Anders Stigebrandt and Jan Aure
MOM (Monitoring-Ongrowing fish farms-Modelling) is a management system which can relate the environmental impact of fish farms to the holding capacity of a site. The holding capacity is defined as the sediment ability to receive organic effluents without causing the benthic impact to exceed predefined levels. The system consists of three integrated parts: 1) environmental quality standards, 2) a standardized monitoring program for checking the environmental conditions on a site according to environmental quality standards, and 3) a mathematical model for predicting the expected environmental impact in relation to farm specifications and information on the hydrography and topography of the site. MOM is described in Ervik and Kupka Hansen (1994: ICES C.M. 1994/F:26).
The present abstract describes a mathematical model that computes maximum fish production under the prerequisite that there has to be acceptable living conditions for benthic animals at the site. This model now constitutes the benthic part of the mathematical model for the MOM system as described above. The building blocks of the model are a new oxygen supply model, computing how bottom sediments and benthic fauna get oxygen from the overlying water, and a new dispersion model for particulate organic waste from the cages in a fish farm. The model will be published in Stigebrandt and Aure (1995: Fisken og Havet, no. 26).
The oxygen supply model computes the oxygen transfer through the turbulent benthic boundary layer. The model probably has general validity but is not yet verified, among other things the values of some constants in the model are not satisfactory determined. Further, the relationship between measured currents and the dimensioning current velocity to be used in the model is still uncertain. These are things that will be clarified during the further development and testing of MOM.
The dispersion model for particular organic waste from the fish cages has provided new and enlightening knowledge on which factors that are most important for the dispersion of the waste. In particular the model focus on dispersion effects of cage size and cage separation. >From the current variability, expressed as the standard deviation s, and estimated sinking times T for the organic waste, the model simulates how waste from a single cage is spread over the bottom. The loading with organic matter is generally maximal below the centre of the cage and decreases with the distance from the centre. The dispersion pattern is displaced by a possible non zero mean current past the cage. The loading of every point of the bottom beneath a marine fish farm is computed as the sum of the contributions from all cages. Simulations using the dispersion model show the following general results: 1) The natural dispersion ability at a site is determined by the 'dispersion length' sT. This may be increased using food with lower sinking speed. 2) The fraction of organic waste from a single cage deposited on the bottom outside the projected area of the cage increases, and the maximum loading beneath the cage decreases, with increasing dispersion length and decreasing area of the cage. 3) In a fish farm, the maximum loading of the sediment with organic waste decreases if the distance between cages is increased and the number of rows of cages is decreased.
The present version of the model for maximum fish production seems to give realistic results. It may also play an important role in the further development of MOM, not least because it provides a structured model of the problem and by that an objective possibility to rank different influencing factors after their relative importance. Computations of maximum fish production are done by a computer program described in Stigebrandt and Aure (1995). Finally, it should be mentioned that in addition to the model for the benthic holding capacity of a fish farm site presented above, the complete MOM model will also contain a model for the environmental conditions in the fish cages and a model ('Fjordmiljø') that computes the far field environmental impact. The foundation of Fjordmiljø is described in Aure and Stigebrandt (1990: Aquaculture, 90:135-156).