Larviculture and Nutrition of Three of Florida’s High Value Food and Stock Enhancement Finfish, Common Snook (Centropomus undecimalis), Florida Pompano (Trachinotus carolinus) and Red Drum (Sciaenops ocellatus)

Abstract

The main objective of this thesis was to gain new insights in three of Florida’s high value food and stock enhancement finfish nutrition (Common snook, Florida pompano and red drum) to improve larviculture protocols. The main bottleneck in snook production is the extremely low larval survival rate, which hinders subsequent research. This work first focused on the source of the larvae by looking at potential nutritional deficiencies in captive broodstock. The lipid composition of wild and captive common snook broodstock were compared to identify disparities and gain the information necessary for the formulation of a suitable diet for captive stocks. Results showed that captive snook lipid content was significantly higher than that of wild fish. However, cholesterol and arachidonic acid (ARA) levels were significantly lower compared to wild broodstock, with potential impact on steroid and prostaglandin production, reproductive behavior and gametogenesis. Eggs from captive broodstock incorporated high docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) levels and low ARA levels. Consequently, ARA/EPA ratio in captive eggs was more than half of that in wild eggs (2.3 ± 0.6 and 0.9 ± 0.1 respectively), with a probable perturbation in eicosanoid production and adverse effects on embryo and larval development. The large differences observed between wild and captive broodstock most likely contributed to the reproductive dysfunctions observed in captive snook broodstock (e.g. incomplete oocyte maturation, low milt production and poor egg quality). In addition, the presence of hydrocarbons was detected in the liver of most of the wild snook sampled. This requires further investigation to identify the source of the contamination, monitor a potential impact on reproductive performances and protect the species habitat. Another major bottleneck in marine fish rearing occurs during the transition from endogenous feeding to exogenous feeding, with mass mortality events linked to inadequate first feeding diets. To gain insight on the early fatty acid requirements and mobilization of pompano and snook larvae, the pattern of conservation and loss of fatty acids from the yolk sac during the endogenous feeding period and subsequent starvation was studied. In both species, fatty acids were utilized as an energy source after hatching. Mono-unsaturated fatty acids were catabolized, while saturated and poly-unsaturated fatty acids were conserved. High levels of arachidonic acid (ARA) in pompano and snook eggs (respectively 2.44 ± 0.1 and 5.43 ± 0.3 % of total fatty acids), as well as selective retention in the unfed larvae, suggested a high dietary requirement for this fatty acid during the early stages of larval development. The effect of an ARA supplementation was therefore investigated in snook larvae at the rotifer feeding stage. Larvae receiving the supplementation did incorporate higher levels of ARA, and DHA/EPA and ARA/EPA ratios were successfully modified to match those observed in wild eggs. No significant improvements in growth or survival were observed, however the success in fatty acid profile modification suggest a possible impact of the supplementation on a longer period of time and a possible effect on stress resistance. Probiotics have been shown to enhance larval performances of several species and this strategy was therefore investigated to evaluate a potential impact on Florida pompano, red drum and common snook larvae. The effect of a commercial mix of Bacillus sp. was studied on larval survival, growth and digestive enzyme activities. Larvae were fed either live feed enriched with Algamac 3050 (Control), Algamac 3050 and probiotics (PB), or the previous diet combined with a daily addition of probiotics to the tank water (PB+). Microbiological analyses were performed at the end of the pompano trial. Numbers of presumptive Vibrio sp. were low and not statistically different between treatments, therefore no additional microbiological analyses were performed on the system. At the end of the pompano and snook trial, standard lengths of larvae from the PB and PB+ treatments were significantly greater than for the control larvae. For both pompano and snook, trypsin specific activity was higher in PB and PB+ larvae compared to the control larvae. Similarly, alkaline phosphatase activity was higher for the pompano larvae fed the PB and PB+ treatments and for the snook larvae fed the PB+ treatment compared to the control larvae. No enhancement of growth or digestive enzymes activities was observed in red drum larvae. Yet, no negative effects were noticed and a longer trial period and the study of additional parameters could reveal different effects. In all three species, survival was not affected by the supplementation; however, stress exposure should be further investigated as the supplementation may strengthen the larvae, especially pompano and snook larvae where the Bacillus sp. supplementation appears to promote growth through an early maturation of the digestive system. Another key challenge in marine fish larval rearing resides in weaning the larvae onto dry micro-diets. This step is commonly concurrent with larvae metamorphosis into juveniles, with extensive morphological and physiological changes that are likely to influence nutritional requirements. In the present project, three microdiets were tested on weaning of Florida pompano larvae: Otohime, Gemma and a reference diet LR803. The experimental system was stocked with 11-day-old larvae, which were co-fed micro-diets and live food from 11 dph to 17 dph then micro-diets only until 28 dph. Survival from 11 dph to 28 dph was similar for all treatments, with an average of 33 %. At the end of the trial, the Gemma larvae were significantly longer and heavier than larvae fed the other diets. Fatty acid composition of the diets and larvae varied significantly between treatments. The Gemma larvae incorporated the lowest amount of eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA) and arachidonic acid (ARA). However, they presented the highest DHA/EPA and ARA/EPA ratios, supporting the concept that the proportions of polyunsaturated fatty acids are of greater importance than their absolute amount. Results from the enzyme analysis showed that fishmeal is a suitable main source of protein for Florida pompano larvae and demonstrated the full functionality of the pancreas at 16 days post hatch. These results provide the basis of a suitable weaning diet for pompano larvae and indicate the possibility of a weaning time prior to 16 days post hatch, which is of high interest in commercial production. Overall, this research provides new data on common snook, pompano and red drum nutritional requirements with results that can be directly applied to help overcome major bottlenecks in the hatchery phase and improve rearing protocols.