UC Davis

The application of otolith (“fish ear stone”) analyses to reconstruct the age, growth, and migration of fishes is integral to our understanding of their population dynamics, life history, and management. The delta smelt (Hypomesus transpacificus) is an endemic migratory forage fish that resides exclusively within the upper San Francisco Estuary (SFE). Historically, this native species was one of the most abundant fishes in the upper SFE and has served as a key indicator of estuarine health. The upper SFE, however, is a highly altered ecosystem, characterized by food web collapse and rapid warming due to climate change. Otolith tools have been broadly applied to understand these effects in delta smelt, however, the effects of changes in environmental conditions on the interpretation of otolith indicators has not been explored. Experimental assessments of how otolith metrics vary among multiple environmental factors are required for their accurate application and interpretation when addressing conservation concerns. Here, we used individually tagged cultured delta smelt of known age to conduct a 70-day experiment examining the additive and interactive effects of temperature (14C and 18C) and food availability (no feed and ad libitum feed) on survival, somatic growth, otolith accretion, and otolith geochemistry. There were significant synergistic effects of temperature and food ration on fish survival. Fish experiencing warm water conditions without food exhibited ~400% higher mortality compared with fish reared at cooler temperatures with ad libitum food. However, survival was equivalent among treatments when sufficient food was available, suggesting that food availability may offset mortality due to heat stress. Treatment effects of otolith accretion rates masked the severity of negative feed and temperature effects on somatic growth metrics. Additionally, we quantified partition coefficients (Dx) and element-to-calcium ratios for the incorporation of eight elements in delta smelt otoliths (7Li, 11B, 23Na, 55Mn, 63Cu, 66Zn, 88Sr, and 137Ba), observing both positive (DNa, DMn) and negative (DBa) correlations with food availability, and negative (DZn, DCu) correlations with temperature, suggesting their utility as potential proxies for environmental conditions. By detailing direct correlations between somatic growth, environmental conditions, and the otolith-based reconstructions used to infer them, we provide insight into both the strengths and limitations of these techniques, and the likely consequences of climate change on the adults of an endangered annual fish species.