Modelling the impact of cod ( Gadus morhua L.) farming in the marine environment—CODMOD

Abstract

The environmental impact of Atlantic cod ( Gadus morhua L.) farming was quantified by assessing the benthic effects of a large cod farm in Shetland, UK and by parameterising a mathematical model based on an existing salmon model (DEPOMOD, [Cromey, C.J., Nickell, T.D., Black, K.D., 2002a. DEPOMOD—Modelling the deposition and biological effects of waste solids from marine cage farms. Aquaculture 214, 211–239.]) to predict solids deposition from farmed cod to the seabed. Benthic physico-chemical and macrofaunal data for three consecutive years provided synoptic data for model validation, allowing model testing throughout the growing cycle, from low biomass to near maximum. A video technique for quantifying the particle size and settling rates of cod faeces was further developed (weighted mean settling velocity for cod faeces was 3.7 cm s − 1 ), and faecal output experiments yielded temporal data on post-prandial faecal output (faecal output was 8.5% of feed input d − 1 by mass), which was lower than faecal output rates that have been used in modelling salmon impacts. Feed settling experiments showed large and significant differences between three proprietary cod feeds. At most benthic stations, there was a good fit between modelled and measured benthic Infaunal Trophic Index (ITI) but it failed to reproduce the moderate to heavy impact observed at an inshore transect. Possible reasons for the underperformance of the model in this area include the hydrographic data available and unreported cage movements in the area of the transect. When these factors were taken into account in the model, performance improved. Surveys of the sea bed surrounding the cod farm indicated that over a cod growing cycle up to near maximum biomass, there is a biological response to organic enrichment of the near-field sediments. This enrichment is not excessive in terms of the biomass present, and is comparable to salmon culture sites of similar scale in similar environments. Provided that species-specific parameters are used for particulate wastes no special considerations for modelling of this relatively new aquaculture species are necessary.