Water exchange and the holding capacity of sites for fish farming in fjords and other inshore areas SPEAR-Training March 16, 2006 Anders Stigebrandt, Univ. of Gothenburg, Sweden

Aim of the course The aim of the course is to teach elements of physical oceanography and fish metabolism needed to estimate the holding capacity of sites for fish farming, in particular sites in fjords and other inshore areas and to compute the holding capacity of a few sites The holding capacity is determined by environmental effects caused by the turnover of organic matter in fish farms, in particular effects on the bottom beneath the farm in the fish cages on the surrounding inshore environment

Fish farm siteSurrounding area Dispersion sub-model Fish sub-model Benthic sub-model Regional water quality model Water quality sub- model

Models needed to estimate holding capacity Fish model to compute the turnover of matter by fish Dispersion model to compute dispersion of particulate matter emitted by a farm. Benthic model to compute the oxygen transport to the sea bed and from that the maximum loading with organic matter that allows a fauna of benthic animals Fish cage model to compute oxygen and ammonium concentrations in the cages Water quality (Secchi depth) model of surface waters in inshore water Water quality (oxygen concentration) model of basin waters in fjords Model of the natural flow of organic matter into basin waters of fjords The models presented during the course are based on first order processes and kept as simple as possible

Two computer programs have been developed 1)FjordEnv - computes water exchange of different strata in fjords and other inshore areas and how this is influenced by changes of e.g. the topography of the mouth. It also computes how outlets of particulate organic matter and nutrients, e.g. from fish farming, influence Secchi Depth and deepwater oxygen conditions in the whole system. 2)MOM – computes the holding capacity of sites for fish farming based on acceptable changes of the state of the local environment (the sea bed) and on expected extreme states with respect to oxygen and ammonium concentrations in the fish cages. The holding capacity is given by the lowest of the estimates from the local and the regional aspects, i.e. the lowest of the estimates given by MOM and FjordEnv, respectively.

Literature Aure, J., and A. Stigebrandt, 1989: On the influence of topographic factors upon the oxygen consumption rate in sill basins of fjords. Est., Coastal and Shelf Science, 28, Aure, J., and A. Stigebrandt, 1990: Quantitative estimates of the eutrophication effects of fish farming on fjords. Aquaculture, 90, (Regional effects) Stigebrandt, A., 1999: Turnover of energy and matter by fish – a general model with application to Salmon. Fisken og Havet, No 5, Stigebrandt, A., J. Aure, A. Ervik, P.K. Hansen, 2004: Regulating the local environmental impact of intensive marine fish farming. III: A model for estimation of the holding capacity in the MOM system (Modelling – Ongrowing fish farm – Monitoring). Aquaculture, 234, (Local effects)

The underlying physics are presented in: Stigebrandt, A., 2001: Fjordenv – a water quality model for fjords and other inshore waters. Rep. C40, Earth Sciences Centre. Gothen- burg University. 41pp. This builds heavily on the following papers: Stigebrandt, A., 1981: A mechanism governing the estuarine circulation in deep, strongly stratified fjords. Estuarine, Coastal and Shelf Science, 13, Stigebrandt, A., and J. Aure, 1989: Vertical mixing in basin waters of fjords. J. Phys. Oceanogr., 19, Stigebrandt, A., 1990: On the response of the horizontal mean vertical density distribution in a fjord to low-frequency density fluctuations in the coastal water. Tellus, 42A, Aure, J., J. Molvaer and A. Stigebrandt, 1996: Observations of inshore water exchange forced by a fluctuating offshore density field. Marine Pollution Bulletin, 33, Stigebrandt, A., 1999: Resistance to barotropic tidal flow in straits by baroclinic wave drag. J. Phys. Oceanogr., 29,