Effects of temperature and subspecies during critical early life history stages of largemouth bass (Micropterus salmoides)

Largemouth bass (LMB), Micropterus salmoides, is the most popular sportfish in the United States, with an expanding global food market. Farmers traditionally raise LMB in earthen ponds that are often plagued with high mortality during the early life history stages. Replacing initial pond stages with intensive indoor culture would streamline production and minimize losses. Our objectives were to (i) identify an optimal thermal regimen for rearing LMB in an indoor recirculation aquaculture system (RAS), (ii) assess the performance of Florida vs. Northern LMB for RAS culture, and (iii) elucidate thermally induced phenotypic changes and inter-linked expression of targeted genes involved in early development. Using RAS, Florida and Northern LMB were raised at 21 degrees C, 24 degrees C, and 27 degrees C. Fish were randomly sampled at 2 to 28 days post-hatch (dph) for total length (TL), body area (BA), myotome height (MH), jaw length (JL), eye diameter (ED), oil droplet area (ODA), and yolk area (YA). Condition index, yolk utilization efficiency (YUE), and yolk utilization rate (YUR) were calculated. Wet weight and survival were taken at 29 dph with expression of targeted genes [growth hormone (gh), insulin -like growth factor 1 (igf1), heat shock protein 70 (hsp70)] determined at 8 and 28 dph. There was a temperature effect on all morphometric traits, where both subspecies increased in size over the temperature gradient, with the largest morphometric traits (TL, BA, MH, JL, ED) detected at 27 degrees C. Larvae had smaller remaining ODA and YA reserves as temperatures increased. Larvae reared at 27 degrees C utilized their yolk at faster rates and were most efficient at converting yolk reserves to body size. Northern LMB had higher YUE than Florida LMB and were typically larger and heavier at 29 dph. LMB reared at 21 degrees C had higher survival than those at 24 or 27 degrees C, while Northern LMB had higher survival than Florida LMB. For Florida LMB, no differences in gh and igf1 were detected across temperatures at 8 dph. However, at 28 dph, these genes were upregulated at 27 degrees C, while expression of hsp70 was downregulated at 24 and 27 degrees C. Northern LMB showed similar expression patterns, with no significant differences detected. In conclusion, the present study suggests that 1) rearing LMB at 27 degrees C may improve growth performance during early ontogeny, and 2) Northern LMB may be selected for faster growth when reared in an indoor RAS.