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

Estuarine food webs are frequently altered by human interventions, including freshwater diversions, toxic compounds, and introduced species. From 1988 through 1991 we examined the external morphological and internal histopathologic condition of larval striped bass (Morone saxatilis) to evaluate the potential importance of starvation to fish recruitment in the San Francisco Bay estuary. During a recent drought (1987-1992), fish populations declined markedly, concurrent with dramatic reductions in phytoplankton and zooplankton food for larval fishes. Such patterns suggest pelagic food is limited during times of low freshwater input; therefore, larval starvation may limit recruitment. However, toxic compounds in agricultural runoff are also less diluted in low-outflow years, enhancing their potential impact. Histopathology enabled us to identify their possible effects. In the laboratory, indices of larval morphology and eye and liver tissue condition reflected starvation after 2 d of food deprivation. From 1988 through 1991 >90% of 980 field-caught specimens were classified morphologically as feeding larvae. Histopathological evaluation indicated that all field-caught specimens (N = 500) had food in their guts and lacked tissue alterations consistent with starvation. However, liver alterations consistent with toxic exposure were seen in 26-30% of the field-caught larvae from 1988 through 1990, dropping to 15% in 1991. While our findings implicate toxic exposure as a factor in the relationship between low freshwater input and poor year-class success of striped bass, reductions of toxic runoff and improvement in larval liver condition in 1991 did not improve larval survival. This suggests the potentially greater importance of interactions with food limitation and predation as well as the futility of pursuing single-factor explanations for recruitment failure. The potential obfuscation of food limitation by toxic exposure also indicates the need for interdisciplinary approaches to distinguishing anthropogenic intervention from estuarine food-web processes.