Modelling trophic interactions, fishing and climatic variations affecting the western English Channel ecosystem

Three trophic models of the Western English Channel were built using the Ecopath with Ecosim (EwE) software. A model for 1994 represented a warm period. Two models represented colder periods in 1973 and in 1985, the later coinciding with a considerable increase in fish and shellfish landings. One of the main observed differences among the models was an increase in primary production that seemed to be reflected in changes in fish biomass. Some parameters estimated indicate that (1) the ecosystem was relatively immature or disturbed, (2) despite the increase fishing effort since the 1970s, the system maturity increased in 1985 and (3) with the increased fishing mortalities operating in the system, this trend was reversed in the 1994 model. Ecosim simulations run from 1973 to 1999 indicated that a bottom-up mechanism plays an important role in the system production. By accounting for primary production changes and by estimating the so-called vulnerability parameters, it was possible to improve the goodness of fit of the model estimates to the available biomass data by about 62 to 68% compared to fitting using only the series of fishing mortalities. The 1994 model was used to explore the effects of the use of the EwE fishing optimization routine on profits, number of jobs and ecosystem structure. An "ideal" mixed policy configuration was found when ecosystem structure was weighted slightly higher than were profits and jobs. This scenario led to an overall reduction in effort but also to increased profits and biodiversity, with almost no lost in the number of jobs. The optimizations also showed that the average trophic level of the catches is quite conservative to changes in ecosystem structure, which contrasted with an estimated biodiversity index from Ecosim that showed huge changes as a function of the weights placed on the policy functions.