HABITAT !Jill FOOD RESOURCE UTILIZATION OF
THREE SPECIES OF EMBIGTOCIDS IN ELKHOPJ;
SLOUGH, MONTEREY BAY, CALIFORNIA
LIBRARY MOSS LANDING MARINE [j\I:,OHAI'orill~t~ PO, Box 450 MosS Landing, Califoml~ "y B~cok2 Stafford &,trim
A thesis
!, submitted iu partiaL
fulfil14nent of the ri-=;qu:i..!:~metlt5 for the degr€E of
Master of Arts ~n t~e DEpart~ent of Biology
California S~~te Universi~Yj FreSDv ACKNOWLEDGMENTS
I wish to express my sincere appreciation to all those who graciously provided their support and assistance throughout all phases of this study.
Thanks are due to Dr. Gregor Cailliet, for his guidance and friendship all these years. All the members of my committee provided comments which greatly improved this thesis. Special thanks to David
Ambrose for his camaraderie in and out of the field and to the many other students of Moss Landing Marine Laboratories who assisted in all phases of the field work. Gary Gillingham, Peter Slattery, Chris Jong,
John Cooper, and Michael Kellogg are all to be thanked for their expertise in helping with much of the invertebrate prey identification.
Jim Barry generously designed the computer programs necessary for handling the data involved in this study. The help of Lynn McMasters in providing all of the figures is greatly acknowledged. Mary Jane
Cavanaugh very adeptly typed the final manuscript.
Finally, I thank my wife Stephanie, not only for her editorial and typing assistance, but most importantly for providing the reason to complete this study.
This study was part of a grant funded by Pacific Gas and Electric
Company. TABLE OF CONTENTS
Page
LIST OF TABLES vii
LIST OF FIGURES ix
INTRODUCTION 1
SEASONALITY, ABUNDANCE, AND REPRODUCTIVE CONDITION 4
Materials and Methods 4
Field and Laboratory Procedures 4
Statistical Analysis Procedures 8
Results ... 10
Distribution and Abundance 10
Size Composition 11
Length-Weight Regressions 16
Fecundity ...... 18
Temperature/Salinity 23
Tagging/Recapture 23
Discussion 23
Distribution and Abundance 23
Length-Weight Regressions 27
Fecundity .. 29
Tagging/Recapture 31
RESOURCE UTILIZATION AND TROPHIC GENERALIZATION 32
Materials and Methods ...... 32 Vl
Page.
Field and Laboratory Procedures 32
Statiscical Analysis Procedures 34
Results ... 36
Cumulative Prey Analyses 36
Feeding Frequency .... 36
Total Number of Prey Categories and Prey Items 36
Major Prey Groupings and Seasonality 39
Dietary Variation Among Locations
Trophic Diversity 47
Dietary Overlap 50
Discussion 56
Cumulative Prey Analysis 56
Feeding Frequency .... 56
~~jor Prey Groupings and Seasonality 57
Dietary Variation Among Locations . 58
Trophic Diversity: Generalists vs. Specialists 62
Dietary Overlap 63
CONCLUSIONS 66
LITERATURE CITED 69
APPENDICES
A. INDEX OF RELATIVE IMPORT_~,CE (IRI) SUMHP~Y FOR CYK~TOGASTER AGGREGATA ...... "/ B. INDEX OF RELATIVE IMPORTANCE (IRI) SUMMARY FOR PHAJIERODON FURCATUS ...... HI
c. INDEX OF RELATI\~ IMPOR~A11CE (IRI) SUli~Y FeR EMBI070CA JAC~SCNI ...... 04 p
LIST OF TABLES
Table Page
1. Number of Tows for Each Date and Location in Elkhorn Slough ...... 6
2. Abundance and Seasonality of Three Surfperch Species from Elkhorn Slough, August 1974-0ctober 1975 . 9
3. Fecundity Data for Cymatogaster aggregata from Elkhorn Slough, August 1974-0ctober 1975 ...... 19
4. Fecundity Data for Phanerodon furcatus from Elkhorn Slough, August 1974-0ctober 1975 ...... 20
5. Fecundity Data for Embiotoca jacksoni from Elkhorn Slough, August 1974-0ctober 1975 ...... 22
6. Percent Similarity Index (PSI) Comparisons of Major Prey Categories by Season for the Bridge Station ..... 40
7. Percent 'Similarity Index (PSI) Comparisons of Major Prey Categories by Season for the Dairies Station ..... 41
8. Percent Similarity Index (PSI) Comparisons of Major Prey Categories by Season for the Kirby Park Station 42
9. Percent Similarity Indices of All Prey Categories for Each Surfperch Species; Within Season, Between Stations. 48
10. Trophic Diversity Summary 49
11. Index of Overlap, Percent Similarity Index (PSI) for Cymatogaster aggregata; Within and Between Stations, and Within and Between Seasons .... 51
12. Index of Overlap, Percent Similarity Index (PSI) for Phanerodon furcatus; Within and Between Stations, and Within and Between Seasons .... 52
13. Index of Overlap, Percent Similarity Index (PSI) for Embiotoca jacksoni; Within and Between Stations, and Within and Between Seasons ...... 54 ....._------
viii
Table Page
14. Percent Similarity Indices (PSI) of Diets Between Cymatogaster aggregata, Phanerodon furcatus, and Embiotoca jacksoni; Within Station, Within Season 55 LIST OF FIGURES
Figure Page
1. Map of Study Area and Sampling Stations ...... 5
2. Length Frequency Histograms for Cymatogaster aggregata by Station and Season .. . 12
3. Length Frequency Histograms for Phanerodon furcatus by Station and Season . . . .. 14
4. Length Frequency Histograms for Embiotoca jacksoni by Station and Season 15
5. The Cumulative Number of Prey Categories from the Pooled Number of Fish for Each Surfperch Species at the Ocean and Bridge Stations ...... 37
6. The Cumulative Number of Prey Categories from the Pooled Number of Fish for Each Surfperch Species at the Dairies and K;rby Park Stations ...... 38 2
Atherinidae. The feeding habits of three spec1es of the Embiotocidae
from Elkhorn Slough (the shiner perch. Cymatogaster aggregata Gibbons
1854, the white seaperch Phanerodon furcatus Girard 1854, and the black
perch Embiotoca jacksoni Agassiz 1853) have not been extensively studied,
but individual species from other locations have been investigated by a great many authors encompassing almost every aspect of their biology.
The geographic distribution of the three species of embiotocids
studied ranges throughout the eastern temperate Pacific. Cymatogaster aggregata, probably the most abundant embiotocid along the California coast, ranges from San Quintin Bay, Baja California, to Port Wrangell,
Alaska, and from the surface to a depth of 146 m (Miller and Lea, 1972).
Phanerodon furcatus, a very common embiotocid north of Point Conception, ranges from Point Cabras, Baja California, to Vancouver Island, British
Columbia, and from the surface to a depth of 43 m (Miller and Lea,
1972). Embiotoca jacksoni, a much more common embiotocid south of Point
Conception, ranges from Point Abreojos, Baja California, to Fort Bragg,
California, and from the surface to a depth of approximately 40 m
(Miller and Lea, 1972).
Studies concerned with resource partitioning have covered a variety of organisms in very diverse habitats (Keast, 1965 and 1970; Cody, 1974;
Schoener, 1974; Macpherson, 1981). Studies centering on the
Embiotocidae have been concerned with several different niche dimensions.
DeMartini (1969) primarily examined the diversity 1n feeding structure morphology for the entire family and related this to habitat and diet.
Alevizon (1975b) looked at the diet and feeding morphology of two geographically isolated populations of Embiotoca lateralis. In another 3
study, both spatial and dietary overlaps were examined for two pairs of
congeners and~. jacksoni and ~. lateralis were found to separate
spatially and overlap morphologically; while Rhacochi1us toxotes and
R. vacca separated morphologically and overlapped spatially (A1evizon,
1975a). Bray and Ebeling (1975) compared diets, habitat preference
and daily activity of two species of embiotocids (P. furcatus and
Brachyistius frenatus) and the senorita (Oxyju1is ca1ifornica) in a kelp-bed off Santa Barbara, California. Hixon (1978) demonstrated the extent of the "realized niche" (Hutchinson, 1957l when compared to the
"fundamental niche" by species removal experiments for .!::.. jacksoni and
E. 1atera1is from Santa Barbara. Ellison et a1. (1979) compared dietary overlaps of five species (~. furcatus, .!::.. jacksoni, ~. vacca, .Hypsurus caryi, and Micrometrus minimus) from King Harbor, California. Finally,
Hixon (1981) has demonstrated how resources were partitioned through the territorial behavior of large, male E. jacksoni off Santa Barbara by experimental habitat manipulations.
The objectives of this study were to provide information on how three species of embiotocids within Elkhorn Slough are able to segregate resources (space, food, and time) sufficiently to allow co-occurrence. In this context, seasonal information on the distribution, abundance, and feeding habits of these three species were inves tigated. SEASONALITY, ABUNDANCE, AND'REPRODUCTIVE CONDITION
Materials and Methods
Field and Laboratory Procedures
Four stations were sampled monthly for a period of 15 months from
August 1974 to October 1975, Three stations (called the "Bridge,"
"Dairies," and "Kirby Park") were within the slough proper, with the most inland station located 6 km from the mouth of the slough, and a fourth station ("Ocean") was just outside the harbor mouth, both to the north and south (Figure I),
The method of sampling was a 4.8 m otter trawl (38.1 mm stretch mesh with 12.7 mm stretch mesh liner). Tows were for either 5- or 10- minute durations into the tidal flow (if any), covering an average distance of 500-800~. Tows of 5 minutes were made from August to mid-December 1974. However, decreases in the winter month catches
(Cailliet et al., 1977) necessitated increasing trawl duration to 10 minutes to provide a sufficient number of individuals for stomach content analysis. The 10-minute trawling time was then retained for the remainder of the study period.
Each of the four stations Was sampled at least once monthly
(Table I), However, those stations with low catches had to be sampled more often to provide adequate numbers of fish. Concurrent with each tow, bottom temperatures were taken with a bucket thermometer (± O.SOC), and salinities were measured with a Goldberg temperature compensated 5
C . ' .. A L I F o R N
A
rigure 1. !-1ap of Stucy A=ea auG SaTI?ling Stations. Deli::ear:io!:.s Inci~2t2 Avera~e ~i5t2nce ~f ~ra~l D~r2ti8n. 6
Table 1. Number of Tows for Each Date and Location in Elkhorn Slough
OCEAN BRIDGE DAIRIES KIRBY PARK 1974 August 1* 2* 1* September 1* 1* 2* October 4* 1* 1* 2* November 4* 1* 1* 1* December 3* 1 3* + 2 3* + 3 1975 January 4 2 4 5 February 4 2 2 2 March 4 2 2 1 April 2 2 4 2 May 3 1 2 1
June 4 1 2 2 July 3 1 2 2 August 1* + 3 1 2 2 September 4 1 1 2 October 4 1 1 2 47 19 32 33
Otter Trawls N = 131 * = 5 minute tows. All other tows 10 minute duration. 7
refractometer (± 0.5 ppt). All sampling occurred during the day between
0800 and 1800 hours and covered all tidal periods.
All embiotocids captured were measured to the nearest mm (SL) and
weighed to the nearest 0.1 g before preservation. In order to assess
size frequency distributions, Cymatogaster aggregata and Embiotoca
jacksoni were grouped into 5 mm size class intervals per season, while
Phanerodon furcatus were grouped into 10 mm intervals per season due to
their larger size (Andersen, 1964). Those fish taken in a healthy
condition in excess of the number required for gut content analyses were
tagged with sequentially numbered and color-coded Floy spaghetti tags
and released. All fish retained for analyses were selected at random.
Specimens were preserved by injection of 10% formalin by means of a
pressurized syringe (Congdon et al., 1975), subsequently rinsed, and
then stored in 50% isopropanol. For larger specimens, the entire gastrointestinal tract and gonads were first removed and then preserved as described above.
At the time of stomach analyses, gonads were removed, patted dry with tissue, and allowed to air dry until all apparent excessive moisture was gone, approximately 5-15 minutes. Gonads were then weighed (± 0.005 g) on an electric balance and the weight recorded.
To assess reproductive condition of females, all ovaries were examined
for embryos. If embryos were found, they were counted and measured
(SL, mm) with an ocular micrometer. Male testes were weighed and it was noted whether they were immature, ripe, or spent.
Catch statistics standardized to 10-minute tows for the total of all fish species were calculated to express mean abundance for each Jt:;~'------
8
embiotocid species at each stati~n and each season. Randomly selected
fish were used to generate the length-weight regressions.
Data from the trawl collections and the gut examinations were
compiled seasonally (August-October 1974, November 1974-January 1975,
February-April 1975, May-July 1975, and August-October 1975) in a manner
similar to Ambrose (1976). These seasonal groupings are justifiable
since they closely approximate the abundance and distributional patterns
of the various fish species, as well as the normal climatic seasonality
of the study area.
Statistical Analysis Procedures
The percent abundance (%A) was calculated as the percentage that
each embiotocid species comprised of the total catch of all fish species
per season at each station (Table 2). The mean number of each
embiotocid species per 10-minute tow (N / tow ± standard error) was also
calculated per season at each station (Table 2).
First order log-log regressions were used to generate the equations
defining the length-weight relationships of the various species. To
determine if there were any sexual differences in regression equations,
an Analysis of Covariance (ANCOVA) comparison was made.
Length at age data of var10US authors were used to describe some
of the distribution patterns and fecundity values. Data from Odenweller
(1975) and Eckmayer (1979) were used for ~. aggregata, while Anderson
and Bryan (1970), Banerjee (1973), and Eckmayer (1979) were used for
P. furcatus, and Isaacson and Isaacson (1966) and Eckmayer (1979) for
E. jacksoni. r
Table 2. Abundance and Seasonality of Three Surfperch Species from Elkhorn Slough, August 1974 October 1975. %A = Percent Surfperch Abundance of all Species per Season; N/to" ± S.E. = Mean Number of Individuals per la-Minute TOI; per Season ±. Standard Error; N = Total Number of Individuals per Season
AUG ~ OCT 1914 HOV 1914 - JAN 1975 FEB ~ APRIL 1975 HAY - JULY 1975 AUG - OCT 1915 ~ Ji/tow is.£. .JL ~ HItOH i S.E. .JL ~ R/tow i S.E. .JL ~ Ii/tow i S.L .JL ~ R/tew i S.L .JL ..1.!L OCEAN ----C:-ll 9gregll ta 0 0 O.B 0.1 i 0.1 I 12.5 3.0 ± 2.5 30 1.4 0.3 t- 0.2 3 34 P. furcatus 5.0 5.0 :t: 0.6 2 6.3 0,4 t 0.4 4 O.B 0.1 t 0.1 I 6.3 1. 5 t 1.0 15 11.1 2.0 t 0.9 14 46 I·~ 0 0 0 0 0 0 UA --;:[j T.J 'T:'G' J8.I! rr:; BRIDGE ~g9regalll 17 .1 76.1 t 62.1 130 0.4 0.3 i 0.3 I 0.7 0.5 :t: 0.4 3 9.4 11. 3 t t.5 34 13.3 31.3 t 13.8 94 J61 P. urcllt'iJ'$ 15.3 109.3 t 72.9 31B 59.3 36.5 t 23.5 146 41.4 28,7 t 15.2 171 36.9 44.7 t 18.9 134 19.7 46.3 t O.A 139 919 r. Jacksonf 11.9 77.3 t 31.4 131 19.9 12.3 t 9.0 49 42.4 29.3 t 19.3 176 24.0 29.0 t 25.1 87 19.1 68.3 t 57,4 105 749 t'XA. !if."'§" 7U ~ 7Q.J 02.T DAIRIES ----c:Gggrega ta 30.3 20,0 :i 10,2 BO 29.6 10.3 :I: 9.6 103 24.0 5.3 t 4.7 41 23,7 19.0 t 15.3 tl4 38.8 43.0 ! 20.9 171 511 P. furcatus 24.2 16.0 :i 10.2 64 4B.3 16.8 i 6.B 16B 32,0 7.0 t 2.1 56 23.9 19.2 f 10.3 tI5 32 .5 36,0 i 6.3 144 547 I·~ 30.3 20.0 t 10.3 BO 9.1 3.2 f 1.7 31 12.0 2.6 ! O.B 11 6.1 5.0 ! 4.6 30 3.6 4.0 i 1.4 16 179 rIA IJlJi IlT.T ~ = 1D KIR8Y PARK c. !!l9regata 62.2 93.6 i 33.2 46B 19.B 5.6 ! 4.7 67 64.5 . 14.0 • 13.B 110 49.9 71.0 t 20.9 379 75.7 58.0! ]].6 34B 13112 P. furcatus I.t 1.6. I.t B 6.2 1.2 i 0.6 14 7.0 2.6 ! 1.5 13 3.2 4.9 :t 2.0 14 2.4 1.B ! O.B II 70 I·~ 11.2 16.8 ! B.2 B4 1.3 0.3 ! 0.2 3 0 0.1 0.2 i 0.1 I I.t 0.8 ! 0.3 5 93 r%A 74.! JI.J 7I.!f IT.'! Tn c. aggrega ta .. 2289 Ii. furcatu$ .. 1582 I·~ • 1021
'" 10
A monthly mean gonosomatic index (G.S.I.) was calculated for females as an indication of reproductive condition according to the following equation from Uiebe (1968):
ovary weight G.S.I. = 100. total body weight x
To determine if species abundance was correlated with either temperature or salinity, linear regression analyses were performed for all stations combined over the IS-month samplin~ period.
Results
Distribution and Abundance
Distinct seasonal abundance and microdistributional patterns were evident in the three species of embiotocids studied. Overall, the 131 otter trawls made from August .l974-Dctober 1975 captured 2,289
Cymatogaster aggregata; 1,582 Phanerodon furcatus; and 1,021 Embiotoca jacksoni (Table 2). Cymatogaster aggregata peaked in the summer (May
July) and early fall (August-October), and had lower abundances in the winter (November-January) and early spring (February-April) (Table 2).
In addition, £. aggregata, the numerically dominant fish in Elkhorn
Slough from August 1974 to June 1976 (Cailliet et al., 1977), was more abundant at Kirby Park, the most inland station, with abundances tapering off progressively seaward (Table 2). Essentially none were found at the Ocean station from August to April.
Phanerodon furcatus exhibited a similar seasonal abundance pattern to that of £. aggregata. High abundances were seen in late summer
(May-July) and fall (August-October), while depressed numbers were ~::;'------
11
observed in spr1ng (February-April) (Table 2). However, the
distributional pattern of P. furcatus for the slough proper was exactly
the reverse of that of C. aggregata, being concentrated at the seaward
Bridge station and tapering off progressively inland (Table 2). Though
present in all seasons at the Ocean station, ~. furcatus was found only
in very low numbers (~2.0 per 10-minute tow) (Table 2).
Seasonal abundance patterns for ~. jacksoni were similar to those
of Q. aggregata, however, their distribution paralleled that of
P. furcatus (Table 2). Peak abundances of ~. jacksoni occurred in the
fall (August-October) of both 1974 and 1975 at all three slough stations,
while the lowest abundance was recorded in late winter (November-January)
(Table 2). Embiotoca jacksoni was confined entirely to the slough
proper, being concentrated in the lower reaches of the slough and
declining in numbers towards Kirby Park (Table 2).
Size Composition
The distribution and abundance of different size classes of
C. aggregata followed a seasonal pattern. From November 1974 to April
1975, there were essentially no individuals smaller than 60 mm (SL) present
at any station (Figure 2). The most inland station, Kirby Park, during
spring had the highest seasonal mean standard length (92 ± 12.1 rom).
Newly spawned Q. aggregata were found at all stations in the summer,
yet Were highly concentrated at Kirby Park, where they were the dominant
size class. Other than the low abundance of young-of-the-year at the
Ocean in Summer (SL = 38 ± 8.5 rom), ~. aggregata was essentially not
present at that station. In the following season (fall 1975), all Cymotogoster oggregoto SO N-3Q N-2 50 N-O N-O N-I IT-SB sr.-3B±B.5 sr.-BO±9.2 40 Ocean 30 20 10
Ul 0 1-1~~~~~~ ~60 ~50 N-113 N-I N-9 N-4B N-94 IT-S4±12.7 SI-9S IT.-B5±9.2 sr.-B4 ±13.5 IT.-73±13.0 >40 Bridge 030 z -20 10 01 ,nIlIlIMb- so N-12B " N·340 lL 50 N-40 N-95 N-42 sr. -BO±B.O IT. -73122.4 tt·roil2.0 0 40 IT.-SI±B.O SI-7S±7.S Dairies 30 20 10 a: ~ 0 I "lr1I H I-- ~ so ,. z 50 N-211 N-76 N-122 N-39B N-354 rr-S519.5 IT- 7S j,7.2 IT.-92±12.1 ST.-so SI.-S4iI6,3 40 :!:255 Kirby 30 Park 20 10 o I dll'IItn - o 30 60 90 120 15 45 75 105 o 30 SO 90 120 15 45 75 105 o 30 60 90 120 S.L. lmm) S.L.lmml S.L. (mm) S.L.lmml S.L.lmm) Aug-Oct'74 Nov'74-Jan'75 Feb-Apr'75 May-Jul'75 Aug-Oct '75
Figure 2. Length Frequency Histograms for Cymatogaster aggregata by Station and Season .... N 13
slough stations were dominated by young-of-the-year fish (Figure 2).
Though lacking data for the spawn of 1974, this same pattern was
evident at the Bridge and Kirby Park stations in fall 1974. With the
exception of five individuals, £. aggregata was not caught in the
slough and nearby Ocean station after reaching 115 mm SL.
Phanerodon furcatus was the only species present at all stations
in all seasons (Table 2, Figure 3), and showed definite size-related
spatial as well as seasonal characteristics (Figure 3). At Kirby Park,
with the exception of three newly spawned individuals, all fish present
over the 15-month sampling period were at least 1+ years old (> 110 mm
SL). The major concentration of breeding adult P. furcatus in the
spring was found at the Bridge, with abundances tapering off with
increasing distance inland. During the previous winter, however, mature
adults were equally distributed between the Bridge and Dairies.
Spawning was initiated in the summer since young-of-the-year fish were
present at all four stations. However, the relative sizes of the number
of newly spawned ~. furcatus at the Bridge and Dairies did not reflect
the relative proportions of the adult populations at those two stations.
By fall 1975, the Bridge had twice the abundance of newly spa~~ed fish
as the Dairies. Tow samples for the Bridge in fall 1974 confirmed the
fact that juvenile 0+ age class fish congregated at the more seawardly
slough station. The maximum size of P. furcatus taken from any station was 230-240 mm SL (Figure 3).
Similar ~. jacksoni size classes occurred at the Bridge and Dairies
ln most of the five seasons (Figure 4). Although no individuals were
taken from Kirby Park ln the spring (Table 2, page 9), there was a very ,
Phanerodon furca/us
50j N·I N'7 N·I N-15 N'22 40 !IT:'131 j SI.·129± j !IT:'156 j IT·149±30.3 j SI.·148±31.9 Oceon 30 20 10 0
50~ N·171 N'216 N·174 N'141 N'144 40 ~ IT.·95±44.9 1 ~ SI.·136±40.8 1 IT'164±23D 1 5.C.·135±43.2 1 n iIT.·106±41.2 Bridge l/l 30 ~ 20 :> Q 10 > 6 0 ;;
50~ N'31 N'169 N'55 N'120 Nol68 40 lIT.·143±44.0 1 iIT.·158±31.4 1 iIT.·168±2e.3 j iIT.·134 ±50.7 1 iIT.132±42.7 Dairies \5 30 20 10 a: lM 0 ::; ~ 50~ N'2 N'IO N'13 N-24 N-Il 40 iIT.·180±29.0 1 SI.·163±19.6 1 !IT.ole2±57.4 1 iIT.oI5e±37.2 1 5.C.oI47±24.7 Kirby 30 Park 20 10 0 20 SO 140 200 260 50 110 ITO 230 290 20 so 140 200 260 50 110 110 230 290 20 SO 140 200 260 S.L. (mm) S.L.(mml S.L.(mml S.L.(mm) S.L. {mml Aug-Oct'74 Nav'74-Jan'75 Feb-Apr'75 May-Jul'75 Aug-Oct'75
Figure 3. Length Frequency Histograms for Phanerodon furcatus by Station and Season +-""' r
Embiofoca jacksoni 60 N-1I6 N-49 j. N-179 N-9O N-204 :1 !f.[.-96±34.2 j !f.[.-150±31.3 !IT:-132±26.B j IT-B4 ±35.4 j ~ !IT.-9B±2B9 Bridge 30 20 '3 10
Figure 4. Length Frequency Histograms for Embiotoca jacksoni by Station and Season ..... en 16 homogeneous mixture of size classes the other four seasons (Figure 4). spawning occurred in summer at the Bridge and Dairies and coincided with
the entry of the dominant 0+ age class into the population (Figure 4).
When considering the growth increment over 3 months, the same size classes (0+) the following season (fall 1975) were again dominant at the
Bridge and Dairies. Although I lack data for the summer 1974 season, a similar pattern was also evident at the Bridge in fall 1974. The first newborn individuals (six fish, 47 rom) to be captured were taken 12 June,
1975, at the Bridge. The smallest seasonal mean standard length (SL =
84 ± 35 rom) was also found at the Bridge in summer 1975 (Figure 4). The largest seasonal mean standard length (SL = 158 ± 21.8 rom), however, was found at Kirby Park in the winter, the Same station which produced the
.lowest abundances throughout the sampling period (Table 2, page 9). The largest !. jacksoni taken from any station, any season was 212 rom SL,
23 August, 1974, at the Bridge.
Length-Weight Regressions
For C. aggregata, regressions were generated for 143 males
(32-104 mm) and 92 females (32-111 rom), producing correlation ("r") coefficients of 0.992 and 0.993 respectively. The ANCOVA indicated no significant difference between male and female equations with respect to slope (F 1 F 05 [2,231] 2.00) or intercept (F 1 F 05 1,232] ca c. = . = ca c. = . = 0.93). Therefore, the length-weight regression equation for fresh individuals generated was formulated using both ~le and female data points, and is as follows: 17
Log W = -4.7812 + 3.1247 (Log L) or
W = 1.7 X 10-5 L3.1247
N = 235
r = 0.992
where W = wet weight (g) and L = standard length (mm).
For P. furcatus, regression equations were generated for 71 males
(49-206 mm) and 98 females (46-219 mm) producing correlation coefficients
of 0.997 and 0.995 respectively. A.~COVA comparison of the two log-log
regressions showed no significant difference between the two with respect
to slope (F . F. [2,165] 1.29) or intercept (F . F. calc = 05 = calc = 05 [1,166] = 0.18). The length-weight equation therefore is derived using
the summed data points of males and females and is as follows:
Log W = -4.7175 + 3.0653 (Log L) or
W = 1.9 X 10-5 L3.0653
N = 169
r = 0.996
Regression equations on 54 male (54-197 rom) and 53 female (53-191 mm)
E. jacksoni produced correlation coefficients of 0.994 and 0.997 respectively. ANCOVA comparison of the two equations again showed no
significant differences between males and females with respect to slope
(F . F. [2,103] 0.43) or intercept (F . F. [1,104] calc = 05 = calc = 05 = 0.08). The length-weight equation therefore uses both male and female values and is as follows:
Log W = -4.7064 + 3.1486 (Log L) or W = 2.0 X 10-5 L3.1486 N = 107 r = 0.995. 18
Fecundity
Spawning for each of the three speC1es of embiotocids studied in
Elkhorn Slough occurred at approximately the same time, from late April
to mid-July, though there were differences among the species as to the onset of the cycle.
Female ~. aggregata in Elkhorn Slough released their young primarily in the months of June and July (only one gravid female was found in August). A total of 787 ~. aggregata embryos were measured from 88 gravid females ranging in size from 73-116 mm SL (Table 3).
Embryos were first observed in ovaries on 12 February (SL = 6.9 ± 1.7 mm) and were not found in any ovaries after 15 August. The mean monthly number of embryos per gravid female steadily declined from February to
July (Table 3). The overall mean number of embryos per gravid female was 8.94 ± 3.01 (range = 4-17). The'two largest embryos found were
36 mm SL taken from a female 100 mm SL and weighing 30.8 g on 9 July.
The smallest free-swimming ~. aggregata captured was 29 mm SL and 0.7 g taken 12 June at Kirby Park. The first evidence of young-of-the-year
~. aggregata at any station was one individual (31 mm SL, 1.0 g) taken
6 June at Kirby Park. The mean G.S.I. (gonosomatic index) values for the months of June and July 1975 were also significantly higher than the prior 4 months, further substantiating the June-July spawning period (Table 3).
Phanerodon furcatus females in Elkhorn Slough released their young from late April to early July. A total of 508 embryos were obtained from 40 gravid females measuring from 137-257 mm SL (Table 4). Embryos were first observed in ovaries on 12 February (SL = 9.4 ± 6.5 mm, from Table 3. Fecundity Data for Cymatogaster aggregata from Elkhorn Slough, August 1974-0ctober 1975. Values Are Expressed as ~Ieans ± Standard Deviation
ADULTS EMBRYOS Mean Mean Mean Mean Mean Month N S. L. (rnrn) Wt. (gms) Ovary Wt.(gms) G.S. I. N Mean N S.L. (rnrn) February 1975 3 101. 7 ± 6.5 31.23 ± fi.20 0.35 ± 0.09 0.90 ± 0.24 42 14.0 ± 2.7 .6.9±I.7 March 1975 19 102.5 ± 10.B 32. B6 ± 9.97 0.67 ± 0.43 1.63 ± 1.00 233 12.3 ± 1.B 9.1 ± 3.1 April 1975 26 B7.7±11.1 20.23 ± B.BO 0.42 ± 0.59 l.B5 ± 1. 5B 220 B.5 ± 2.7 10.0 ± 3.5 May 1975 14 B9.4 ± 12.4 21.74 ± 10.35 2.3B ± 3.55 7.55 ± B.76 II 111 7.9 ± 2.2 17.4±B.B June 1975 14 B9.9 ± 5.3 21. 73 ± 5.14 4.33 ± 2.36 19.23 ± B.45 104 7.4±1.2 2B.5 ± 4.4 Jul y 1975 11 B9.B ± B.7 24.32 ± B.52 4.70 ± 2.36 1B.75 ± 4.03 72 6.6 ± 2.5 30.3 ± 3.3 AU9ust 1975 1 91.0 29.00 6.44 22.00 5 5.0 34.0 ± 1. 0 BB 7B7
..... '" ·······7~.!l'1 r .. <.< .. /ii'.' '!~~.~
Table 4. Fecundity Data for Phanerodon furcatus from Elkhorn Slough, August 1974-0ctober 1975. Values Are Expressed as ~Ieans ± Standard Deviation
ADULTS EMBRYOS Mean Mean Mean Mean Mean Month N S. L. (mrn) Wt.(gms) Ovary Wt. (gms) G.S .J. N Mean N S.L.(mrn)
February 1975 8 208.4 ± 38.8 296.26 ± 142.71 1.47 ± 1.75 0.43 ± 0.33 103 12.9 ± 5.4 8.1 ± 5.3
March 1975 7 174.0 ± 23.7 162.03 ± 82.22 1.20 ± 1.32 0.59 ± 0.39 99 2B.4 ± 37.6 7.8 ± 4.5
Apri 1 1975 6 176.5 ± 34.4 162.63 ± 96.91 8.42 ± 10.56 3.5B ± 3.33 89 14.8 ± 10.2 23.4 ± 10,3
May 1975 11 17B.9 ± 17.2 165.82 ± 52 ..23 14.66 ± 13.01 7.61 ± 5.11 143 12.1 ± 3.8 33.7 ± 9.1 June 1975 7 16B.6 ± 23.9 141.49 ± 62.66 12.65 ± 9.17 B.12 ± 3.54 64 9.1 ± 3.4 39.0 ± 6.4 July 1975 1 197.0 235.7 31.77 13.22 - 10 10 51. 0 ± 1.0 40 508
N >-' i?c'_--IiIIIIIiII------~
21
five females) and were not seen in any female ovaries after 9 July
(Table 4). As with C. aggregata, there was a general decline ~n mean
monthly number of embryos per gravid female from February through
August (Table 4). The overall mean number of embryos per gravid female
was 12.51 ± 6.49 (range = 5-34). The largest embryos found within an
ovary were three individuals 52 rom SL, taken 9 July from a female
197 rom and weighing 235.7 g. The smallest free-swimming P. furcatus
captured was 34 rom SL taken 24 April at Kirby Park; this individual was
also the first evidence of young-of-the-year ~. furcatus at any of the
four stations. Spawning size therefore probably ranges from 34 to
approximately 52 rom SL. Recalling the June through July spawning time
of ~. aggregata, there appeared to be a lag period of approximately
5 weeks between the onset of spawning for P. furcatus and C. aggregata.
Newborn E. jacksoni in Elkhorn Slough were found during early
summer (June-July) when the embryos were approximately 45 rom SL or
larger (Figure 4). A total of 203 ~. jacksoni embryos were collected
from 18 gravid females ranging in size from 135-170 rom SL (Table 5).
The overall mean number of embryos per gravid female was 11.28 ± 3.01
(range = 7-18). The paucity of gravid females is due in part to the
relatively subdued abundances of ~. jacksoni within Elkhorn Slough from
January through August (Table 2, page 9). Embryos were first observed
in one female 151 rom SL and 142.7 g on 30 December, 1974 (14 embryos,
SL = 4.4 ± 0.1 rom) and were not found after 12 June (Table 5). The
largest embryo found within an intact ovary was 42 mm SL, taken 12 June
from a female ~. jacksoni 160 rom SL and 189.8 g. The smallest
free-swimming individuals captured (six) were 47 rom SL at the Bridge, '~1'V':,~
Table 5. Fecundity Data for Embiotoca jacksoni from Elkhorn Slough, August 1974-0ctober 1975. Values are Expressed as Means ± Standard Deviation. The Two Sample Sizes in Parentheses Are for the Number of Individuals of the Month's Sample for Ifhich a G.S.I. Value Could Be Calculated
ADULTS EMBRYOS Mean Mean Mean Mean Mean Month N S.L.(mm) Wt. (gms) Ovary Wt.(gms) G.S.1. N Mean N S.L.(mm) December 1974 1 151.0 142.70 0.34 0.24 14 14.0 4.4 ± 0.1 January 1975 B 155.6 ± 10.9 NA 0.4B ± 0.25 NA 102 l2.B ± 2.9 7.4±3.6 (4) 155.0 ± 14.9 15B.48 ± 44.96 0.31 ± 0.15 0.19 ± 0.04 (54) 13.5 ± 3.1 6.5 ± 1.8
February 1975 2 156.5 ± 2.1 NA 0.90 ± 0.11 NA 22 11.0 ± 1.4 14.4 ± 1.6 (1) 155.0 154.90 0.82 0.53 (10) 10.0 13.3 March 1975 2 146.5 ± 3.5 138.20 ± 15.41 1.26 ± 0.35 0.91 ± 0.15 15 7.5 ± 0.7 17.7 ± 1.3 April 1975 3 154.0 ± 7.9 152.60 ± 23.87 10.56 ± 1.00 6.97 ± 0.57 26 8.7 ± 0.6 36.9 ± 2.5 May 1975 1 167.0 197.80 23.44 11.85 15 15.0 37.9 ± 2.5 June 1975 1 160.0 189.80 15.77 8.31 - 9 9.0 40.2 ± 1.0 18 203
N N 23
12 June. Since the largest embryos found were smaller than the minimum
size of 47 rom there was justification for assuming that the spawning
period extended beyond June (the last observed gravid female).
Temperature/Salinity
Bottom temperatures were found to range from a low of lloe in
December at the Bridge and Dairies to a high of 21.7°e in September at
Kirby Park. Salinities ranges from a low of 28.5 ppt in February at the
Bridge (a time of high precipitation and high land run-off) to a high of
38.0 ppt in September at Kirby Park (evidently due to high aerial temperatures and long water residence time resulting in evaporation).
Abundances of any of the three species of embiotocids were not significantly correlated with either temperature or salinity.
Tagging/Recapture
A total of 96 ~. aggregata, 380~. furcatus, and 350 ~. jacksoni were tagged and released in Elkhorn Slough over" the 15-month period from August 1974 to October 1975. Of the 836 tagged fish, only six
~. jacksoni were recovered (0.72%) from subsequent otter trawls or local fishermen. The mean number of days to recapture was 75.8 ± 56.6
(standard deviation), with all fish being taken from the location at which they were tagged. Information on growth was not obtained.
Discussion
Distribution and Abundance
The microdistribution and abundance of these three embiotocid species varied with season and size classes, however, the- relative roles 24 of each variable is uncertain. Seasonal abundance patterns of
Cvmatogaster aggregata were similar to those patterns described from
Southern California in that high numbers of individuals were found ~n summer and early fall and low numbers in winter (Bane and Robinson,
1970; Allen and Horn, 1975). Apparently, low winter abundances were due
to an offshore migration of the majority of the population, while a small percentage remained within the slough. Bane and Robinson (1970) reported a similar winter migration for ~. aggregata in Newport Bay,
California, with reproductively mature fish entering the bay in the spring to spawn.
Size composition helps explain SOme of the distribution and abundance patterns of ~. aggregata. The spr~ng concentration of individuals at Kirby Park (seasonal mean standard length = 92 ± 12.1 ~) were evidently mature, reproductive adults (Eigenmann, 1894; Hubbs, 1921;
Hubbs and Hubbs, 1954; Gordon, 1965; Wilson and Mi11eman, 1969; Ander~on and Bryan, 1970; Bane and Robinson, 1970; Shaw, 1971; Shaw et al., 1974;
Odenwe11er, 1975; Ec~yer, 1979; Darling et al., 1980). Therefore, following the low winter abundances, adult, reproductive ~. aggregata enter the slough in spring and congregate at Kirby Park, where the following season they release their young.
It is apparent that adult ~. aggregata emigrate from Elkhorn
Slough after thEir second year since lIS mID SL individuals were the largest size class captured. Bane and Robinson (1970) found that the population of ~. aggregata in Newport Bay, California, was similarly composed of primarily 0+ and 1+ age class individuals. Odenwellzr
(1975) found that C. aggregata £ro~ Anaheim Bay, California, grew to 25
approximately 117 rom (SL); an age of 2.5 years. Wilson and "Milleman
(1969), however, reported that 115 rom (SL) fish in Yaquina Bay, Oregon, were 3+ years old. Anderson and Bryan (1970) found that 115 rom (SL)
fish from Humboldt Bay, California, were approximately 4+ years old.
Wilson and Milleman (1969) suggested that maturity may be dependent on size rather than age. If this is the case, then increased prey densities or increased growth from warmer water temperatures might account for the length at age discrepancies found in the maximum length sizes of other studies.
The seasonal abundance patterns of Phanerodon furcatus within
Elkhorn Slough, were similar to those of ~. aggregata but P. furcatus was more abundant seaward and not subject to the same magnitude of seasonal fluctuations as ~. aggregata (Table 2, page 9). Phanerodon furcatus in the slough apparently do not undergo the winter migration into offshore deeper water that ~. aggregata exhibit and therefore appear to be year-round residents of the slough. In the winter Season mature, reprOductive individuals were equally dispersed between the Bridge and the Dairies. However, in the spring just prior to spawning most reproductive adults were concentrated at the Bridge. Once spawning occurred (late April-early July), and newborn P. furcatus were found at all stations, there was apparently a migration of these young-of-the year fish to the Bridge the following season (August-October 1975)
(Figure 3, page 14). As with adult populations of ~. aggregata and
~. furcatus, there also appeared to be a spatial separation of newly spawned fish, with juvenile ~. furcatus at the Bridge and juvenile
C. aggregata at the Dairies. Since P. furcatus as large as 230-240 rom, iF' ...... ;.:. fIIIII'------~~---
26
which are a minimum of 6 years old (Anderson and Bryan, 1970; Banerjee,
1973), were found within the slough; it appears that ~. furcatus not only
occur within the slough year-round, but may spend a majority of their
lifespan there as well.
The seasonal abundance of Embiotoca jacksoni within the slough was
similar to C. aggregata: high fall concentrations and reduced winter
populations. The low winter abundance of ~. jacksoni within the slough
may also be an indication of an offshore migration into deeper water.
However, since none of the offshore trawl samples in 15 months collected
any ~. jacksoni, this migration must be rapid and to some other area
than the immediate offshore Ocean station. The rather homogeneous
mixture of size classes of ~. jacksoni at Kirby Park, in combination
with the low number of individuals, is indicative that the upper reaches
of the slough are neither a prime feeding nor spawning area~ for
E. jacksoni.
Spatial overlap of ~. jacksoni and ~. furcatus appeared maximized
since both species were very abundant at the Bridge station. When the
young-of-the-year ~. jacksoni enter the population in the summer, their
abundance and subsequent growth at the Bridge the following season
(August-October 1975) is reflective of the initial parental population
levels. One possible explanation for the seaward residency of juvenile
E. jacksoni is the extensive Zostera beds on the mudflats of the Bridge
station and the probable predator protection that these eelgrass beds
afford to newborn, cryptically colored fish. As with ~. furcatus, it
appears that ~. jacksoni utilize Elkhorn Slough for a good portion of
their lives, since individuals of sizes between 210 and 215 rom (SL) ---
27
occurred there, which by conservative estimates must be 4+ years old
(Isaacson and Isaacson, 1966; Eckmayer, 1979).
Length~'eight Regressions
Comparisons with other studies indicate that length-weight
relationships of ~. aggregata from Elkhorn Slough are more similar to
populations from Northern California than Southern California (Anderson
and Bryan, 1970; Odenweller, 1975; and Eckmayer, 1979). For a given
length, fish from Humboldt Bay (Anderson and Bryan, 1970) were found to
weigh slightly less than the fish from Elkhorn Slough. The minor
variations between the two equations might be due to latitudinal
differences in water temperatures or differences in methodology. There
is a substantial difference between the equation generated herein and
that formulated by Odenweller (1975). Cymatogaster aggregata from
Anaheim Bay, California (Odenweller, 1975), were found to be heavier for
a given length than fish from Elkhorn Slough. The equation presented
for male ~. aggregata by Eckmayer (1979) (also Anaheim Bay) ~s m close
agreement with the equation presented here; however, the equation
representing females (N = 122) would generate values that would be
somewhat lighter for a given length than what was found for fish from
Elkhorn Slough.
The length-weight equation generated for ~. furcatus from Elkhorn
Slough is similar to populations from Northern and Southern California
(Anderson and Bryan, 1970; Banerjee, 1973; and Eckmayer, 1979). The
equation that was generated for ~. furcatus in Elkhorn Slough is
relatively close to the equation produced by Anderson and Bryan (1970). 28
nD~ever, as was the case for female ~. aggregata, !. furcatus from the
slough tend to be somewhat heavier than the fish from Humboldt Bay.
since Anderson and Bryan (1970) did not find any sex-related growth
differentials until fish reached their second year, this may explain why
there is no difference between the male and female equations in the
current study. Similarly, Banerjee (1973) found no sex-weight differences for 0- to 3-year-old fish from Tomales Bay, California, while
P. furcatus of the same size from Elkhorn Slough were found to be heavier. Part of the sex-weight discrepancy may be accounted for by different methodologies, since some studies use fork length and others standard length. The separate equations that Eckmayer (1979) generated for males and females are in almost exact agreement with the equation presented by Anderson and Bryan (1970), and therefore rather close to the one presented herein. Part of the discrepancy between the current study and those found in the literature may possibly be due to inadequate sample sizes, or that not enough older fish were analyzed in order to complete the equation.
The length-weight equation presented for ~. jacksoni is in very close agreement to the relationship Eckmayer (1979) described for male
~. jacksoni from Anaheim Bay, California. This similarity to Eckmayer's
(1979) data is probably due to the lack of a sufficient number of heavier, gravid females. However, Isaacson and Isaacson (1966) also found no difference in growth rates for E. jacksoni from Long Beach
Harbor and San Francisco Bay. 29 fecundity
Apparently C. aggregata from more northern latitudes give birth to larger individuals than fish from the more southern part of the range.
Wilson and Milleman (1969) reported that ~. aggregata from Yaquina Bay,
Oregon, were 39-49 mm TL (total length) at birth. Having first converted measurements from TL to SL, the difference reported for size at birth may be due to discrepancies in the measurement methods (i.e., shrinkage from preservation).
There is good agreement with other studies of ~. aggregata concerning the mean number of embryos per gravid female and the ranges of those means. Wilson and Milleman (1969) found exactly the same mean number of embryos (8.9) per gravid female. The finding for ~. aggregata
,that the number of embryos per female is a direct function of increasing female size is in agreement with several authors (Hubbs, 1921; Gordon,
1965; Wiebe, 1968; Wilson and Milleman, 1969).
Spawning time of C. aggregata, however, does vary with latitude, normally occurring later in the colder, more northern part of the range.
Studies concerned with fish from Southern California (Bane and Robinson,
1970; Odenweller, 1975) show that ~. aggregata spawn there in the spring
(March-May) while the studies of ~. aggregata from Elkhorn Slough,
Northern California (Shaw et al., 1974), Oregon (Wilson and Milleman,
1969), and British Columbia (Gordon~ 1965; W,iebe, 1968) show these more northerly populations tend to be summer spawners. This difference could easily be explained by the earlier warming of the more southerly waters
(Terry and Stephens, 1976).
The spawning time for P. furcatus further helps to separate 30 populations of C. aggregata and P. furcatus. Phanerodon furcatus
initiate spawning approximately 5 weeks earlier in the season (late
April-early July) than do C. aggregata (June-July). Adult populations of both species are centered at opposite ends of the slough during spawning season, as are their newly spawned offspring; ~. aggregata at
Kirby Park and ~. furcatus at the Bridge and Dairies. This spatial and temporal separation may reduce any potential competition that might arise if other niche dimensions overlapped.
For ~. furcatus, other studies report somewhat different results regarding the mean number of embryos per gravid female, the mean size at birth, and spawning time. Goldberg (1978) found that P. furcatus from
El Segundo, California, spawned from late May to early July with a mean of 6.9 embryos per female (range = 3-11) and a mean size of 53 rom SL at birth. Banerjee (1971) working in Tomales Bay, California, found the mean number of embryos to be 19.6 (range = 8-32) and a mean size at birth of 43.5 rom SL; spawning time ranged from ea~lyMay-mid-August (1 month later than that found in the current study). Though my sample size from
Elkhorn Slough is small, it does show more variation than was found in other studies: overall mean number of embryos per gravid female
12.51 ± 6.49 (range = 5-34), spawning size 34 to approximately 52 rom SL, with spawning occurring from late April to early July. Goldberg (1978) did find a positive correlation for the number of embroys per gravid female, in that larger fish spawning earlier in the season had a higher number of embryos than did smaller fish. However, this correlation was not readily apparent in the current study.
Fecundity values obtained for E. jacksoni from this study may 31 suffer from small sample size, yet there are certain valuable comparisons with other studies. The overall mean number of embryos and the range of that mean are certainly comparable to those estimates produced by
Isaacson and Isaacson (1966) for E. jacksoni from Long Beach and San
Francisco, California, and those of Behrens (1977) for Half Moon Bay,
California. However, both size at birth and the time of spawning differ between this study and those of the two previous authors. Size at birth in Elkhorn Slough was estimated to be at least 42-47 mm SL, 14% less than that found by Isaacson and Isaacson (1966) and Behrens (1977).
This could be due to the small sample size; however, six 47 mm SL free swimming individuals were taken, strongly suggesting that the estimate is valid. The other point of disparity is the time of spawning.
Isaacson and Isaacson (1966), since they did not find any modal progression in the growth of embryos, assumed that E. jacksoni from
Long Beach and San Francisco spawned all year long. It is hard to account for the major time difference found·between my estimate of
June and July for spawning and that of Behrens (1977), who found that in
Half Moon Bay (approximately 60 miles to the north of Elkhorn Slough),
E. jacksoni spawned from mid-September to mid-November.
Tagging/Recapture
It is fairly evident from direct observation that there is a high tagging mortality associated with ~. aggregata and therefore only a minimal number of ~. aggregata individuals were tagged. No growth information was obtained for ~. jacksoni due to the discrepancies of size reported at time of recapture. RESOURCE UTILIZATION AND TROPHIC GENERALIZATION
Materials and Methods
Field and Laboratory Procedures
The collection methods, sampling sites and times, and preservation techniques were the same as previously mentioned. For laboratory analysis the entire gut was removed by incisions between the esophagus and anal opening. The gut was then cut longitudinally and with the contents intact, stomach fullness was subjectively scored as: 0 = 0%, 1 = 25%,
2 = 50%, 3 = 75%, and 4 = 100% (DeWitt and Cai11iet, 1972). State of digestion was scored as: 1 = very finely digested, nothing recognizable; 2 = moderately digested material, Some recognizable parts;
3 = little digested, some undigested material; 4 = undigested, whole animals (DeWitt and Cai11iet, 1972). Prey items were then removed, identified to lowest possible taxa, measured (± 0.05 mm) using an ocular micrometer, and counted.
Percent volume of each prey category was then subjectively estimated by eye, considering the contents of the entire gut as unity (Bray and
Ebeling, 1975). This procedure was followed since a high proportion of the prey items were found to be in various states of digestion. Due to the large percentage of unidentifiable digested material and inorganic debris that the various species ingested, gravimetric or displacement volume estimates would have been highly impractical. The inherent biases of the subjective estimates were minimized in that only one 33
person was involved in the analysis of a large number of fish Over a
long period of time. Since prey with various states of digestion were
often found in one gut, much of the undigestable material was spread
throughout the intestine, and many fish without material in the stomach
had food in the intestine; prey items were examined and counted from
the entire length of the gastrointestinal tract.
Even though many of the prey items within the intestine were
highly fragmented, they usually could be identified to the family or
generic level. If digestion was too far advanced, identifi~ation was
made only to class (i.e., Crustacea, Mollusca, &~d Polychaete). Some
digested polychaetes were not identified to family or genus since there
were massive numbers of fragments and for the purposes of this study
WEre considered "unidentifiable." To provide the most conservative
estimate of these fragments and others, a standaraizatiou of counts was
established for specific groups. Perica~idean crustaceans snd
polychaetes were counted by the number of haads, bivalves by attached
hinges, and decapod crustaceans by carapaces.
Identifications of difficult prey species were verified or
accomplished by several invertebrate taxonomists at Mess Landing ~~rine
Laboratories. Voucher specimens were kept for all of the individual
prey categories. To ensure a representative sampling of the mejor ?rey
categories that were eaten by each fish during eech season and at each
loca~ion, randomly pooled gut contents of each fish species at each
station were plotted agains~ the cumulative fiurnber of prey categories
and the point at which the cu~ve leveled off was ~Dted. l 34
Statistical Analysis Procedures
The Index of Relative Importance (IRI) was calculated for each prey
category for food-containing fish as a combination of the numerical and volumetric importance and frequency of occurrence (Pinkas et al., 1971).
To calculate the most conservative estimates for the above index, no cut-off points were established for abundance or frequency of occurrence of the various prey categories. The numerical importance
(%N) of a particular prey item was the percentage ratio of its abundance relative to the total abundance of all items ln one stomach.
The volumetric importance (%V) was its average percent estimated volume.
The percent frequency of occurrence (%FO) was the percentage of fish found to contain at least one individual of a particular prey category.
The IRI value was calculated by summing the percent numerical and volumetric values and multiplying that sum by the percent frequency of occurrence: IRI = (%N + %V) %FO. Values of the IRI range from 0-20,000.
To facilitate comparisons between each pair of species, seasons or locations, IRI values for each group were converted to percentages.
Digested material and sediment debris were eliminated as prey categories from all comparisons.
Several indices of prey diversity were calculated for each species at each station during each sampling period. One diversity index was the information function:
H' = - ~ p. ln p., 1 1 where p. = n. /N (Shannon and Weaver, 1963). An estimate of evenness 1 1 35 was obtained using:
H' JI = H' max where H' 1n Sand S number of species (after Pie1ou, 1969). An max = = estimate of the extent to which individual prey categories dominated each seasonal sample was calculated as percent dominance: 2 %D = p. , :L after Odum (1971). An estimate of niche breadth was calculated following
Levins (1968):
1 B =
The Percent Similarity Index (PSI) (Sanders, 1960; Silver, 1975;
McEachran et a1., 1976) was used to compare the degree of similarity of prey arrays for different predators between stations as well as between seasons:
PSI = ~min p. , :L where min p. = the smaller value in comparison between two prey arrays :L of a particular prey category. Values range from 0, indicating complete un:Lqueness to 1.00, indicating total similarity. Since all prey categories identified were retained in the prey analysis, regardless of
%IRI composition, overlap was arbitrarily deemed significant at the 0.40 level. Cai11iet and Barry (1978) have shown the PSI index to be the most appropriate and simplest index for comparisons of predators that feed on unequal numbers and proportions of prey items. To test for inter- specific dietary overlaps, comparisons were made only for within station, and within season. 36
Results
cumulative Prey Analyses
Cumulative prey curves for all bnt the Ocean-caught fish leveled
off well below the number of guts analyzed (Figures 5 and 6). At the
Ocean station, however, there was an insufficient number of both
Cymatogaster aggregata and Phanerodon furcatus caught to guarantee an
adequate sample size'of guts, therefore fish from that station will not
be discussed further. In general terms, the number of prey species
utilized by each of the embiotocid species decreased from the Bridge to
Kirby Park and therefore the number of predator stomachs needed to
adequately describe the respective prey arrays also decreased with
increasing distance inland (Figures 5 and 6).
The number of fish examined (with contents), the total number of
prey categories per season, the total number of prey items per season,
along with the calculated IRI value, %IRI value, and rank of all prey
categories are presented in Appendices A-C.
Feeding Frequency
There were very few empty stomachs found for any of the embiotocid
species throughout the year. Only 6.6% of the 547 C. aggregata, 0.9% of
the 347 P. furcatus and 0.8% of the 242 Embiotoca jacksoni gastro-
intestinal tracts Were empty. No differences were found in the
percentage of empty guts among localities for any species.
Total Number of Prey Categories and Prey Items
A total of 163 prey categories and 194,986 prey items were identified I 60 (J) w ~40 J Phanerodon {urcafus C) w OCEAN l- e{ 20 o .....--..- Cymatoqasfer aqqreqafo >- IJJ 111__-.-_-..__..,.-_....._--, D: Or I j , j , n. 0 20 40 60 80 100 Ii..o lr IIJ 80 ~ Embiofoco jacksoni III ";J: :i: P{urcafus :l • ~60 laJ ___ C. oqgregafa > .....--I}-/ BRIDGE I- 40
o -I " r i -,---, ,------T .------1 o 20 40 60 80 100 120 140 160 180 200 POOLED NUMBER OF STOMACHS
Figure 5. The Cumulative Number of Prey Categories from the Pooled Number of Fi.h for J<:ach W Sl1rfperch fipecies at the Oeean anc.l Bridge StatiollB --.J ~."'_·~C.,, 61 \>~' ;-~-L!i ..... ,;.'."-".,.",, ;.- ....2"'9<'" j''¥N 2:1' t"6i H'ire *Ciierliiiitil pent-W ,Lk <+9'*\[-' ':'6- : <;(. -54 It r ;:C"·7 ;":-p.'; 'ff) -:¥\W-:,!' r;i-<',::,-?; -YW!jt;t,W "I \,''71, ~
(J) 80 I.IJ Pfurcotus 0:: ....-r- E.jocksoni, II g60 .-r-'-=--~ I.IJ J. .r .-Jo____=__ C oggregoto ~ 040 >- DAIRIES I.IJ g: 20 lI.. o 01' iii iii I f5 0 20 40 60 80 100 120 140 m :::E :::l C. oggregoto Z 60 w > ~ 40 ...J Pfurcotus :::l ~ E.;ocksoni KIRBY PARK ~ 20 o
a F iii iii i i J i II o 20 40 60 80 100 120 140 160 180 200 220 240 POOLED NUMBER OF STOMACHS
Figure 6. The Cumulative Number of Prey Categories from the Pooled Number of Fish for Each Surfperch Species at the Dairies and Kirby Park Stations W <:Xl 39
from the diets of ~. aggregata, ~. furcatus, and ~. jacksoni from all
stations over the IS-month sampling period. Cymatogaster aggregata
(N = 472) fed on a total of 97 prey categories and had a mean of 257.9
prey items per gut. Phanerodon furcatus (N = 280) fed on a total of 112
prey categories and had a mean of 102 items per gut. Embiotoca
jacksoni (N = 209) fed on 99 different prey categories and averaged
213.9 prey items per gut. The very high number of prey items for
~. aggregata was due to concentrated feeding on blooms of harpacticoid
copepods during the summer of 1975.
Major Prey Groupings and Seasonality
All three spec~es of embiotocids exhibited subtle seasonal shifts
in diet. Cymatogaster aggregata were most abundant at Kirby Park
throughout the 15-month sampling period (Table 2, page 9). The feeding
habits of ~. aggregata differed more seasonally at this station than at
either the Bridge or Dairies (Tables 6, 7, and 8). At Kirby Park, the
diet of C. aggregata was composed principally of polychaetes and
pericaridean crustaceans. However, the relative proportions of each of
these major prey categories changed seasonally. In fall 1974, (4S-'/OIH'" ~. aggregataAate almost exclusively gammarid amphipods (91%) (Table 8).
During the winter season, their diet shifted and polychaetes comprised
the major portion (49%), followed by gammarid amphipods (38%) and <~c:/,~-/'<(J------?.-!:,:~) copepods (11%). In the spring~'miscellaneous species of pericaridean
crustacea (primarioy Isopoda), gammarid amphipods, and copepods composed
approximately 60% of the diet, while polychaetes comprised 29%. In the I - I .. (80-- (C~H'_ S_~~t-l 0-' summer, Qopepoda)dominated the diet of C. aggregata (60%). The diets in ((j~'-L~I=,:C ~'/2) /Dy)
.... ------40
Table 6. Percent Similarity Index (PSI) Comparisons of }Iajor Prey Categories by Season for the Bridge Station. Values Are Expressed as Percent Index of Relative Importance (%IRI x 100), August 1974-0ctober 1975 (PSI = ~ min p.) ~
, BRIDGE 1 <.".~- /'1 ( f·.~ If; Cymatogaster ag9regata Aug - Oct Nov - Jan Feb - April May - Ju~r Aug - OCt .J .It.'.'} .f,e__ (.1 _ ;i.f:Li/j-i C"j::(,~, ;,,,,I-c', ,./ . Polychaeta 89.77 47.48 14.70 0.41 18.07 Misc. Annelida 0 1.16 0 0 0 Copepoda 1.62 49.42 71.34 76.53 57.81 Gaamarfdea 4.04 0.73 7.03 4.72 2.31 Caprell1dea 1.29 0.08 0 0.08 0.19 Misc. Crustacea 0.64 0.69 0.87 10.80 15.93 Decapoda 0 0 3.84 0.01 0.25 Gastropoda 0 0.01 0 0 0 Bivalvia 0.17 0.01 0 0 0.68 Fish (fragments + eggs) 0 0.01 1.20 0 0 Miscellaneous 2.50 0.42 1.05 7.45 4.76 1i"7IT iG2T ;r-;;-s- if72S" if72S"
PSI Aug - OCt Y5. Nov - Jan -sr.I4 Nov - Jan vs. Feb - April ···76.13 Feb - April V5. Hay - July'" 78.40 May - July vs. Aug - Oct · 76.18
Phanerodon furcatus
Polychaeta 2.06 85.08 61.92· 12.78 30.66 Misc. Annelida 0 0 0 0 0 Copepoda 0.07 0.02 0 25.46 0.25. Ganrnarfdea 3.65 11.42 19.7i 28.17 19.81', Caprell1dea 93.37 2.38 3.63 26.83 29.81 Misc. Crustacea 0.05 0.26 0.05 2.86 3.72 Decapoda 0 0.20 1.13 2.84 1.01 Gastropoda 0 0 0.02 0 0.01 Bivalvia 0.82 0.50 6.48 0.52 10.47 Fish (fragments + eggs) 0 0.01 5.62 0 0 Miscellaneous 0.02 0.14 1.42 0.39 4.21 N7I N • 48 "if749 if72S" N • 25
PSI Aug - Oct Y5. Nov - Jan 8.68 Nov - Jan vs. Feb - Apri 1 76.62 Feb - April ys. May- July'" 38.23 May - July V5. Aug - Oct · 64.45 Embiotoca jacksoni Polychaeta 47.56 84.01 70.65 17.72 53.60 Misc. Annelida 0 0 0 0 0 Copepoda 0.06 0.10 0.18 26.62 0.30 Gartmaridea 15.72 14.07 26.88 42.29 20.01 Caprell idea 33.36 1.12 0.56 4.17 5.19 Misc. Crustacea 3.10· 0.20 0.24 8.28 3.85 Decapoda 0.01 0.02 0.56 0.16 0.27 Gastropoda 0.01 0 0 0.03 0.11 Bivalvia 0.14 0.03 0.09 0.24 2.98 Fish (fragments + eggs) 0 0.04 0.01 0 0 Miscellaneous 0.06 0.42 0.80 0.89 13.69 N = 25 N • 26 N = 23 ~ ~
PSI Aug - Oct vs. Nov - Jan 63.11 Nov - Jan vs. Feb - April 86.06 Feb - April YS. May - July = 46.63 May - July vs. Aug - Oct 47.37 41
Table 7. Percent Similarity Index (PSI) Comparisons of Major Prey Categories by Season for the Dairies Station. Values Are Expressed as Percent Index of Relative Importance (%IRI x 100), August 1974-0ctober 1975 (PSI = "2: min p.) 1.
DAIRIES
Cymatogaster a9greqata Aug .. OCt Nov .. Jan Feb .. April May .. July Aug - Oct ,'L-V"C ,"-,~) j ,'.'", ;i.",(,. f!C" Polychaeta 14.20 30.89 7.10 0.10 17 .30 H1sc. Annelida 3.17 0 0 0 0 Copepoda 63.81 51.08 55.97 57.18 62.83 Galllllarf dea 0 3.62 15.22 22.69 12.20 Capre111dea 0 0 0 0.04 0.15 H1sc. Crustacea 9.59 13.53 12.15 10.80 6.37 Decapoda 0 0.10 1.17 0 0.02 Gastropoda 0 0.01 0.01 0 0 Bivalvia 0.18 0 0.53 0.02 0.10 Fish (fragments + eggs) 0 0 0.22 0.37 0 Miscellaneous 9.04 0.81 7.72 8.78 1.04 w-;;-rr ;r;;qa ~ ~ jf7"g
Aug - OCt Ys. Nov _ Jan 78.85 Hov - Jan vs. Feb .. April · 74.87 Feb - April vs. Hay - July ·• 90.05 Hay .. July YS. Aug - Oct · 77.32 Phanerodon furcatus Polychaeta 27.89 66.53 56 ..59 0.75 51.05 Misc. Annelida 0 0 0 0 0 Copepoda 0 0 0.02 0.32 0.30 GaRIMr1dea 0.37 16.74 34.06 62.81 20.59 Coprell1dea 0 3.36 2.49 4.08 0.01 -Misc. Crustacea 66.86 0.26 0.83 3.34 0.69 Decapoda 4.88 0.82 0.66 6.27 22.45 Gastropoda 0 0 0 0.04 0.01 Bivalvia 0 0.81 0.95 5.43 1.43 Fish (fragments + eggs) 0 11.42 ;'.79 6.30 0 Hi seellaneou5 0 0.07 0.59 10.69 3.52 ~ ;r-.;;J4 ~ jf7"g ~
PSI Aug - Oct YS. Nov .. Jan 2D4 Nov .. Jan vs. Feb - April 81.41 Feb - April YS. May .. July·· 44.14 May .. July Ys. Aug .. Oct · 33.57
Emb1otoca jacksonf Polychaeta 5.42 49.66 11.25 0.03 23.49 Misc. Annel1da 0 0 0 0 0 Copepoda 0 0.04 0.06 3.10 0.99 Galmlar1 dea 0.96 45.58 78.41 79.26 53.34 Caprell idea 0 0.07 0.10 0.60 0.07 H1sc. Crustacea 0 1.56 6.03 15.96 12.96 Decapoda 64.47 2.43 0.26 0.36 2.85 Gastropoda 0 0 0 0 0 Bivalvia 4.38 0.03 0.14 0.23 0.30 Fish (fragments + eggs) 0 0.09 1.18 0.01 0 Miscellaneous 4.77 0.57 2.61 0.43 5.97 ;r;;s JGli N7TI jf7"g jf7"g PSI Aug - Oct vs. Nov - Jan 9:TI Nov - Jan vs. Feb - April 59.47 Feb - April V5. May - July = 65.47 Hay - July V5. Aug - Oct 68.43 r 42
Table 8. Percent Similarity Index (PSI) Comparisons of Major Prey Categories by Season for the Kirby Park Station. Values Are Expressed as Percent Index of Relative Importance (%IRI x 100), August 1974 october 1975 (PSI = ~ min p.) ~
KIRBY PARK
Cymatoq8ster 8ggregata Aug - OCt Nov - Jan Feb - April Hay - Ju~r Aug - Oct .;.~-, ..., toj ,....,.....,:::.} 'co::! . Polychaeta 4.9B 48 .• 87-.) 28.54 2.19 72.03 Misc. Annelida 0.71 0.01 0.02 0 COpepoda 1.7B 11.21. 9.70 6~:l~~ 3.29 Garrmar1dea 91. 94; :372a·' 18.90 6.46 5.09 Caprel11dea 0 0 0 0 0 Misc. Crustacea 0.27 0.21 30.88 10.45 . 0.83 Decapoda 0 0.04 0.01 0 0.06 Gastropoda 0 0 0 0 0 Bivalvia 0 0 0.26 0.04 0.01 Fish (fragments + eggs) 0 0 2.05 0.13 0 Hi see11 aneous 0.32 1.88 10.38 20.12 18.70 if"'i6il rr;;zr 1f748 ~ ~
Aug - OCt vs. Hov - Jan 45.08 Noy - Jan Ys. Feb - April · 59.25 Feb - April vs. May - July ·a 39.37 May - July YS. Au9 - Oct · 30.11 Phanel"'Odon furcatus
Polychaeta 0 0 0.04 0.08 68.56 Misc. Annelida 0 0 0 0 0 Copepoda 0 0 0 0 0.06 Gamlaridea 41.19 0.12 0 1.84 3.27 Capre111dea 0 0 0 0 0 Misc. Crustacea 0 0 0.04 0.27 0.35 Decapoda 58.B1 0 5.13 0.95 3.56 Gastropoda 0 0 0 0.02 0.22 Bivalvia 0 B1.91 18.75 92.35 2.68 Fish (fragments + eggs) 0 17 .9B 75.88 0.99 0 Hi see11 aneous 0 0 0.16 3.52 21.28 "ff"i"2 N;.-.r H = 12 il"723 H = 11
Aug - OCt Ys. Nov - Jan 0.12 Noy - Jan ys. Feb - April 36.73 Feb - April ys. Hay - July ·= 20.93 May - July YS. Aug - Oct .9.36
Emb1otoca Jackson1 Polychaeta 0.02 74.09 0 15.88 Mise. Annel ida 0 0 0 0 Copepoda 0 0 0 1.02 Gammaridea 96.23 25.91 16.06 30.96 Caprellidea 0 0 0 0 Misc. Crustacea 0.02 0 0 3.75 Decapoda 2.82 0 67.00 33.36 Gastropoda 0 0 0 0 Bivalvia 0.22 0 0 0 Fish (fragments + eggs) 0 0 0 0 Miscellaneous 0.70 0 14.29 14.75 1f7IB ~ "tf7O 1f7Z ii"7I
Aug - Oct vs. Nov - Jan 25.93 Nov - Jan vs. Feb - April Feb - April vs. May - July = May - July vs. Aug - Oct 64.00 43
the falls of 1974 and 1975 were not very similar, since ~. aggregata fed
predominantly on polychaetes (72%) in 1975 while in 1974 they had fed
almost entirely on gammarid amphipods (91%). The relatively large %IRI values obtained in the Miscellaneous category for summer and fall 1975 were due to a detrital component. The highest degree of similarity was
~' "", ,.'" ..5!j e ii'~t;:2f-' ':: ,.;'-tf',t, ) found between winter and spfing"diets for ~. aggregata at Kirby Park
(59%) (Table 8). The least amount of seasonal dietary similarity for c. aggregata was found between summer and fall of 1975 (30%).
Unlike ~. aggregata, the area of the slough where P. furcatus were most abundant was at the most seaward station, the Bridge (Table 2, page 9). Phanerodon furcatus at the Bridge exhibited a pronounced seasonality in their diet (Table 6). At the Bridge, ~. furcatus fed primarily on polychaetes and pericaridean crustaceans,however, decapod· crustaceans, bivalves, and certain species of fish eggs did contribute to the diet as well (Table 6). In the winter and spring, polychaetes dominated the prey of P. furcatus at the Bridge (85% and 62% respectively). In the summer, there was a strong shift to pericaridean crustaceans (26% copepods, 28% gammarid amphipods, and 26% caprellid amphipods), with polychaetes contributing only 13%. In fall 1975,
P. furcatus fed equally on polychaetes (31%) and caprellid amphipods
(30%), followed next in importance by gammarid amphipods (20%) and small bivalves (11%) (Table 6). As with C. aggregata at Kirby Park, the highest similarity in diet for P. furcatus at the Bridge was between winter and spring (77%) (Table 6). The least amount of similarity, however, was between spring and summer (38%), not summer and fall as for C. aggregata. ------~
44
Embiotoca jacksoni in Elkhorn Slough were also most abundant at
the seaward Bridge station (Table 2, page 9), and their diets and the
relative seasonal proportions of the major prey categories Were very
similar to those of !. furcatus (Table 6, page 40). At the Bridge,
E. jacksoni fed almost entirely on polychaetes and crustaceans. In the
fall of 1974, polychaetes were the most important single prey category
(48%), followed by caprellid (33%) and gammarid amphipods (16%)
(Table 6). In the winter and spring, ~. jacksoni fed almost totally on
polychaetes (84% and 71% respectively), with the remainder of the diet
being composed principally of gammarid amphipods (14% and 27%
respectively). In the summer, there was a strong shift towards a
pericaridean crustacean dominated prey array (42% gammarid amphipods,
8% miscellaneous crustaceans, and 4% caprellid amphipods), supplemented
by polychaetes (18%). In the fall of 1975, ~. jacksoni shifted back to I, a polychaete-dominated diet (54%, similar to fall 1974), followed by gammarid amphipods (20%). As with ~. aggregata·and !. furcatus, i ~. jacksoni diets were most similar between winter and spr~ng (86%) J (Table 6). As with P. furcatus, the strongest seasonal shift in diets ~ was between the spring and summer seasons.
,. Dietary Variation Among Locations
Cymatogaster aggregata in Elkhorn Slough fed primarily on an
assortment of small pericaridean crustaceans and polychaetes. In fall
1974, the diet varied with location. The most important prey category
at the Bridge was the opheliid polychaete Armandia brevis (Appendix A),
while at the Dairies it was benthic harpacticoid copepods, and at Kirby 45
Park, the epibenthic gammarid amphipod Corophium sp .. In the winter,
harpacticoids and unidentifiable polychaetes were the primary food items
of ~. aggregata at both the Bridge and Dairies. However, at Kirby Park
the spionid polychaete Streblospio benedicti and the amphipod Corophium
sp. were most important, followed by harpacticoids. In the spring,
there was a distinct difference in diets between two stations. At the
Dairies, harpacticoids, the gammarid amphipod Aoroides columbiae, and
the cumacean Cumella vulgaris were the most important. At Kirby Park,
C. vulgaris comprised the major portion of the diet, followed closely by
S. benedicti, Corophium sp., and harpacticoids. In the summer,
C. aggregata at all stations fed almost exclusively on small
pericaridean crustaceans and polychaetes were insignificant. Of the
crustacean component, harpacticoids were the primary prey category at
all stations. The only difference among locations were that harpac
tieDids were followed in importance by the isopod Munna ubiquita at the
Bridge, while at the Dairies, it was ~. columbiae, and at Kirby Park, a detrital component and ~. vulgaris were more important ~n the diet. In
fall 1975, ~. aggregata at the Bridge fed much more in the water column
since calanoid copepods and cyprid larvae were important prey
categories, yet the polychaete ~. brevis was still present. At the
Dairies, as 1n the previous summer season, harpacticoids were important
prey, followed by ~. columbiae and A. brevis. Finally, at Kirby Park,
~. aggregata switched in the fall (1975) to a predominantly polychaete diet, with ~. benedicti, Capitella capitata and unidentifiable
polychaetes being the major prey categories consumed.
The major prey categories of P. furcatus in Elkhorn Slough were •
46
polychaetes, small crustaceans, and bivalves. At the Bridge in the winter, !. furcatus fed very heavily on the polychaete A. brevis and to
a much lesser extent on A. columbiae (Appendix B). At the Dairies, the primary prey categories were unidentifiable polychaetes and the demersal, adhesive eggs of the jacksmelt Atherinopsis californiensis.
In the spring, there were no differences among locations in feeding for
the Bridge and Dairies since A. brevis and A. columbiae were both
top-ranked prey items. Also, the fish at the Bridge had the same diet as those from the preceding winter season. In the summer at the Bridge,
P. furcatus shifted to a predominantly pericaridean crustacean diet of approximately equal portions of ~. columbiae, Caprella sp., and harpac ticoid copepods. At the Dairies, ~. columbiae was the primary food item, while at Kirby Park, P. furcatus fed almost totally on.bivalves, with the venerid clam Gemma gemma also of major importance. In fall 1975,
P. furcatus at the Bridge fed equally on Caprella sp. and ~. brevis, followed next in importance by the gammarid amphipod Jassa falcata. At the Dairies, ~. brevis was the overwhelmingly dominant prey category consumed, followed by approximately equal amounts of A. columbiae and the shore crab Hemigrapsus oregonensis.
Embiotoca jacksani found in Elkhorn Slough were primarily polychaete and small crustacean feeders. At the Bridge in fall 1974,
~. jacksoni fed principally on Caprella sp. and unidentifiable polychaetes (Appendix C). At Kirby Park, prey selection was confined almost entirely to Corophium sp .. In the winter at the Bridge, ~. brevis overwhelmingly dominated the diet followed by ~. columbiae, while at the
Dairies, though the principal food items did not change, their 47 proportions were more equal. In the spring at the Bridge, dominant prey items and ranks were identical to the winter. In the summer season at the Bridge, ~. jacksoni fed primarily on A. columbiae, followed closely
1n importance by harpacticoids and A. brevis. At the Dairies,
A. columbiae dominated the diet in conjunction with the isopod
Ianiropsis analoga. Finally, in fall 1975, the top two prey categories at the Bridge and Dairies were the same, but their respective ranks were switched between locations, with ~. brevis being most important at the
Bridge and ~. columbiae at the Dairies.
Within season comparisons of Percent Similarity. Index values for each embiotocid species between stations indicated that the prey arrays from the Bridge and Dairies were most similar to one another, those from the Dairies and Kirby Park were next, and those from the Bridge and
Kirby Park were the least similar (Table 9).
Trophic Diversity
Seasonal fluctuations in trophic diversity of ~. aggregata appeared to be greatest at Kirby Park. Richness (R') and evenness (3') increased in spring to a peak of 2.00 and 0.54 respectively, at Kirby Park
(Table 10). Other than fall 1974 at the Bridge and Kirby Park,
~. aggregata had relatively high values of trophic diversity for all seasons, all stations.
For P. furcatus and E. jacksoni, trophic diversity values were relatively high at both the Bridge and Dairies in all seasons (Table 10).
Richness and evenness for P. furcatus increased in fall 1975 at the
Bridge to a peak of 1.97 and 0.54 respectively. Diversity for 48
Table 9. Percent Similarity Indices of All Prey Categories for Each Surfperch Species; Within Season, Between Stations. -- = One or Both Stations with Insufficient Number of Fish for Comparison
Bridge - Dairies Bridge - Kirby Park Dairies - Kirby Park Cymatogaster aggregata Aug - Dct 1974 3.0 2.4 4.5 NaY 1974 - Jan 1975 7B.3 18.2 17.9 Feb - April 1975 26.2 May - July 1975 74.8 69.6 71.4 AU9 - Oct 1975 21.0 12.1 11.8
I Phanerodon furcatus I AU9 - Oct 1974 ,! NaY 1974 - Jan 1975 12.1 Feb - April 1975 82.3 13.3 i May - July 1975 35.8 2.1 11.2 I Aug - Oct 1975 39.5 8.2
Embiotoca jacksoni Aug - Oct 1974 7.2 Noy 1974 - Jan 1975 63.9 Feb - April 1975 May - July 1915 47.0 I,j Aug - Oct 1975 48.4 Table 10. Trophic Diversity Summary. N = Number of Stomachs Analyzed (with contents); S = Total Number of Prey Categories; H' = Seasonal Mean Shannon-Weaver Trophic Diversity of Individual Stomachs Combined; JI = S~asonal }Iean Evenness Component of Diversity of Individual Stomachs Combined; %D = Seasonal Mean Percent Dominance of Individual Stomachs Combined; B = Seasonal Mean Niche Breadth of Individual Stomachs Combined; -- =. Insufficient Sample Size
BRIDGE DAIRIES KIRBY PARK !L S .JL _J_'_ ..1Q.. _B_ !L S _H_'_ L ..1Q.. _B_ !L i.. .JL _J_'_ ..1Q.. _B_ Cymatogaster 8 regata AU9 - Oct 19y4 22 14 0.54 0.20 0.80 1.25 17 16 1.50 0.54 0.38 2.65 68 29 0.46 0.14 0.83 1.20 Nov 1974 - Jan 1975 21 29 1.36 0.40 0.34 2.92 48 26 1.28 0.39 0.37 2.74 27 16 1. 30 0.47 0.34 2.97 Feb - April 1975 -- 20' 41 1.77 0.4B 0.31 3.22 48 41 2.00 0.54 0.18 5.50 May - July 1975 25 25 1.07 0.33 0.55 I.B2 27 26 1.31 0.40 0.39 2.58 64 33 1.28 0.37 0.41 2.44 AU9 - Oct 1975 25 35 1.44 0.41 0.37 2.71 25 28 1.37 0.41 0.42 2.36 35 26 1.69 0.52 0.25 4.07 Phanerodon furcatus AU9 - Oct 1974 -- -- Nov 1974 - Jan 1975 48 38 0.93 0.26 0.62 1.62 34 24 1.25 0.39 0.42 2.40 Feb - April 1975 49 42 1.51 0.40 0.36 2.75 26 41 1.15 0.31 0.43 2.35 May - July 1975 25 34 1.73 0.49 0.22 4.49 25 30 1.63 0.4B 0.39 2.56 23 25 0.99 0.31 0.52 1. 94 AU9 - Oct 1975 25 39 1.97 0.54 0.20 4.92 25 32 1.52 0.44 0.29 3.51 Embiotoca jackson! AU9 - Oct 1974 25 23 1. 39 0.44 0.34 2.98 -- 18 12 0.21 O.OB 0.93 1.08 Nov 1974 - Jan 1975 26 47 0.89 0.23 0.62 1.61 17 34 1.16 0.33 0.41 2.46 Feb - April 1975 23 43 0.89 0.24 0.54 1.84 -- 0 May - July 1975 25 40 1.82 0.49 0.23 4.42 25 28 1.15 0.35 0.51 1.95 AU9 - Oct 1975 25 44 1.65 0.44 0.32 3.17 25 35 1.66 0.47 0.31 3.19
..,. '" 50
P. furcatus, from spring through fall 1975, decreased with increasing
distance inland; this, however, was not the case for ~. aggregata nor
~. jacksoni. For!. jacksoni, richness and evenness increased to a
peak of 1.82 and 0.49 respectively, at the Bridge.
Dietary Overlap
Of the within station comparisons for ~. aggregata, fish from the
Dairies consistently had the highest similarity to themselves over the
five-season sampling period (Table 11). The Bridge station produced
the least amount of overlap in prey arrays between seasons. Of the within season comparisons, the summer (May-July) exhibited the most uniform diet composition between stations, while in fall 1974 and 1975
(August-October) prey similarity between stations was very different.
Of the between station, between season comparisons, the Bridge and
Dairies were the most similar in diet composition. The "s tationll effect
on the amount of similarity appeared stronger than the "seasonal" effect
since 6 out of 10 comparisons of within station, between seasons had
> .40 similarity, while only 4 out of 13 comparisons of within season, between stations showed> .40 similarity (Table 11).
Phanerodon furcatus from the Bridge station were generally more similar between seasons than those either from the Dairies or Kirby
Park, especially in winter and spring (Table 12). The between station comparisons of P. furcatus from the Bridge and Dairies, not only within season, but between seasons as well, were consistently more similar to one another than those from the Dairies and Kirby Park. The
"stationll effect on P. furcatus, though not as large as that for 51
Table 11. Index of Overlap, Percent Similarity Index (PSI) for Cymatogaster aggregata; Within and Between Stations, and Within and Between Seasons. B = Bridge, D = Dairies, and KP = Kirby Park; -- = One or Both Stations with Insufficient Number of Fish for Comparison
Aug - Oct '74 Nov '74 - Jan '75 Feb - April '75 May - July '75 AU9 - Oct '75 8 0 KP 8 0 KP 8 0 KP 8 0 KP 8 0 KP ... ~ '" ....u <> 0 .03 • '" ..,"- .02 .05 """ on ~ .17 .63 .05 ..,..0: '" , ... 0 .06 .68 .04 .78 ~ ..,"- .03 .19 .45 .18 .18 > z0
on ~ '" ------~.. -0- 0 .57 .62 .19 -- "", . "- .Q .., .13 .10 .53 -- .26 ...."
on ~ -- .71 .19 ,., '" ~ .., 0 -- .83 .23 .75 ", ..,., ..,"- -- .69 .36 .70 .71 :>:
on ~ '" .09 .21 .08 .... u <>, 0 .74 .75 .63 .21 '" "-.., .12 .13 .27 .12 .12 """ ...
52
Table 12. Index of Overlap, Percent Similarity Index (PSI) for Phanerodon furcatus; Within and Between Stations, and Uithin and Between Seasons. B- Bridge, D : Dairies, and KP : Kirby Park; -- : One or Both Stations with Insufficient Number of Fish for Comparison
Aug - Oct '74 Nov '74 - Jan '75 Feb - April '75 May - July '75 Aug - Oct '75 6 a KP 6 a KP 6 0 KP 6 0 KP 6 a KP .,. !:' '" ....u '", 0 -- '" Q. -- -- """ "" !:''" c ------., '" ", .,. 0 ------.12 !:' Q. ------> "" -- -- z0 '" !:' '" .76 .16 -- ~ -.:: c- o .70 .10 -- .82 "" • Q. .D..... "" .01 .07 -- .13 --
!:''" '" .34 .42 .04 >, ~ ., 0 .35 .42 .16 .36 ", Q. >, .12 .04 .20 .02 .11 ::l:" ""
!:''" '" .46 .19 .06 ....u 0 '", .35 .32 .06 .40 Q. '" .06 .17 .09 .06 -- """ ""
-----liilIIIIl_ 53 r1 /, ~ ~. aggregata, was still apparent ~n that 3 out of 8 of the within ~' station, between season comparisons were> .40, while 2 out of 7 of the
within season comparisons were> .40 (Table 13).
Intraspecific comparisons of diet similarity showed that ~. jacksoni
from the Bridge and Dairies were similar most seaSOns (Table 13).
"Station" effect was once again dominant over "season" in that 6 out of 7
of the within station comparisons versus 3 out of 4 of the within season
comparisons were ~ .40 (Table 13).
Cymatogaster aggregata and ~. furcatus, and ~. aggregata and
E. jacksoni were less similar in comparisons of prey arrays than
P. furcatus and ~. jacksoni (Table 14). For C. aggregata and ~. furcatus,
only lout of every 10 comparisons of PSI values were> .40 (Table 14).
The relative amount of similarity in prey composition between the two
species was generally higher at the Dairies than either the Bridge or
Kirby Park, with the exception of one seasonal overlap value (fall 1975
at Kirby Park). Similarity values for fish from the Bridge and Dairies
were somewhat elevated in summer. Lowest values for Dairies and Kirby
Park occurred in the spring (February-April). l'hen the prey of
~. aggregata were compared with those of ~. jacksoni, only 2 out of 10
comparisons were ~ .40 (Table 14). At the Bridge (as was the case for
~. aggregata and P. furcatus), the highest similarity in prey arrays
occurred in the summer. However, at Kirby Park in fall 1974 the PSI
was .91, an unusually high value relative to the others that were
determined for that particular predator species pair comparison.
The diets of P. furcatus and ~. jacksoni were very similar, and
7 out of 8 comparisons were> .40 (Table 14). Overlap was much higher 54
Table 13. Index of Overlap, Percent Similarity Index (PSI) for Embiotoca jacksoni; Within and Between Stations, and Within and Between Seasons. B = Bridge, D = Dairies, and I~ = Kirby Park; -- = One or 50th i Stations with Insufficient Number of Fish for Comparison > ~ p j f AU9 - Oct '74 Nov '74 - Jan '75 Feb - April '75 May - July '75 AU9 - Oct '75 ~ B 0 KP B 0 KP B 0 KP B 0 KP B 0 KP J
~ ~ ~ " '" ....u 0, 0 -- "- .07 «"'" "" --
~ ~ .14 -- .03 ..,"~ '" , 0 .13 -- .04 .64 ~ ~ "------> "" -- z0
~ ~ '" .84 .77 -- ~ ~... 0. 0 .26 .59 -- -- «, "- -- -- ""..... "" ------
~ ...- '" .45 .54 -- >, :;.., 0 .28 .78 -- .47 , "- >, m ------:E ""
~ ~ '" .47 .24 -- .... u 0 0 .64 .63 .- .48 , "- -- -- .- «"'" "" -- -- 55
Table 14. Percent Similarity Indices (PSI) of Diets Between Cymatogaster aggregata, Phanerodon furcatus, and Embiotoca jacksoni; Within Station, Within Season. - - One or Both Species with Insufficient Number of Fish for Comparison
BRIDGE DAIRIES KIRBY PARK f. aggregata YS. P. furcatus Aug - Oct 1974 Nov 1974 - Jan 1975 .22 .26 Feb - April 1975 .16 .03 May - July 1975 .31 .34 .06 Aug - Oct 1975 .20 .25 .59 f. aggregata YS. f. jacksoni Aug - Oct 1974 .07 .91 Noy 1974 - Jan 1975 .19 .06 Feb - April 1975 .23 May - July 1975 .40 .30 .30 Aug - Oct 1975 .19 .30
P. furcatus YS. f. jacksoni Aug - Oct 1974 Noy 1974 - Jan 1975 .95 .09 Feb - April 1975 .78 .48 May - July 1975 .70 .67 Aug - Oct 1975 .49 .47 56 at the Bridge when compared with the Dairies in winter and spring.
Summer and fall 1975 overlaps between the two stations were approximately equal. Prey category utilization by the two predator species at the Bridge in winter was almost identical (PSI; .95).
Discussion
Cumulative Prey Analysis
It is essential in predator-prey studies that the adequacy of sample size be established. Hurtubia (1973) and Hoffman (1978) have used the approach of cumulative diversities for the determination of necessary sample size. Since the cumulative prey curves used in this study leveled off well below the number of guts analyzed, it is evident that an adequate sample size was acquired.
The findings that the number of prey species' utilized by the three species of embiotocids, and the number of guts necessary for a random sampling of those prey species, decreased with increasing distance inland from the Bridge are identical to those reported by Ambrose (1976) for the Bothidae and Pleuronectidae.
Feeding Frequency
The paucity of empty stomachs found in the three spec~es of embiotocids suggests that prey are abundant in Elkhorn Slough, and that fish are ~onstantly feeding; this agrees with other fish feeding studies
(Talent, 1973; Ambrose, 1976) there and with benthic invertebrate sampling (Nybakken and Jong, 1977).
Several authors (Hobson, 1971; Bray and Ebeling, 1975; Terry, 1975; r 57
Ebeling and Bray, 1976; Ellison et al., 1979) have observed different
species of embiotocids to be diurnal feeders and have suggested that
they are visual predators. If this is true for the three species ~n
this study, then there would be a strong advantage to daytime feeding in
consistently, highly turbid waters (such as Elkhorn Slough) in order to
utilize any amount of available light.
Major Prey Groupings and Seasonality
It appears that the three species of embiotocids within Elkhorn
Slough exhibit a seasonal, opportunistic feeding behavior. It is
evident that the primary major prey categories of all three species
are polychaetes and various small crustaceans. As such, the three
major species of embitocids within Elkhorn Slough, Cymatogaster
aggregata, Phanerodon furcatus, and Embiotoca jacksoni certainly fall
into the feeding guild described by Ebeling and Bray (197.6) as "small
mouthed, microcarnivores. It
Evidence for the opportunistic mode of feeding comes from several
different sources. There is a wide range in values of percent
dominance (Table 10, page 49), indicating these embiotocid species are
able to utilize either a broad-based food supply or a rather narrow-based
one. Richness (HI) and evenness (J') do fluctuate seasonally (Table 10),
indicating there is a seasonal change in the relative abundances of the
var~ous prey categories. Since there is a change in the dominant prey
categories with season (Appendices A-C), and these changes are
synchronous with the respective prey abundances from benthic core data
(Nybakken and Jong, 1977), then opportunism in feeding is indicated. 58
Cymatogaster aggregata at Kirby Park do not appear to have one
preferred prey category since they readily "switch" between polychaetes and iii ~ small pericaridean crustaceans, depending on the season. The respective
i" ~ seasonal dominance of these higher categories is in close agreement with
abundance data from benthic cores taken at .the same station (Nybakken and
Jong, 1977). It therefore appears that C. aggregata in Elkhorn Slough
are feeding in proportion to the relative abundances of benthic epifaunal
and infaunal prey. Both f. furcatus and ~. jacksoni at the.Bridge
station exhibit similar seasonal shifts in their major prey categories
between polychaetes and small crustaceans. As was the case with
£. aggregata, these seasonal peaks in prey arrays of particular prey
categories are highly synchronous with prey abundances from benthic core
data (Nybakken and Jong, 1977).
Dietary Variation Among Locations
Diets in this study varied with season and location, and undoubtedly
much of this variation as well can be qualitatively attributed to the
distribution and seasonal abundance of the various prey species (Ambrose,
1976; Nybakken and Jong, 1977). Streblospio benedicti, a small, surface,
tube-dwelling polychaete and Capitella capitata, a small, burrowing
polychaete were found to be very abundant at Kirby Park and decreased ~n
abundance towards the Bridge (Nybakken and Jong, 1977). Armandia brevis
similarly is a small, surface-dwelling polychaete, much more active than
either S. benedicti or £. capitata as it will swim up into the water
column (Hermans, 1966). Abundance of A. brevis was concentrated at the
Bridge and decreased with increasing distance inland (Nybakken and - 59
Jong, 1977). Benthic core data for number of individuals of lumped
species of polychaetes suggested locational variation in seasonal blooms
(Nybakken and Jong, 1977). Polychaetes at the Bridge were found to
bloom in winter (October-January); at the Dairies this bloom was somewhat
delayed, occurring in winter and early spring (November-March); while at
Kirby Park the period of peak abundance was even more restricted
(January-March).
Corophium sp. and Aoroides columbiae, the two dominant gammarid
amphipods fed upon by embiotocids in the slough, are small, slow
moving, surface tube-dwellers. Corophium sp. dominated the upper
reaches of the slough, while A. columbiae was much more abundant at the
Bridge and Dairies (Ambrose, 1976). The cumacean Cumella vulgaris,
and benthic harpacticoids were found throughout the slough. Abundance
'data'for lumped species of crustaceans indicated a locational variation
similar to polychaetes, though not as large in magnitude (Nybakken and
Jong, 1977). At the Bridge, crustaceans bloomed from late spring
through summer (April-July); at the Dairies this bloom occurred somewhat earlier (February-March); while at Kirby Park it extended from spring
through summer (February-July).
Seasonal opportunism in feeding by ~. aggregata was indicated since the dominant prey species utilized at each station varied with season. In the late fall and winter, ~. aggregata fed predominantly upon harpacticoids, the polychaetes A. brevis and ~. benedicti, and the epibenthic amphipod, Corophium sp., depending on location. With the onset of spring, prey consisted primarily of mixed pericaridean crustaceans (harpacticoids, gammarid amphipods, and a cumacean). f
ill .5 Jf 60 ~ Throughout the summer, their diet was dominated by epibenthic harpacti-
coids. Presumably, harpacticoids were unusually abundant and their
availability precluded any other prey category from dominating the diet.
In the fall 1975, dominant prey arrays varied widely among stations.
At the Bridge the top two prey categories of ~. aggregata were
calanoid copepods and cyprid larvae, and they may have taken advantage
of this planktonic food brought in on flood tides. However, at Kirby
Park, ~. benedicti and ~. capitata were the most important food items,
possibly due to a late summer bloom and subsequent settling out of
larval forms of both species (C. Jong, pers. comm.).
Phanerodon furcatus within Elkhorn Slough exhibited the same
apparent seasonal opportunism as did ~. aggregata, yet the dominant
prey species utilized were different. Concordant with the winter bloom
of polychaetes at the Bridge and Dairies, P. furcatus fed predominantly
upon polychaetes. Possibly due to either a continued availability of
polychaetes or selection, A. brevis continued to dominate the diet of
P. furcatus into spring. A complete switch in diet occurred in summer;
pericaridean crustaceans were very abundant throughout the slough, yet
they were utilized primarily at the Bridge and Dairies stations, while
the small introduced bivalve Gemma gemma was eaten more frequently at
Kirby Park. In fall 1975, the dominance of A. brevis in the diet of
P. furcatus at the Dairies was reflective of the high relative abundance
of polychaetes and the low relative abundance of crustaceans from
benthic core data (Nybakken and Jong, 1977).
Embiotoca jacksoni was very similar to ~. furcatus, not only in
its distribution, but in utilization of dominant prey species as well. 61
Dietary variation between locations was minimized for ~. jacksoni, S1nce
blooms of ~. brevis in the winter and A. columbiae in the summer occurred with approximately equal abundance at the Bridge and Dairies stations
during the respective seasons (C. Jong, pers. corom.; G. Gillingham,
pers. corom.).
The results of this study for each of the embiotocid spec~es varied
from those of different authors. Whether or not other studies found a
seasonality in feeding seemed to depend to a large extent on sample
size, and if there was a seasonality factor associated with those
potentially utilizable prey species. "Boothe (1965) (N =" 107) found
that ~. aggregata in San Francisco Bay fed on very similar prey species
to those found from Elkhorn Slough; gammarid amphipods (Photis and
Corophium) were the most important prey all year, followed by cumaceans, bivalves (Q. geroma) and polychaetes. Gordon (1965) (N = 217) found a somewhat different diet for ~. aggregata from British Columbia in that mussels and algae were the dominant prey catego~ies utilized. Bane and
Robinson (1970) (N = 39) determined that green algae and crustaceans
(primarily garomarid amphipods) were the most important food items of
~. aggregata, yet they found seasonality was not a factor in feeding.
Cymatogaster aggregata in Anaheim Bay (Odenweller, 1975) (N = 138), fed
in a seasonal manner similar to ~. aggregata in Elkhorn Slough
(zooplankton in summer and polychaetes in winter). In a study of
P. furcatus from a kelp bed off Santa Barbara, Bray and Ebeling (1975)
(N = 55) found algal-encrusting bryozoans to be the dominant food item; macruran shrimp and amphipods were less important. Seasonality in feeding was not found to be of any importance (Bray and Ebeling, 1975), 62 presumably due to the continued abundance of bryozoans. As in the current study, Baldwin (1970) (N = 73) found that !. jacksoni from San
Luis Obispo fed primarily on gammarid amphipods and polychaetes. In
King Harbor, California, Terry (1975) found the diets of P. furcatus
(N = 17) and E. jacksoni (N = 20) to be somewhat distinct in that
E. jacksoni fed primarily on gammarid amphipods, with only minor amounts of polychaetes and bryozoans, while f. furcatus ate equal amounts of the three categories. In Del Mar, California, Quast (1968) determined that f. furcatus (N = 18) and !. jacksoni (N = 90) ate basically the same major prey categories (benthic crustaceans, polychaetes, and bivalves), except that in the dominant items, ranks were switched. Finally, Ellison et al. (1979), differing from the current study, stated that P. furcatus and E. jacksoni had significantly different diets; P. furcatus fed on gammarid amphipods, isopods, and gastropods, while E. jacksoni ate primarily amphipods and the polychaete Armandia.
Trophic Diversity: Generalists vs. Specialists
All three species of embiotocids were found to be generalized microcarnivore, seasonally opportunistic feeders. Seasonal opportunism has been supported in this study by the exploitation of broad-based food resource and the seasonal "switching" in dominance of the major prey species within that food resource. Evidence for trophic generalization comes mainly from three different areas: overall fish size, the evolution of different feeding morphologies, and the breadth or diversity of food types consumed. It has been reported that generally larger organisms might have a more diverse diet owing to the capability to 63
consume a wider range of prey sizes (Springer, 1960; Keast, 1970;
Schoener, 1974). This apparently holds true for P. furcatus from
Elkhorn Slough, since they consumed more total prey categories than
either C. aggregata or ~. jacksoni, and their trophic diversities
increased steadily from winter 1974 through fall 1975 at the Bridge
station.
Both Tarp (1952) and DeMartini (1969) considered ~. jacksoni to be
the most primitive and, hence, unspecialized member of the Embiotocidae.
DeMartini (1969) predicted ~. jacksoni would display a very catholic
diet based on the unspecialized nature of its feeding morphology (lip
S1ze, jaw teeth, and pharyngeal teeth). This is evidenced by the broad
types of major prey categories consumed (epibenthic crustaceans,
epifaunal and infaunal polychaetes, and bivalve molluscs) as well as the
relatively high seasonal mean trophic diversity values. Cymatogaster
aggregata should also be considered a trophic generalist, not only due to
the relatively high diversity values found in this study, but its feeding
morphology as well. The delicate and closely spaced jaw teeth, in
combination with finely meshed gill rakers, allow ~. aggregata to
alternate between feeding modes of benthic picking and filter-feeding
thus increasing the diversity of food types consumed.
Dietary Overlap
Apparently ~. aggregata has utilized not only spatial, but dietary
separation as a means of avoiding species interactions, such as
competition. Predictably, with wide spatial separation and abundant
food, interspecific dietary overlaps should be high (Keast, 1965; Zaret
. 64
d Rand, 1971), yet this was not the case when either P. furcatus or
. jacksoni were compared with C. aggregata. Presumably this lack of
igh dietary overlap may be partly due to differences in overall body
size, different feeding morphologies, or different feeding behaviors.
when P. furcatus and ~. jacksoni were compared, high diet overlap
found (three out of four of the comparable seasons at the Bridge
similarity values> .70).
Apparently for P. furcatus and ~. jacksoni, neither spatial nor
dietary separations are the factors minimizing their interspecific
interactions. Schoener (1974) has suggested habitat dimensions to be
more important in separating niches than food dimensions, and food to
be more important in separating niches than temporal dimensions.
Separation of niches on a temporal dimension, however, should not be
the operational mechanism either, since both species are considered to
be visual, diurnal predators. Two possibilities exist that would
explain this apparent concurrent utilization of space and food: (1) a niche dimension along which the two species could segregate has not been measured; or (2) if resources (in this case space and/or prey abundance) are not in short supply, then organisms may utilize those resources without a detrimental effect on one another (Pianka, 1978).
The second possibility seems the most reasonable explanation since utilizable prey species within the slough were abundant, based on the very low percentages of empty guts found in all species and invertebrate benthic core data as well.
Apparently all three species of embiotocids are able to co-exist within Elkhorn Slough for a variety of reasons. Cymatogaster aggregata 65
not only eats different food items but usually lives elsewhere in the
slough than either I. furcatus or !. jacksoni. Based on an abundant
food resource, P. furcatus and E. jacksoni are able not only to live In
the same place in the slough, but usually consume the same prey items as
well .
..------CONCLUSIONS
1. Seasonal abundance patterns for all three species of embiotocids were similar with high numbers of individuals in summer and fall, low numbers in winter.
2. Microdistributional patterns within the slough differed between species. Cymatogaster aggregatawere most abundant in the upper portion of the slough, ~. furcatus and ~. jacksoni were most abundant seaward.
3. Length-weight relations of each species showed similarities with different geographical populations. Cymatogaster aggregata from the slough were more similar to populations from Northern California than
Southern California. Phanerqdon furcatus were similar to both Northern and Southern California populations. Embiotoca jacksoni were more similar to populations from Southern California than Northern California.
No differences were found in growth rates of males or females for any species.
4. All three species are summer spawners. Reproductive data for
C. aggregata are in close agreement with other studies. The number of embryos per female is a direct function of increasing female size.
Spawning time varies with latitude, normally occurring later in the colder, more northern part of the range. Reproductive data for
P. furcatus differs from other studies in number of embryos per female, size at birth, and spawning time. Reproductive data for ~. jacksoni agrees with other studies in number of embryos per female, yet differs in size at birth and spawning time. 67
5. The number of prey specles utilized by the three species of
embiotocids, and the number of guts necessary for a random sampling of
those prey species, decreased with increasing distance inland from the
Bridge station.
6. Very few empty guts were found, suggesting either somewhat continuous feeding and/or that prey were abundant in Elkhorn Slough.
7. Feeding data indicated all three species were opportunistic feeders, diets varying with season and location. The primary major prey categories for each embiotocid were polychaetes (mainly eaten in winter) and various small crustaceans (mainly eaten in summer).
8. The variations in diets for each species was qualitatively attributed to the distribution and seasonal abundance of the various prey species.
9. Diets of P. furcatus and~. jacksoni were very similar, while the dominant prey of ~. aggregata differed from either preceding specles.
10. Evidence for trophic generalism in each"species Comes mainly from three different areas: overall fish size, the evolution of different feeding morphologies, and the breadth or diversity of food types consumed.
11. Apparently~. aggregata has utilized spatial and dietary separation as a means of minimizing species interactions, such as competition.
13. Apparently P. furcatus and ~. jacksoni have not utilized spatial and dietary separation as a means of minimizing species interactions. 68
14. Due to the apparent abundance of prey, ~. furcatus and
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INDEX OF RELATIVE IMPORTANCE (IRI)
SUMMARY FOR CYMATOGASTER AGGREGATA 78
,,", tltmd ... ""Ill ... r __lftl'.....u. 1.01 0.01 ~:'I~ O.U • 0.01 1.13 O.M .0.01 £1~.Ulo.. """ ...1 o.n " n ...... I ~. " " ,lnp.ol.I.th... .'"rt>tlhd., 15.5 0.11 • ~.Ol 0.15 • 0.01 .. Trlcl,'loll " .. • 0.01 _ '.61 o.al 10 n.n O.ll 0.01 u _ u """",,.1... '.51 ..• " _ltolO " 01lpc,,"UU n.S7 0 ••0 I~.M D.55 , 0.51 0.01 ..• ~.19 "Ir'\I' ... 111.11 2.D!I CUlul1l4.. I..I.....J 2.01 ..• IS 1.11 11 ID.l.'-; O.BO • t.o~It.o". ""~U.U !le.IS 5 5.1J It lIS.•' 1.fi1 ~I_l ... O.~l 15.5• 1"""...t.1 0.5' ...... l4u 1""1 ,.1 0.11 0.01 'lot,....,.... ~I"" ltCllI... , .• "I 1111.7J .• n.n A....ftdl ...... 10 "'.n .... " "",llOdae.'" 1..,._t.1 "",..,1"'.1""._'.1 O.l' n._I.hll. " hili,"", {...I_'.I _.11 u 0.11 0.01 14.11 0.11 "'Iroo.. "",I " , ••ocl.HI ... _ ...1,.".. ' ...e...... fIC!II ... O.lt U " ~ Stnbllli~l. _Ietl 1.11 ..• 11 :m._1 1 10.11 .. ~ It ••:11 .. lD5l.n 11.55 S,lll... I""I_'.1 i.n f .._ '"",1 '.M " Ao'thlO""" " Crr" .. "" , ..ft 10 ..... CI."",,,,"-II. on..eo4t 7.11 0.01 0.01 II ll.n 0.10 U l.ts 0.01 £oqoIoII_QrdII_U 0.5' 0.01 " " Cth llII " 10.:10 0:11 I~ 0.<1 • 0.01 I~ i.)II '.M 0.11 t.ol . C cl11C1t'< 117.15 1.71 C)'C' I" ...... «1""1.. U1.t!I lUS.1J II.OJ In.tl 1.11 nU•.ll n.ll U7.n 1.05 1lIo 1CJI CI ll'O _ •• ,f r1.~ £0.., .... lwlto. c••, ...n) " " " " " u " 79 mllllUI _ '~«ll 1975 M' _ .lUll IJ1'~ DoII«lts trill' ~U< MillO 1.8H IB.lil Tout _r or ~.... c..."",n.. " " -.,. of nth (•••1".. l,,1 '" CO",''',, I <' " " .. 80 ~ • IlCfOUI I f15 n,.. Cl'1l1C (la, NR """ CA ...... L!.J..:. .!L.!:..L. ~u ..L.!..:.L !!.:.!.:.L ""- ...!.:!..:.l:. .!l..:.!.:.L. n •• _I~I"""... 0.23 • 1'1.01 n l.n '.M lI.1I ~ 0.01 n .• --_n,.".,...... Q'r1 " ~hr.r1... ". n ..,...I.'u.... 0.41 0.01 n .• ~-T_lIm. Tritt..l", l.n 0.01 '-" '.01 lIt.o:l l.ll -,Ipv_h " "'-114.0-- " lI'l.,...-u"'...,1_ ....In..... ,.n '.M iZ%.IO I.:: 11'.1lI la.JQ ....tt.lIl_ 1_1_•. 1 Conull. ~..1tnl .}.O.ea 1.21 " , Ul.\1 LM tHJ.rt 10.n l~.....,... {...... n.l • __loIN ( ...hMolt.) "".-.11 .,",,"1 ""',... l.'l ..~ m _1._H 111l2.lI:' I.n 1&0.21 •.n . ""Ih•.,cl... {...I...... l l.U '.01 • •.u • a.~ n ..I,....A. {"'_t.: " '.n .... " h_l.lt ••. ..• • a.DI lui 1,.-... 1...'-.1 Ln n "'.~ •.n I'II,-n I,,., '.M 0••\ 1'1.01 :t.1 ,. _hIli " "-1,.-.. _,_.,.u S~I."II --.«. 11.10 •.m 17 .•1 0.1' nu.n D." S'IIl.... iI_'_'.1 " W-I...... ' " LU ~ ~_. i .. I " - tnllCKaM I_ i m." '.Zt " "..... D.ll • 0.01 1:1.51 ... a.,J 1'1.01 Zl.S S.II ~ n.n •.n ,,-EorfoIo'I_~- .... Cll_.,,,, w::!.IS U.C1 CIt..... II. Crcl:.__act"..... •.n • 0.01 n.s lOonKtIC'. M." o.n • _.M '!lI.n rn.1'IIl' ,.n ...... n e.u • D.al .. Ct ....,,..,. n.....", ~ 0.01 n .. ~ ~., C.....l.,..-- UU.•] II." , lal." ••ft .. ~ 1O.G1 "'tcntt'Kll_II...... uonh O.U 1I,.11loKa& .." --,.11 ». " I ~ ,." .. ~ I ~1.... ( ....11MM.1 ~11...1,.rI, '.H ,.~ U n.n .. ~ " 1.:! ,.~ 1_1~ I ~11._I. " 11.11 a.lo I '.n .. ~ _.ft - m., ,-. " •.» o 0.1n ~" .. c...vw..... (..'_.1 ~ I :..1....." _h.,. •.n 00.111 n .• j .. u I _H. '." I t ••tlwlU 13." ,.n rw.1'I 1.1. 0 " I ,I -"1_10.,_.] '.0 •.n 1.1' n I 0.15 " O.Gl n .• -,-~"" _*-",,,,,1- .." ...... 1_'.. a'.11 ••ft ~." 1.11 , n .• •.n ~I_U. l"I.~t •.n It.M 0.1. n ~." '.~ Atrl•• lr1_ ';·'1 I "• AII_ta_w 11 •.l"I ~.l' ;~.jO ~.U ,. ,~ , ... _ O.G\ ,),," "'c.au ~.Zl n.1] " ,IIdU.I...... ,n. _"-" , ~ll.n...... U.1I G.ll , :1.11 G.10 oec.._l_ .n _Oltl " " _ t.KI...... lui, --_a,"," , ...,_.) ~ ... • :.GI u I ll.n 0.\1 l.tl ~ In~..,.l_ .. '.H :.~ ~.~. " " .." _';rntn •...,..."..11 " " I~ ...11 ...--.,. ~- Gin.... 2ft ..... " ..I••• n." _.ll G.;I 0.15 ~ 0.01 ~., .",.I.. u_ .." ~111""'C...l_.1 " " •.....,. '.1I,.... II. 1,....1..- ~__... l ...._.! ~.,. ,." :a.~ ~. ~.--- "-.I~_"f1""" (....."11_'_11.,,.,.) •.n <0.131 lJ.t •.n • 1t.01 U.S lI""""'~.....1 ~u -~.....'1.__ .. G.II < G.Gl U 1.10 G.Cl n 0'...... _ [~ ... 1.1.'1 O.ll .....011-... 1...... "1'111 ___ " 1.1-~..1 __ ,.~ G.~I n :m.'!3 1.'1 -~J-.l__• ~.n I.U ".11 0.11 IIM.n U.M , oJ .JI O.JS ".• " Ol ..._.~d 12~7 .:2 n.~ , ;'C.I~ " ~.~ ""." 1.'c lcul _.t 1_.'_' .....,. ll.a1 '.0"17 I.UJ 1.7n In.. I_ .f ...... , Cc-"" u _ at "h~ " " t..._ '... n c.....-u! " " n APPENDIX B INDEX OF RELATIVE IMPORTANCE (IRI) SUMMARY FOR PHANERODON FURCATUS 82 ..rlrD !L.!..,L ...... ft·...." ...... ~ ~-- ••.01 • •.al K -,., ...... '." • •.0' II a.lI ~:'II~ to,,,,.·b- . O\...... -,.,.~ • •.a. " 1...... hr•• ~. a.I/ 0.01 II .0.0' L" 0.01 1tl.S -- .." 1.U •••1 II _Itl'_.".. l,tl.l~ ...",,,.U lUI." 1I.1I Il.U ..10 Il - a.~r _ ..U... C ,...,.! '.0 11 II ..... •.11 .0 I 1.11 [...".n·",.c _··! , I] 11.IlI O.IS \l e..OlUlh.- <"".UU " ..~ , .. "n lI.S a.1i _''''''.I '_'·! " I ':: a.1I -'.....•10 1..'_'·1 n ""l"""""h .,.-.11...',,, II i /.11 _ C ·./ ..• a.1l " .., " lIlt.1l )l.U Iln.11 'i.1I :::::::':'r,,:.__ •. , ...""*"',... (...... ) ••U •.01 I " 11.' a.1I •••1 II II.JIl.. ...1""''''/''''_'.1 1.11 ..• " ~11"'/""_'.1 j 1.11 I'll" I.M .. ~.I' " ..• ..• ~.ll 0.11 11 1_1$jl'''''''(''''_'.1,._.". II" IS." (»_1 ' ~- " N '.n ¢.ol...... H.' I 1.10 .- " • ~.Ol [re'_'''' .. '.n 0.' "'F7OCUCO'" '.N n a.1I [Iffi....'. [tn"_" fl.,.II. 0.11 "'II1Cl ,-., '.n • ••al n C,..... , ...... 11 ,,"I"'.. 0.01 H.'" _.-.-...... " II.S •.11 .a.or,' a.O'! • a.•l ... t.oll. _I..rb 1.11 11 o.n .0.01 II ~.U • •.al ·U.l 1.1I •.~ 11 AIOU.I_I. __,. coli"""'''' T_'__11_. L. II 1.11 a••1 It ""'...... u.~ •• U 11 a.l'I • D.... 11 •.U U.S ,-~._".-...I,..., -"...._.,..-... (...,_.. ) ""' 10_'..,. a.1t 1l a.." • •.a' '.0 -...... ,...,.. .." • o.n • a.OI a.Ol 14 _,..' (..,_t.) 1.11 II 4/.11 ••SI "".n a.Sl -,-_n C..,..... l ~""_''''''''''' 11.11 •.It 11 1'.a! ••IJ • o.al ~ Iu-.. ... -., ,-,.. _." "'1.'1 1.14 lloOl.to \l.U 111.11 .• "I.~ S.II ".11 e.-t_ . . a.1t Ao.I.. ,.,_. '.n :1r==:":::.-.'-:;"' h to. n .." ~ ,." ..• ,.Ico.. '.n •G••r •.. -.,.,-h ....Il.Io ..__,.,... • -...... ·D.M ... 1 ' ..'_•• 1 ~'h . IM.I1 1.11 IlI.IlI I." lll./ll 1••1 In.O'! tou.... ",- . .a.al » ..• ...... [ ••' 10 .. ¢.o'''_'''''' •.n .a.lII 11 ...' 1_ • a.al •.'1 • •. ~1 1'1.' _ 1..._'.1 •.. ~.K I." a.l:I II 1.11'." ..• ".11 I '.0 o. a.a' _ " ~, " - .. » .• ...... _,... a.•' • a.Ol '.n ·.. ".. .0.01 " e-:to"" _,...... 1' n I -,""...... " a••' "In a.1S '1... '.n ]l.1I 0.'0 10 ' ••OCU ..•.11 • a.~l " • a.•' ..II.... _ .. ~"I "'.. .. ·..•. •.. -_-...11 . "I ." • ••1 T ,., . ...111 ••1••"..-...... 1101"""""" ..,..... <...., ....." "I " ....1. 0.0 '.n 11.11 ,." •.n •.11 tonl" ' '_'.1 I _,[1,-.. ,..,_'..•.1'11. I Cr7o'- coli...... ' ...... ", , ,_•. ! ! a.Ol I'lI.S .., • a.ol 11 "_1,-... Qoo l ...,_,.) I 1.'/ ••ll'! IS I -ft ... • ~.01 ••0' ...... " • .." \ " a.lI • 0.01 H (¢OJ_U-'.'<-<- • •.01 I1.S [dol-...'''' , . ... llooO',,"'''' I ••IS .a.al )f 'hu.__10 Il ...... , ••w •.01 1.11 , 'lJ.'a L. 1.11 --- •• ~.li :tn... •.0' U O.al • •.01 'h''''' '...' '.l ~ .•l II 1.11 [11, • .,...,u.... _" I I . ", ....•••,IT.·_,... ..11 " ... ! l.1I TO'rTn"'" _. 0.0' .a.OI :10.11 11,.,_" '0.0' 11 0.11 '0.'" /. 1.'" 0.01 " . Do'...... 1,'1'." '.n I' .... 0.01" U." •.SI .. ".11 a.J' l' ~,_.- .. ".0 '~ a.ll • a.o' ~,,...... ", ·/l;!.lo 11.'1.." , Sl11.'1 IiO.lXl ..S/." '0./0 II i lJl!.11 I 1...'_.11..'.'..... '_."... ·.ll' I.U' '.J" 1...... ,_, •• _,c.. ••.,.".. n .. _,., ... ,~ ~ li'~ ! 1 ,1 I ,I'll- ! ~!"r! -·- . f - - i f .,.,,1} --I, I'}" i t::::: gc If'I';BfW~ ~i~1iE!I"111!~J~1Ij'lljll'=!£ilff~f-;!!!f'l!i~!II!-!~ji!i11i!fiif~{1[!I~I!!![';f~;' '~'illf !1 ~' 'IIi' j':'! !,i_I, ~ ~ !H? i!;· 1!!iJj'l!ll"'I~ :n' , ~JI:~f.. :t :!t ",I ..III'tl HI;.!! I hi" fl;'lh;,i ",JI',I-j:I"I! ;j' 'I' , li~! ;: iIl"iH'i :!~= ... {l" ·i~:!;? ~i=iigi; l~~~ 3tl c'=tE! i !~ = ~;; i fii!!i~fi=l"i?!~! ff ? ~ I:H . .:1' --f''"' -, l!-H;o _OJ' '11- 0HiIT 1--', 'i': -. -I'!!:l-,-' " l;' -- H • I' -I: l, 0 - ,-- il-l,,' 0 11 11-'1 Ii. I !l ~E q ! :..i U:-il!:....s-;:.. 1~ ~ r.~:..:: g ;:! r. ! it f I j!! ! r ;: ;::::l!r. g ! 1 .. : .j : =i 'S-!. § :: ~~~~ , , ~~ - ~i , !:oK: , ':ilI b ': ~~ ~ ;::~l:: :t ;: ; ;; ~~ ~ : .== = :;: ;: "2 I ;; .. " .. , .. :t:;e~ , ; l:!~ ~;: ••~:. •• ~ ~ ~~ ~;;i ~ ~ ~ a 1:: = • i:::w h ; :: i:: ; ag .. ::::::= , ~~ ::: co Co> APPENDIX C INDEX OF RELATIVE IMPORTANCE (IRI) SUMMARY FOR EMBIOTOCA JACKSONI 85 ~T _ oen:-lI II" .,~ !L!.L "" _.-fo....t.lhn 1.10 0.01 ~;'·I~ I'::, ..• tIo''''M.-'0. 1." •.u " .1.'''''''1.1-,...... , • ~.~l U.S T._II••,o T'lda"'" I ." - •.u • o.DI n .• '.u .D.DI n.s -_II""--(l.-cull 011.-.. tl.n •.n 11 UZ.Jl l.lll IOS.1O D.n ...Ire"""-..rn,"._ I." [,0-,..111,.. h"_I.l 1.~1 a.m n &..11..,11 .. II.M D.D'I 11 I." D.DI ~ . 'n D.DI 11 ..~wrll . n .• ...... _tra '" •.u • D.al "ur-roll .''''''''1'...,... " nl.U """.ollo ...... h " llIu.n m.• ::£'1_.'.. (...,...... ! • 11.1l5 ~::::l::::::U " S.,.I (...,_t.l ' .....I t._leU •.u n ~~_...... - n.~ D.lo II I.'" •.n 11 D.n • 0.01 ~.S o.SO .0.01 n.' •.c.I...U. D.ll • a.C! U.S 0.50 • D.DI n .• t,.,_Ua 1.11 D.Dl n.s .....-.ICIII"" 1.11 \l.ll ~.ct1 " l.]] '.M a.Jl ~ .D.Ol..• tl...... '. " O.DI U.S D.SO n".• tlM"1"""h ".,.11. '.M a.Dl lS t"...U I...... D.n • a.DI 01.' "'I""",,~_11,-,_10 ,.n ~1".(..._.1 ~II. "".,"ri_ a.lI • D.C! U.S n.ll 0.11 T...,oOK.. 1.51 0.01 n.s D.50 n .• -_toAIh • 0.01 " _0. D.II D.DI 1l U.Sl D.lIl 1S 'l.ll' a.17 I.eo_"'la_'.1...11 ... .D.DI U.S '.nI.n D.Dl 10 1.'4 1.11 D.DI ..• ,.-1...... _'... n n.S(! 0.11 .. _., ZID.11 ,.m " ".0111.'.. '.11 D.at 11 n.'l D.H u _.-_ ..._1_1_.1 If'.!'! , '.M D.Ol lS 1.11 ..• ~""''''''''''I""l... •.u tlI.S J.n •.u .." D.DI n ..- ·.n O.Dl tlI " ...... 1_1•• ~ I1I:.S1 1I.1l 1 »Ol.1t n .• ~I... I •• .• '.M • l'I.r. l.n S )II.U _ ", ..• • llUI.n n .• '.n _lI._l_ '.n • D.DI " • ,, u .• ,\I,'. <:r1_ " D.DI ..• D.DI 11 ..•.u D.DI n.'" ,.• D.m n 1.n D.DI .. 1"~I" n.n •.n "" ... "'luu .. 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T....-n'd.l.m"",,, ·D.Dl .,.., 1> ts &:11.10.." •. n Do' , .." ~'-'-" •.w ''''.11 ,. "• ~'''''''' ..' ...... 1 '1.U lnl.ll II.!! To.., -..., , 1 , 1'_ 1,1'0 T...,_.' ,c.t_ .. " .. " _. of '''" 1_'.... (_ItO <0", ..,.1 " " " 86 ~~lfwt_u. -'tn....,. 0." • < 0.01 ~... , 00.0\ n ,." 'hl,...,ori~_' .• " 1~-.....11.,..1. 00.01 111"04'010 • ~ o.n '.MO.IS !i1 ...... 1t " L" .0.01 " -_HoI& " " " al1,.o:J1oou ...1",...... 'U _,_ f1.lS 0." o.OJ n l.n '.M 0.01 " " t..'u'U... l...' ....I.1 u.tIl 0.01 u ,.n " ...... t.,I..11o eo..hab lQ.U o.n n " ,." 0.01 " n.1l " " ~1...n1 ... 1.01 o.m _1)111,. " ...... U ... ".n 0.11 u 0.' Ll7 n "U"...... " Hal... 1I.1t 0.10 u II.SO llM.l1 , """' tAU.OIl s.c.Jl , l1tA.lI 1111.:16 "• lIOll." ....,."....,. .I l_t.l I.U o.m V '.M "',...I4H("'_•.1 I.U 0.01 " •.• " S.ll ~1·"·I ...I...t.l ..0.!1 " " ""'...' 1..101_1.1 0.'1 00.01 " Stn\ll t. _Ic~ o.n 00.01 n 0.11 lMpM ,--, .0.'1 00.0\ ..• ,." ro 0.' " ~- " en,taC..... < O.aT o.n .a.OI 'l.S ,.u ,." •.ClI_,... o.n " o.n < 0.0\ U.5 " (1'<1_'<11 o.u n.ll •.m ro., '.M IlO.,-.;t.I..loIa '.M %0.01 '50.%1 n.tf a.n Ill," 0.16 lin1 '. 1.11 0.0\ " 0.11 < O.OJ •" tl ,...," no",n. O.lf 00•.01 " c I...... < 0.01 i.1l '.M • 1'.11 '.m " "',_lIo_tloIlOU""'" a.n • a.C\ "&!.5 O.V < 0.01 '- 'ou\ -... .f l ..t ...... 1 ,..., 1'_ l,i71 ,.~ &.Sll 1.1~ 1...I_.f,...,0..,.,.., ... " .. " _f.f 'u" r...' ..... (."" ""0'...."1 " " "