Trophic model of a lightly exploited cod-dominated ecosystem

A 27 compartment model of the food-web in Sørfjord, northern Norway was balanced using the mass-balance model Ecopath based on data from Sørfjord and adjacent fjord areas. Sørfjord is a lightly exploited high-latitude fjord ecosystem with cod as the main piscivore. During 1993–1996, the biomass of planktivore fish was low, and the pelagic community in the model was dominated by herbivore zooplankton, mainly copepods and krill, chaetognaths and microzooplankton. Carbon flows were dominated by the pelagic lower trophic level groups and the components of the microbial food-web. Flows originating from detritus amounted to 53% of the total carbon flow. The model benthic community was dominated by detrivores, with benthic invertebrate predators as important secondary consumers. Model output values for the benthic groups were sensitive to the input values of the cod and other fish groups. Macroalgae constituted a major part of the living biomass and was an important source for detritus, but did not provide a major input to herbivores. Cod, phytoplankton and krill were the major keystone groups and had large trophic impacts on the other groups. Herbivorous krill constituted much (>50%) of the cod diet, resulting in a low trophic level (TL=3.6) of cod in Sørfjord compared to TL of cod in other ecosystems ranging from 3.7 to 4.4. Cod biomass, production and yield were relatively high compared to other boreal and high-latitude ecosystems with cod populations. Model transfer efficiencies averaged 21.7% from trophic levels II–IV. The low level of exploitation may enhance long-lived benthic groups that not only directly support higher trophic levels but also contribute to a broad food base that supports a relatively high production of cod.