Research

Research conducted by the University of Oxford claims that the rapid expansion of fish farms in the Mediterranean Sea is killing wide swaths of Posidonia oceanica, a seagrass that’s endemic to the Mediterranean and vital to biodiversity and carbon sequestration in the region.

A University of Oxford study has documented ecological degradation of Posidonia oceanica meadows in Poros, Greece, linked to long-term fish farming impacts extending up to 14 years after farm removal and as far as 900 metres from former sites.

Overabundance of fish farms is a risk to both the marine environment and the economy. By Christos Logaras

Independent journalist Francesco De Augustinis explores the impact of the Area of Industrial Development of Aquaculture planned for Aetolia-Akarnania in Western Greece, where fish cages will occupy 100 hectares and another 100 hectares will be used for other activities. Inhabitants of the islands and villages protest against the fish farms and any further expansion project. In the past ten years they have watched their sea become contaminated and poor in natural resources. The industrial fisheries at Western Greece have also had an impact on Posidonia meadows, which is now absent or very deteriorated.

Finfish aquaculture has damaged seagrass meadows worldwide as wastes from the farms can kill these habitat-forming plants. In Cyprus, the Mediterranean endemic Posidonia oceanica is at its upper thermal limits yet forms extensive meadows all around the island. Understanding this under-studied isolated population may be important for the long-term survival of the species given that the region is warming rapidly. When fish farming began around Cyprus in the mid-nineties, cages were moored above seagrass beds, but as production expanded they were moved into deeper water further away from the meadows. Here, we monitored the deepest edge of meadows near fish farms that had been moved into deeper waters as well as at a decommissioned farm site. Four P. oceanica monitoring systems were set up using methods developed by the Posidonia Monitoring Network. Seagrass % coverage, shoot density, % of plagiotropic rhizomes, shoot exposure, leaf morphometry, and sediment organic matter content and grain size were monitored at 11 fixed plots within each system, in 2012–2014 and in 2017. Expansion at the lower depth limit of seagrass meadows was recorded at all monitoring sites. Most other P. oceanicadescriptors either did not change significantly or declined. Declines were most pronounced at a site that was far from mariculture activities but close to other anthropogenic pressures. The most important predictor affecting P. oceanicawas depth. Monitoring using fixed plots allowed direct comparisons of descriptors over time, removes patchiness and intra-meadow variability increasing our understanding of seagrass dynamics and ecosystem integrity. It seems that moving fish farms away from P. oceanica has helped ensure meadow recovery at the deepest margins of their distribution, an important success story given that these meadows are at the upper thermal limits of the species.

Over the last 3 decades fish cage aquaculture has increased exponentially in the eastern Mediterranean Sea and has induced various levels of environmental change in coastal waters. The most apparent negative changes involve the degradation of the native seagrass Posidonia oceanica (L.) Delile. Our study examined the effects of fish cage aquaculture on seagrass health, sea urchin density and herbivore feeding pressure in shallow seagrass patches in the coastal waters of 2 Greek islands (Evia and Crete) between February and May of 2008. Evia and Crete represent a wide range of fish farming intensities, from small-scale (75 t yr−1) to large-scale (1000 tyr−1) fish production, respectively. On both islands, the seagrass variables, shoot productivity, standing crop and leaf morphometrics (length and width) were significantly lower (p < 0.0001) inimpacted seagrass patches adjacent to fish farms compared with control patches. In addition, significantly higher sea urchin densities and herbivore feeding pressure (percentage of shoots with herbivore grazing scars) were found in impacted patches on both islands. Higher leaf tissue nitrogen and epiphyte loads were also found in impacted patches, but these increases could not be attributed to fish farming alone. Our results show that negative effects on seagrass patches can occur as a result of fish farming at both small-scale and large-scale intensities and that increased sea urchin densities and feeding pressures are important indirect effects of coastal aquaculture on these islands.

Offshore fish farming is predicted to increase in the near future driven by the lack of coastal space. In this review I discuss the environmental issues of offshore farming from experience in coastal farms. Even more so than in coastal farms, a rapid and wide dispersal of dissolved waste prod- ucts is predicted for offshore farms. Despite wider dispersal of particulate waste products, fast sinking rates of feed pellets and faeces suggest organic enrichment of the bottom sediments in farm vicinities (hundreds of meters), although at lower loading rates than coastal farms. The benthic response to organic enrichment is unpredictable due to lack of knowledge from shelf areas. Most shelf sediments are considered carbon limited and fish farm waste products may stimulate the benthic communities, but due to the sparse abundance and absence of pollutant-tolerant benthic fauna, the capacity of ben- thic communities to assimilate organic matter may be limited. Instead, microbial decomposition of waste products could become important, leading to increased oxygen demand and accumulation of sulfides in the sediments. This may negatively affect benthic biodiversity. Interactions with wild fish (aggregation, genetic impacts, spreading of disease and parasites) are expected, but difficult to pre- dict, as the composition of species attracted to offshore farms will be different from that of species attracted to coastal farms. Escapees are potentially a high risk due to farm failure under rough weather conditions in the open sea. The carbon footprint of farming offshore will increase (transportation) and the ecological footprint (fishing feed) will remain a severe constraint, as in coastal farming. Offshore farming is subject to high costs of operation, including those for monitoring environmental conditions. Research should focus on interactions with wild fish populations, mapping of sensitive benthic habitats and experimental studies of the response of benthic habitats to organic enrichments.