New research from the Mediterranean suggests that marine ecosystems are disturbed by the organic food and faeces waste from fish farms, even when the pollutants themselves can no longer be detected. The study recommends calculating an individual pollution 'carrying capacity' for each type of marine habitat, and provides the first published estimate for a maërl habitat.
Marine aquaculture is growing and is under increasing pressure to become more sustainable. Particulate organic wastes from farms may spread out several hundred metres around farms, accumulating in seabed sediment and altering its geochemistry and biology. The environmental impact of the pollution varies with the farming method, the depth and current conditions, and with the seabed communities themselves - different habitats have different sensitivities to contamination.
This study focused on the impact of a Spanish sea bream farm in the Western Mediterranean on maërl beds. Maërl is a slow-growing, unattached seaweed (or 'coralline algae') with a hard, chalk skeleton, important as a nursery for many commercial fisheries and scallop beds. It is listed in Annex V of the Habitats Directive1 (species whose removal from the wild may be restricted). Maërl is known to be little affected by industrial pollutants, such as metals, but is very sensitive to fine sediments and organic particulates. Since lost beds rarely recover, maërl should be considered a non-renewable resource.
The researchers measured sedimentation rates of nutrients found in organic waste - organic carbon, nitrogen and phosphorus - in sediment traps at 0, 20, 120 and 600m from a fish cage on the farm. The chemical and biological status of the sediment was assessed at each of these distances. Seabed macrofauna (tiny organisms) were sampled at the different distances to gauge ecological health, based on features such as abundance, biomass, trophic groups and species richness.
Levels of sediment declined with distance from the farm, and reflected the chemical status.There were signs of ecosystem disruption at close proximities. For example, maërl within 20m of the cage were grey (a sign of stress) and smaller than those at 120m. The sediment data suggested that the farm had no influence at 120m distance, but there were biological clues that the ecosystem at this location was disturbed. For example, algal density was lower at 120m than at 600m. The abundance and diversity of macrofauna declined rapidly close to the farm, where grazing species were absent, indicating that organic pollution was changing the ecosystem function.
The study suggests that maërl beds respond differently to other ecologies which have been the focus of most other similar studies, such as unvegetated beds in salmon farming regions. It therefore proposes defining a 'carrying capacity' for each type of habitat - the level of organic pollutant input below which no ecological disturbance is expected. The carrying capacity of the maërl ecosystem was defined as the sedimentation rate (0.087 grams of carbon per square metre per day) at the distance from the farm where ecosystem diversity stabilised. This is the first time such a parameter has been given, but the authors suggest measuring it for each seabed habitat to help environmental protection agencies set limits for aquaculture.
This study demonstrates that fish farm impact might be underestimated if it is only assessed using waste dispersion data.