Seriolid fishes, such as the longfin yellowtail, Seriola rivoliana, comprise one of the most valuable finfish groups for U.S. aquaculture development; both in RAS systems, and offshore. However, there is a major constraint to increasing hatchery production and selective breeding programs for Seriolids: hatchery produced offspring show poor fecundity and low larval viability. To address these impediments, first generation hatchery-bred S. rivoliana juveniles were reared to sexual maturity under distinct diet regimes and exposed to thermal stimuli.
Longfin yellowtail, were initially acquired at 1-2 grams and kept in tanks that received natural daylight and flow through, filtered seawater at ambient ocean temperature (26oC). After an initial 7 month grow-out period, fish were divided into groups at two sites, the Hawai‘i Pacific University’s Oceanic Institute (OI) and Ocean Era Inc. (OE), to begin a diet trial (month 0). One group at each site was fed a commercial 8 mm pellet diet, and the other, a gelatinized diet of homogenized fresh seafood and vitamins (gel diet). By month 6, fish fed a gel diet were transitioned to a whole fresh diet of chopped fish and squid; the pellet diet remained unchanged. Subsets of fish were periodically sampled. On two occasions, fish were employed in parallel trials to examine effects of a freshwater dip and fasting on physiological parameters of stress and growth, respectively. By month 12, fish were terminally sampled at OI; by month 14, some fish were terminally sampled at OE and the remaining fish were individually tagged and exposed to a thermal treatment consisting of two 3-day exposures to 22 oC. Weights were taken monthly from fish at both sites and feed conversion ratios (FCR) were adjusted to biomass removal and corrected for moisture content. Hepatosomatic index (HSI) was calculated when fish were sampled, at months 0, 3, and 7 at both sites, and months 12 and 14 at OI and OE, respectively.
Initial analyses of morphometric data indicate a consistent effect of diet at both sites. Fish fed the gel diet grew slower, had higher FCRs, and lower HSIs than those fed a commercial pellet diet. Based on their low performance, fish that were on a gel diet were transitioned to a whole fresh diet. Over the following 6 months, growth rates of fish fed a whole fresh diet were higher than the gel diet phase and similar to those of pellet fed controls at each respective site. The onset of reproductive maturity was first observed in 1-year-old fish while nearly all individuals were mature at 1.5-years-old. While a transient drop in temperature did not affect morphometric traits, transcriptional analyses of pathways associated with growth and reproduction are underway to assess thermal effects. Together, these results indicate that yellowtail recovered growth rates on a fresh diet despite initially performing marginally on a gel diet. Further identification of key phsyiological traits associated with performance in captivity will inform the production of yellowtail broodstock and lay the foundation for developing selective breeding strategies.
[Supported by NOAA (Award no. NA22OAR4170108)]