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.
Aquatic ecosystems around the world face serious threats from anthropogenic contaminants. Results from 8 years of field and laboratory investigations indicate that sublethal contaminant exposure is occurring in the early life stages of striped bass in the San Francisco Estuary, a population in continual decline since its initial collapse during the 1970s. Biologically significant levels of polychlorinated biphenyls, polybrominated diphenyl ethers, and current-use/legacy pesticides were found in all egg samples from river-collected fish. Developmental changes previously unseen with standard methods were detected with a technique using the principles of unbiased stereology. Abnormal yolk utilization, brain and liver development, and overall growth were observed in larvae from river-collected fish. Histopathological analyses confirmed and identified developmental alterations. Using this methodology enabled us to present a conclusive line of evidence for the maternal transfer of xenobiotics and their adverse effects on larval striped bass in this estuary.Morone saxatilis ͉ contaminants ͉ biomarkers ͉ histopathology ͉ unbiased stereology
There is no standard method to determine the applicability of otolith Sr/Ca ratio to reconstructing estuary use. We have developed a novel method to determine the response of otolith Sr/Ca to changes in water Sr/Ca and salinity in San Francisco Estuary (California, USA). We perform correlated, spatially resolved Sr/Ca and Sr isotope measurements using otoliths from adult striped bass ( Morone saxatilis ) in the San Francisco Estuary to estimate the otolith–water Sr/Ca partition coefficient (DSr = 0.305 ± 0.009). DSr did not vary significantly with salinity, and therefore the salinity–otolith Sr/Ca model was constructed by substituting the partition coefficient into the nonlinear salinity–water Sr/Ca mixing model for the system. The model demonstrates that the primary factor controlling the response of Sr/Ca to salinity is the Ca concentration in the freshwater source flowing into the estuary. A concentration of 60 ppm Ca is an approximate threshold below which estuary Sr/Ca increases rapidly to near the marine Sr/Ca at low salinities (5‰–15‰), thereby providing sharp delineation of estuary entrance, but little to no discrimination among higher salinity habitats. Our approach provides a general framework for assessing the potential utility of Sr/Ca in estuarine systems and specifically for the San Francisco Estuary.
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