Focusing on a Norwegian fjord, the authors of the study found that white urchins (Gracilechinus acutus) were 10 times more abundant (38 urchins m−2) at sites close to fish cages than at control sites (4 urchins m−2). In such densities the urchins were forming “barrens” – areas of the seabed in which the echinoderms dominated at the expense of other species due to their overgrazing on kelp.
As the authors of the paper, which was published in Aquaculture Environment Interactions, suggest, while regulators tend to consider the acute impacts of organic and nitrogenous wastes close to farms, open cage farms can have a much wider impact and that “the energy-rich trophic subsidy that aquaculture provides may create cascades with influences over broader spatial scales”.
Although, the authors concede, urchin barrens did exist in the Norwegian fjords before the advent of intensive finfish production, the results of their study suggest that the barrens are likely to have proliferated since the 1970s. However, they add that it is now essential to establish accurate baseline data in order to assess how the barrens are evolving.
As the authors conclude: “An understanding of localised impacts may be adequate where aquaculture operations are small. However, when aquaculture operates at high regional densities, diffuse effects can become additive and influence change on a much broader spatial scale, where unfortunately, baseline data are often lacking. As aquaculture continues to expand, it is critical to capture a robust environmental baseline through which broad-scale changes can be evaluated in the future. Further exploration of the interaction between G. acutus and finfish aquaculture is warranted to fully assess and subsequently mitigate any consequences for broader ecosystem function in the Norwegian fjords.”