LARVAL STRIPED BASS CONDITION IN A DROUGHT-STRICKEN ESTUARY - EVALUATING PELAGIC FOOD-WEB LIMITATION

Estuarine food webs are frequently altered by human interventions, inc luding freshwater diversions, toxic compounds, and introduced species. From 1988 through 1991 we examined the external morphological and int ernal histopathologic condition of larval striped bass (Morone saxatil is) to evaluate the potential importance of starvation to fish recruit ment in the San Francisco Bay estuary. During a recent drought (1987-1 992), fish populations declined markedly, concurrent with dramatic red uctions in phytoplankton and zooplankton food for larval fishes. Such patterns suggest pelagic food is limited during times of low freshwate r input; therefore, larval starvation may limit recruitment. However, toxic compounds in agricultural runoff are also less diluted in low-ou tflow years, enhancing their potential impact. Histopathology enabled us to identify their possible effects. In the laboratory, indices of l arval morphology and eye and liver tissue condition reflected starvati on after 2 d of food deprivation. From 1988 through 1991 >90% of 980 f ield-caught specimens were classified morphologically as feeding larva e. Histopathological evaluation indicated that all field-caught specim ens (N = 500) had food in their guts and lacked tissue alterations con sistent with starvation. However, liver alterations consistent with to xic exposure were seen in 26-30% of the field-caught larvae from 1988 through 1990, dropping to 15% in 1991. While our findings implicate to xic exposure as a factor in the relationship between low freshwater in put and poor year-class success of striped bass, reductions of toxic r unoff and improvement in larval liver condition in 1991 did not improv e larval survival. This suggests the potentially greater importance of interactions with food limitation and predation as well as the futili ty of pursuing single-factor explanations for recruitment failure. The potential obfuscation of food limitation by toxic exposure also indic ates the need for interdisciplinary approaches to distinguishing anthr opogenic intervention from estuarine food-web processes.