Assessment of some chemical parameters in marine sediments exposed to offshore cage fish farming influence:: a pilot study

This work investigates changes in some chemical parameters in sediments adjacent to an offshore fish farm (gilthead sea bream, Sparus aurata and sea bass, Dicentrarchus labrax) in the western Mediterranean Sea (San Pedro del Pinatar. Murcia. SE Spain). Sampling was designed taking into account previous hydrographic and waste dispersion studies, so samples were taken underneath the cages, 100, 200 and 500 in north and south. and at a control station 1.8 km south from facilities. The organic matter (OM). total nitrogen (TN) and total phosphorus (TP) content were analysed during a 1-year production period. No increase in the organic matter and nitrogen content was observed between samples taken front the same station. However. the phosphorus content showed a seasonal pattern underneath the cages. consisting in a fall after the lower growth and food intake period (winter, temperature <15 degreesC), and a progressive increase until the end of the study. Depth (36 m), average current speed (7.85 cm s(-1)) low biomass production (200 tonnes per year) and short life of the fish farm (1 year) would have contributed to the small changes observed. The nearness of other pollutant sources such as a floodway stream and a domestic sewage outlet might be related with the space-time variations observed in the selected parameters. The need to complement these with other physical. chemical and biological parameters is clear in order to provide a picture of the influence of the fish farming activity and to differentiate any such influence from environmental impact sources in the same area. (C) 2004 Elsevier B.V. All rights reserved.