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Scottish Executive Locational Guidelines for Fish Farming

Predicted Levels of Nutrient Enhancement and Benthic Impact

Fisheries Research Services, Marine Laboratory, Aberdeen

Document format: pdf

Publisher: Fisheries Research Services (Scotland)

Creation date: 2002

The Scottish Executive document 'Locational Guidelines for the Authorisation of Marine Fish Farms in Scottish Waters' provides guidance to the aquaculture industry and regulatory bodies on the environmental suitability of coastal areas for fish farming. This guidance is based on advice from Scottish Natural Heritage and Fisheries Research Services (FRS), relating to the relative environmental sensitivity of local areas supporting aquaculture. The purpose of this document is to describe the predictive modelling approach used to inform the FRS component of this advice, through estimation of the levels of nutrient enhancement and benthic impact arising from current marine finfish cultivation in sea lochs (and similar semi-enclosed water bodies).

A simple box model is used to predict the level of enhancement of soluble nutrient nitrogen from fish farming sources, treating nitrogen as a conservative substance. This model is a function of the flushing rate of a sea loch, the total consented biomass of all the finfish farms in the loch and the nitrogen source rate. A mass balance model was used to estimate the rate of release of nitrogen at 48.2 kgN per tonne of salmon produced per year. These data are used to calculate an equilibrium concentration enhancement (ECE) for nitrogen, expressed in µmol l-1. Non-salmonid species are accounted for in this model, by the application of appropriate 'species factors' to correct for the different rates of soluble nitrogen release from the farming of such species.

Benthic impact is estimated using a modified 'Gowen' model to predict the area of seabed impacted by the deposition of organic matter in the form of solid waste from finfish farms. Using the modified technique described, the along- and across-loch dispersion of solid waste is estimated for each farm in a sea loch. These distances, together with an estimate of the surface area of the farms are used to predict an elliptical area of seabed impacted by organic carbon deposition. An appropriate distribution of particle settling velocities and associated quantities of excreted carbon are applied in order to estimate areas of seabed impacted by different levels of organic carbon deposition within the impacted area. The results of this model are used to estimate the total percentage area of the seabed of a loch impacted by a level of enhanced organic carbon deposition greater than 0.70 kgC m y. Above this critical value, it has been shown that the infaunal diversity of sediments is reduced, and the seabed can be considered degraded.

The models described here predict the relative levels of nutrient enhancement and percentage areas of seabed degraded by organic carbon deposition for 111 sea lochs. The results of both models are scaled from 0 – 5, and the two scaled values are added together to provide a single combined index. On the basis of this combined index, areas are designated as Category 1, 2 or 3, where Category 1 areas are considered to be the most environmentally sensitive to further fish farming development due to high predicted levels of nutrient enhancement and / or benthic impact.

Key Words: Scotland • UK • Environmental Model • Benthic Impacts of Aquaculture • Water Quality