Skin detachment from salmon and rainbow trout

Abstract

A survey aimed at relevant stakeholders highlighted loose skin as a quality concern since 2019. The issue is typically detected after storage, with indications of higher prevalence during late spring-summer in Southern-West regions, while during late summer-autumn in Northern regions. The issue seems transient, i.e. skin seems to reattach after a period. Industry statistics and experiments found no correlation between common delousing methods and loose skin. Stress prevention is crucial for fish welfare, also given the observed vulnerability of the adipose tissue under the skin to collapsing under mechanical pressure. In each case of loose skin, a glossy liquid was found between the skin and muscle, being mainly composed of fat (69%), with a higher content of 18:1n-9 and 18:2n-6 (found in plant oils), and a lower content of 22:6n-3 (found in fish oil), compared with the skeletal muscle. Also, the amino acid composition varied, indicating selective leakage. We were unable to obtain rainbow trout with loose skin, but surprisingly large differences in collagen composition were found between the species. No indications of increased activity of degrading enzymes were detected, but the extent of lipid peroxidation was higher in salmon with loose skin compared with salmon with reattached skin, as well as in soft muscle in contrast to firm muscle. Microscopic examinations did not reveal any explicit signs of abnormal connective tissue structure in salmon with loose skin. Instead, variations in the morphology of myofibers were observed. Further, transcriptomics revealed substantial changes in the muscle transcriptome, including a large array of inflammatory genes in fish with loose skin. It is concluded that the issue of loose skin arises while the fish is alive, is associated with soft muscle, and becomes apparent after a certain period post-slaughter. In addition to novel knowledge on loose skin, the project provided useful knowledge that can be utilized in future research on soft texture and fillet gaping.