Fish farm effluents cause metabolic depression, reducing energy stores and growth in the reef-forming coral Lophelia pertusa

Abstract

Cold-water corals (CWCs) have come under increasing pressure from human activities over the last decades. Of particular concern in Norway is the potential impact of open net pen aquaculture on CWC reefs formed by Lophelia pertusa, a threatened and declining habitat. We conducted a 1 yr in situ transplantation experiment and corresponding field measurements of 2 reefs located close to fish farms to elucidate the impacts of particulate organic waste released from the farms on coral colonies. Our study provided new evidence of negative impacts of organic effluents on L. pertusa ecophysiology. After 1 yr, both naturally occurring and transplanted corals, at distances ranging from 250 m to 1 km downstream of what would be regarded as an average-sized Norwegian fish farm, exhibited depressed metabolic rates compared to corals outside the main depositional footprint of the farms. The metabolic depression impeded energy acquisition, reducing growth and energy reserves by up to 70 and 50%, respectively. No clear threshold for significant biological impact could be detected along a distance gradient. Instead, a gradual decrease in metabolic rates, growth and lipid reserves occurred with increasing modelled sedimentation rate of organic waste from the farm. The strong statistical correlation between oxygen consumption, growth, energy stores and sedimentation rates implies that predictions of the short-term impact of aquaculture effluents on L. pertusa ecophysiology and CWC reef development may be achievable in the future. This would significantly improve the ability of management to make informed decisions on the licensing of new farms near CWC reefs.