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Rearing temperature and parentage influence development of early post-hatch Longfin smelt (Spirinchus thaleichthys) in captive culture

Abstract

The wild abundance of Longfin smelt (Spirinchus thaleichthys, LFS) in the San Francisco Estuary (SFE) has declined to less than 1% of pre-1980’s numbers. As a result, they have been listed as “threatened” under the California Endangered Species Act since 2009. In an effort to avoid localized species extinction, a captive culture program was created for LFS through the Fish Conservation Culture Laboratory at the University of California, Davis. Larviculture methods for LFS are still being optimized to ensure a successful culture program. For my MS thesis, I tested the impacts of rearing temperature on early post-hatch LFS larvae from three different parental clutches to identify which rearing temperatures are best for successful development of these early life stages. I reared newly fertilized embryos and post-hatch, non-feeding yolk-sac larvae in freshwater (0 ppt) at 10, 12, or 15°C until 3 days post-hatch (dph). I measured larval morphometrics (total length, eye diameter, yolk sac area), and mortality for all groups. I was the first to evaluate the incidence and severity of jaw and spinal malformations for cultured LFS. Larvae reared at 15°C were smaller (mean ± SE; 0.52 ± 0.0040 mm), significantly younger at hatch (13.1 ± 0.1 days post-fertilization; dpf), and had slightly higher mortality and increased spinal malformations compared to other rearing temperatures. Embryos reared at 10 and 12°C were of similar size (10℃: 0.59 ± 0.0030 mm, 12℃: 0.56 ± 0.0030 mm), but 10°C larvae were significantly older at hatch (10℃: 18.4 ± 0.05 dpf, 12℃: 15.8 ± 0.1 dpf). Clutch had a much more variable impact on response metrics, markedly in size, suggesting that parentage may mediate developmental responses to unfavorable environmental conditions and change. No incidences of jaw malformation and a very small percentage of spinal malformations were observed amongst all treatment groups. Overall, the novel results described in this study suggest that parentage modifies the impacts of rearing temperature on larval development and survival. As a result, I make recommendations to broaden the impact of future LFS and other sensitive larval fish studies on conservation larviculture techniques. While our understanding of factors impeding LFS culture is limited, this study represents a significant step towards identifying more biologically efficient larviculture and conservation management techniques for this and other critically threatened fish species.

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