Salmon farm nutrients increase numbers of damaging sea urchins

With large networks of Atlantic salmon farming sea-cages spread throughout Norway’s fjords, the environmental impacts of increased organic and nitrogenous wastes surrounding farms has been questioned. The white urchin Gracilechinus acutus is an ecosystem engineer within fjords, and high densities of urchins can shift the ecosystem into urchin barrens, which can have cascading effects and change biodiversity. Dr. Camille White found that urchin barrens around aquaculture sites were 10 times more abundant and 15 mm smaller compared to urchins found at sites without sea-cages. In laboratory experiments, Camille also tested if urchin diets influenced by aquaculture waste affected reproductive outputs compared to natural diets. She found that while urchins fed aquafeed diets had gonad indices 3 times larger than urchins fed with a natural diet, their reproduction was compromised, with lower fertilisation success and lower larval survival. However, due to higher densities of urchins found at farming sites, the overall larval outputs at farming sites was five times higher than sites without sea-cages. Therefore, aquaculture waste can influence fjord ecosystems by stimulating aggregations of urchins, causing the formation of urchin barrens and altering natural ecosystems.

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