Environmental issues of fish farming in offshore waters: perspectives, concerns and research needs

M Holmer

doi:10.3354/aei00007

Abstract

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.

Highlights

Aquaculture has the potential to play a major role in feeding the human population in the future (Duarte et al 2009)
This review aims at identifying knowledge gaps on the road to an environmentally sustainable ‘blue revolution’ in offshore locations
The key question that remains is if mariculture will expand into offshore locations

Summary

INTRODUCTION

Aquaculture has the potential to play a major role in feeding the human population in the future (Duarte et al 2009). Due to low nutrient concentrations, water quality is beneficial for farming at offshore sites compared with coastal areas, reducing the risk of phytoplankton blooms, including those of toxic species (Pelegri et al 2006). The vast majority of benthic habitats in shelf areas are, dominated by bare sediments, ranging from fine-grained muddy sediments under weak currents to coarse-grained shell sand under strong currents (Gage 1996) These deep benthic communities (fauna and microbial) are generally carbon limited, controlled by low input of organic matter from the overlying water column (Gage 1996). Escapees (incl. spawn) interact with wild fish, affecting gene pools, and compete for habitat

Introduction of species into new habitats

Findings

CONCLUSIONS