Growth, foraging, and health effects of turbidity on captive, sub-adult Delta Smelt

Food limitation is considered a major cause for the decline of Delta Smelt ( Hypomesus transpacificus ) and other pelagic forage fishes in the Sacramento-San Joaquin Delta and San Francisco Estuary. However, reversing the decline of pelagic productivity in this heavily modified and invaded estuary will be difficult and may instead benefit invasive species. A potentially less difficult, more targeted strategy may be increasing turbidity, which, like pelagic productivity, has declined in the SFE. Turbidity is essential for larval Delta Smelt to initiate feeding and is strongly associated with catch of older, wild Delta Smelt. Here, we examined its influence on a range of endpoints in sub-adult Delta Smelt in two, month-long experiments. Cultured, sub-adult Delta Smelt were housed in replicate buckets, fed live prey ( Artemia ), and exposed to varying levels of turbidity from additions of phytoplankton ( Nannochloropsis sp.) or clay particles (bentonite). In the first (low light) experiment, no benefit of turbidity due to Nannochloropsis (up to 5 NTU) was detected in the absence of Artemia , and only a modest increase in liver glycogen (20%) was detected in the presence of Artemia with added turbidity (1 NTU). In the second (moderate light) experiment, fish gained 75% more weight, had improved body condition, and had elevated liver glycogen (41%) in bentonite treatments up to 25 NTU compared to clear water. For the same three endpoints, fish showed positive but not statistically significant responses to the Nannochloropsis treatments up to 10 NTU. Cortisol and stomach fullness results suggest that the benefits of turbidity were associated with a combination of decreased stress and improved foraging in turbid water. Overall, our results indicate that turbidity from clay particles up to 25 NTU strongly benefits sub-adult Delta Smelt.