Stable Isotopes as Tracers to Constrain the Predictions of Environmental Models on Effluents from Fin Fish Cage Farms

P. M. S. Monteiro

Sea Fisheries Research Institute, Private Bag X2, Roggebaai, 8012, Cape Town, 8012, South Africa.

Internet: monteiro@sp.sfri.ac.za

Abstract

Quantitative assessment and modelling of the impact of fish farms on the organic load and composition of marine sediments is often a problem because, especially in the context of a second input, it is difficult to establish a direct link between the source and the sediment data. Stable isotopes (Carbon ¹³C, Nitrogen ¹⁵N and Sulphur ³⁴S) may offer a solution to this problem as a result of the link between the variability of the natural abundance of light and heavy isotopes and trophic position in the food web. The benefits of using stable isotopes are particularly evident when two and if possible three are simultaneously measured introducing sufficient independent parameters to solve enough mass balance equations for the number of inputs.

The intensive use of stable isotopes to study the trophic dynamics of marine food webs is a relatively recent phenomenon but it has proven very useful to support or question existing views on trophic dynamics in often complex systems. Its use has been extended to the study of the far-field impacts of land based organic discharges (sewage) on sediment biogeochemistry and benthic food webs. The objectives of this presentation are:

• to discuss how variability of the natural isotope abundance in marine food webs is driven by trophic position
• to show, using a spreadsheet generated model, how near and far field impacts from fish farms can be constrained.
• discuss different techniques with which to analyse organic and isotope data: (hyperbolic mixing model, histograms, scatter plots)

The use of these techniques were illustrated with a case study where the appearance and development of an Ulva sp bloom in Saldanha Bay, South Africa was linked to a fish meal factory discharge using stable isotopes. Stable isotopes are not only useful tracers but also help to understand the ecological interactions which occur as a result of enhanced organic loading of marine systems which are the cause of eutrophication. It is therefore a "tool box" of techniques which though still under utilised in marine aquaculture, offers clear benefits to constrain environmental impacts and assess near and far field model predictions.