Contaminant and food limitation stress in an endangered estuarine fish

Abstract

The abundance of Delta Smelt (Hypomesus transpacificus), a fish species endemic to the upper San Francisco Estuary (SFE), is declining. Several causes for the population decline have been proposed, including food limitation and contaminant effects. Here, using juvenile Delta Smelt collected from throughout their range, we measured a suite of indices across three levels of biological organization (cellular, organ, individual) that reflect fish condition at temporal scales ranging from hours to weeks. Using these indices, the relative conditions of fish collected from five regions in the SFE were compared: Cache Slough, Sacramento River Deep Water Ship Channel, Confluence, Suisun Bay and Suisun Marsh. Fish sampled from Suisun Bay and, to a lesser extent the Confluence, exhibited relatively poor short-term nutritional and growth indices and morphometric condition, while fish from the freshwater regions of the estuary, and Cache Slough in particular, exhibited the most apparent histopathological signs of contaminant exposure. In contrast, fish from the Suisun Marsh region exhibited higher short-term nutrition and growth indices, and better morphometric and histopathological condition. For instance, fish collected from Suisun Marsh had a mean stomach fullness, expressed as a percentage of fish weight, that was 3.4-fold higher than fish collected from Suisun Bay, while also exhibiting an incidence of histopathological lesions that was 11-fold lower than fish collected from Cache Slough. Thus, our findings support the hypothesis that multiple stressors, including food limitation and contaminants, are contributing to the decline of Delta Smelt, and that these stressors influence Delta Smelt heterogeneously across space.

Science Advancements Key to Increasing Management Value of Life Stage Monitoring Networks for Endangered Sacramento River Winter-Run Chinook Salmon in California

“…In Delta Smelt, Hammock et al (2015) used suites of indices across levels of biological organization (cellular, organ, individual) to assess fish condition at temporal scales ranging from hours to weeks to understand short-term habitat-specific condition and longer-term cumulative condition across life stages and habitat regions. For example, stomach fullness, RNA-DNA ratios, and plasma insulin-like growth factor-I reflect recent foraging success and shortterm nutritional status (Buckley 1979;Beckman et al 1998;MacFarlane and Norton 2002).…”

“…• Suites of indices assess condition during short temporal scales (hours to weeks) and longer-term cumulative condition (Hammock et al 2015) • Identifies foraging success and nutritional reserves (Beckman et al 1998;MacFarlane 2010) Example Applications…”

“…• Otolith-derived growth rates during freshwater rearing explain variability in salmon ocean survival (Woodson et al 2010) • Condition indices across levels of organization (cellular, organ, individual) for Delta Smelt varied among Delta regions (Hammock et al 2015) https://doi.org/10.15447/sfews.2017v15iss3art1…”

Effects of freshwater flow and phytoplankton biomass on growth, reproduction, and spatial subsidies of the estuarine copepod Pseudodiaptomus forbesi

“…However, the decline in delta smelt is due at least in part to food limitation at various life stages (Bennett, 2005;Slater & Baxter, 2014;Hammock et al, 2015). Primary productivity and abundance of the copepod prey of this fish have been low since * 1987 mainly because of grazing by the clam Potamocorbula amurensis, introduced in 1986 (Cloern & Jassby, 2012).…”

“…The energy available for growth and reproduction may also be reduced by heightened metabolic demands for osmoregulation or in response to stress (Lee et al, 2013;Hammock et al, 2015). Thus, food limitation can be aggravated by stressful environmental conditions.…”