A comparative study of sediments under a marine cage farm at Gran Canaria Island (Spain). Preliminary results

Abstract

This paper presents the results of a comparative study of sediments beneath an offshore cage farm for on-growing gilthead seabream (Sparus aurata L.) over a 1-year period, from the start of farming operations. The cage system was located at Melenara Bay, on the East coast of Gran Canaria Island, Canary Islands. Samples were collected from three zones: underneath the cages (two sampling points), 60 (four sampling points) and 200 m (four sampling points) from the cages. Sediment samples were analysed for particle size, organic matter, nitrogen, and phosphorus contents. Statistical analyses of all parameters showed no significant differences between samples within the same zones. When different zones were compared, there were no significant differences for organic matter and phosphorus content, and minor differences for particle size. However, the average nitrogen content of the sediments underneath the cages was significantly greater than the other two zones. There were temporal variations in the two nutrients in all zones, with a trend towards the recovering of initial values, except for nitrogen beneath the cages. In general terms, the physical and chemical characteristics of sediments studied were little affected by the operation of the farm over the period of this study. This could be explained by the fact that the farm was in its first year of operation, its small size (80 Mt output per year), and the high average velocity of water currents (about 6 cm/s).