Larval Striped Bass Condition in a Drought‐Stricken Estuary: Evaluating Pelagic Food‐Web Limitation

Abstract

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.

Maternal transfer of xenobiotics and effects on larval striped bass in the San Francisco Estuary

“…This method enabled us to detect developmental changes previously unseen using standard methods. Typically, morphological studies of the early life stages of fish use linear measurements, such as standard length, body depth at the pectoral fins, and body depth at the anus, to determine size and growth relationships (14,39,40). However, in our opinion these types of metrics do not provide an accurate representation of growth and development.…”

“…The most recent step change raises significant concerns because it has occurred during a period of moderate weather conditions and river flows that typically result in at least minor population recovery (12). The past 3 decades of monitoring data and various studies suggest that some factors causing the striped bass population decline occur in the early life stages and before the 38-mm young of the year index (3,13,14), an index that has been used to accurately measure population abundance of striped bass for more than 40 years (15).…”

“…However, there is evidence that starvation is not occurring (14) and that the combined effects of water diversions and introduced species do not account entirely for the recent declines in the population (12).…”

“…Effects of contaminants on the early life stages of striped bass have been documented since the late 1980s (3,14), but comprehensive studies investigating contaminant concentrations, exposure, and effect have not been conducted to this date.…”

Integrating contaminant responses in indicator saltmarsh species

“…For example, if an alteration in growth or reproduction is observed, managers have little information to determine why that alteration may have occurred or what its signiWcance might be. We believe that if early warning, mechanistically-based indicators can be used to detect the stressors that managers are most concerned about, then improvements in the protection of aquatic life within a given wetland can be achieved (Cherr, 2002;Van Dam et al, 1998;Bennett et al, 1995). The indicators described here are part of an extensive set of integrated mechanistic indicators in marsh plants and animals at multiple spatial scales and levels of biological organization.…”