Mortality and reduced photosynthetic performance in sugar kelp Saccharina latissima caused by the salmon-lice therapeutant hydrogen peroxide

Abstract

Sugar kelp Saccharina latissima is one of the dominating kelp species on sheltered sublittoral sites along the Norwegian coastline, providing important habitats and ecosystem services. In finfish aquaculture in Norway, infections caused by salmon lice Lepeophtheirus salmonis are one of the biggest challenges the industry is currently facing, and presently the most-used therapeutant is hydrogen peroxide (H 2 O 2 ). Commonly, this treatment involves bathing the fish in a solution of approximately 1700 mg H 2 O 2 l -1 seawater before releasing the solution into the surrounding waters. The present study was conducted to increase the knowledge on how these H 2 O 2 emissions from farm operations could impact juvenile S. latissima. This was achieved by determining the effects of a 1 h exposure to realistic H 2 O 2 levels on S. latissima mortality and photosyn - thesis. Effects on photosynthesis were determined by incubating plants at 3 time intervals postexposure. Toxicity potentials including lethal concentration for 50% of the population (LC 50 ) and effective concentration (EC 50 ) for photosynthetic capacity (P MAX ) and efficiency (α) were determined based on these data. Juvenile S. latissima was highly sensitive, having an LC 50 of 80.7 mg H 2 O 2 l -1 , which is less than 5% of the dose commonly used at farms and emitted to the environment. A concentration of 85 mg l -1 caused an immediate 90% reduction in both P MAX and α. The EC 50 was found to be 27.8 and 35.4 mg l -1 for P MAX and α, respectively. This may indicate that natural S. latissima populations in the vicinity of fish farms can be negatively affected by H 2 O 2 bath treatments.