172
3.2
Exploited Ecosystems
3.2.1 Benthic Effects of Aggregate
Extractions
The effects of sediment dredging on benthic
groups consumed by sole and plaice in the
Eastern English Channel have been assessed.
Benthic species were grouped according to their
diet or mobility, and analyses were performed at
the group level. First, a BACI (before-after control impact) analysis was conducted to determine
whether dredging impacted benthos abundance.
This approach allowed determining relationships
between dredging intensity and decreases in benthic abundance and quantifying the recovery rate
of the studied benthic groups. At the group level,
it was shown that impacts of dredging are maximal from 6 to 12 months after dredging, these
impacts being either positive (opportunistic
groups) or negative. Concerning mobility,
impacts of dredging were maximal on burrowers.
On the contrary, mobile species seemed to be less
impacted. Carnivorous species were amongst the
most impacted trophic groups. Detritivores
seemed to be positively impacted by dredging, at
least for a few months after dredging. It was not
possible to link increases or decreases in abundance after dredging to a recovery time, increases
in abundance being linked to strong year effects.
3.2.2 Spatial Interactions Involving
Fishing Fleets and Other
Sectors of Activity
Discrete choice models building in a random utility function (RUMs) were developed to determine how fi shing effort is allocated spatially and
temporally by a selection of French, English and
Dutch fl eets fi shing in the Eastern English
Channel. Results showed that fi shers tended generally to adhere to past annual fi shing practices
and to the areas where they experienced high revenues and also that French dredgers strongly
interacted spatially with English vessels.
Furthermore, results indicated that maritime traffi c, aggregate activity and restricted areas negatively impact the choice of many of the fl eets
under investigation. Other spatially explicit statistical analyses have been conducted to evaluate,
separately, the impacts of aggregate extraction
and maritime traffi c. The effects of both aggregate extraction intensity and the proximity to
dredging sites on the distribution of fi shing effort
were investigated, for a broad selection of French
and English demersal fl eets operating in the
Eastern Channel (Marchal et al. 2014 ). The most
striking result is that neither dredging intensity
nor the proximity to the extraction site had a
major deterring effect on fi shing activities. To the
contrary, the fi shing effort of dredgers and potters
could be larger on aggregate sites than in their
close neighbourhood, whilst the fi shing effort of
netters has increased substantially in the impacted
area. The attraction of fi shing fl eets is likely due
to a local and temporary concentration of target
species. However, knowledge on the vulnerability and life-history characteristics of these species to aggregate extractions suggests that
overextending the licensed areas would be detrimental to them and to their related fi sheries in the
longer term. Maritime traffi c seems to be a perturbation for the fi shing activities. However, in
the case of the red mullet fi shery, vessels do not
avoid traffi c lanes when they expect high fi sh
densities. They then may take the risk of fi shing
inside the traffi c lanes or in areas of high marine
traffi c densities. An effort has then been made to
validate the outcomes of fl eet dynamics models
by administering a survey to French fi shers operating in the Eastern English Channel. Although
French fi shers generally did not feel constrained
in the amount of space they had available for fi shing, some mentioned shipping lanes, aggregate
extractions and competition with other fi shers as
constraining factors.
4
From Modelling Ecosystem
Compartments to Holistic
Ecosystem Modelling
Three modelling approach patterns have been
pursued concurrently to model the impact of
human activities on the Eastern Channel at the
scale of the ecosystem, using the models ISISFish, OSMOSE and ATLANTIS. An ISIS-Fish
model was developed, including two fl atfi sh
P. Marchal et al.
3.2
Exploited Ecosystems
3.2.1 Benthic Effects of Aggregate
Extractions
The effects of sediment dredging on benthic
groups consumed by sole and plaice in the
Eastern English Channel have been assessed.
Benthic species were grouped according to their
diet or mobility, and analyses were performed at
the group level. First, a BACI (before-after control impact) analysis was conducted to determine
whether dredging impacted benthos abundance.
This approach allowed determining relationships
between dredging intensity and decreases in benthic abundance and quantifying the recovery rate
of the studied benthic groups. At the group level,
it was shown that impacts of dredging are maximal from 6 to 12 months after dredging, these
impacts being either positive (opportunistic
groups) or negative. Concerning mobility,
impacts of dredging were maximal on burrowers.
On the contrary, mobile species seemed to be less
impacted. Carnivorous species were amongst the
most impacted trophic groups. Detritivores
seemed to be positively impacted by dredging, at
least for a few months after dredging. It was not
possible to link increases or decreases in abundance after dredging to a recovery time, increases
in abundance being linked to strong year effects.
3.2.2 Spatial Interactions Involving
Fishing Fleets and Other
Sectors of Activity
Discrete choice models building in a random utility function (RUMs) were developed to determine how fi shing effort is allocated spatially and
temporally by a selection of French, English and
Dutch fl eets fi shing in the Eastern English
Channel. Results showed that fi shers tended generally to adhere to past annual fi shing practices
and to the areas where they experienced high revenues and also that French dredgers strongly
interacted spatially with English vessels.
Furthermore, results indicated that maritime traffi c, aggregate activity and restricted areas negatively impact the choice of many of the fl eets
under investigation. Other spatially explicit statistical analyses have been conducted to evaluate,
separately, the impacts of aggregate extraction
and maritime traffi c. The effects of both aggregate extraction intensity and the proximity to
dredging sites on the distribution of fi shing effort
were investigated, for a broad selection of French
and English demersal fl eets operating in the
Eastern Channel (Marchal et al. 2014 ). The most
striking result is that neither dredging intensity
nor the proximity to the extraction site had a
major deterring effect on fi shing activities. To the
contrary, the fi shing effort of dredgers and potters
could be larger on aggregate sites than in their
close neighbourhood, whilst the fi shing effort of
netters has increased substantially in the impacted
area. The attraction of fi shing fl eets is likely due
to a local and temporary concentration of target
species. However, knowledge on the vulnerability and life-history characteristics of these species to aggregate extractions suggests that
overextending the licensed areas would be detrimental to them and to their related fi sheries in the
longer term. Maritime traffi c seems to be a perturbation for the fi shing activities. However, in
the case of the red mullet fi shery, vessels do not
avoid traffi c lanes when they expect high fi sh
densities. They then may take the risk of fi shing
inside the traffi c lanes or in areas of high marine
traffi c densities. An effort has then been made to
validate the outcomes of fl eet dynamics models
by administering a survey to French fi shers operating in the Eastern English Channel. Although
French fi shers generally did not feel constrained
in the amount of space they had available for fi shing, some mentioned shipping lanes, aggregate
extractions and competition with other fi shers as
constraining factors.
4
From Modelling Ecosystem
Compartments to Holistic
Ecosystem Modelling
Three modelling approach patterns have been
pursued concurrently to model the impact of
human activities on the Eastern Channel at the
scale of the ecosystem, using the models ISISFish, OSMOSE and ATLANTIS. An ISIS-Fish
model was developed, including two fl atfi sh
P. Marchal et al.