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Fishery Bulletin 93(2). 1995 



kuloo, 1967; Doroshev, 1970; Davies, 1973; Morgan et 

 al., 1981). Temperature also strongly influences the 

 development of striped bass eggs and larvae (Polgar 

 et al., 1976; Rogers et al., 1977; Rogers and Westin, 

 1981), the generation times and turnover rates of 

 zooplankton prey (Heinle, 1969), and probably the 

 consumption rates of predators. 



Previous studies on striped bass recruitment vari- 

 ability averaged larval growth and mortality rates 

 over multiple daily cohorts (Polgar, 1977; Dey, 1981; 

 Kernehan et al., 1981; Uphoff, 1989; Setzler-Hamil- 

 ton et al. 2 ; Low 3 ), thereby obscuring relationships 

 with environmental factors. The presence of daily in- 

 crements on otoliths allows accurate estimates of lar- 

 val hatchdates, growth, and survival (Methot, 1983; 

 Crecco and Savoy, 1985; Essig and Cole, 1986; Leak 

 and Houde, 1987; Rice et al., 1987, a and b) and can 

 provide valuable information about processes and 

 factors affecting recruitment. Otolith-based esti- 

 mates of larval growth histories have the potential 

 to detect the subtle changes in growth rate that could 

 cause order-of-magnitude variability in recruitment 

 (Houde, 1987, 1989). 



We hypothesize that variable striped bass recruit- 

 ments are generated by variable growth and survival 

 rates of cohorts of larvae produced during a two- 

 month period of highly variable environmental con- 

 ditions. In a related paper (Rutherford et al. 4 ), re- 

 sults of a 3-year study of striped bass larval dynam- 

 ics in the Potomac River and Upper Chesapeake Bay 

 indicated that, on an annual basis, mean larval abun- 

 dances and ratios of weight-specific growth to instan- 

 taneous mortality rate (G/Z) are correlated with ju- 

 venile recruitment indices, indicating that recruit- 

 ment level is fixed during the larval stage. In this 

 study, an analysis of daily cohort abundances and 

 vital rates (growth and mortality) of larvae is pre- 

 sented to describe the process of year-class forma- 

 tion in striped bass and to illustrate how a primary 

 variable, temperature, affects cohort-specific growth, 

 survival, and recruitment potential in Chesapeake Bay. 



Methods 



Striped bass eggs and larvae were sampled every 3 

 to 7 days from April to June in the Potomac River 

 (1987-89) and in the Upper Bay (1988 and 1989; 



Rutherford et al. 4 ; Fig.l). Methods of collecting 

 ichthyoplankton and environmental data are de- 

 scribed briefly below and in more detail by Houde 

 and Rutherford 5 and Rutherford et al. 4 



Eggs and larvae were collected in duplicated ob- 

 lique tows of a paired 60-cm bongo sampler, with 333- 

 and 505-um mesh nets. Larval abundances based 

 upon the 505-um mesh size, 60-cm bongo net samples 

 were adjusted for extrusion of some small larvae by 

 comparing catches with those in a paired, 333-um 

 mesh net. Adjustments for gear avoidance by large 

 larvae were made by comparing 60-cm net catches 

 during daylight with a series of night catches made 

 on the same dates, and by comparing the 60-cm net 

 catches with those in a 2-m 2 Tucker trawl at the same 

 stations and dates. The 2-m 2 Tucker trawl (700-um 

 mesh) also provided collections of large larvae in late- 

 season surveys. Riverwide abundances of eggs and 

 larvae were estimated by multiplying mean station 

 densities by the river volume which those stations 

 represented. Egg abundances also were estimated 

 and are reported in Houde and Rutherford ( 1992). 



Densities of zooplankton that were potential prey 

 for striped bass larvae were estimated from pumped 

 water samples taken near-bottom, mid-depth, and 

 at surface and filtered onto a 53-um screen, or taken 

 by vertical lifts of a 20-cm, 53-um mesh plankton net. 

 Temperature, pH, and salinity were measured at all 

 stations on each survey. Conductivity was measured 

 in 1988 and 1989, and turbidity and light were mea- 

 sured in 1987 and 1988. In addition, gauges at dams 

 upstream of the spawning grounds provided continu- 

 ous temperature measurements. 



Larval ages, hatch dates, and age-frequency dis- 

 tributions were estimated from an analysis of sagit- 

 tal otolith increments, which are deposited daily in 

 striped bass (Jones and Brothers, 1987; Secor and 

 Dean, 1989). Otolith increments were counted at 

 least three times and the mean of the last two counts 

 was used to estimate age. Aged larvae were grouped 

 into 3-day periods (cohorts) based upon their hatch 

 dates. Three-day cohorts were designated because 

 95% confidence intervals around mean ages indicated 

 a 3-day range. Otolith-aged larvae from represented 

 length classes were used to estimate the proportions 

 of unaged larvae in each 0.5-mm length class that 

 fell within each one-day age class. These 'age-length 



3 Low, A. F. 1986. Striped bass egg and larva survey in the Sacra- 

 mento-San Joaquin estuary. California Dep. Fish and Game. 

 Unreferenced report, November 1986. Available: California 

 Department of Fish and Game, Stockton, CA 95205. 



4 Rutherford, E. S., E. D. Houde, and R. M. Nyman. Relation of 

 larval stage growth and mortality to recruitment of striped bass, 

 Morone saxatilis, in the Chesapeake Bay. Unpubl. manuscr. 



5 Houde, E. D., and E. S. Rutherford. 1992. Egg production, 

 spawning biomass and factors influencing recruitment of striped 

 bass in the Potomac River and Upper Chesapeake Bay. Rep. to 

 Maryland Dep. Natl. Resour., Contract No. CB89-C01-003. Univ. 

 Maryland, Center for Environmental and Estuarine Studies, 

 Ref. No. [UMCEESJCBL 92-017, 313 p. Available: University of 

 Maryland Center for Environmental and Estuarine Studies, 

 Chesapeake Biological Laboratory, 1 Williams St., Solomons, 

 MD 20688. 



