Modelling food web interactions, variation in plankton production, and fisheries in the western English Channel ecosystem

To explore the contributions that fishing, trophic interactions and plankton production make to explanations of the observed variation of higher trophic (principally fish) levels in the western English Channel ecosystem, Ecosim simulations were run from 1973 to 1999 using the most complete data set yet assembled. The results indicate that a bottom-up mechanism plays an important role in the system production. Inclusion of a primary producer biomass forcing term, estimated from empirical data, improved the goodness of fit of the model estimates to the available biomass data by about 25% compared to fitting using only the series of fishing mortalities. Model fitting was further improved by changing the so-called vulnerability parameters, causing an overall improvement of 62% in explained variation. Incorporating the new vulnerability values, the model was used to estimate a primary production anomaly function to replace the primary producer biomass forcing in driving the model simulations. In this scenario, the model estimated a series of values for primary producer abundance that approximated the empirical data, but gave lower estimates than were observed towards the end of the period. This version also gave a better fitting to the zooplankton abundance data and generally improved the fitting to all functional groups.