Forsøksdyr: Factors affecting maximum neutral buoyancy depth of Atlantic salmon

Godkjenningsdato 18.07.2018

Atlantic salmon possess an open swim bladder that is refilled by swallowing atmospheric air at the water's surface for buoyancy regulation. To what extent salmon can fill their swim bladders is unknown. Yet, knowing this would enable calculations of the maximum depth that they could attain neutral buoyancy, which is expected to be an important determinant of their swimming depth behaviour. Lipid content and size are strongly correlated in salmon and this would influence buoyancy, as fat is less dense than water. Furthermore, with depth-based cage technologies aimed at salmon lice prevention forcing fish to swim deep, knwing their maximum depth of neutral buoyancy may be an important consideration in ensuring farmed fish welfare.

We therefore plan to conduct an experiment using an Increased Excess Mass Test (IEMT) to investigate maximum swim bladder volumes of Atlantic salmon at a range of sizes attained in a standard production cycle. We also propose to conduct an IEMT on salmon fed with a high, average and low fat diet to investigate the effect of lipid content on buoyancy. Further, we will perform an IEMT on salmon that have full and empty stomachs and how this influences swim bladder capacity by manipulating access to food in the short term.

We expect that the animals in our experiment will be subjected to minimal distress. The IEMT measurement was first developed by Pflugrath et al. (2012). For each fish involves externally attaching one magnet while the fish is under sedation, and then once the fish has recovered in a tank, attaching additional magnets until it is unable to achieve neutrally buoyancy by refilling its swim bladder with air. The fish will only experience negative buoyancy for a brief period.

We plan to use 30 individuals for each of: 4 size classes (100, 500, 1000 g, and 3000 g), three diet groups (high, average and low fat) and three groups of salmon fed different amounts (100%, 50% and 0% of a daily ration), totalling 300 fish.

We have investigated ways of replacing, reducing and improving our methods, however IEMTs on live animals as outlined in this application are the most scientifically robust way of gathering information on the maximum swim bladder volume of Atlantic salmon. A previous study attempted to estimate swim bladder volume with a rupture test on swim bladders removed from fish, but this gave unrealistic and widely variable results. The IEMT test with magnets was deemed to be more appropriate and accurate. Reduction and improvement of the experiment, to ensure the fish will experience minimal distress, has been achieved by using anaesthetic during surgery and minimising handling to add magnets as the fish pick up the magnets themselves.