Abstract
The threat of salmon lice remains the largest obstacle for further growth of the Norwegian salmon aquaculture industry, where large amounts of delousing operations are performed every year to keep the lice at bay. The use of ballan wrasse as a biological delousing agent has been increasing in popularity over the last years in the salmon aquaculture industry due to a high delousing efficiency, no resistance development and smaller effects on salmon welfare compared to chemical and mechanical delousing. The demand has mainly been met by fisheries, however, there is a large interest in successful aquaculture of the species to secure year-round availability of high quality fish. The rearing of ballan wrasse has proven difficult, with bottlenecks occurring in the larval stages. As for many other marine fish, ballan wrasse larvae are small and little developed upon hatching and do not accept inert feed. Thus, live feed must be supplied in adequate quantities and qualities to secure growth and survival. The traditional feeding regime of rotifers followed by Artemia before weaning onto formulated feed seems to be suboptimal for the ballan wrasse larvae, where large mortalities, low growth and stress tolerance and deformities are prevalent in cultivated larvae. Improved growth and development has been observed when the fish are fed on copepods during the larval stages compared to rotifers and Artemia, likely due to the nutritional composition which is more adapted to the requirements of marine fish larvae. Copepods have recently become commercially available, in addition to a new live feed organism, namely cryopreserved cirriped nauplii. These nauplii have been suggested to have a similar nutritional composition as copepod, and thus may be able to replace rotifers and Artemia as live prey in the rearing of ballan wrasse. Thus, the present study was conducted to evaluate the effect of feeding ballan wrasse larvae with copepods and/or cirripeds compared to the traditional diet of rotifers followed by Artemia. A start-feeding experiment was performed, where ballan wrasse larvae were fed either copepods, experimental small cirripeds or rotifers from 4-18 dph and either Artemia or large cirripeds from 18-32 dph, followed by weaning onto the same formulated feed from 32-48 dph. As the gut and liver in developing fish larvae respond sensitively to dietary changes, histological examination of biomarkers of nutritional condition in the gut and liver tissue was performed. In addition, growth and survival were assessed. The different live feed organisms varied greatly in their quality as live feed for ballan wrasse larvae, where feeding with copepods as the first live feed gave significantly better survival than larvae fed either rotifers or experimental small cirripeds as their first diet regardless of the second live feed offered. Growth was similar between larvae fed copepods and rotifers, but larvae receiving experimental small cirripeds were significantly smaller than all other treatments throughout the experimental period. The nutritional condition of the larvae was reflected in the gut and liver histology, where feeding with copepods followed by Artemia resulted in a rapid development of both gut and liver tissue during the live feed period, but weaning to formulated feed induced some degeneration of the gut tissue in the form of shortened microvilli. Copepods followed by large cirripeds resulted in similar liver histology but a slightly slower development of gut tissue during the live feed period compared to the other copepod group, but weaning onto formulated feed showed pronounced effects in the form of large hepatocytes and amount of glycogen vacuoles in the liver, and taller villi, epithelial height and microvillus height than the other treatments. Although larvae fed experimental cirripeds displayed signs of starvation after the first live feed period in the form of a reduced hepatonuclear size in the liver as well as shortened gut epithelial height and microvilli, signs of starvation were reduced after the second live feed period. Larvae fed rotifers followed by Artemia displayed histopathological alterations such as accumulation of lipid droplets in the liver tissue and enterocytes as well as small and indistinct hepatocyte cells with shrunken nuclei after weaning to formulated feed. Most measured parameters of the gut and liver could be related to standard length with some variation between treatments. The relationship between gut and liver histology and growth rate was less clear.
It is concluded that feeding with copepods resulted in the best growth, survival and nutritional condition of ballan wrasse larvae, and thus may be a viable option to replace rotifers in aquaculture of the species. The best results were obtained by feeding copepods followed by cirripeds, where a large surface area of the gut as well as high amount of energy reserves in the liver indicated a high metabolic capability and resistance to starvation, as well as a good utilization of the formulated feed.