Longfin smelt (Spirinchus thaleichthys) were once one of the most abundant fish species in the San Francisco Estuary (SFE), California. However, over the last two decades, their abundance has plummeted to <1% of historical values, contributing to their being listed as ‘threatened’ under the California Endangered Species Act. Developing a captive culture of longfin smelt to be used as a sustainable source of research specimens and for use in supplementing wild populations is one conservation management tool that is currently under development. Unfortunately, high larval mortality has hindered the development of a captive culture and no individuals in captivity have ever survived to reproductive age. To improve culture methods for longfin smelt, we measured the responses of larvae to two environmental conditions that correlate strongly with field abundances of longfin smelt in the SFE: temperature and salinity.
For temperature, we incubated and hatched embryos at three different temperatures (9, 12, 15?) and measured larval heart rate through acute stepwise increases in temperature to identify their Arrhenius breakpoint temperature, a proxy for a fish’ thermal optimum, and the temperature at which heart rate peaks, a proxy for a fish’s thermal limit. We also measured larval growth and yolk resorption rates at these temperatures to corroborate the findings of our cardiac metrics. For salinity, we reared yolk-sac larvae in five different salinities (0.4, 5, 10, 20, 32 parts per thousand [ppt]) and measured their osmotic and ionic balance, growth, and yolk resorption rates over time to identify their upper and lower salinity tolerances, and potential salinities to use in culture.
Our results indicate that culturing conditions between 9-14? and 5-10 ppt salinity would be appropriate for longfin smelt larvae. We found that 12-14? maximized the proportion of individuals at their thermal optimum, which correlated with the highest growth rates measured at 12?. However, we also found that larvae reared at 9? ultimately grew to the same size as those at 12?. For salinity, we found that larvae experienced disruption to osmotic and ionic balance at 20 and 32 ppt and that larvae in 0.4 ppt had stalled yolk resorption rates. This correlated with the decreased growth at these salinities compared to 5 and 10 ppt. Using these results, larviculture methods were modified and improved survival in culture to the highest rates achieved thus far.