Introduction
Lumpfish (Cyclopterus lumpus) is used as a cleaner fish against sea lice infestations in salmon aquaculture. It is widely used in Canada, Norway, Scotland, and Faroe Islands due to its better tolerance to cold water. However, their welfare is still poor as high mortalities occur during the deployment in sea cages. A tailored diet, that covers the nutritional requirements, is essential to provide good welfare, improve survival and maintain their delousing efficacy. Long chain polyunsaturated fatty acids (PUFA) such as docosahexaenoic acid (DHA, 22:6n-3) and eicosapentaenoic acid EPA (20:5n-3) are required for growth as well as development and reproduction (Sargent et al., 1999).This study aims to evaluate the effect of five experimental feeds with diverging EPA+DHA levels on juvenile lumpfish to investigate the effects on fish performance, lipid and fatty acid metabolism as well as stress response.
Materials and Methods
Lumpfish, approximately 20 ± 2 g (mean ±SD), were sourced from Nesvík Marine Centre (Faroe Islands) and transferred into an experimental flow through system. Each tank was stocked with 24 lumpfish and was randomly assigned to one out of six diets, in quadruplicate. A basal extruded diet was coated with either rapeseed oil (RO), krill oil (KO) (Aker Biomarine, Norway) or a blend of both oils, generating five experimental feeds with decreasing EPA+DHA levels (22.8-5.6 % total fatty acid; 0KO, 25KO, 50KO, 75KO, 100KO, respectively). A commercial diet was also used as a control (COM). All feeds were produced by Havsbrun (Faroe Islands). Feed was manually delivered at a feeding rate of 2.5% of their body weight. Uneaten pellets were daily syphoned and weighed out in order to record daily feed intake. Four sampling points were carried out throughout the trial which lasted for 52 days: a basal sampling (S0), two nutritional samplings (S1 and S2; 21 and 47 days, respectively) and a stress challenge (S3).
During S1and S2, fish were measured for morphometric data, as well as liver and viscera weight. Three whole fish were stored at -20° for proximate analysis, while ; whole intestine, brain, gills, eyes and liver were dissected from two fish and stored at -20° for lipid analyses. At S3, fish were exposed to an acute stressor (chasing and confinement), after which, fish were left to recover and sampled 1 hour and 6 hours after the stress for morphometric data and plasma cortisol. Linear models were used to investigate the effects of the diets, while a post hoc Tukey HSD test was performed to identify differences between dietary treatments. Principal component analysis (PCA) was performed on fatty acid profile of tissues.
Results and Discussion
Preliminary data analysis showed no significant differences in growth parameters, survival and condition indices, in both S1 and S2, between dietary treatments. Despite this, significant differences were found in the mean cumulative feed intake, with fish fed 25 – 100KO inclusion had a higher feed intake throughout the trial (on average 18.1 g), while fish fed the commercial diet had a lower feed intake (10.1 g ± 3.7). There was an interaction in terms of whole body lipid content between diet and time, indicating that some feeds (COM and 75KO) increased the lipid content of the fish faster than the others. Fish fed 25KO had the highest liver lipid content, while those fed 100KO had the lowest. There was also an effect of diet on the whole intestine lipid content (, with fish fed COM showing the highest lipid content in this tissue, while those fed 100KO and 0KO had the lowest. Significant differences were observed in liver between the dietary treatments in terms of fatty acid composition. Liver fatty acid profile reflected dietary input (Betancor et al., 2014), as can be observed in the PCA (Fig.1). The increasing dietary inclusion of EPA and DHA lead to an increase of n-3 fatty acids in the liver, while fish fed diets with high rapeseed oil inclusion (50KO, 25KO, 0KO), displayed higher levels of oleic acid (18:1n-9) and n-6 fatty acids such as 18:2n-6. Significant differences were found between dietary treatments in plasma cortisol 6 hours after the stress, where fish fed a higher inclusion of krill oil (100KO, 75KO, 50KO), had significant lower levels of cortisol. In agreement, fish fed vegetable oils showed higher cortisol plasma levels than fish fed a fish oil rich diet (Perez-Sanchez et al., 2013).
References
Pérez-Sánchez, J., Borrel, M., Bermejo-Nogales, A., Benedito-Palos, L., Saera-Vila, A., Calduch-Giner, J. A., & Kaushik, S. (2013). Dietary oils mediate cortisol kinetics and the hepatic mRNA expression profile of stress-responsive genes in gilthead sea bream (Sparus aurata) exposed to crowding stress. Implications on energy homeostasis and stress susceptibility. Comparative Biochemistry and Physiology Part D: Genomics and Proteomics, 8(2), 123-130.
Betancor, M. B., Howarth, F. J., Glencross, B. D., & Tocher, D. R. (2014). Influence of dietary docosahexaenoic acid in combination with other long-chain polyunsaturated fatty acids on expression of biosynthesis genes and phospholipid fatty acid compositions in tissues of post-smolt Atlantic salmon (Salmo salar). Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology, 172, 74-89.
Sargent, J., Bell, G., McEvoy, L., Tocher, D., & Estevez, A. (1999). Recent developments in the essential fatty acid nutrition of fish. Aquaculture, 177(1-4), 191-199.
Funding
University of Stirling, Fiskaaling, Bakkafrost, Havsbrun, Aker Biomarine