Newly discovered spawning and recruitment of threatened Longfin Smelt in restored and underexplored tidal wetlands

Effects of Salinity on Fertilization, Hatching, and Larval Performance of Longfin Smelt Spirinchus thaleichthys

Understanding the spawning and rearing habitats of fishes is critical to effective fisheries management and conservation. Longfin smelt Spirinchus thaleichthys is an imperiled migratory fish that is believed to spawn and rear in habitats of varying salinities; however, optimal conditions for each stage remain unknown. Here, we examined the effects of variation in salinity on egg fertilization, hatch success, and larval growth and survival. Eggs that were fertilized in freshwater (0.4 ppt) exhibited a significantly higher fertilization rate (81%) than those fertilized in brackish water (62% at 5 ppt), with no detectible effects of fish origin or female size. In contrast to fertilization rates, once the eggs were fertilized, their hatching rates were not affected by the fertilization salinity, incubation salinity, nor their interaction; however, hatching success and larval survival both increased with increasing maternal body mass. Larval growth rate appeared to be independent of salinity and maternal size. Taken together, the results indicate that fertilization is possible at a range of salinities, but optimal at lower salinities for longfin smelt; however, embryos and larvae can perform well across a range of salinities. Furthermore, results indicated that larger mothers produced high-quality offspring, a finding that supports the “bigger is better” paradigm in fisheries science and management. These results likely explain, in part, the spawning and rearing behaviors of wild longfin smelt and suggest that the conservation culture program would likely be optimized by utilizing freshwater fertilization and larger females as broodstock.

Growth, osmoregulation and ionoregulation of longfin smelt (Spirinchus thaleichthys) yolk-sac larvae at different salinities

Longfin smelt (Spirinchus thaleichthys) is a threatened anadromous fish species that spawns in freshwater to moderately brackish (i.e. 5-10 ppt) reaches of the upper San Francisco Estuary and has declined to ~1% of its pre-1980s abundances. Despite 50+ years of population monitoring, the efficacy of 10+ years of conservation efforts for longfin smelt remain uncertain due to a limited understanding of how the species responds to environmental variation, such as salinity. For example, high mortality during larval stages has prevented culture efforts from closing the life cycle in captivity. Here, we investigated the effects of salinity on longfin smelt yolk-sac larvae. Newly hatched larvae from four single-pair crosses were acutely transferred to and reared at salinities of 0.4, 5, 10, 20 or 32 ppt. We compared whole-body water and sodium ion (Na⁺) content, notochord length and yolk-sac volume at 12, 24, 48, 72, and 96 hours post-transfer for each salinity treatment. We found that larvae maintained osmotic and ionic balance at 0.4-10 ppt, whereas salinities ˃10 ppt resulted in decreased water and increased whole-body Na⁺ content. We also found that larvae grew largest and survived the longest when reared at 5 and 10 ppt, respectively, and that yolk resorption stalled at 0.4 ppt. Finally, there were significant but small interclutch variations in responses to different salinities, with clutch accounting for <8% of the variance in our statistical models. Overall, our results indicate that longfin smelt yolk-sac larvae likely perform best at moderately brackish conditions, thus yielding a mechanism that explains their distribution in field surveys and providing key information for future conservation efforts.

Captive Rearing of Longfin Smelt Spirinchus thaleichthys: First Attempt of Weaning Cultured Juveniles to Dry Feed

The rapid decline of longfin smelt Spirinchus thaleichthys, a threatened euryhaline forage fish in California, is a serious concern for scientists and resource managers. To recover and conserve this species, a captive culture program was initiated, focusing on the collection, captive rearing and breeding of wild broodstock, and the rearing of their offspring. Although progress has been made in the collection of broodstock and the production and culturing of larvae, no studies have evaluated the rearing of juvenile life stages in captivity. The present study examines methodological considerations for culturing F1 juvenile longfin smelt, specifically, the first efforts toward weaning juveniles to a dry commercial pellet feed. Cultured juvenile longfin smelt were fed live Artemia only or co-fed Artemia and dry feed for 62 days, and the effects of feed type on juvenile survival, growth, body condition, and fatty acid profiles were examined. No significant differences were observed between feeding treatments, despite an 80% reduction in Artemia in the co-feeding treatment. Furthermore, examination of fish stomach contents at the end of the trial confirmed the transition to dry feed. This is the first study to indicate successful feeding by longfin smelt on dry commercial pellets, and suggests that juvenile longfin smelt can be fully weaned onto dry feeds. Results of this study are critical for closing the lifecycle of longfin smelt in captivity and developing a successful conservation culture program for this imperiled species.

Resistance and resilience of pelagic and littoral fishes to drought in the San Francisco Estuary

Many estuarine ecosystems and the fish communities that inhabit them have undergone substantial changes in the past several decades, largely due to multiple interacting stressors that are often of anthropogenic origin. Few are more impactful than droughts, which are predicted to increase in both frequency and severity with climate change. In this study, we examined over five decades of fish monitoring data from the San Francisco Estuary, California, USA, to evaluate the resistance and resilience of fish communities to disturbance from prolonged drought events. High resistance was defined by the lack of decline in species occurrence from a wet to a subsequent drought period, while high resilience was defined by the increase in species occurrence from a drought to a subsequent wet period. We found some unifying themes connecting the multiple drought events over the 50-yr period. Pelagic fishes consistently declined during droughts (low resistance), but exhibit a considerable amount of resiliency and often rebound in the subsequent wet years. However, full recovery does not occur in all wet years following droughts, leading to permanently lower baseline numbers for some pelagic fishes over time. In contrast, littoral fishes seem to be more resistant to drought and may even increase in occurrence during dry years. Based on the consistent detrimental effects of drought on pelagic fishes within the San Francisco Estuary and the inability of these fish populations to recover in some years, we conclude that freshwater flow remains a crucial but not sufficient management tool for the conservation of estuarine biodiversity.