As marine protected areas (MPAs) are increasingly being utilised as a tool for fishery
management, their impact on the food web needs to be fully understood. However, little is
known about the effect of MPAs on fish assemblages, especially in the presence of
different life history and ecological traits. Modelling the observed changes in fish
population structures may provide a mechanistic understanding of fish assemblage dynamics.
In addition, modelling allows a quantitative estimate of MPA spill-over. To achieve this
purpose, we adapted an existing ecosystem model, OSMOSE (Object-oriented simulator of
marine biodiversity exploitation), to the specific case of the presence of fish with
multiple life histories. The adapted model can manage 4 main categories of life history
identified in an estuary MPA: fish that (1) spend their entire life cycle locally, (2) are
present only as juveniles, (3) enter the area as juveniles and stay permanently except
during reproduction periods, which occur outside the estuary, and (4) are present
occasionally and for a short time for foraging purposes. To take into account these
specific life-history traits, the OSMOSE code was modified. This modelling approach was
developed in the context of the Bamboung Bolong MPA, located in a mangrove area in the
Sine-Saloum Delta, Senegal. This was the ideal case to develop our approach as there has
been scientific monitoring of the fish population structure inside the MPA before fishery
closure, providing a reference state, and continuous monitoring since the closure.
Ecologically similar species were pooled by trophic traits into 15 groups that represented
97% of the total biomass. Lower trophic levels (LTL) were represented by 6 compartments.
The biomass of the model species was calibrated to reproduce the reference situation
before fishery closure. Model predictions of fish assemblage changes after fishery closure
corresponding to the Bamboung MPA creation scenario were compared to field observations;
in most cases the model reproduces observed changes in biomass (at least in direction). We
suggest the existence of a “sanctuary effect”, that was not taken into account in the
model, this could explain the observed increase in biomass of top predators not reproduced
by the model. Finally, the annual MPA fish spill-over was estimated at 11 tons (~33% of
the fish biomass) from the model output, mainly due to diffusive effects.