There is growing concern that predation by juvenile (ages 1-3) Striped Bass Morone saxatilis may negatively affect the population dynamics of Delta Smelt Hypomesus transpacificus, an imperiled species listed as threatened under the U.S. Endangered Species Act and endangered under the state of California's Endangered Species Act. We explored the potential predator-prey interaction between juvenile Striped Bass and Delta Smelt in California's San Francisco Estuary. Specifically, we addressed two study questions. (1) Is juvenile Striped Bass abundance correlated with Delta Smelt survival? (2) What is the estimated peak monthly consumption of Delta Smelt by juvenile Striped Bass during summer? We addressed the first study question using regression techniques and the second with functional responses to estimate per capita Striped Bass consumption of Delta Smelt using Delta Smelt biomass densities estimated from trawl surveys as input to the functional responses. We found no evidence for a correlation between juvenile Striped Bass abundance and Delta Smelt survival. Based upon available data, we estimated that consumption of Delta Smelt may range from a level not discernibly different from 0 g·Striped Bass −1 ·month up to about 11 g·Striped Bass −1 ·month −1 . These are the first estimates of potential Striped Bass consumption of Delta Smelt.

Like several other fishes in the pelagic community of the upper San Francisco Estuary, age‐0 striped bass Morone saxatilis have shown a major decline based on a midwater trawl sampling program that has been conducted for over 40 years. We hypothesized that the apparent decline in age‐0 striped bass might be partially attributable to a behavioral shift away from the channels sampled by the trawls. We found no evidence of an upstream–downstream shift in age‐0 distribution. Instead, age‐0 striped bass distribution remains closely associated with the low‐salinity zone of the estuary. However, the survey data suggest a substantial long‐term distribution shift away from channels and toward shoal areas. The hypothesis that young striped bass are undersampled by midwater trawls is supported by modeling of demographic patterns, which showed that the decline in numbers of age‐0 fish was not consistent with increasing trends in age‐1 fish. We hypothesize that reduced food availability in pelagic habitat is a major cause of apparent behavioral shifts by age‐0 striped bass and some native fishes. Nonetheless, the magnitude of the shift toward shoal habitat does not appear to fully account for the extreme decline in age‐0 striped bass abundance.

Striped bass are both a major predator of native fishes and support a recreational fishery in the San Francisco Estuary (the estuary). Quantifying their demands on their prey is important for understanding long-term trends of fish in the estuary. In this study, we: (i) applied a bioenergetics model of sub-adult (age 1 and age 2) and adult (age 3+) striped bass (Morone saxatilis) to quantify long-term consumption patterns from 1969 through 2004 in the estuary; (ii) developed a method to estimate the abundances of sub-adult striped bass; (iii) evaluated how consumption varied by age and gender; and (iv) identified factors that affect the resulting consumption estimates. On a 'per capita' basis, modeled individual prey fish consumption increased after 1990, and individual total and prey fish consumption by age-2 striped bass increased after 1994. Conversely, individual total and prey fish consumption by adult striped bass decreased over the period analyzed. This decline in individual consumption over the study period was related to a decline in mean length at age of adults. As expected, long-term trends in population consumption (total and prey fish) by all ages of striped bass (ages 1 through 6) closely followed their respective population-abundance trends. Population total consumption and prey fish-specific consumption by sub-adult striped bass was found to be similar to the population consumption by adult striped bass, largely because of the high abundance of sub-adults. Unlike adult striped bass that may emigrate and forage in the Pacific Ocean, the majority of sub-adult striped bass reside within the estuary; hence, consumption by the relatively abundant sub-adult population may have significant effects upon their estuarine prey species.

Various methods have been developed to mitigate the adverse effects of the Federal Columbia River Power System on juvenile Pacific salmon out-migrating through the Columbia River basin. In this study, we found that hatchery-reared spring Chinook salmon Oncorhynchus tshawytscha in the river are in varying degrees of health, which may affect delayed mortality and the assessment of the effectiveness of management actions to recover listed stocks (e.g., barging fish downstream versus leaving fish in the river). A laboratory disease challenge with Listonella anguillarum was completed on fish from Rapid River Hatchery and Dworshak National Fish Hatchery (NFH) with different out-migration histories: (1) transported by barge, (2) removed from the river before barging, or (3) left to travel in-river. Barged fish from Rapid River Hatchery experienced less mortality than fish from Dworshak NFH. No statistical differences were found between the hatcheries with fish that had in-river out-migration histories. We suggest that the stressors and low survival associated with out-migration through the hydropower system eliminated any differences that could have been present. However, 18-25% of the fish that were barged or collected before barging died in the laboratory before the disease challenge, compared with less than 2% of those that traveled in-river. Owing to disproportionate prechallenge mortality, the disease-challenged populations may have been biased; thus, they were also considered together with the prechallenge mortalities. The synthesis of prechallenge and disease-challenged mortalities and health characteristics evaluated during out-migration indicated that the benefit of barging was not consistent between the hatcheries. This finding agrees with adult survival and delayed mortality estimates for the individual hatcheries determined from adult returns. The results suggest that the health status of fish and their history before entering the hydropower system (hatchery of origin and out-migration path) are critical variables affecting the conclusions drawn from studies that evaluate mitigation strategies.

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