The impact of sea cage salmon farming on the benthic environment of Big Glory Bay, Stewart Island
Edwards, John Mark Raymond
Rearing salmon in sea cages can have pronounced localised effects on benthic faunal assemblages and the physicochemical parameters of their environment. These effects result from sedimentation of pelleted feed and the metabolic wastes of the salmon. To the detriment of the environment, the three sites studied were situated in sheltered, shallow, relatively enclosed body of water with limited water circulation. A survey approach was used in this study to examine changes in macrobenthic community composition and sediment physicochemical parameters. Measurements were taken to quantify the amount and composition of wastes sedimenting out around the cultures. These assessments were made with distance from the cage sites, and using a temporal series of abandoned sites, to measure the spatial impact of the sites and the progress towards recovery after cessation of organic input. Complete defaunation occurred under the cages in the anoxic, sulphide rich sediment. The sediment in the proximity of the cages had very high total organic matter, total nitrogen and total phosphorus contents and substantially increased oxygen demand. Statistically significant effects were measured in physicochemical parameters to 30 m from the edge of the occupied cages. The macrobenthic community close to the cages is of low diversity with a pronounced numeric dominance of the most abundant taxa, generally deposit feeding polychaetes, between which there are limited possible interactions. A site disused for twentyeight months showed some recovery in sediment physicochemical parameters and macrobenthic community composition at 30 m on the transects radiating from the sites. However the area which was located directly beneath the culture was azoic with a considerable anaerobic accumulation of organic matter releasing gas bubbles as a result of methanogenesis.
Advisor: Jillett, John; Thrush, Simon
Degree Name: Master of Science
Degree Discipline: Zoology
Publisher: University of Otago
Research Type: Thesis