Long-term welfare effects of repeated warm water treatments on Atlantic salmon (Salmo salar)

Lene Moltumyr,Jonatan Nilsson,Angelico Madaro,Tore Seternes,Fredrik Agerup Winger,Ivar Rønnestad,Lars Helge Stien

doi:10.1016/j.aquaculture.2021.737670

Abstract

Warm water treatment has in recent years become widely used for ridding salmonids of sea lice in aquaculture although the consequences of the treatment for fish welfare are not adequately investigated. The objective of this study was to document potential long-term welfare effects of repeated warm water treatments on Atlantic salmon (Salmo salar). Five weeks after a baseline welfare indicator scoring, non-anaesthetised Atlantic salmon (w¯ = 1379 ± 313 g, n = 332) were treated individually in a chamber with seawater at a temperature of 34 °C (warm water treatment) or 9 °C (control treatment) for 30 s. The treatment was repeated after 23–24 days. During the second treatment, a subset of the fish was video recorded for behavioural analysis. Seventeen to eighteen days after the second treatment, welfare indicators were again scored, and organ samples were taken for histopathological examination. The repeated warm water treatments resulted in a significantly increased prevalence and/or severity of scale losses, snout wounds, various eye problems, and active fin injuries as well as a significantly reduced specific growth rate. The fish displayed an immediate, strong behavioural reaction when exposed to warm water, which was probably the main cause of the detected injuries.

Highlights

Warm water treatment has in recent years become widely used for ridding salmonids of sea lice in aquaculture (Overton et al, 2018; Cer bule and Godfroid, 2020)
One fish from each treatment group were euthanised at the second treatment due to punctured eyes and consequent blindness
Two fish in the warm water group died in the period between the second treatment and the final welfare indicator scoring

Summary

Introduction

Warm water treatment has in recent years become widely used for ridding salmonids of sea lice in aquaculture (Overton et al, 2018; Cer bule and Godfroid, 2020). There are various industrial warm water treatment systems, but the general operational procedure is to fast the fish for 2–7 days, crowd them in the sea cage, and pump them past a dewatering strainer into a treatment «chamber» where they are exposed to seawater at a temperature of 28–34 ◦C for 20–30 s (Holan et al, 2017; Noble et al, 2018; Haram, 2020). The consequences of warm water treatment for fish welfare are, not adequately investigated (Noble et al, 2018; Gismervik et al, 2020), and ever since this delousing method was taken into use, there have been anecdotal and scientific reports of high mortality (Overton et al, 2018; Walde et al, 2021) and various injuries (Gismervik et al, 2018; Poppe et al, 2018; Gismervik et al, 2019b, 2020) after treatment. Water temperature changes initiate behavioural and physiological acclimatisation processes that can last from days to weeks depending on the magnitude of the temperature change (Brett and Groves, 1979; Jobling, 1994)

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Discussion

Conclusion