Structure, fonctionnement, évolution des communautés benthiques ...

tel-00009359, version 1 - 1 Jun 2005
Chapitre 3 - Fonctionnement du réseau trophique benthique de la Grande Vasière
that N. norvegicus is an opportunistic carnivore, which exploits both benthic (mollusks and
polychaetes) and bathypelagic (decapod larvae, Euphausiacea, copepods) prey (Lagardère 1977).
Because these findings are in agreement with those obtained by Jennings et al. (2002) in the Celtic sea
it can be deduced that the diet of N. norvegicus is quite homogeneous over a large geographical area.
However, differences in individual sizes and individual weights in the two studies limit comparisons.
It can be concluded that N. norvegicus and M. rugosa are two competitors throughout their life cycle
and occupy the same trophic niche in the ecosystem. Selective fishing of one of these two “sister
species” (N. norvegicus) could induce serious consequences in a long term evolution of the
N. norvegicus populations. The hypothesis should be that N. norvegicus populations could be
progressively replaced by those of M. rugosa which could secondly prevent or limit a N. norvegicus
stock reconstitution, either by a natural recruitment or by fishery management. Thus M. rugosa and
N. norvegicus by occupying the sediment compete for space and by feeding on the same preys
compete for food, and moreover, adults of M. rugosa are potential predators of N. norvegicus juveniles
and could thus limit recruitment.
The main features of the trophic interactions in which N. norvegicus and M. merluccius are
involved have been summarized in figure 8. Predator-prey interactions explain the transition from one
trophic level to the other and trophic competition explains the position of species within one group. It
can be noticed that M. merluccius adults have no competitor which implies that a decrease in the
M. merluccius population by fisheries impact should induce either a drastic preys biomass. In
reference to N. norvegicus the consequences of overfishing should result in either an increase of the
M. rugosa biomass and/or an increase of the preys.
Figure 8: Trophic relationships of M. merluccius and N. norvegicus and their main prey and competitors.
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