Yanagitsuru Yuzo R, Daza Itza Y, Lewis Levi S, Hobbs James A, Hung Tien-Chieh, Connon Richard E, Fangue Nann A
Department of Wildlife, Fish, and Conservation Biology, University of California, Davis, CA, 95616, USA.
Fish Conservation and Culture Laboratory, Department of Biological and Agricultural Engineering, University of California, Davis, CA 95616, USA.
Conserv Physiol. 2022 Jul 3;10(1):coac041. doi: 10.1093/conphys/coac041. eCollection 2022.
Longfin smelt () 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.
长鳍胡瓜鱼是一种濒危溯河产卵鱼类，在旧金山河口上游的淡水至中度咸淡水（即5 - 10ppt）水域产卵，其数量已降至20世纪80年代前丰度的约1%。尽管进行了50多年的种群监测，但由于对该物种如何应对环境变化（如盐度）的了解有限，长达10多年的长鳍胡瓜鱼保护工作的成效仍不确定。例如，幼体阶段的高死亡率阻碍了人工养殖实现其生命周期的封闭。在此，我们研究了盐度对长鳍胡瓜鱼卵黄囊幼体的影响。将来自四个单对杂交的新孵化幼体急性转移至盐度为0.4、5、10、20或32ppt的环境中饲养。我们比较了每种盐度处理在转移后12、24、48、72和96小时时的全身体水和钠离子（Na）含量、脊索长度和卵黄囊体积。我们发现，幼体在0.4 - 10ppt时维持渗透和离子平衡，而盐度大于10ppt会导致水分减少和全身体Na含量增加。我们还发现，幼体分别在5和10ppt饲养时生长最大且存活时间最长，并且在0.4ppt时卵黄吸收停滞。最后，不同盐度下的反应存在显著但较小的窝间差异，窝在我们的统计模型中占方差的比例小于8%。总体而言，我们的结果表明，长鳍胡瓜鱼卵黄囊幼体在中度咸淡水条件下可能表现最佳，从而产生一种机制来解释它们在野外调查中的分布，并为未来的保护工作提供关键信息。
