The expansion of ballan wrasse farming, used as a biological control against sea lice in Atlantic salmon, is constrained by the slow growth rate in the species and extended period required to reach deployment size. Rearing temperature and diets are the two main growth limiting factors in fish. In this study, farmed ballan wrasse juveniles were reared at 10, 13 and 16 ºC over a period of 3 months and fed two different commercial diets commonly used in marine finfish, Otohime S2 and BioMar Symbio. At the end of the trial, fish growth was +125, +75 and +25 % compared to their initial weight in 16, 13 and 10 ºC treatments, respectively. It was suggested that temperatures above 16 ºC may promote growth even further. Furthermore, feed conversion ratio was significantly improved in fish reared at 16 °C. However, diets did not impact on any of the growth performance indicators although a significantly higher daily feed intake was observed in fish fed BioMar Symbio. Importantly, no significant effects of temperature and diets on mortality and condition factor were observed. No differences were found in the fish (whole-body) macronutrient composition between diets. Analysis of the protein, lipid and energy digestibility revealed lower apparent digestibility coefficients than normally observed in marine species, suggesting the diet formulation is not optimised for the species. Finally, fish reared at 10 ºC showed increased hepatosomatic index, suggesting fat storage in the liver under cold temperatures. These results showed that the production cycle could be shortened by more than 4 months in fish reared at 16 ºC. This could contribute to increase hatchery productivity and meet demand from the salmon production sector while reducing costs associated with the nursery phase although maintaining a constant high temperature would increase operational costs.

Wild copepods are the main natural diet of marine finfish and they meet the larvae's requirements in phospholipids and essential fatty acids (EFA). While Artemia nauplii are an easier and more reliable live feed to produce in hatcheries for marine fish larvae than wild zooplankton, enrichment products commercially used lack phospholipids and essential longchain polyunsaturated fatty acids (LC-PUFA). This is particularly true for docosahexaenoic acid (DHA) within their polar lipid fraction (PLDHA), which is critical to the survival and good development of the larvae. In this study, we showed that it is possible to increase the levels of phospholipids and DHA within the PL fraction of Artemia nauplii using marine lecithin through a process referred to as "boosting". A cheaper alternative to marine lecithin, soya lecithin, was also tested but resulted only in a significant increase of the phospholipid content of the nauplii with no positive effect on the essential LC-PUFA levels, due to the absence of LC-PUFA in the soya lecithin. This study also showed that the levels of PLDHA in the Artemia boosted with marine lecithin did not reflect the levels of PLDHA in the lecithin, highlighting there the complexity of the boosting process. Finally, chilling enriched Artemia nauplii at 5 ºC for up to 10 h did not impact on their nutritional quality post-enrichment. Ultimately, this study proposes innovative and sound enrichment strategies to produce Artemia nauplii rich in EFA and/or PL, similarly to that of the wild copepods' lipid profile.

The production of high-quality marine fish fry is limited by the low survival observed during the larval phase, which is often attributed to dietary deficiencies of the diets at first feeding. Despite progress made with live feed (i.e. rotifers, Artemia), enrichments in essential fatty acids for marine fish larvae, little is known on the micronutrient requirements such as selenium (Se). Se is a critical component of several enzymes maintaining important biological functions such as cellular oxidation, and therefore plays a key role in oxidative and stress status of marine larvae. The levels of Se found in the larvae's natural diet (i.e. copepods) is generally higher than those of the enriched live preys used in hatcheries. This study aimed at establishing a protocol to enrich Artemia nauplii with Se using different inorganic (sodium selenite) and organic (selenoyeast). Results indicated that the use of dissolved sodium selenite, an alternative inorganic and cheaper form of Se, did not increase the levels of Se in the nauplii. However, the use of selenoyeast (Sel-Plex) confirmed that it is possible to enrich the nauplii with targeted levels of Se, since this process followed a dose-response pattern with Se enrichment ranging from 1.7 to 12.4 mg kg-1. In addition, the supplementation of Sel-Plex to the regular enrichment product did not impact on lipids and fatty acids enrichment irrespective of the dose dispensed. Overall, this study contributes to the refinement of the live prey enrichment protocols that are critical to the success of marine finfish larviculture protocols.

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