The University of Maryland System Ref. No. [UMCEESI-CBL 92-017 - Center for Environmental and Estuarine Studies Chesapeake Biological Laboratory So lomons, MD 20688-0038 ,- Egg Production, Spawning Biomass and Factors Influencing Recruitment of Striped Bass in the Potomac River and Upper Chesapeake Bay REPORT TO: Maryland Department of Natural Resources Tidewater Administrat ion Tawes State Off ice Bui lding 580 Taylor Avenue Annapolis, MD 21401 Contract Number: CB89-001-003 E.D. Houde and E.S. Rutherford The University of Maryland System Center for Environmental and Estuarine Studies Chesapeake Biological Laboratory P.O. Box 38 Solomons, MD 20688 January 1992 TABLE OF CONTENTS EXTENDEDABSTRACT .............................. 1 INTRODUCTION ................................ 5 STUDY AREAS AND SURVEYS ........................... 7 EggsandLarvae ............................ 7 PotomacRiver .......................... 7 Upper Bay ............................ 7 Late Stage Larvae ........................... 7 PotomacRiver .......................... 7 Upper Bay ............................ 7 METHODS ................................... 14 Sampling and Samples ......................... 14 Net Tows ............................ 14 Environmental Data .......................... 16 Zooplankton Densities ......................... 17 1988Samples .......................... 17 1989Samples .......................... 17 Ichthyoplankton Identification .................... 18 Otolith Preparation and Analysis ................... 18 Back-Calculat ion of Lengths-at-Age .................. 19 AdultFemaleData ........................... 20 ANALYSIS .................................. 22 Egg and Larvae Abundances ....................... 22 EggProduction ............................ 22 Spawning Biomass and Age-Specific Spawning Potential ......... 25 Larvae Length-Frequency Distributions and Adjustments ......... 27 Age.LengthKeys ..... .,...................... 27 Larvae Age-Frequency Distributions .................. 28 Estimating Growth Rates ........................ 29 Aggregated Sample Mean Growth Rate ............... 29 Individual Cohort Growth Rates ................. 29 Back-Calculation of Larval Growth Rates and Variability ..... 30 Mortality Rate Estimates ....................... 30 Cohort Hatch Dates .......................... 32 RESULTS ................................... 33 1989 Environmental Data Summaries ................... 33 Potomac River .......................... 33 Temperature ........................ 33 Salinity and Conductivity ................. 33 pH ............................ 33 Rainfall and River Discharge ............... 37 Upper Bay ............................ 37 Temperature ........................ 37 Salinity and Conductivity ................. 37 pH ............................ 42 Rainf a1 1 and River Discharge ............... 42 Dissolved Oxygen ..................... 42 Egg and Larvae Data Summaries. 1989 .................. 42 PotomacRiver .......................... 47 Upper Bay ............................ 59 Potomac River and Upper Bay Comparisons ............. 72 Zooplankton Densities ......................... 73 Potomac River .......................... 73 Upper Bay ............................ 82 Striped Bass Egg Production and Spawning Biomass ........... 86 PotomacRiver .......................... 86 iii Upper Bay ............................ 93 Numbers and Spawning Biomass of Adult Females. and Age-Specif ic Egg Production .............................. 98 PotomacRiver .......................... 98 Upper Bay ............................100 Potomac River vs . Upper Bay ................... 104 Potential and Actual Egg Production ...............104 Potomac River .......................105 UpperBay .........................111 Age-Specific Female Contributions to Egg Production....... 111 Larva Length-Otolith Length Relationship ...............113 Potornac River ..........................115 Upper Bay ............................115 Larva Length Frequencies .......................118 Potomac River.......................... 118 Potomac River Tucker Trawl Catches ............120 Upper Bay ............................128 Upper Bay Tucker Trawl Catches ..............128 Age-Length Relationships .......................135 Larva Age Frequencies .........................135 Potomac River 1989 ....................... 135 UpperBay1989 .........................137 Growth ................................146 Potomac River ..........................146 Aggregated Sample Mean Growth Rate ............146 Cohort -Specific Growth Rates ............... 146 Back-Calculated Growth ..................149 Upper Bay ............................ 153 Aggregated Sample Mean Growth Rate ............153 Cohort-Specific Growth Rates ...............153 Back-Calculated Growth ..................157 Environmental Factors and Growth ................161 Mortality ...............................166 Potomac River ..........................166 Upper Bay ............................166 G/Z Ratio ...............................171 Potomac River ..........................171 Upper Bay ............................171 Estimated Abundance of Striped Bass Larvae at 8.0 mm SL ....; ...174 Potomac River ..........................174 Upper Bay ............................181 DISCUSSION .................................186 LITERA'rlIRE CITED ..............................215 APPENDICES .................................220 ACKNOWLEDGMENTS We thank the following persons for assistance in carrying out the research and producing the report. Assistance with field collections in the Potomac River was provided by J.H. Cowan, Jr., J. Gooch, K. Langley, D. Monteleone, B. Moser, R. Nyman, A. Vazquez, and C. Zastrow. In the Upper Bay, the Maryland DNR Resources and Monitoring Division carried out the field collections, which were coordinated by R. Takacs. The Fisheries Division of Maryland DNR provided gillnet catch and effort data on adult striped bass. We especially thank T. Sminkey and S. Early for these data and P. Jones for helpful discussions related to the data. Several people assisted with sorting, identifying and measuring fish larvae and zooplankton samples. They include L. Beaven, C. Cooksey, S. Dorsey, K. Langley, D. Lytle, L. Linley, and J. MacGregor. Helpful discussions on use of otoliths in age and growth analyses and on striped bass larvae ecology were contributed by D. Secor and J.H. Cowan, Jr. C. Zastrow assisted in preparation of tables and graphs. L. Fernandez word processed the report. EXTENDED ABSTRACT Ichthyoplankton surveys in the Potomac River and Upper Chesapeake Bay were carried out in 1989 to estimate striped bass egg productions, age- specific spawning biomasses of adult females, cohort-specif ic larval growth and mortality rates, and hatch dates of 8.0 mm larvae survivors. Possible consequences to recruitment of environmental factors were examined in 1989 and for data collected in 1987-1988. In 1989, 18 surveys in the Potomac River and 14 surveys in the Upper Bay were completed in the 3 Apri 1 to 15 June period. Primary ichthyoplankton samplers were a 60-cm, 333-pn mesh plankton net and a 2 mZ, 700-pn mesh Tucker trawl. Spawning by striped bass, which is strongly influenced by water temperature, commenced on 2 April and was essentially completed by 29 May in the Potomac River. In the Upper Bay, spawning commenced on 10 April and extended into June. The temporal and spatial occurrences and distributions of eggs and larvae In both spawning areas are described and discussed in relation to environmental factors (temperature, rainfall, river discharge, pH, conductivity, zooplankton abundances) . 'The estimated 1989 egg productions were: Potomac River -- 11.33 x lo9 (S.E. 1.96 x lo9) and Upper Bay -- 14.57 x lo9 (S.E. 5.69 x lo9). Corresponding female biomasses were: Potomac River -- 55,501 kg; Upper Bay -- 70,808 kg. The 1982 year-class females contributed the highest percentage of eggs in each system: Potomac River -- 41.8%; Upper Bay -- 54.7%. Egg production estimates in the Potomac River from 1987-1989 did not differ significantly among years, ranging from 6.65 x lo9 to 11.33 x lo9, but increased by eight- fold in the Upper Bay in 1989 (14.57 x lo9) compared to 1988 (1.78 x 10'). Spawning was more protracted in 1989 than in either 1987 or 1988 in the Potomac, or 1988 in the Upper Bay. Females older than age 10, which had contributed substantial ly to Potomac egg product ion in 1987 and 1988, apparently were absent in 1989. A small percentage of females older than age 1 10 continued to contribute modestly (4.5%) to Upper Bay egg production in 1989. The gillnet catches of adult females, when compared with cumulative egg production based upon ichthyoplankton collect ions, indicated that female daily catch-per-unit-effort did not provide an accurate index of daily egg product ion. Larvae length-frequency distributions were converted to age-frequency distributions from otolith-aged subsamples and derived age-length keys. Mean annual growth rates of striped bass larvae in 1989, based upon a stable-age distribution assumption, were 0.20 mm dm' in the Potomac River and 0.29 mm d-' in the Upper Bay. Back-calculated, 3-day cohort growth rates from otolith increment analysis ranged from 0.11 to 0.36 nlm d-' in the Potomac River and from 0.18 to 0.36 mm d-' in the Upper Bay. Growth rates of Potomac larvae in 1989 were similar to those observed in 1987, but higher than 1988 rates. Mean mortality rates, based upon the stable-age distribution assumption for 1989, were