Introduction
Atlantic wolffish (Anarhichas lupus) has been proposed as a cold-water species in aquaculture 1. However, in order to explore the commercial farming potential of the species, we need more knowledge about its nutritional and digestive physiology, nutrient requirements and feeding protocols for optimal growth and welfare. Previous studies in fish on protein digestion, including a few on Atlantic wolfish, show that the enzymatic activity of intestinal trypsin and chymotrypsin is well correlated with growth, and feed conversion efficiency 2–4. Digestion leads to nutrient uptake and increased plasma concentrations of amino acids, glucose and lipoproteins. Plasma amino acid and glucose concentrations of fed Atlantic wolffish has not been measured. In other fish species dependent on feeding strategy and intestinal morphology, digestion of protein and amino acid uptake result in increased free amino acid plasma concentration short after feeding 5. Further, lipase activity and fat absorption have not been examined in Atlantic wolffish. Free fatty acids (FFA) plasma concentration is known to vary between species and have seasonal and circadian variations 6,7.
The aim of the current study was to gain knowledge about the diurnal and nocturnal activity of digestion of protein and lipids in Atlantic wolffish.
Material and method
Atlantic wolffish juveniles were kept in re-circulating water systems with artificial seawater at 10˚C, and artificial 12:12-h light-dark photoperiod. The fish was fed in excess to visual satisfaction once a day for 4 weeks in the morning (MF) (8 am), or evening (EF) (8 pm) (Skretting Amber Neptune grade 2.0, Skretting, Stavanger, Norway). The fish were fasted 24 h before sampling. On the sampling day, the fish were fed once and after 30 min, uneaten feed was collected. Sampling of chyme, intestinal scrapings and plasma started 1h post feeding with subsequent sampling after 7, 13, 19 and 25 h for each feeding regime. Intestinal enzymatic activity of trypsin, chymotrypsin and lipase as well as plasma profiles of free amino acids (FAA), free fatty acids (FFA) and glucose were measured.
Results and conclusion
In both feeding groups, there was no difference in trypsin and chymotrypsin activity in the proximal mid-intestine over time (Fig 1 A and B), whereas there was higher of chymotrypsin activity in the distal mid-intestine in MF fish 1 h post feeding (not shown), and lipase activity remained un-changed during the whole experimental period (not shown). In MF fish, there was a decrease in trypsin and chymotrypsin activity in the proximal mid-intestine after 19 h that was correlated with a drop in hepatic total FAA concentration and glucose, suggesting that the fish goes into a digestive resting state during the dark cycle (Fig. 1 C and D). This effect was not seen in EF fish that had sustained enzyme activity and high plasma FAA, and glucose concentrations through the experimental period.
In conclusion, Atlantic wolffish has a higher and more stable trypsin and chymotrypsin activity along with increased plasma FAA and glucose plasma concentration if they are fed in the evening compared to if they are fed in the morning. Since feeding time influence the digestive ability and nutrient uptake over time after an ingested meal this information could be used to optimize the feeding strategy for Atlantic wolffish. Future studies should aim for examining the long-term growth effect under different feeding regimes and continuous feeding.
References
1. Moksness, E. & Pavlov, D. a. Management by life cycle of wolffish, Anarhichas lupus L., a new species for cold-water aquaculture: a technical paper. Aquac. Res. 27, 865–883 (1996).
2. Rungruangsak-Torrissen, K., Pringle, G. M., Moss, R. & Houlihan, D. F. Effects of varying rearing temperatures on expression of different trypsin isozymes, feed conversion efficiency and growth in Atlantic salmon (Salmo salar L.). Fish Physiol. Biochem. 19, 247–255 (1998).
3. Rungruangsak-Torrissen, K., Moss, R., Andresen, L. H., Berg, A. & Waagbø, R. Different expressions of trypsin and chymotrypsin in relation to growth in Atlantic salmon (Salmo salar L.). Fish Physiol. Biochem. 32, 7–23 (2006).
4. Lamarre, S. G., Le François, N. R., Falk-Petersen, I. B. & Blier, P. U. Can digestive and metabolic enzyme activity levels predict growth rate and survival of newly hatched Atlantic wolffish (Anarhichas lupus Olafsen)? Aquac. Res. 35, 608–613 (2004).
5. Mai, K., Xue, M., Xie, S. Q. & Kaushik, S. J. Fish nutrition - Protein and amino acids. (Academic press, 2022).
6. Larsson, Å. & Fänge, R. Cholesterol and free fatty acids (FFA) in the blood of marine fish. Comp. Biochem. Physiol. -- Part B Biochem. 57, 191–196 (1977).
7. Plisetskaya, E. Fatty acid levels in blood of cyclostomes and fish. Environ. Biol. Fishes 5, 273–290 (1980).