truncation of larval densities 

 beginning in the vicinity of Maryland 

 Point (Md. P.) and extending down- 

 stream. This suggested to us, and 

 was later supported by work of 

 Ulanowicz and Polgar (1980), that 

 late stage larvae in this area were 

 subjected to conditions which 

 resulted in higher than normal 

 mortality rates. 



Subsequent comparisons of strip- 

 ed bass ichthyoplankton distribu- 

 tions, zooplankton abundances, and 

 food habits of larval striped bass 

 provided evidence which suggested 

 that quantity and distribution of 

 zooplankton in relation to the first 

 larval feeding stages may be a key 

 factor in recruitment success. In a 

 general fashion, larval abundance and 

 the number of food items per larval 

 stomach declined with the densities 

 of food items (zooplankton) in a 

 down-estuary direction (Figure 3) . 

 The sharpest gradients in zooplankton 

 densities coincided quite well with 

 sharp declines in late stage larval 

 densities. Moreover, it was found 

 that striped bass larvae fed upon the 

 largest prey items they could 

 capture. Using Jacobs' (1974) modi- 

 fication of Ivlev's Electivity Index: 



D =^±JL 



r+p - 2rp 



where D is the selectivity index, 

 r is the proportion of a given 

 food type in the feeder ration, 

 and p is the proportion of the 

 same food in the zooplankton, larval 

 striped bass showed a positive selec- 

 tion for adult Eurytemora af finis , 

 cyclopoid adults and copepodites, and 

 the cladoceran, Bosmina longirostris 

 and a negative selection for 

 copepod nauplii and most rotifer 



species (Table 1). Since the abund- 

 ance of the favored-prey species was 

 similar to the general zooplankton 

 abundance pattern, Beaven and 

 Mihursky (1979) concluded that food 

 may have been limiting for striped 

 bass larvae in the lower reaches of 

 the spawning area. 



ENVIRONMENTAL INFLUENCES 

 IN RECRUITMENT SUCCESS 



To this point, we have built a 

 case which suggests that recruitment 

 success is determined by the end of 

 the larval stage and that position in 

 the estuary where spawning takes 

 place and zooplankton abundance are 

 important factors regulating this 

 process. In this section we attempt 

 to show that these factors are in 

 turn influenced by several climatic 

 variables . 



Several authors have success- 

 fully related internal ecosystem 

 characteristic to the behavior of ex- 

 trinsic variables (Menzel et al. 

 1966, Aleem 1972). Copeland (1966) 

 found that fishery yields in some 

 Texas bays increase in years of above 

 average river flow. Menzel et al. 

 (1966) showed similar trends for oys- 

 ter stocks in Apalachicola Bay, Flor- 

 ida. Heinle et al. (1975) concluded 

 that colder than normal winters en- 

 hance zooplankton and juvenile fish 

 recruitment in the Patuxent River, 

 Maryland. Sutcliffe et al. (1976) and 

 Sutcliffe et al. (1977) demonstrated 

 significant correlations between 

 catches of 17 species of commercial 

 marine fish and shellfish and sea 

 temperatures in the Gulf of Maine. 



More to the point, in the 

 California Delta larger year-classes 

 of striped bass seem to result from 



154 



