Effects of fish farm effluents on kelp forest ecosystems: Kelp performance, associated species, and habitats

Sea cage production of salmonid fish delivers a large amount of waste products to the marine environment including dissolved nutrients, organic particles, and chemicals. Recent developments in the Norwegian aquaculture industry have led to large-scale fish farms being increasingly situated in dispersive environments where kelp forests dominate the shallow hardbottom habitats. The potential impacts of farm effluents on kelp forests are poorly understood, yet important to clarify with kelp forests currently declining worldwide. To assess the overall effect of fish farming effluents on the performance of first-year kelp, I conducted an in-situ translocation study with Saccharina latissima. I found that growing near fish farms did not improve or reduce the growth performance of first-year S. latissima, or affect the plants’ nutrient-reserves. I tested the toxicity of a common therapeutant used to treat salmon for sea lice infections (hydrogen peroxide) and found that juvenile S. latissima plants are highly sensitive to this chemical. By comparing determined toxicity potentials (e.g. LC50) with model predictions I found that S. latissima forests growing near fish farms may be exposed to lethal concentrations of hydrogen peroxide. In a dispersive coastal system, I evaluated the presence of large-scale fish farms during high and low production on three different communities associated with Laminaria hyperborea plants; the lamina epiphytic community, the stipe epiphytic community, and the holdfasts macrofaunal community. Where the lamina and stipe communities would receive the farm-nutrients from the water column, the holdfasts were anticipated to trap farm-derived organic particles. For both the epiphytic load on laminas and the macrofaunal community in holdfasts, I found no differences between reference sites and farm sites that could be directly related to fish farm effluent loads, suggesting that these communities are resilient to farm-impacts in dispersive environments. The stipe epiphytic community showed a clear response to fish farm effluents, with an increase in bryozoan and opportunistic macroalgae biomass, lower habitat heterogeneity and a shift in community structure near farms that were at high production. This demonstrates that proximity to fish farms can result in a small but significant alteration of L. hyperborea as a habitat. This thesis reveals the impacts that different waste products from salmonid production can have on kelp forests. The impact of fish farm effluents on the performance and health status of kelp species and communities associated with the kelp forest is, however, far from fully understood. Results gained from this thesis should be utilized to set management directions but also to set the path for future focus areas in research into the environmental effects of fish farming.