 1 i . ■ * ■ » * c c «cPd m a u u o m H r H r o oi o H r t ' Hf m H r H r s >* H •t H in * » o o o n i 1 0 1 0 o o 4 0 cn o O O C n 00 O C a IN I . * w » r ~ r O n crv o o o o in 101 0 01 n n O J O a o Ho□ o rH 1 ■ . * ■ » r O a3 o H r o o o 04 o l I . . + ■ » . ■ . o o o 1 0 o O Q O on m m n 3 o IN 1 J ) i r~ H r H r ' O n o o « t ' H r o □ fN n n n H r o o + + i ■ . ■ 0 < i P I' o o O' 01 m O 01 r~ H * 1 n i 4 0 D < n n - m ■ f i o r-r m 4 0 H r □ H O C
Table 2.9,-summary of cruisa-specific, weekly egg production (x 10ID) and variance estimates (x 101*) , survey area hydrography and parameters used to calculate seasonal production estimates during 1990*
CRUISE DATE DAYS PRODUCTION VARIANCE TEMP. SAL. 4 1 MAY 7.5 2.67 6. 58 15.5-16.7 17.6-21.8
5 B MAY 7.0 0.24 0.72 16.9-17.6 16.1-19.7
6 15 MAY 8.0 2. 15 5. 87 17.3-18.3 17.6-22.0
S 24 MAY 7.5 2 . 86 2. 18 17.0-10.6 17.7-25.1
9 31 MAY 3. 5 0.36 0.42 18.4-19.1 17.7-21.8 C6
Figure *13 Frequency distribution of egg catches made during the 1990 ichthyoplankton survey. Aouanbajj tueajsd «7
Figure 2-14 Stations sampled during 1990 cruises that contained eggs of black drum. CRUISE a CRUISE 9 25 MAY 1 JU N <8
Figure .15 Cruise-specific estimates of egg production versus temperature (°C) during 1990 cruises. Temperature (C) o w o CM t- i- LO o Cruise Cruise Date 1 May May 1 8 May May 15 24 May 31 May
o
(CHOI) uoi.pnpojd 66g 69
Figure t.16 Ichthyoplankton survey grid* Triangles are stations occupied during eight surveys, apri1-May 1991. Some stations were occupied more than once, 76° 10' 76° 00' 37
27.5
MATTAWOMAN CR
SAND SHOAL INLET
CAPE ChARL ES
SHIP SHOAL ISLAND 70 season, 60% (n - 4B) of all samples (r> =■ BO) contained eggs of black drum (Figure 2.17). station-specific estimates of abundance ranged from 0.01-15.B eggs/m1 with the maximum value observed in strata B on 14 Hay (Table 2.10). Similar to 1990, few eggs were collected in strata D or in the extreme lower A strata. Figure 2.18 depicts all stations sampled during the 1991 spawning season where eggs of black drum were collected. While eggs of black drum also were collected throughout the survey area in 1991 65% of all production during 1991 was observed in the A and B strata. Cruise estimates of production in 1991 ranged from
0.07-1.28 x 10IU (Table 2.11). Peak egg production in 1991 occurred on 9 May (Figure 2.19). Annual Egg Production and Spavner Biomass-Total egg production estimated during the 1990 spawning season was 5.2 x ID10 + 2,5 x 10*. Based upon fecundity values reported from the
Gulf of Mexico (Hieland and Wilson 1991; Fitzhugh et al. 1993), the female biomass required to produce the observed number of eggs was 38,731 kg. Total egg production for the 1991 season
(3.70 x 10,fl ± 2.4 x 10*) translates into a female biomass of
2 5,482 kg (Table 2.12). Examination of 95% confidence intervals indicates that there was a significant difference in egg production between years. Additional Coiiections-During several cruises in 1990, three fixed stations in seaside inlets (Sand Shoal and Ship Shoal
Inlets) and off Fisherman Island were sampled. Black drum eggs were identified in some of these collections. Eggs first ■># m n n r ' O fM o CD 1—1 c0 m D n O r - * ■ ■ I * ■ 1 * » 1 * f * ■ l ■ f f 1 in n r t n r-l in ID C"1 o IO M fM in H CM i—i in (Ji in n n n IN 119 rH
1/1 in fM
Table 2.11.-Summary of cruise-specific, weekly egg production (x 1010) and variance estimates (x 10u) , survey area hydrography and parameters used to calculate seasonal production estimates during 1991.
CRUISE DATE DAYS PRODUCTION VARIANCE TEMP. SAL.
5 29 APR a. 5 1. 21 5.61 14*4-15.0 19.3-27.5 6 9 MAY 7.5 1.70 3.31 17.0-18.0 16,3-24.3
7 14 MAY 6,0 1.45 5. 17 18.6-19.0 16.0-25.7
9 22 MAY 7.5 0.09 0. 44 19.1-19.7 19.7-26.0
10 28 MAY 3 . 5 0.46 0. 36 21.3-23,2 17.9-24.2 73 Table 3.12.-Data summary for 1990 and 1991 egg production and population biomass estimates of black drum in lower Chesapeake Bay.
YEAR 1990 1991 TOTAL DAYS 33 .5 33.0 KEAN DENSITY 2 . 34 1 . GO TOTAL AREA 587 km* 587 km1 MEAN WEIGHT 25 kg 2 5 kg SPAWNING FREQUENCY* 3,5 d 3.5 d BATCH FECUNDITY** 3*5 x 10* 3-5 x 10s TOTAL FECUNDITY*** 3*3 x 101 3-3 x 10T EGGS/KILOGRAM 1.3 X 10e 1.3 x 10* TOTAL PRODUCTION 5,2 x 1 0 10 3.7 x 10to POPULATION BIOMASS 77,563 kq 56,061 kg I TOTAL VARIANCE 1.6 x 1016 1.5 X 10J* 95% CONFIDENCE 2*5 x ID8 2.4 x 10* INTERVALS
** - Calculated from Nieland and Wilson (1993) regression
equation- 128,684 x Total Weight (kg) + 257,588.
*** Batch Fecundity x Frequency 74
Figure 2*17 Frequency distribution of egg catches made during the 1991 ichthyoplankton survey. Egg Egg Density
AouanbaJd luaaiad 75
Figure 2.18 Stations sampled during 1991 cruises that contained eggs of black drum. UJ5
o--. ,-’> i '-'M • * I ' V ' V !
fi' *^:0vA.ut .j p ^ ■■ V ; ■c -a * ■ - * ♦ . -\ ♦ ♦* ^ ♦ * ♦ * ! IC ulLU <> n <*> o
'■'s * V L t • !*,! -4. -1 \i. :, if .IS; H-V, }• MAY 29 APR
v. 22 CRUISE 9 CRUISE 10
' ♦ *
=3 < £E (Ji ( J r\l 7<
Figure .19 Cruise-specific estimates of egg production versus temperature (°C) during 1991 cruises. TEMPERATURE (eC) CRUISE DATE CRUISE
(okh) NOJionaoud 0 0 3 77 appeared in samples on 24 April 1990, earlier than observed in Bay collections. While this finding indicates that some spawning probably occurs on the seaside of the eastern shore, the magnitude of spawning and the size of the spawning stock is presently unknown.
-Distribution and Abundance of -Larvae-*Densities of pre flexion and flexion larvae identified as black drum in 1990 (n - 13) and 1991 (n - 3) ranged from 0.02 - 0.3 mJ; however, there were many additional larvae (yolk-sac) within samples that could not be identified to species. Thus, actual densities of larvae of black drum may have been higher. Specimens that were identified to species ranged in size from 2.2 - 5.1 mm NL.
Larger specimens of other sciaenid larvae that were identifiable to species were limited to silver perch, Balrdiella chrysourtt and weakfish, cynoscion regal is. All sciaenid larvae were collected within the A and B strata during both 1990 and 1991 ichthyoplankton cruises.
DISCUSSION
Black drum reportedly spawn from March-Way in Chesapeake Bay
(Frisbie 1961; Joseph et al. 1964) when surface water temperatures are 14.0-20.2 (Richards 1973). Although our collections did not begin until 1 April during both years, we saw no evidence of spawning until late April (1990 Eastern Shore;
1991 lower Chesapeake Bay) or early Hay (1990 and 1991 lower
Chesapeake Bay). Although Joseph et al, (1964) collected no eggs 70 of black drum in early June, the collection of eggs in late Hay during the present study suggest that spawning may continue into June during some years.
Data from the present study concerning the spatial aspects of spawning of black drum are consistent with the observations of
Joseph et al. (1964) who collected eggs of black drum both in the
Bay, just north of Kiptopeke Beach, and seaside in the barrier island inlets. Joseph et al. (1964), however, curtailed sampling north of the city of Cape Charles due to the high concentrations of ctenophores that made sampling impractical. These authors did not calculate egg densities from their samples, but they did report collecting a maximum number of about 2,000 eggs in a 5 min tow of a 1-m net. More recently, Cowan et al. (1992) found highest densities of eggs of black drum during the period 20-29
May when densities averaged 3.2/m), Their highest single-day abundance was 6.4 eggs/m3 (23 May 1990).
The accurate delineation of the black drum spawning grounds is crucial to obtaining an estimate of egg production and spawning stock size, it is likely that the survey area effectively encompassed the spawning grounds of black drum since overall egg abundances were lowest in the southern half of stratum A and in all of stratum D. Large black drum do occur elsewhere in Chesapeake Bay during spring and may be abundant in the Maryland portion of the bay (Maryland Department of Natural
Resources, technical report). However, direct evidence of spawning activity is lacking. Therefore, based on these results 79 And the absence of additional published reports of eggs in the plankton, black drum probably do not spawn elsewhere within Chesapeake Bay. The collection of black drum in gill nets on the seaside eastern shore is well documented (Virginia Marine
Resources Commission statistics department; Babko 1991). Local fisherman indicate that these fish arrive 2-3 weeks prior to their occurrence in the Bay and are gravid. Black drum eggs collected during 1990 at seaside inlets provide the first direct evidence of spawning on the seaside eastern shore. however, the size of the seaside population, the spatio-temporal extent of their spawning activity and the degree to which seaside and bayside epawners may mix is presently unknown. Results of the two Lagrangian time-series experiments indicate that black drum egg production is highest during a protracted period from midnight to mid-morning but may occur throughout the day. These findings differ from studies in the Gulf of Mexico (Holt et al. 1985; Saucier and Baltz 1992; Fitzhugh et al. 1993) that suggested that black drum spawn at dusk (1600 - 2000). In Chesapeake Bay, spawning activity of black drum apparently was minimal at dusk. While Holt et al. (19B5) staged field caught eggs to arrive at their estimates, Fitzhugh et al. (1993) relied on the presence of post-ovulatory follicles (POFs) to assign spawning times. Post-ovulatory follicles are identifiable in histological sections for 24-48 h or longer depending on sea-water temperature (Hunter and Macewicz 1985; Hunter et al. 1986). Fitzhugh et al, (1993) suggested that black drum POFs could persist for 32 h at 19-22°C. Consequently, assigning times of spawning times based upon the presence and ages of POF's may be misleading. so Saucier and Baltz (1992} utilized hydrophones and recorded drumming sounds to estimate time of spawning. Whether all black drum spawning activity is accompanied by sound production is unknown. Nonetheless, notable differences apparently exist between the dial periodicity of spawning of black drum in Chesapeake Bay and the Gulf of Mexico. While these differences may simply be regional, limited temporal coverage in sampling (sunset + 3h) by Holt et al. (1985) and Saucier and Baltz (1992) could explain their failure to collect early staged eggs or locate drumming aggregations during other times of the day. The observation that spawning of black drum occurs throughout the day may have prevented the identification of daily cohorts that could be followed throughout the experiment. With a priori knowledge of daily spawning periodicity, a more precise egg ageing/staging criteria could improve the accuracy of identifying distinct cohorts, Nonetheless, hourly cohorts, defined from back-calculated spawning times, could be followed for some part of each experiment and enabled the calculation of rough estimates of egg mortality. Unfortunately, densities of eggs rarely exhibited a steady decline over the two experiments. While one explanation for this event may be patchiness of eggs in the plankton {McGurk 1984) , the observation that the water column was well-mixed suggests that both advection and dispersion of eggs into and out of the sample area was possible during both years (ie.-McGurk 1986, 1989, Kim and Bang 1990, Davis et al. 1991). During the 1990 and 1991 experiments, individual cohort mortalities ranged from 55% after 12 h in 1990 to 95% for cohorts followed for 19 and 25 h in 1991. Consequently, 80-98% of an individual cohort (i.e.- 40 h incubation time) may be removed during the egg stage. Furthermore, if these rates are similar during the yolk-sac stage, survival to first-feeding is potentially very low. Other estimates of mortality of eggs reported for fishes who spawn pelagic eggs are rather consistent 01 between 60 and 801 d 1 but may be as high as 95% d 1, Olney et al. (1990) used an average dally value of 68% for their egg production estimates of striped bass, Morons saxatilis t in Virginia rivers. Recently, Doreey (1993) estimated that bay anchovy (Anchoa jaltchllli) die at 90-95% d 1 in the mesohaline portion of Chesapeake Bay. Frisbie (1961) suggested that the low and variable recruitment of black drum in Chesapeake Bay could be attributable to potentially lethal environmental conditions to eggs and larvae at the northern extent of the spawning range of black drum. However, hatching experiments conducted at sea during the 1991 time-series experiment produced estimates of 85-100% viability after 24-3 6 h suggesting that eggs could have hatched with time. Cowan et al. (1992), who also examined black drum egg viability, reported that 50% of the eggs used in their meeocosm experiments during 1990 were viable, attributing their low hatchability to the handling procedures necessary in correctly stocking their enclosures. There are no published records of larvae of black drum collected in Chesapeake Bay. Cowan et al. (1992) hypothesized that poor recruitment of black drum in Chesapeake Bay results from their temporally short and spatially limited spawning season that overlaps with abundance peaks of gelatinous, zooplankton predators. Results of enclosure experiments in the Bay by Cowan et al. (1992) indicate that in the absence of predators, nearly 7% of larvae can survive to 10 days post hatch at ambient zooplankton prey concentrations (200 prey l'1) . our results suggest that viability of eggs is high and survival of larvae to first-feeding stages does occur in the presence of gelatinous predators in Chesapeake Bay, however, the numbers of survivors is probably very low. Generally, egg production surveys sample over an expansive area and contain numerous zero values (ie.-Pennington and Berrien 1985, Kendall and Picquelle 1990) which require special treatment 82 (ie.-Delta distribution, Pennington and Berrien 19B5)„ Ichthyoplankton samples taken during the black drum spawning seasons In 1990 and 1991, however, contained relatively few zero values. Consequently, when zero values occurred, they were treated equally to positive stations in all calculations of strata-speciflc abundance. Between 1990 and 1991, egg production values declined. Calculated values of daily egg production of black drum in Chesapeake Bay (1990, 4.32 x 101 - 4.03 x IQ’; 1991, 1.44 x 10* - 2.56 x 10y) probably are conservative because densities of eggs are reported at the time of collection (N,) rather than an estimate of the number of eggs present at the time of spawning adjusted for mortality (NJ . Based upon daily mortality rates of from 60-97% d 1, an estimated time of spawning of 0600 h and an average time of collection of 1200 h, estimates of strata- specif ic egg production could be calculated using N0 and would be 20-4 5% greater than those reported. At any rate, estimates of egg production for black drum are also low when compared to other species whose daily egg production values have been calculated to be several orders of magnitude higher than black drum (e.g., Houde 1977; Berrien et al. 19B1; Kendall and Picquelle 1992). These estimates, however, are for pelagic fishes with large stock size and spawning biomass. Based on seasonal biomass estimates and the average weight of fish in the stock (27.27 kg), a conservative estimate of the spawning population of black drum in 1990 and 1991 was 2,B44 and 2,055 individuals respectively. Comparisons of commercial landings during the 1990 and 1991 spawning season (Aprll-June) and estimates of spawning stock size indicate that 4 5% and 861 of the spawning stock were caught during 1990 and 1991. Estimates of variance in egg production studies are inherently large as a result of the expansive areas sampled and the probability of a high percentage of zero values due to plankton patchiness. Notwithstanding these intrinsic biases, 83 variance estimates for black drum during 1990 and 1991 do not take into account all the possible sources of variability in the samples. Other sources of variability may include seasonal changes in egg mortality, daily variability in spawning intensity, and weekly variations in the areas of spawning intensity. Furthermore, variability in egg size over the spawning season could create identification problems for samples identified using egg diameter analysis (Chapter I) . Although these first order estimates of population size should be viewed with caution, they do indicate that the spawning stock in lower Chesapeake Bay is relatively small when compared to other closely related species who generally realize seasonally strong recruitment (ie.-apot, Leiostomus ranthurus and weakfish, Cynoscian regalis). Additionally, reported landings estimates appear meager when compared to other commercially harvested fishes in Chesapeake Bay. For example, reported commercial landings of spot and weakfish, Cynoscion regalis, in L99Q and 1991 were from 0.7-1.1 million kg compared to 37-49,ooO kg for black drum during those years (Virginia Marine Resources Commission; 1991 Annual Summary). The spatio-temporally limited spawning season of black drum, combined with a comparatively small spawning stock size, high rates of egg mortality and fishing pressure suggests that the overall contribution of Chesapeake Bay black drum to the mid- Atlantic stock is minimal. Consequently, it is also unlikely that the Chesapeake Bay black drum represent a self-sustaining population. Calculations made on a realistic, seasonal estimate of egg production (1 x 1010) using conservative estimates of mortality (egg stage to first feeding larvae (Z = 0.90 d 1), first feeding larvae to juveniles (Z = 0.50 d 1), juvenile to fall emigration (Z = 0.03 d l) , fall emigration to maturity at age V (0.15 yr1), and age V to bay population median age of XXV (Z = 0.15 yr1)} results in the return of eight (B) age XXV black drum from a single seasons spawning event. Subsequently, the B4
mechanisms by which this population maintains Its relatively stable size remains to be identified. As most of the largest individual sciaenld species (i.e., red drum, Scla&nops ocellatus; spotted seatrout, Cyr>o&don n&bulosue; spot, Leiostomus xanthurus; croaker, Hicropogonias undulatus and weakfish) occur towards the northern extent of their range, it is likely that the Bay population of black drum is a mixed stock that is recruited into by the oldest and largest fish along the mid-Atlantic region during offshore, overwintering migrations. While further studies are needed to determine the extent of spawning on the seaside of Virginia's Eastern Shore and the fidelity of these large fish to the Chesapeake Bay on an inter-annual basis, these results raise interesting management questions in regards to the typical, interjurisdictional management schemes of the Atlantic States Marine Fisheries Council. as
Bp*tio-tem poral »bundt&oa And distribution of gelatinous ioopl«nkton pr*dit«ra and their potential iapiot on a o i a u i d 0 9 9 pray during spring in lower Cbaaapaika Bay
INTRODUCTION
Predation is likely the most important component of mortality for marine fish eggs and yolk-sac larvae because starvation is inconsequential during these early life stages (Monteleone and Duguay 1968; Bailey and Houde 1969; Govoni and Olney 1991)- While many vertebrate and invertebrate taxa prey on the early life stages of fishes {see Bailey and Houde 1969 for review), reliable evaluations of predation have been hampered by technical and interpretive problems. Specific impediments to the study of predation on ichthyoplankton were addressed by Bailey and Houde {1969) and Govoni and Olney (1991) and include; (1) difficulty in the identification of prey in the guts of predators; (2) difficulty obtaining quantitative estimates of prey consumption; {3} large variability in estimates of abundance of predators and prey; (4) net feeding of prey by predators in collecting devices that may lead to over-estimates of predation; and (5) difficulties in assessing small-scale, spatio-temporal patterns of co-occurrence, necessary to interpret oscillations in the abundance of predators and prey that are not due to predation
{Frank and Leggett 1985).
Soft-bodied, gelatinous zooplankton are well documented predators of zooplankton and ichthyoplankton (i.e., Lebour 1922,
1923; Killer 1974; Burrell and Van Engle 1976; Mountford 1979; as Alvarino 1980; Aldridge 1984; Purcell 1985, 1989, 1990, 1991,
1992; Konteleone and Duguay 1988; Govoni and Olney 1991; Cowan et al. 1992). Many scyphomedusae, cubomedusae, lobate ctenophores and hydromedusae are actively swimming predators and exhibit a type-I functional response to variable prey density that may result in significant impacts on populations of fish eggs and larvae when predator densities are high (ie.-Purcell 1985;
Monteleone and Duguay 1988) . Previous studies of potential predation by gelatinous zooplankton have relied on analysis of gut contents, digestion times and predator abundance (see Purcell
1985 for review), with results indicating that the impact of these predators can be high but variable. For example, Moller
(1990) estimated predation rates by the scyphomedusae, Aurelia aurita, on the standing stock of Atlantic herring larvae to be from 2.6-4,4% d 1. Purcell (1981, 1990) reported that Rhizophy&a sysanhardti consumed 28.3% of the fish larvae in the Gulf of
California in July and August, while from 28-86% of Pacific herring could be consumed by the hydromedusa, Aeguora victoria, during sampling periods off British Columbia.
Two gelatinous zooplankton taxa, the hydromedusa Nemopsis bschei and ctenophore Mnentlopsis leicfyi, are reported to reach peak, annual abundance during spring and summer in Chesapeake Bay
(Miller 1974; Burrell and Van Engel 1976; Verity 1987; Govoni and
Olney 1991; Cowan et al. 1992; Purcell and Nemazie 1992). While few studies have focused on the ecology of these species in the
Bay, recent studies indicate they may be Important predators of 07 local zooplankton and ichthyoplankton. Baird and Ulanowicz f1989) suggested that predator control by ctenophores in late spring represented the primary change in the seasonal dynamics of
Chesapeake Bay. Govoni and Olney (1990) examined the potential predation of M. leidyi on eggs of the bay anchovy, Anchoa mitchilli, within and outside the Chesapeake Bay plume, concluding that potential predation (0-174 eggs/tn’/d'1) was highest in unstratified, well-mixed water masses in early summer.
Burrell and Van Engel (1976) identified fish eggs and larvae in the stomadaea of M. leldyl collected in the York River, Virginia and reported that total zooplankton and ichthyoplankton abundance was inversely proportional to volumes of ctenophores.
The impact of hydromedusae as predators on zooplankton and ichthyoplankton populations in Chesapeake Bay has only recently been studied. However, since these taxa may comprise a major fraction of the spring plankton community (Larson 198 7; Kat&akis and Conover 1991; Purcell and Nemazie 1992), their effect on prey communities could be important. The only study to focus on the ecology of S. bachei (Purcell and Nemazie 1992) reported that densities in the upper reaches of Chesapeake Bay were high (0,1-
132 medusae mJ>) and concluded that N . bachei was the most important gelatinous predator on zooplankton populations during early spring when they were the dominant gelatinous zooplankton.
Assessment of the impact of gelatinous predators on zooplankton and/or ichthyoplankton populations requires quantitative measures of predator abundance as well as diel a« activity and distribution patterns, factors that have been reported in very few studies (Burrell and Van Engle 1976, Purcell 1985) „ Gelatinous zooplankton, especially ctenophores, are difficult to identify and enumerate from traditional, preserved plankton samples due to their fragility (Purcell 1988; Olney and
Houde 1993). Consequently, non-destructive methods of sampling are desirable for these taxa.
Olney and Houde (1993) evaluated the utility of in situ silhouette photography to sample estuarine zooplankton and ichthyoplankton in Chesapeake Bay, and concluded that the camera- net system provided reliable density estimates for many common planktonic taxa including hydromedusae and fish eggs. While both adult and cydippid stage ctenophores could be enumerated in photographic records, they were absent or indistinguishable in the paired, preserved sample and the accuracy of density estimates by the camera-net system could not be assessed. The results of Olney and Houde (1993), however, did indicate that the camera-net system held promise for studies of gelatinous zooplankton distribution and abundance. The camera-net also provided depth discrete abundance and hydrographic information at scales finer (< 1 m) than those attainable with traditional gear, permitting the assessment of spatial overlap between predators and prey in an individual cast of the gear. While the camera-net has been used to examine egg production of striped bass, Horone saxatilis, in a tributary of the Chesapeake Bay (Field 1990) , and to sample eggs of walleye pollock, Theragra calcogramtna, in the 89 Gulf of Alaska (Reed et al. 198B) its' full utility has yet to be realized in a multi-species, ecological study.
Black drum, Pogonias cromis, enter the lower Chesapeake Bay in early spring and spawn over a restricted period from late April through Hay (Joseph et al. 1964; Chapter II). However, evidence of recruitment in the form of young-of-the-year is negligible or zero during most years. Frisbie (1961) suggested that Chesapeake Bay lies near the northern geographic limit of the reproductive range of black drum and that environmental conditions, particularly low water temperature, may limit survival. Joseph et al. (1964) collected many black drum eggs in areas where gelatinous predators were uncommon, but commented that large concentrations of M . leidyi caused them to curtail sampling north of the city of Cape Charles, Virginia. Subsequently, Cowan et al. (1992) conducted experiments in 2.2 m5 enclosures off the city of Cape Charles in 1990, concluding that potential predation by n. bachei and W. leidyi was high with 38% of the water column cleared daily of fish eggs and larvae at a single station in lower Chesapeake Bay. While evidence of controlled or limited recruitment of black drum by gelatinous zooplankton is mounting, questions remain concerning their effect over the entire spawning area and to what extent predator-prey populations are spatio-temporally coincident. Jn situ silhouette photography was used to examine the ecology of ubiquitous gelatinous zooplankton and their potential impact or correlation with apparently limited, seasonal 90 recruitment of black drum in lower Chesapeake Bay. My objectives were to 1) describe the vertical and horizontal distribution and abundance of gelatinous zooplankton during the black drum spawning season; 2} describe small-scale patterns of spatio- temporal co-occurrence or separation between gelatinous
zooplankton and sciaenid eggs under variable hydrographic, diel and tidal conditions and; 3} estimate the Impact of predation on the standing stock of black drum eggs during spring 1990 and 1991 to address the hypothesis of Cowan et al. (1992} that gelatinous predators may limit recruitment. Findings were based on the results of ichthyoplankton surveys conducted in lower Chesapeake Bay in the vicinity of historic, black drum spawning grounds during spring 1990 and 1991.
MATERIALS AND METHODS
Field Method*:
Data were derived from weekly zooplankton surveys that were conducted during spring (1 April to 1 June) in 1990 and 1991 and described in Chapter II. Samples were obtained with a camera-net system consisting of paired 335 micron nets fitted to a rigid frame {Figure 3.1) (see Olney and Houde 1993 for a complete gear description). The frame supported a Benthos Model 3 73 camera, hydrographic sensors and flowmeters. As plankton passed through an open chamber in the camera, photographs were taken at 1-s intervals, and temperature, conductivity and depth recorded for each frame. Individual casts resulted in a film strip *1
Figure 3 * 1 Diagram of the paired-net towing frame. Labelled instruments are: A) electronic flowmeters; B) conductivity-time-depth (CTD) probe; C) inclinometer; D) plankton camera. ,...E 92 {approximately 7.6m; 300 observations) and a standard preserved sample. All deployments of the camera system were 5-min, stepped oblique tows. During cruise 10 (1991) when ctenophores were abundant, paired net collections (n - 13) were fixed in a It chromic acid solution (Gosner 1971) to facilitate counting of ctenophores aboard ship. These data were used in comparisons with camera derived ctenophore densities.
Two Lagrangian time-series experiments were conducted during
1990 and 1991, the sample design and area are described in
Chapter II. Data were analyzed to examine changes in diel vertical water column position of predators and prey.
Computational Methods;
Gear Efficiency.-Differences between camera-derived and net collection ctenophore densities were estimated using a paired t- test (alpha = 0.05}. Paired t-tests also were employed to test for differences between camera and net estimates of sciaenid egg density. Analysis of variance (ANOVA) was employed to test for differences within and among the four sample strata.
Estimation of Abundance.-Ichthyoplankton and gelatinous zooplankton were enumerated from film records examined in the laboratory under a stereomicroscope at low (6X) power. Eggs identified as sciaenids were spherical and possessed one or more oil globule(s) (Johnson 197B, Chapter I}. Resolution of the camera prevented specific egg identifications from films, however, identity was determined in paired preserved samples using morphology and genetic techniques (chapter I)* Ctenophores and hydromedusae were identified using the Keys of Gosner (1971).
Life stages of ctenophores were separated based on morphology with all tentaculate forms designated cydippids and all lobate individuals designated adults. Only counts of adult ctenophores were used to calculate predation rates. Hydromedusae were measured with an ocular micrometer and separated based on size. Since photographs were not life-sized images, ocular units in l mm were estimated using animals of known size (i.e., the adult stage of the copepod, Acartia tonea = 1.0 mm). Hydromedusae, those < 5 mm bell height (BH) , were considered small while those > 5 mm BH were designated large. Only large hydromedusae were considered in the analysis of predation. Densities of individual taxa (A) were calculated using;
A ----- ^ --- £1) F(0 , 0062m3)
Where N was the total number of an individual taxa in a single film record, F was the total number of frames in an individual cast of the gear and 0.0062 mJ was the volume photographed in a single frame. Average stratum-specific density was calculated for each cruise to describe spatio-temporal variability in predator-prey abundance, Relative amounts of variation among stratum-specific densities of gelatinous predators were compared using the coefficient of variation (CV) by; v * — X 1 0 0 ( 2) 7
Vertical Distribution.-Vertical distributions of predators and prey were determined in relation to water column stratification (ie.- thermocline, halocline). Vertical position in the water column was estimated at all stations where the number of individual predators exceeded station depth. Because effort in each 1-m depth interval was variable, the proportion of effort at each depth interval was calculated as f/total frames, where f was the number of frames at any 1-m depth interval.
Depth-specific density estimates (PJ were adjusted by the proportion of effort at each 1-m depth interval. Center of mass
(Z„) was calculated at each station usin9 the method of Frank et al. (1993):
(3)
Where P, was the proportion of ctenophores, hydromedusae or eggs in the ith depth interval and was the average depth of the ith station.
Variability in centers of mass (Za) of predators and prey was determined during the 24-h Lagrangian time-serles experiments conducted Kay 1990 and 1991. Data were examined to describe any diel and/or tidal patterns that may influence spatio-temporal coincidence between gelatinous zooplankton and sciaenid eggs. 95 Distances between Predators and Prey.-Distances (m) between
individual sciaenld eggs and their nearest potential predator
were calculated to assess small-scale spatio-temporal co
occurrence. Distances separating individual frames were variable depending on volume filtered that was variable during an individual deployment (range = 30-90 cm1/sec) . Therefore, calculations of distance separated used an average flow determined between frames that contained predators and prey.
Separation distances were calculated as:
Djstance(m) =_L^L[£L (4) 100
Where F = frames between prey and the nearest predator, c = flow
(cm/frame) averaged between the two individual observations.
Distances of separation could be interpreted differently depending on when a target was photographed and the density of predator taxa. For example, if a predator was photographed at 4- m upon descent of the gear as it first reached the 4-m depth
interval, and a eciaenid egg was photographed 30 frames later at 4.2 m as the gear continued to descend, the two taxa would be separated by IB m using GO as a value of c in equation (4) .
Alternatively, if a predator was photographed at 4 m on descent, and an egg was photographed at the same depth (but during the final ascent by the gear to the surface), the number of frames separating the two taxa would be high (F - 100-200), and estimates of distance large (60-120 m>■ Consequently, additional 96 information {i.e., center of mass) was needed for analysis when
separation distances were greater than station depth.
Impact of Gelatinous Zooplankton on Fish Egg Populations.-To
estimate the potential impact of predation by hydromedusae and ctenophores on the black drum egg population, the abundance of predator taxa in each stratum during a single cruise was estimated by:
P ■ (C) (k) {A) (5)
Where C = average predator density in a stratum, k = number of
stations in a stratum and A = area of a single station (2.6 x 10* mJ) . Gelatinous predator abundance (P) was multiplied by the number of eggs consumed per predator day to estimate potential, station-specific impact. Consumption rates were taken from Cowan et al. (1992) who reported values for both H. leidyi and w. bachei on black drum and bay anchovy eggs in 2.2 m* enclosures in
lower Chesapeake Bay during spring. Average number consumed pred1 d 1 estimated from stocking density and mortality rates was
2,5 day1 for N, bachei and 1.9 day1 for M. leidyi. Stratum- specific estimates of egg consumption were adjusted for the observation that black drum eggs comprised 6% of the total
ichthyoplankton collected in 1990 (Cowan et al. 1992). Adjusted consumption values were divided by total, stratum-specific, black drum egg production (Chapter II) to estimate proportion consumed.
Humber of prey consumed per predator per day is variable and *7 wholly dependent upon prey density. Consequently, an additional measure of predator Impact was made using clearance rates (to estimate the proportion of each stratum that could be cleared of prey over time} (Monteleone and Duquay 1988; Cowan et al. 1992;
Cowan and Houde 1992) that are independent of prey density.
Clearance rates reported in the above studies were calculated from predators held in enclosures that ranged in volume from
0.0015-3.2 mJ, and results were variable dependent on container volume and predator size. Clearance rates estimated for M. leidyi ranged from 13-168 1 d 1. For the purposes of this analysis, a mean reported by Cowan et al. (1992; 49.8 1 d 1) was used because it was derived from experiments conducted at the same time and in the same study area of the present investigation. Furthermore, predator sizes, though not directly measured in the present study, were similar; and the enclosures employed by Cowan et al. (1992) were large (2.2 m*) , possibly minimizing container effects. Cowan et al. (1992) reported the only estimate of clearance rates available for bachei (range -
11.1-199.4 1 d 1; mean = 61.4 1 d 1) . However, while estimates of predator impact using clearance rates are independent of prey density, they assume that each predator clears a unique water volume. Consequently, predation potential is likely overestimated when predator densities are high (i.e. > lo m5) .
Therefore, when greater than 100% of the water column could be cleared, life expectancy, or the time before an egg may be expected to encounter a predator is more realistic. 9ft
REfiULTA Gear Ef/iciency.-No significant difference was detected between paired observations (n = 13) of densities of adult M. ieidyi in preserved and camera-net collections (Paired t-test) collected during cruise 10, 1991. Analysis of variance (ANo v a ) conducted on differences between preserved and camera-net densities (n - 4) in each stratum (n - 3) also revealed no significant difference in regards to location.
Paired observations of sciaenid egg densities from camera and net collections made during 1990 (n “ 1?) were significantly different (P < 0,05) with camera derived densities generally being greater* No significant difference was detected between paired comparisons in 1991 (n = 14).
Distribution and Abundance of Gelatinous Predators
Gelatinous Eooplankton occurred in 951 and 85% of all stations sampled during regular surveys in 1990 (n = 112) and 1991 (n -
144). Nemopsis bachei and adult M . leldyi comprised > 95% of the gelatinous zooplankton community sampled during 1990 and 1991.
Cydippids were not identified to species. Other ctenophores {Beroe ovata and PJeurobrachia pilous) , scyphomedusae (Cbrysoura quinquicirrha) , and hydromedusae (BougainvilJ ia carolinensis) were also identified in collections but were uncommon*
Settops is bache i,-Small N, bachei occurred at similar densities between years with peak density (1990 =31 m J; 1991 = 7 m’3) observed on 16 April and l April respectively (Table 3.1).
Small N. bachei were uncommon in collections after temperatures 99
Tab la 3.1.-Stratum-specific mean and range of density {//mJ} of email N bachei collected in 1990 and 1991 with number of positive etationa (+) and coefficient of variation (CV) estimated for each stratum.
1 9 9 0
CRUISE DATE STRATUM MEAN RANGE {+) CV
2 16 APRIL A 4 2-11 4 1.0 6 13 1-24 4 0.9 C 15 8-27 4 0.6 D 91 42-150 4 0.6 TOTAL 31 1-150 16 1.4
3 24 APRIL A 2 0-6 1 — S 2 0-8 1 - c 0 - 0 - D 6 6-19 2 1.5 TOTAL 3 0-19 4 2.1
4 1 MAY A 1 0—3 2 1.2 B 3 0-10 1 - C 7 0—3 0 1 - D 11 0-44 1 - TOTAL 6 0-44 5 2.3
5 8 MAY NO SMALL HYDROMEDUSAE COLLECTED
6 15 MAY A 0 — 0 — B 1 0-3 2 1. 5 C 3 0-10 1 - D 0 - 0 - TOTAL 1 0-10 3 3.0
9 24 MAY A 0,2 0-1 1 — B 5 0-20 1 - C 0 - 0 - D 0 - 0 - TOTAL 2 0-20 2 1.6 9 31 MAY NO SMALL HYDROMEDUSAE COLLECTED 100 Table l,-(Continued). 1 9 9 1
CRUISE DATE STRATUM MEAN RANGE ( + ) CV
1 1 APRIL A 1 0-3 3 0.9 B 3 0-4 2 0.9 C 4 0-9 3 1.1 D 20 1-54 4 1.2 TOTAL 7 0-54 12 1.9
2 8 APRIL A 0. 3 0-1 2 1. 0 B 1 0-3 2 1. 3 C 2 0-3 3 0.9 D. 4 1-9 4 1, 1 TOTAL 2 0-9 11 1,5
3 15 APRIL A 1 0-2 2 1.2 B 1 0-2 3 0,9 C 1 0-1 3 1.0 D 13 3-27 4 0.0 TOTAL 4 0-27 12 1.2
4 22 APRIL A 1 0-2 3 0,9 B 4 0-12 3 1.2 C 4 2-B 4 0.7 D 7 0-13 2 1,0 TOTAL 4 0-13 12 1. 1
5 29 APRIL A 0 . 2 0-1 1 B 2 1-4 4 0.6 C 1 0-1 2 1.1 D 2 0-4 3 0.9 TOTAL 1 0-4 10 1.1
6 9 MAY A O.l 0-0.5 1 - B 0.3 0-1 1 - C 0.4 0-1 2 1.4 D 0.4 0-2 1 - TOTAL 0.3 0-2 5 1.8
7 15 MAY A 0 — 0 - B 0 - 0 - C 0 - 0 - 1 D 0,3 0 H* 1 - TOTAL 0. 1 0-1
NO SMALL HYDROMEDUSAE COLLECTED DURING CRUISES 9 AND 10 101 dropped between 16 and 23 April and were uncommon or absent from samples taken in May (Figure 3.2).
Large N. bachei (> 5 mm) were most abundant on 16 April and
1 May, 1990 when average, cruise-specific densities were 177 and 155 m J, respectively (Table 3.2). Large N . bachei were most abundant in northern strata during all cruises except on 16 April and 15 May when densities were greatest in the A and B strata, respectively. The greatest, single station estimate of density was observed at D on 1 May (83 3 m'1) . Mean densities of large N. bachei on 24 April were an order of magnitude lower (25 m 3} than estimates made during the previous and succeeding cruises.
Overall abundance of large N. bachei increased throughout April, peaking on 1 May (Figure 3.2) after which densities steadily declined.
Densities of large W. bachei in 1991 were one to two orders of magnitude less than samples taken during 1990 (Table 3.2).
During all collections in 1991, stratum-specific densities of adult N. bachei never exceeded 3 m'3 (Figure 3.2).
ffnemiopffis lGidyl. -Cydippids were collected throughout the survey area in 1990 (Table 3.3). Lowest densities were observed from 16 April to 8 May (range - 7-18 m'J> , but increased during the last three cruises on (range = 35-39 m J) (Figure 3.3).
Greatest, single station density (208 a 3) occurred in the B stratum on 24 May. Centers of abundance were variable throughout
1990 with highest densities measured in the B (n = 1), C (n = 3) and D (n = 3) strata. 102 Table 3,2.-Stratum-specific mean and range of density (#/&*) of large N bachei collected in 1990 and 1991 with number of positive stations ( + ) and coefficient of variation (CV) estimated for each stratum.
1990
CRUISE DATE STRATUM MEAN RANGE {+) CV
2 16 APRIL A 0 - 0 - B 67 269 1 - C 9 8-28 2 1.5 D 233 27-659 4 1.3 TOTAL 177 8-659 7 1.3
3 24 APRIL A 4 0-9 3 0.8 B 8 3-12 4 0.5 C 54 11-76 4 0.5 D 34 14-66 4 0.7 TOTAL 25 0-76 15 1. 1
4 1 MAY A 2 1-3 4 0.6 B 58 12-142 3 1. 1 C 249 58-606 4 0.9 D 342 67-833 ___ 4 1.0 TOTAL 155 0-833 15 1.6
5 8 MAY A 1 0-2 3 0.9 B 33 1-90 4 1.2 C 85 9-132 4 0.6 D 249 136-413 4 0. 5 TOTAL 92 0-413 15 1. 3
6 15 MAY A 15 0-56 3 1.8 B 2 0-8 2 1.9 C 14 4-24 4 0.6 D 33 21-62 4 0.6 TOTAL 16 0-62 13 1.2
8 24 MAY A 2 1-3 3 0.6 B 4 1-8 4 0. 7 C 77 6-265 4 1.6 D 68 12-141 4 0.9 TOTAL 40 1-265 15 1.8
9 31 MAY A 5 0-13 3 1.2 B 9 0-34 2 1.9 C 34 1-81 4 1.1 D 53 9-113 _ 4 0.8 TOTAL 25 0-113 13 1.3 Table 2 ,(Continued}. 1991
CRUISE DATE STRATUM MEAN RANGE ( + ) CV
1 1 APRIL NO LARGE ti bachei COLLECTED
2 8 APRIL A 0 - 0 - B 0.1 0-1 1 - C 0.1 0-1 1 - D 0 0 - TOTAL 0.1 0-1 2 1.1
3 16 APRIL ONE POSITIVE COLLECTION IN C STRATUM DENSITY =» 0.53/a'5
4 22 APRIL A 0.1 0-1 1 _ B 3 0-10 1 - C 0 - 0 - D 0.1 0-1 1 - TOTAL 1 0-10 3 3.3
5 29 APRIL ONE POSITIVE COLLECTION IN C STRATUM DENSITY - 0.46/a5
6 9 MAY A 0 - —- B 0.4 0-2 1 - C 0.3 0-1 1 - D 1 0-2 2 1.6 TOTAL 0.3 0-2 4 2.1
15 HAY ONE POSITIVE COLLECTION IN A, B AND C STRATA DENSITY - 0.5/m’\
22 MAY ONE POSITIVE COLLECTION IN A. C AND D STRATA DENSITY = 1*1 m 1 AT D; 0,5 HT AT A AND C. 10 28 HAY ONE POSITIVE COLLECTION IN B, C AND 0 STRATA DENSITY - 0.6 a 5 AT D, 1.3 W AT C , AND 0.8 m 5 AT B. 104 Table 3, 3. -St return-spec if ic mean and range of density (//&*) of cydippld stage ctenophorea collected in 1990 with number of positive stations (+) and coefficient of variation (CV> for each stratum. 1990
CRUISE DATE STRATUM MEAN RANGE <+) CV
2 16 APRIL A 4 2-5 4 0.4 B 7 1-19 4 1.2 C 2 1-5 3 1.1 D . 16 1-54 3 1.6 TOTAL a 0-54 14 1.7
3 24 APRIL A 11 1-20 4 0.7 B 9 0-23 2 1.2 C 49 4-140 4 1.3 D 2 0-6 3 1.3 TOTAL 18 0-140 13 2.0
4 1 MAY A 3 0-11 3 1.7 B 9 2-26 4 1.3 C 10 0-28 2 1. 3 D 5 0-19 1 — TOTAL 7 0-28 10 1.5 5 8 KAY A 4 0-6 3 0.8 B 4 0-10 3 1. 3 C 25 0-75 3 1.4 ... D 6 0-12 2 1.1 TOTAL 9 0-75 11 2.0
6 15 MAY A 12 0-37 3 1.5 B 16 0-38 3 1.1 C 21 5-39 4 0.7 D 25 7-46 4 0,6 TOTAL 18 0-46 14 0.8 8 24 HAY A 30 25-35 4 0.2 B 24 0-66 4 1.3 C 89 B-208 4 1.0
_D 56 0-90 ... 3 0.7 TOTAL 35 0-208 13 1.6 9 31 MAY A 17 2-28 4 0.6 B 12 0-33 3 1.1 C 33 14-70 4 0.7 D 93 20-204 4 0.9 TOTAL 39 0-204 15 1. 3 109 Tabl* 3.3-(Continued) 1991
CRUISE DATE STRATUM MEAN RANGE ( + ) CV
3 16 APRIL A 0 0 B 5 1-16 4 1.4 C 2 1-3 4 0.4 D 7 1-14 4 1.0 TOTAL 4 0-16 12 1.5 4 2 2 APRIL A 5 0-12 3 1. 1 B 33 3-72 4 0.9 C 21 1-46 4 0,9 D 23 0-37 3 0,6 TOTAL 20 0-72 14 1.0 5 29 APRIL A 2 0-6 3 1.4 B 14 0-45 4 1.4 C 5 0-9 3 1.0 D 7 5-0 4 0.3 TOTAL 7 0-4 5 14 1-5 6 9 MAY A 10 2-24 4 1.1 B 26 IB-44 4 0.4 C 14 5-23 4 0.6 D 20 10-37 4 0.6 TOTAL 18 2-44 16 0.7 7 15 MAY A 2 1-3 4 0.5 B 6 1-15 4 1.0 C 4 1-0 4 0.8 D 32 16-50 4 0.6 TOTAL 11 1-50 16 1.4 9 22 MAY A 10 1-18 4 0.0 B 7 3-13 4 0.6 C 14 B-22 4 0.4 D 20 5-50 4 1,0 TOTAL 13 1-50 16 0.9 10 28 MAY A 96 27-100 4 0.8 B 27 14-46 4 0.6 C 27 16-36 4 0.3 D 44 30-60 4 0,3 TOTAL 49 14-100 16 1.0 IOC
Figure 3.2 Abundance estimates of small and large Nemopsie bachei collected during cruises in 1990 and 1991. 1990
200 : 20
150 H | 15 H H m l 100 * H 10 > tn z LU m o 50
0 J - ^ K -- —I W . 0 4/9 4/16 4/23 5/1 5/8 5/15 5/25 5/31 CRUISE DATE i■ADULTS -"(LARVAE •TEMPERATURE
1991 50 25
40 20 H ro m + £ E 30 15 m ? H
0 J □ -ill _ 4/1 4/8 4/16 4/22 4/29 5/9 5/15 5/22 5/28 CHUISE DATE IC;LARGE SMALL • TEMPERATURE 107
Figure 3*3 Abundance estimates of adult and cydippid stage ctenophores collected during cruises in 1990 and 1991 * 1990 200 20
150 15 m n £ E 'u m 33 t£ 100 > c/) 10 —I a LU ID Q m 50 o
0 .— - 4 i - ■ - 0 4/9 4/16 4/23 5/1 5/8 5/15 5/25 5/31 CRUISE DATE !■ ADULTS :iCYDIPPIDS •TEMPERATURE
1991 50 25
40 ! ! 20 m n £ £ ~u 30 * 15 m £ J? 5 z 20 10 C LU X Q m o 10 i 0 ■ -P 0 4/1 4/8 4/16 4/22 4/29 5/9 5/15 5/22 5/28 CRUISE DATE lADULTS PiCYDIPPIDS • TEMPERATURE ice Cydippids wore absent from samples collected on 1 and 8
April 1991 and were collected sporadically from 16 April through
22 Hay (range - 4-20/m,J) . Greatest abundance was observed on the last cruise (28 Hay) when average density, at all stations, was
49 m* (Table 3.3) and water column temperature exceeded 2CPC
(Figure 3.3). The highest single station estimate of density was measured in A on 28 Kay (180 m 1) ■
Adult M. leidyi occurred sporadically in collections until 24 and 31 Kay 1990 when they were most common in the D stratum
(Table 3.4). No adult M , leidyi were collected during the 8 May cruise period and densities peaked on 25 May (Figure 3.3).
Adult M. leidyi were observed throughout 1991, however, densities were low (< 3 m 3) until mid-May (Table 3,4). Average survey area densities of H . leidyi increased from 6 to 16 m 3 between 15 and 2 2 Hay and increased only slightly during the last cruise (Figure 3,3). The highest stratum-specific (31 m 3) and single station estimate of density (58 m J) of lobate M . leidyi occurred in the D stratum on 2 2 May 1991,
Gelatinous predators (both large N . bachei and lobate M . leidyi) were generally least abundant in the A and B strata. During 1990 when tf. bachei was the dominant gelatinous zooplankton, greatest abundance was observed in the C and p strata with the exception of 16 April (Figure 3.4). Samples collected in 1991 contained mostly M. leidyi that were more evenly distributed throughout the four strata but still appeared to prefer the northern sections of the survey area (Figure 3.5). tot Table 3.4.-Stratum-specific mean and range of density (#/ro*) of lobate M, leidyi collected in 1990 with number of positive stations (+} and coefficient of variation (CV) for each stratum.
1990
CRUISE DATE STRATUM MEAN RANGE (+) CV
2 16 APRIL A 0 _ 0 B 0 .1 0-0.4 1 - C 0 - 0 - D 2 0-6 2 1,2 TOTAL 1 0-6 3 1,4 3 24 APRIL A 8 0-17 2 1.2 B 4 0-9 2 1.2 C 1 0-5 1 - D 0 - 0 - TOTAL 3 0-17 5 1.8
4 1 MAY A 5 0-9 3 1 . 0 B 0 - 0 - C 13 0-47 2 1. 7 D 14 0-50 3 1.7 TOTAL 8 0-50 8 2 . 0 5 8 MAY HO M. leidyi COLLECTED
6 15 MAY A 0 - 0 - B 0.1 0-1 1 - C 5 0-15 2 1. 4 P 6 0-17 3 1,3 TOTAL 3 0-17 6 2 , 0
8 24 MAY A 5 2-8 3 0.7 B 12 0-30 3 1. 1 C 20 5-41 4 0.8 D 95 24-200 4 0.9 TOTAL 35 0-200 13 1.6 9 31 MAY A 11 0-36 3 1. 5 B 11 0-40 3 1.9 C 20 0-5B 3 1. 4 D 24 15-28 4 0.3 TOTAL 16 0-58 13 1.1 110
Table 3,4-{Continued) 1091
CRUISE DATE STRATUM MEAN RANGE <+) CV
1 1 APRIL A 0. 5 0-1 2 1. 3 B 0.2 0-1 1 - C 0.5 0-1 3 0.9 D 0.4 0-1 3 0.8 TOTAL 0.4 0-1 9 1. 1
2 B APRIL A 0 — 0 - B 0 - 0 - C 0.3 0-0.5 2 1. 0 D 1 0.5-1 ... 4 0. 5 TOTAL 0.2 0-1 6 1. 6
3 16 APRIL ONE POSITIVE COLLECTION IN THE C STRATUM DENSITY = 1.0
4 22 APRIL A 0.1 0-1 1 - B 0.3 0-1 2 1.0 C 0.3 0-1 2 1.0 D _Q,2 p-1 1 - TOTAL 0.2 0-1 6 1. 6
5 29 APRIL A 0 — 0 — B 0.1 0-1 1 - C 0. 1 0-1 1 - D 2 0-6 2 1.5 TOTAL 0.5 0-6 4 3 . 1
6 9 MAY A 2 0-4 3 0.8 B 2 1-3 4 0.7 C 2 0-3 3 0.8 D 1 1-2 4 0.6 TOTAL 2 0-4 14 0.7
7 15 MAY A 5 2-8 4 0. 5 B 4 1-5 4 0. 5 C 8 4-12 4 0.4 D . 6 _ 2-9 4 0,6 TOTAL 1-12 16 0.6 111
Table 3.4.-Continued, 1991
CRUISE DATE STRATUM MEAN RANGE < + ) CV
9 22 HAY A 10 3-16 4 0.6 B 10 4-26 4 1.1 C 13 1-25 4 0.8 D 30 19-58 4 0.6 TOTAL 16 1-58 16 0.9
10 28 MAY A 12 5-21 4 0.7 B 16 3-45 4 1.2 C 19 7-39 4 0.8 D 20 6-48 4 0,9 TOTAL 17 3-4B 16 0.9 112
Figure 3.4 Stratum-specific abundance of adult hydromedusae and ctenophores during cruises in 1990. B in £ CM in CO Q s ! I in . £ LO LD J- 55 < Q o CO LU ■ IT) CO : 1 ! g occ 55 co
m £ 55 < : i
i I I O o o o o O o o CO CM (eUJ) A1ISN3Q 113 Figure 3.S Stratum-specific abundance of adult hydromedusae and ctenophores during cruises in 1991. if) ■ LU O) ^ CM LU ^ CO => IT CM O CM 't ! t£> A A Strata Strata *C Strata Strata i s? in o in o m in co co CM CM (tW) A1ISN3Q 114 The greatest difference in stratum-specific distribution in 1991 was observed on 22 Hay when lobate M. leidyl were nearly twice as abundant in the D stratum. Survey Area Hydrography, -Vertical profiles taken at each station were less saline in the up-Bay direction and were largely isothermal with only l-2flC differences observed between surface and bottom. Kaloclines (defined as an observed change of 2 p/OD or greater in 2 m) occurred sporadically in all strata. When present, haloclines generally occurred mid-water at depths ranging from 1-7 m. Stations with depths of 6 m or less were we11-mixed. Hydrographic conditions observed in lower Chesapeake Bay in spring 1990, are presented in Table 3.5 to describe seasonal surface to bottom physical characteristics. Vertical temperature profiles taken in 1991, were similar to those described in 1990. Surface to bottom salinity discontinuities were observed in all strata and were mid-water (4-6 m) with some as deep as 8-10 m. All stations less than 6 m were well-mixed. Cruise-specific hydrographic conditions presented in Table 3.5 illustrate general surface to bottom physical characteristics observed in 1991. Vertical Distribution of Gelatinous Zooplankton.-Centers of mass {ZB) calculated for both large JY. bachei and lobate M, leidyi during daylight hours (0830-1820) in 1990 ranged from 2.fi ll.5 m (Table 3.6). Examination of vertical distributions during stratified conditions indicated that centers of mass were generally mid-water and could occur either within, above or below m Table 3.5,-Ranges of hydrographic conditions and sample depths at stations occupied in lower Chesapeake Bay during spring 1990 and 1991. E Q I g I W < u n i W u W pj h w s ■V [L, B > U w to w □ & E M h O C M W £ P M > > M E CX ) U . C S W ■ > t h ■1 * j H r IQ i l i— n i N f i n H r n o i n i H r H t H t O C H V H r H t O H N ( H r H CTi N I H ■ 01 10 H r X(X (X H n i-f iJ l f H 1 — 1 1 1 1 1 1 1 l i 1 1 » . i • * * * 4 + m n i o t n i M C n i M C 1 0 M C ~ r M C o o m i o o o O C rH O H 1 4 4 • ■ 4 4 4 4 . H r 1 ft —l i— 1 0 n i > 4 0 1 o D H I t n i m l r J o —i i— —1 1— ■tf f I i i O C ' O N C ^ H 1 — 1 — i 1 1 4 ■ * » «- 0 < n i o 1-1 t O C n i —1 1— t~ 4 0 4 0 3 0 —i i— n i y(N ( ’y O C 0 4 4 0 ■—I N I l—l O C 10 H n i N I N f > r M C N f m —t i— H r • 1 1 1 1 1 1 l ■ 4 4 4 4 ■ 4 ■ 4-1 —1 1— l C D C ^4 4-1 D I N | n i H 01 —1 4— m H c N f H r O B O C ~ r « 1 1— " f N ( N I io r*i H >-1 1 1 1 1 1 4 4 4 4 • 4 ■ > 4 ■ ■ 0 1 O C H ( |N —1 1— l*> 0 ( IP —1 1— D C H r A n i i-4 H 01 10 M 4-1 c 4 > i N f Q C O N f 4? i-l 1 — 0 l 1 1 1 H r O C N f O —1 4— o t 0 % —1 4— o n i n H t m —1 4— 1 4— h £ o —1 4— O C N I 1 4— O i 0 —1 4— M i-l O ■ O C 1 1 1 1 1 4 i 4 ■ 4 A H * H w i U> i H E >1 0*1 l C H r N f 0 4 - H ■ 4-1 < < a0* , 6M J .-5 rl , J a s s s s s g 4-1 n o H n i o h < 1 0 H 4-1 o 1 0 O N N f « N f 4-f pinn^in^r'ff'O H r 101 o t 0 1 n 1 I 1 1 1 i l 4 4 i i 4 i 4 4 4 t 4 ■ ■ \D ~ t N < I P r-H n H n i H H ■ < tk I-l H i n N < * N n n k o m * c H r > □ H + r 4 4 » * ■ • + ■ ' J J■ ' ‘ ■ * c O C I ■— n i N I 0 V r-i 0 1 O N I O H i N f n i <-l <-l 1 0 l r i r H r 1 ■— 1 i— 1-1 1 0 p i O C —1 1— 0 1 H ■-I H H i 1 1 I 1 1 I I 1 1 4 4 ■ H i N f N C N f r r O C p m N I n i r4 - r N f n N r-l IN Lf> N I H i —l i— N f N I p H H -■ n + * » i 4 ■ t I - r O o t - r ? 4 N I m r-' 1 4— H r 1 0 y 0 1 o t —ir)4^ 4 ) r i i— f r Q\ 0 c 4-f 1 4— H r N f N —1 4— H r 1 t 1 1 « 4 i i 4 i ■ * ■ 4 4 4 * H r O C N f H - r 0 < - r t f i H r O H H i H r y 0 1 f 4 o 0 0 V N I o N E H i H i H 1 4 m i 4 + i 4 4 ■ ■ > N f ~ r H r 0 1 n i t r m |-~ H r H r N f 4 0 N I N I N f N I n i N f o •* 1 0 o * H r * 0 - P l i ■ 1 1 4 4 » 4 4 4 ■ » N f m N < Q N f H t O f O C 0 1 —1 4— O M I y r Q O C n i H i —1H H r 1 4— n Q n i O S N ( j n H ■ f I m o m H r 1 1 1 t 1 1 1 1 4 4 ■ 4 H r 1 0 O C N I N C O C N f —l i— i f —1 4— 0 1 0 1 H 4 0 N I N f 1 I k 4 ■ 4 > ■ Table 3.6.-Stratum-specific center of mass (ZJ for predatory gelatinous zooplanXton during spring 1990. Halocline depth (H^) during stratified conditions is represented by a range in depth while homogeneous conditions are represented by a a single, station depth value. N.D. = no data, * = number of predators < station depth. 1 0 z. M w fN § H w N N N >* in O' £ in (-1 Cl < < • The rarity or absence of gelatinous zooplankton during spring 1991 precluded assessment of vertical distribution during cruises 1-6 and center of mass (zB) could only be calculated for lobate H. leidyl during cruises 7-10. Centers of mass were within or below the halocline in all but one profile during cruises 7-10 (n = 19 of 20} and were mid-water during well-mixed conditions (n “ 11} (Table 3.8}* Spatial Co-occurrence of Predators and Sciaenid Egg Frey.- Gelatlnous predator densities were variable in samples containing eggs in 1990 (range - 0-341 m J) (Table 3.9). Camera-derived estimates of sciaenid egg densities ranged from 0.5-5.1 m 5 (mean = 1.9 m 3} with 58% occurring in the A and B strata. Sciaenid eggs were routinely most abundant in samples with few or no predators. Exceptions occurred at stations 061 and 062 during cruise 8 when egg densities were 4.4 and 1.8 tn- and predator na Table 3.7-Comparison of the depth distributions of N. bachei, M* leidyl and cydippids at 5 m intervals in all samples from cruises 6,8 and 9 {1990) where taxa co-occurred. CRUISE DATE TAXA 0-5 m 5-10 m 10-15 m 6 15 MAY N . bachsi 66 29 3 N ■ leidyi 55 40 5 8 22 MAY N > bachei 11 aa 1 M. IsIdyl 30 66 4 cydippids 47 52 1 9 31 MAY N. bachei 72 27 1 M, laldyi 76 24 0 cydippids 73 27 0 * rt C □ n n r* in m f" 4 1 1 1 i 4 H in * * * n n r- in 4 (A rl r> « n 4 * i i ■ i ■ ft IS MS N n n VJ 4 n M r- 8 3 “ ? ? r- n- n n- >-i 4 ( 1 4 o >H ♦ in 10 H * * n 4-1 4 a\ o o in t4 in o 4 4 ■ ■ m 4 r tvj in r- r* m n 4 * K> CO i n d > 4 O i 1 1 l 4 o !E f N rH 4 4 m * n 4 * * rj H (N 10 4 o cn CN H o IN 4 ■ ■ ■ 4 4 4 * ■ £ H ■ tsJ f N in 10 n U) U) 10 10 N +3 -H O 10 A <41 *3 in NJ 10 in p> 1 1 1 i 1 IN H 4 n n 4 m in r-' -ti >-) rl H 41 « Ji 10 *0 t 4 10 in CO 4 fN H +J K 4) +3 +> 4) o 4 4 ■ ■ ■ 4 4 c tr & u u N 4— 1 hO fN m 4 ID BO U Cl < C D a A B u a d'H T( VI <4 Vl 0 > >< h £ £ I m CRUISE DATE STATION PREY PREDATOR 4 1 MAY A13 4.5 0 . 5 B43 0.5 11. 6 B67 5.1 0 . 0 5 8 MAY A10 1.3 1.3 B83 0.6 0. 6 C36 0.5 107 . 5 6 15 MAY A06 2.6 1.5 B34 0.5 0,5 B50 4.1 0.5 B75 3 . 1 0,0 D87 0,6 22 . 0 8 24 MAY A70 0.5 6.7 B04 1.3 51. 7 B67 l.fl 2 . 4 C4 3 1.2 294. 1 C51 0,5 43.7 D44 0.7 40.4 DG1 4.4 340. 9 D62 1.0 214 . 7 9 31 MAY B36 3.4 2 .1 B69 0.7 0 . 0 C2 2 0.7 46.1 C65 3.1 80. 7 C7 5 1. 2 67.0 121 densities were 341 and 215 m J respectively. Center of mass Predators were absent in all samples containing eggs during Cruise 5 (29 April). While camera-derived densities of sciaenid eggs were greater than 1990 estimates (range - 0.5-14.0 m 1 (mean - 2.3 n J), there were fewer positive stations (25%). Centers of mass for sciaenid eggs and predators during 1991 Indicated a high incidence of co-occurrence (Table 3.12). Sciaenid eggs were located mid-water under unstratified conditions and occurred above, below or within the halocline under stratified conditions during cruise 5 and 6 when predators were rare or absent. Sciaenid eggs and predators co-occurred in samples taken during cruises 7-10 and only at a single station (station C27, cruise 10) were eggs and predators spatially segregated by a hydrographic boundary. Densities of black drum eggs from preserved samples (Chapter II) and camera-derived predator densities during 1990 were 122 Table 3.10.-Centers of mass (ZB) of sciaenid eggs and gelatinous predators at each station where eggs were identified in films during 1990, with station depth (Z) and halocline depth (HJ , when present. All values are reported in meters. 2™ CRUISE DATE STATION EGGS PREDATORS Z H, 4 1 HAY A13 9.8 13.5 19 3-4 B67 13.8 0 19 4-6 B4 3 11.0 8 . 0 12 5-7 5 B HAY A54 1.0 0 9 8-9 A10 9.0 0 12 6-8 B8 3 10.0 0 10 6-8 C36 5.0 6. 5 9 6-8 6 15 HAY A06 2.6 0 7 _ B75 8.7 0 12 - B50 13.2 0 18 - B34 3 . 0 0 11 - D87 4.0 4.9 8 — 8 22 HAY B04 5.4 5.9 11 6-8 B67 11.0 0 17 - C4 3 8.0 5.0 B 2-4 C51 12 . 0 7.1 16 4-6 D44 4.0 8.3 12 7-9 D81 6.0 6.5 10 7-9 D62 4.0 6.5 10 7-9 DB7 4.0 4.9 8 — 9 31 HAY B3G 5.4 0 7 — B69 7.0 0 9 - C22 1.0 2.6 4 - C65 3.0 4.0 7 - C75 6.0 4.7 6 — 133 Table 3.11.-Site-specific sciaenid egg prey end gelatinous predator densities (//in1) during 1991. CRUISE DATE s t a t i o n PREY PREDATOR 5 29 APR A20 1.0 0.0 A4 3 0.5 0.0 A71 2.0 0.0 B57 0, 5 0.0 B67 2 . 1 0.0 6 9 MAY AOG 14 . 0 1. 7 A63 0*5 0.0 021 1. 4 2. 1 D37 1.5 2.6 7 15 HAY A14 2 . 8 3.4 A2G 0.5 8.4 A79 1.1 6.4 AB4 5 . 5 2 . 2 B44 2.9 4.4 BB2 3 .fl 0. 8 9 22 MAY AB2 1. 7 2.8 B59 0 . 6 3 . 6 10 28 MAY A52 1. 6 5.6 B74 1.4 12 . 3 C27 0.7 21.8 124 Table 3 .12.-Centers of mass (ZB) of sciaenid eggs and gelatinous predators at each station where eggs were Identified in films during 1990, with station depth (Z) and halocline depth (HJ , when present, All values are reported in meters. Z- CRUISE DATE STATION EGGS PREDATORS Z H* 5 24 APRIL A71 1.0 0 7 2*4 A43 4.0 0 11 - A20 3.4 0 19 - B69 4.4 0 10 - B67 9.2 0 14 - B57 13,0 0 14 2-4 6 a MAY AOS 5,3 0 7 2-4 A63 2.0 0 6 2-4 D21 6.0 0 10 7-9 D37 0.7 0 10 7-9 7 15 MAY A14 3 .1 3.4 4 — A26 3 . 0 5, 7 11 - A79 3.0 2. 6 7 2-4 AS 4 3 . 0 0 6 - 844 5.7 3. 5 8 1-3 B82 7. 3 0 9 — 9 22 HAY A82 2.7 0 7 - B59 9.0 0 13 - 10 28 MAY A52 3.0 0 11 - B74 5.0 0 9 - C27 1.0 6.8 10 2- 13 5 compared and indicate that overall egg densities were highest in strata with relatively low predator abundance (5-40 m 1) (Figure 3.6}. During peak spawning on 15 Hay 1990, egg densities were highest in the B stratum where predator densities were low. Higher predator densities (20-240 m 3} in the upper strata (C and D) were generally accompanied by the lowest egg densities (0.01- 0 . 27 m 1} . Densities of predators (M. leidyi) were lower in all strata sampled in 1991, however, egg densities from preserved samples were similar between years (Figure 3.7). Predators were absent or occurred in very low numbers (< 5 m 3) during cruises 5 (29 April) and 6 (9 Hay) when sciaenid eggs first appeared in collections. Densities of predators during peak spawning (15 May} (Chapter 11} rose slightly but did not exceed 5 m 1, Egg densities increased from 22 May - 28 May in all strata as predator densities increased. Lagranglan fcime-series analysis of predator-prey distribution and abundance.-Centers of mass and densities of S. bachel, M . leidyi and sciaenid eggs varied over the 24 h time- series experiment in 1990. Large N . bachei and lobate M. lejefyi occurred in collections but densities were usually low except for an order of magnitude increase at 1615 hrs (Table 3.13). Centers of mass calculated for both predators and prey (Figure 3.8) were similar with vertical distributions being nearer the surface during mid-morning and progressively deeper throughout the day. Centers of mass of predators and prey over a tidal cycle revealed 13 ft Table 3.13-Densities of predator* and sciaenid eggs at each time interval during the 1990 time-series experiment with tidal stage and halocline depth in meters (HdJ . Abbreviations are: Emax - maximum ebb current, Fmax - maximum flood current, Sf - slack before flood, Se = slack before ebb, Ftnid - early flood. DENSITY TIMETIDE N bachel M leidyi Eggs Hd 1000 Sf 0 0 14.3 5-7 1313 Fmax 0.5 0.5 2 . 7 5-7 1615 Se 6.5 11.3 11.9 9 1925 Emax 0 1.1 5.9 2 -4 2226 Sf 0 0.5 13.1 4-6 0054 Fmid 0 0 2 . 5 4-6 0739 Emax 0 3.5 4 . 6 5-7 12 7 Figure 3.6 Stratum-specific preserved, sciaenid egg density versus camera derived predator density during 1990. A-Stratum B-Stratum t )AINQOOH {tw) O O A1ISN3Q A1ISN3Q dOlVQ^bd CRUISE DATE CRUISE DATE 120 Figure 3-7 Stratum-specific preserved, sciaenid egg density versus camera derived predator density during 1991. A Stratum B Stratum <(w) E U JS3 3 9 9 (EIU)AJJSN3Q AilSN3d aOi.VQ3dd 129 Figure 3,8 Comparisons of center of mass (3") for gelatinous predators and sciaenid eggs during the 1990 Lagrangian time-series experiment. PREY CO (O cv ■ T a o 0 01 O 8 - I LU CD 1 * * PREDATORS ♦ E G G S ! I O • l CO to C\J SyOXVQ3Hd 110 smaller differences in Zm during slack tides than during maximum current (Figure 3*9}. Sciaenid egg densities varied with no consistent relationship to predator density in 1990 (Figure 3.10). Increasing egg and predator densities occurred mid-day (1313 h to 1615 h) taut were followed by declines in both taxa at 1925 h. Declining egg density from 1615 to 1925 occurred when densities of predators were high and differences in centers of mass were less than 1 m in an unstratified water column (Figure 3.11}. Densities of eggs increased from 1925 to 2226 when predator densities were low. Although predator densities were also low between 2226 and 0054 h, centers of mass were within 1 m and egg densities declined. Densities of eggs steadily increased from 0054 to 1111 when predators were rare or absent and differences in center of mass exceeded 3.5m. Sciaenid eggs were collected sporadically in samples during the 1991 experiment (Table 3.14} occurring at 1000, 2 000, and 2300 h. Egg occurrence generally coincided with high abundance of gelatinous zooplankton. When present, eggs generally followed the same, diel vertical distribution as predators (Figure 3.12). Both taxa were deepest from mid-morning to early afternoon, occurring mid-water the rest of the day. Examination of vertical distributions with regards to tidal stage Bhowed that centers of mass were closest during 2 of 3 slack tides observed, however, lack of eggs in most samples confound interpretation (Figure 3.13). Trends in sciaenid egg and predator densities over time 131 Table 3,14-Densities of predators and sciaenid eggs at each time interval during the 1991 time-series experiment with tidal stage and halocline depth in meters (Hd). Abbreviations are: Em ax - maximum ebb current, Fmax - maximum flood current, St = slack before flood, Se = slack before ebb. DENSITY TIME TIDE tt bachei M leidyi Eggs Hd 0715 Emax 0 21.1 0 9 1000 Sf 0 28.9 1.5 13 14 00 Fmax 0 4.4 0 19 1615 Se 0 3.4 0.6 5 2000 Emax 0 14. 5 2 .9 10 2300 Sf 0.5 28.9 1. 1 3-5 0200 Fmax 0.6 1. 1 0 3-5 0500 Se 0 17. 6 0 2-4 4-6 0820 Emax 0 25.8 0 2-4 4-6 133 Figure 3.9 Comparisons of center of mass {Z") for gelatinous predators and sciaenid eggs with regards to predicted tidal stage during the 1990 Lagrangian time-series experiment. SSVW 30 H31N30 133 Figure 1,10 Densities of sciaenid eggs and gelatinous predators during the 1990 Lagrangian time-series experiment. o * ■ CO or-. o LO DENSITY PREDATOR m CM o> in S EGG DENSITY DENSITY EGG CO CO o o o in in CM (cW) A1ISN3Q 134 Figure 3.1 Densities of gelatinous predators and sciaenid eggs with differences in center of mass (Z“) during the 1990 Lagrangian time-series experiment. Zm (m) EGG DENSITY ■ PREDATOR DENSITY ^DISTANCE DENSITY PREDATOR ■ DENSITY EGG (cW) A1ISN3Q 135 Figure 3.12 Coiuparieone of center of mass {Z") for gelatinous predators and sciaenid eggs during the 1991 Lagrangian tiae-series experiment. PREY (Zm) 00 10 CM ° o I • !1 *CM O O O CM O O C0 Si • ■ UJ (- O • O 00 o ■ PREDATORS • EGGS • PREDATORS ■ * s s o o + to o CM CO to CM (WZ) SdOlVQBUd 13* Figure 3.13 Comparisons of center of mass (Z“*) for gelatinous predators and sciaenid eggs with regards to predicted tidal stage during the 1991 Lagrangian time-series experiment. M SSVW do H31N30 137 were difficult to discern due to the scarcity of eggs. Predator densities peaked during the late morning at both the start and conclusion of the experiment and at midnight (Figure 3.14). Peak predator abundance at 1000 and 2 300 corresponded with slack tides while high abundance at 082 0 corresponded with maximum ebb. While sciaenid egg densities were always low, they declined to zero after times of peak predator density (1000 and 2300) and were already zero at 0820 the following morning. Differences in centers of mass ranged from o.s-4.9 m with the greatest spatio- temporal occurrence estimated when predator density was low (1800 h) (Figure 3,15). Distances Between Predators and Prey,-Distances (m) separating sciaenid eggs from their nearest potential predator ranged from 0 (same frame) to 108 m in samples where eggs and predators co-occurred in 1990 and 1991. While separation distances less than station depth may indicate close spatial coincidence, larger distances were difficult to interpret. Average distance of separation decreased progressing up Bay from stratum A to D (Table 3.15) in 1990, corresponding with increased predator density. During 1991, however, lower predator densities resulted in larger separation distances, with only 8 of 72 observations < 1 m. Average distance of separation between sciaenid eggs and predators varied during the 1990 time-series experiment ranging from 2-60 m over 24 hrs, Examination Of average distance separated, centers of mass and tidal stage revealed some general 13fl Table 3,15-Stratum-specific distances (in) between predators and prey during 1990. Data are presented as numbers of observations and mean distances for separation less than 1 m and greater than 1 m. 1990 Strata Separation Number Average Separation A <1 0 0.4 >1 a 18.3 B <1 11 0.1 >1 32 20.4 C <1 10 0 . 0 >1 1 9.7 D <1 10 0.2 >1 2 1.5 1991 Strata Separation Number Average Separation A <1 4 0.4 >1 45 20.1 B <1 3 0.1 >1 14 23.9 C <1 0 - >1 1 11.6 D <1 1 0.7 >1 4 41. 5 139 Figure 3.14 Densities of sciaenid eggs and gelatinous predators during the 1991 Lagrangian time-series experiment. UD O LD 8 CM CM in o ♦ s CO O I o o CM O LU * ICO — CM t— , o T'¥ <82 • EGGS • ■PREDATORS EGGS O O O *4 O° m i o in o in o in o o rt CM CM (elu) A1ISN3Q 140 Figure 3.15 Densities of gelatinous predators and sciaenid eggs with differences in center of mass (Z“) during the 1991 Lagrangian time-series experiment. Z m (m) 00 CO 00 LU O z : < oO CD <2 O Q o 0 o ocn £ CO o z o LU 3 O o GC o CO s R W/^:W//s///w:;Z;, Y few) A ± I S N 3 Q 141 trends (Figure 3.16). Taxa were usually more segregated during predicted periods of strong currents. During slack before ebb, when centers of mass were similar, separation distances were minimal. However, during slack before flood centers of mass were similar but separation distances were large (> 4 0 m} and predator densities were low (< 1 m 3). Closer examination of CTD data at this station revealed that predators were collected on descent of the gear, while eggs were photographed on ascent. Therefore, similar depths of occurrence suggest that separation distances were closer than 4 0 m. Higher densities of predators during the 1991 time-series experiment were accompanied by lower separation distances at all times. While the shortest separation distance occurred during slack before flood, greatest separation was observed during slack before ebb (Figure 3.17). Predator Clearance Pates and Potential Prey Consumption.- Extreme fluctuations in predator abundance during the study period, resulted in a variable proportion of the number of eggs present that could be consumed every day. During the period of black drum spawning in 1990 when the dominant gelatinous zooplanktar was N, Jbachei the percentage of eggs in each strata that could potentially be consumed ranged from 1 to 100% (Table 3,16), Greatest impact occurred in the northern strata except in A on 31 Hay. Least impact was observed during peak spawning in the A and B strata on 1 May and 15 May. Declining abundance of gelatinous predators in 1991, n * TS H 0 0 £ 5 - o ai <0 01 H t" O O tflNOO O ri n O n to y) nj O Ol f l O WHO 01 v o o m O o (SI V n O ^ so 1-1 o r-i *r o 01 0 B H t 4-1 H- u 3 * D. C « C O 0 C ■H H ~H 0 -n +j o — X fi U 1+4 c a ■h** a,e c 01 r-l n o Co 0 0 O t" H tft O kO h n t" rS n in r-l r* m th o Oi i-l 4J b 3 O r-l (0 BQ o o n 0 1 o o o M o o I N ill o o h n a k ti *G ♦ o * * B tiia *h o o o O O Q O o o o o O O O O O O O O □ - ^ > V V v Ij — if) — i *-i Ij tji C TJ O bi 4J C <013 V 4J H V 0 0 C *0 IH 41 H *H It O 0 0 Ih| ,w—s. rs (SI 1^ 1^ r-l r- in (Si Figure 3.16 Influence of tidal stage on center of mass SSVINdOtidlNdO 144 Figure 3.17 Influence of tidal stage on center of mass fZ"1) and distance separated (m} of gelatinous predators and sciaenid eggs during the 1991 Lagrangian time- series experiment. DISTANCE (m) to m m c\i t % ~ b h LU O' CJ £ CO " b/jt < 't oQ < p I- _ a LU o Q LU tr Q. O' • • PREDATORS «EGGS ^DISTANCE ~ b * >S' , I CM CO to CM (w) SSVW dO H 31N30 145 resulting in a concurrent decline in the proportion of the observed eggs consumed (Table 3.17). During peak black drum spawning (8 and 15 May, Chapter II), declines in production based on consumption rates were minimal (< 5%). Greatest impact on egg production occurred on 22 May when egg abundance was low. Although egg production increased on the final cruise in 1991 (28 May) when predator abundance was maximal, strata with the greatest production were least affected. DISCUSSION The camera-net system was an effective method for sampling lobate ctenophores based on comparisons with traditional net samples. Stalling of gelatinous taxa along the sides of the net may aid in explaining why preserved sample densities were generally higher than films (Olney and Houde 1993). Differences between camera and preserved sciaenid egg densities were significant in 1990, but were non-significant in 1991. While these results are in contrast to olney and Houde (1993) , who found no differences among collections of fish eggs, they may be explained by the relative rarity of sciaenid eggs in the present study. During 1990, when differences were significant, eggs were more dispersed throughout the survey area and low density collections were common. Consequently, eggs were very uncommon in films. During 1991, when more eggs were counted in fewer collections, the camera-net was more efficient. Therefore, examination of small-scale patterns of vertical, Table 3.17.-Stratum-specific estimates of egg production obtained from paired, preserved collections {Chapter II) and predator abundance (Predator), proportion of the vater column cleared (Clearance), potential number of eggs eaten (Consumed) and percentage of egg production eaten (Percent) in 1991. Abundance and consumption estimates are expressed as numbers of individuals x 10* „ -r ■d -0 4J £ O V u +J Q i4 I -p +J £ u CO u 4) n 4) 4) o M U c Id □ c m c 41 o M § ki 2 (A IV n id 3 u e u 01 4 n & o o a M fi o « H H O 4 D I" 0 O 0 I" D I- ■ t t u a n CM m ID4IO OlTlrlO o o o o O O O O O Q O u? in ic Cl oooo o o ro - f t i n t o o o o o o o o oooo n i—I n Ci oo o o 4 l ft ft 4 ■ in 4 4 n no * ft ft IN CV densities were high. Although ctenophores (predominately M* leidyi) have historically been reported as the dominant gelatinous zooplankton collected during spring in lower Chesapeake Bay (Joseph et al. 1964 ; Miller 1974; Verity 1978; Govoni and Olney 1991; Cowan et al. 1992}, this was not the case during spring 1990, Whether lack of competition, favorable conditions for polypoid feeding and growth or some other factor favored the proliferation of N . bachei from mid-April through mid-May in 1990 is unclear. However, densities at single stations were sometimes 1-2 orders of magnitude higher than those previously reported (Cowan et al. 1992; Purcell and Nemazie 1993). The observation that small N. b&chel were less abundant than larger animals may simply be related to sample time. Peak polypoid release of medusae probably occurred in March before collecting began in April and earlier sampling may have resulted in the collection of more small forms. Alternating patterns of occurrence of N , bachel and M. leidyi were probably not produced by seasonal temperature- salinlty variations, since conditions were similar between years. High abundance of N. bachel in 1990 may have been an unusual phenomenon related to unknown conditions that were favorable to feeding and settling of the polyp stage during the previous season. Conditions during 1990 were also favorable to the production of M. leidyi as evidenced by densities being higher 148 than those measured in 1991 even in the absence of It, bacbei. While seasonal or annual cycles of abundance of N. bachei were not dictated by water temperature, low densities in late April 1990 corresponded to a sharp decline in water temperature. Low water temperatures were observed to weaken the swimming movements of M. leidyi, in the laboratory, and caused them to sink to the bottom (Hiller 1974) . While this response may not be applicable to ff. bachelt similar reactions may have affected vulnerability to the sampling gear. Cydipplds abundance was greatest after abundance of N . bachei began to decline in 1990, suggesting that competition for food may have partly limited the production of cydippids by adult ctenophores. Abundance of cydippids may also have been limited by the predatory impact of large N. bachei and M. leidyi, a tenable argument since all taxa co-occurred in samples and had similar depth distributions. Nemopsls bachei were rare or absent in 1991 col lections. Lack of information concerning polypoid mortality and feeding conditions and the rarity or absence of predatory or competing gelatinous taxa preclude an explanation of why densities of ft. bachei were 1-3 orders of magnitude lower than those observed in 1990. Low abundance of If. bachei may be expected to have resulted in less competition and subsequently higher cydippid production by lobate M, leidyi. Actually, the opposite occurred with densities of M. leidyi being less than those measured in 1990. For whatever reason, cydippid production was delayed in 149 1991, peaking during the last cruise (28 May). Ctenophore predators (ie,-scyphomedusae and fierce ovate), that may have limited M, leidyi abundance in 1990 and 1991, were rare or absent throughout the sampling period. Zooplankton prey concentrations during spring 1990 and 1991 were not measured but may have contributed to these differences. At any rate, factors affecting the timing of gelatinous predator production may be important to the subsequent survival of eggs produced by spring-spawning fishes in lower Chesapeake Bay. Large and small W, bachei were consistently more abundant in the northern sections of the survey area in 1990 and 1991. Although bachei are tolerant to a wide range of salinity (Calder 1971, Burrell and Van Engel 1976), they may prefer the consistently lower salinities of this region. Information concerning the sessile, polyp stage of N . bachei is unavailable (Calder 1971, Purcell and Nemazie 1993). Center of mass for gelatinous zooplankton was typically located at a mid-water depth despite the presence or absence of a halocline. These data indicate that gelatinous zooplankton occurred throughout the water column under the hydrographic conditions sampled in lower Chesapeake Bay in spring. In contrast, Govoni and Olney (1991) found highest abundance of M . leidyi in subsurface, high salinity waters. These investigators, however, were working in the area of the Chesapeake Bay plume where high salinity, coastal shelf water underlies warmer, low salinity Chesapeake Bay water, stratification probably acts as a 150 more formidable boundary in the region of the plume. Tidally induced vacillations between stratified and unstratified conditions in the lower Bay may also help to account for differences in the two regions. Patterns of diel vertical distributions of gelatinous zooplankton were variable during the two Lagrangian time-series studies. Centers of mass were located at mid-water, near surface and near bottom but distributions were not consistent with any diel period. Ho detectable pattern was evident in r e g a r d s to tidal stage. In contrast, Miller (1974) examined diel vertical distribution of M . leidyi in the Pamlico River, North Carolina and reported that they had a "marked" preference for surface waters during the day and were distributed at all depths at night. Centers of mass calculated for gelatinous zooplankton predators and sciaenid eggs indicated that the two taxa were spatially coincident when they co-occurred. While distances between eggs and predators in 1990 were variable and difficult to interpret, they indicate that potential encounter was lowest in the A and B strata where egg production was highest. Gelatinous zooplankton predators were more uniformly distributed throughout the survey area in 1991. While separation distances between predators and prey were shorter, average predator densities were an order of magnitude less in all strata. Consequently, in areas where densities of eggs were highest (strata A and B) , eggs were least likely to encounter a 151 predator. The actual distance between a predator and an individual prey item that would result in egg mortality is unknown. While predation rates based on clearance volumes and potential egg consumption were calculated on a daily time frame, estimates of distances between predators are instantaneous and could change over time. Based upon reported swimming speeds (0.12 cm sec1) and clearance rates (ie.- 61 1 d (, Cowan and Houde 1992), an Individual predator could travel 156 m and clear 91.7 1 of water, in 36 hrs, the incubation time of an individual egg (Joseph et al. 1964; Chapter II). Consequently, mobile gelatinous taxa probably have the potential to encounter any sciaenid egg when taxa co-occur. The probability of this event, however, certainly diminishes when predator abundance is low, Diel depth distributions of gelatinous zooplankton and sciaenid eggs followed similar patterns during both years, findings that may be unusual owing to the fact that gelatinous zooplankton are actively swimming and eggs are passive particles. Declines in egg density during the experiments were not necessarily accompanied by high densities of predators. In fact, large declines in egg density followed samples lacking gelatinous predators. During the 1990 experiment, increase in densities of eggs from midnight to mid-morning when predator densities were variable may be explained by the observation that adult black drum spawn during this time in lower Chesapeake Bay (Chapter II). Declining egg abundance in samples containing few or no predators 152 may be the result of either sampling error (ie.-failure to follow the initial patch of eggs) or additional predators that are capable of avoiding collecting devices. For example, during spring in lower Chesapeake Bay, menhaden (Brevoortla tyrannus), a large planktivore, are extremely abundant in lower Chesapeake Bay and a major fishery focuses effort in the area of black drum spawning (personal observation). Consequently, these fish may be important predators of spring-spawning species in lower Chesapeake Bay. Inverse correlations between densities of fish eggs/larvae and predators may be interpreted differently dependent upon the sampling gear used and the hydrography of the water column sampled (Frank and Leggett 1985, Frank 1988, Govoni and Olney 1991). Negative correlations in predator-prey abundance may not necessarily indicate trophic interactions if reciprocal oscillations in abundance are due to the occupation of distinct water masses (Frank and Leggett 1985). However, if predators and prey are found to co-occur and a gear type is selected that samples predators and prey with near equal efficiency, results may be meaningfully interpreted (Frank 1988). In the present study, predators and prey were shown not to be separated by distinct water masses, and the camera-net system is likely an efficient gear for both predator-prey taxa (Olney and Houde 1993). Consequently, interpretations of predator Impact on sciaenid eggs in this study were deemed appropriate. Overall predator abundance in 1990 was greatest during peak 153 spawning. Immediate predator impact on egg production, however, appeared to be low, an observation attributable to low predator density in the areas of greatest production. However, given that net transport in the lover Bay is up-bay and the duration of the egg and yolk-sac larval stages is nearly 3-4 days, eggs spawned in the lower Bay could potentially be transported into the upper survey area where predator densities were high. Thus, the observation that egg production estimates for black drum were higher in southern Bay samples in 1990 (Chapter II) may be explained by the impact of predation in the more northern strata. Other factors potentially involved in the variable production estimates include spawning location and frequency. Historically, the commercial and recreational fishery for black drum has been centered in the lower Bay and the seaside of Virginia's eastern shore (Joseph et al. 1964, Chapter II, V.M.R.C, Annual Landings Summary). Consequently, failure to collect eggs of black drum in northern strata, even at stations where predator densities were low, may be a function of adult spawning locality. Adult spawning behavior may also aid in explaining variable, cruise- specific egg production estimates. Declines in egg production without changes in the gelatinous predator community may be the result of variable spawning effort. Black drum are batch spawners (see Chapter II] and while lunar cycle was not found to dictate spawning activity (Chapter II), variable, weekly egg production may be due, in part, to some unknown factor that influences spawning intensity. 154 Depressed gelatinous predator abundance apparently reduced their overall impact on the black drum egg population in 1991. Densities of N. bachei were negligible during the spawning season in 1991, consequently, mortality induced by gelatinous predators was mostly imposed by M. leidyi. Egg production declined dramatically on 22 May when average densities of predators was 10 m 1. Although these densities are an order of magnitude lower than those observed at the end of the 1990 season, ctenophores may potentially clear a much higher volume of water. Irregardless, overall declines in egg production in 1991 may have been more related to adult spawning activity/periodicity rather than predation by large gelatinous predators. 155 SUMMARY The population of black drum that enters Chesapeake Bay in spring is comprised of relatively few, large adults whose annual egg production (Chapter II) is comparable to the dally egg production of many fishes that spawn pelagic eggs (ie.-Houde 1977, Lo 1985, Kendall and Picquelle 1990). "Normal11 surface to bottom hydrographic conditions and ambient prey concentrations found on the spawning grounds are suitable for the survival of early life stages to at least 10 days post-hatch (Cowan et al. 1992, chapter II). Peak egg production of black drum occurs in the lowest portion of Chesapeake Bay where potential encounter with gelatinous predators is least likely (Chapter II, present study), an observation consistent between years with vastly different predator communities. However, net up-bay transport makes it more likely that an egg or yolk-sac larvae will encounter a predator some time during these early life stages. Consequently, a relatively small number of black drum eggs are spawned in lower Chesapeake Bay in spring and a potentially large fraction of those may be consumed by predators. Although gelatinous zooplankton predators may account for a large but variable fraction of the number of eggs consumed, unexplained sources of mortality may be substantial (Houde and Cowan 1994). While the remaining fraction of black drum eggs may hatch, yolk- sac and pre-flexion larvae are subjected to high rates of mortality, as are the products of any pelagic spawner (see Bailey 151 and Houda 198B for review) . The proportion of larvae that do survive, settle in remote, shallow, muddy backwaters where sampling is extremely difficult (Joseph et al. 1964, Olney and. Daniel 1991}. Small juvenile black drum grow rapidly in nursery areas, reaching 160-210 mm SL by early fall (olney and Daniel 1991} and larger fish are sporadically reported in pound net catches near the mouth of the Bay in October and November (personal communication with Local fisherman and Dave Boyd Virginia Marine Resources Commission) possibly on migration to overwinter in more southern waters. Reports of age II black drum returning to Chesapeake Bay the following spring are undocumented. 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Wiener, B.W. 1965 in McLane, A.J., ed. 1965. McClane's Standard Fishing Encyclopedia. Holt, Rhinehart and Winston, Inc., New York. 1057 pp. Zastrow, C.E., E.D. Houde and E.H. Saunders. 19BS. Quality of striped bass, (Morons saratiiis), eggs in relation to river source and female size. ICES 19B8 ELHS/No, 60. 167 VITA Louli Broiddua Du Ia I, ill Born in Hartford, Connecticutt on February 14, 1963. Graduated from the O'naal School in Southern Pines, North Carolina, 19B1, Received a B.A. in Biology from Wake Forest University in 1985. Entered the graduate program at the College of Charleston, Grice Marine Laboratory in August 1985 and received a M.S. in 1988. In August of 1988, he entered the college of William and Mary, Virginia Institute of Marine Science, School of Marine Science. He married Ruth in June 1989.