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J. Robertson et al.
two periods 1977-1986 and 1987-1994, to reflect changes in the observed trends in diet
and landings from the different fisheries and establish possible changes in food
availability in the two major periods.
Results indicate that: up to 60% of the fish component of the diet of human
consumption fish species consists of sandeels and Norway pout in approximately equal
amounts; and that there was a 30% decline in overall biomass consumption in the
second period 1987-1994, coincident with changes in the transfer efficiencies between
trophic levels and the loss of several trophic interactions. The observed changes were
largely associated with the dynamics of the top predators, suggesting that a trophic
trickle had appeared in the ecosystem, and that prey switching and shifts in the
suitabilities of the predatory fish species had occurred. One potential reason may be
that because sandeel and Norway pout are the two species which are consumed in the
greatest quantities, there may be direct competition between the industrial fishery and
the human consumption fishery.
The potential for recruitment and growth overfishing was also analysed in
terms of biomass thresholds; it was concluded that whilst the major human
consumption stocks in the North Sea do not show any significant signs of recruitment
failure, most of them are certainly at risk.
The interpretation of these results is compounded by the level of over quota
landings, by-catches and discards of human consumption species within the industrial
and human consumption fishery. Results from interviews with the industry indicate
that levels are as high as 60-70% in some areas and times of year, with an overall level
of not less that 30%. To fully assess the impacts of this and the industrial fisheries on
human consumption species three scenarios were examined using the MSVPA and
ECOPATH models: i) a 30% increase in the historical landings of human consumption
species for the 1987-1994 period; ii) a forward MSVPA run using these revised data
and with a 50% decrease in the industrial fisheries (i.e., Norway pout, herring and
sandeel) and based on the revised 1991 stomach survey data; and iii) another run with
50% decrease in the industrial fisheries, using 1977-1986 ecosystem structure as a
starting condition.
However, given the large uncertainty in estimating the potential reaction of
fleets to any large changes in regulations, forward projections of the impacts of
reducing the industrial fishery were not undertaken.
Results show that the introduction of the 30% over quota landings and
discards into the models have a far larger and more widespread effect on the transfer
efficiencies between trophic levels compared to a reduction in the industrial fisheries
under reported 1994 conditions. The removal of 50% of the industrial fishery has the
effect of reducing the transfer efficiency of the trophic level where these species exist,
suggesting that many of the gains would be dissipated rather than passed on up
through the ecosystem. However, any changes that have been observed in the
ecosystem over the past two decades appear to be attributed not only to shifts in
predator-prey interactions but also to the higher levels of extraction of top predators.
The overall conclusion of this analysis is that the high levels of over quota
landings and discards from the roundfish fishery will mitigate against the full effect of
J. Robertson et al.
two periods 1977-1986 and 1987-1994, to reflect changes in the observed trends in diet
and landings from the different fisheries and establish possible changes in food
availability in the two major periods.
Results indicate that: up to 60% of the fish component of the diet of human
consumption fish species consists of sandeels and Norway pout in approximately equal
amounts; and that there was a 30% decline in overall biomass consumption in the
second period 1987-1994, coincident with changes in the transfer efficiencies between
trophic levels and the loss of several trophic interactions. The observed changes were
largely associated with the dynamics of the top predators, suggesting that a trophic
trickle had appeared in the ecosystem, and that prey switching and shifts in the
suitabilities of the predatory fish species had occurred. One potential reason may be
that because sandeel and Norway pout are the two species which are consumed in the
greatest quantities, there may be direct competition between the industrial fishery and
the human consumption fishery.
The potential for recruitment and growth overfishing was also analysed in
terms of biomass thresholds; it was concluded that whilst the major human
consumption stocks in the North Sea do not show any significant signs of recruitment
failure, most of them are certainly at risk.
The interpretation of these results is compounded by the level of over quota
landings, by-catches and discards of human consumption species within the industrial
and human consumption fishery. Results from interviews with the industry indicate
that levels are as high as 60-70% in some areas and times of year, with an overall level
of not less that 30%. To fully assess the impacts of this and the industrial fisheries on
human consumption species three scenarios were examined using the MSVPA and
ECOPATH models: i) a 30% increase in the historical landings of human consumption
species for the 1987-1994 period; ii) a forward MSVPA run using these revised data
and with a 50% decrease in the industrial fisheries (i.e., Norway pout, herring and
sandeel) and based on the revised 1991 stomach survey data; and iii) another run with
50% decrease in the industrial fisheries, using 1977-1986 ecosystem structure as a
starting condition.
However, given the large uncertainty in estimating the potential reaction of
fleets to any large changes in regulations, forward projections of the impacts of
reducing the industrial fishery were not undertaken.
Results show that the introduction of the 30% over quota landings and
discards into the models have a far larger and more widespread effect on the transfer
efficiencies between trophic levels compared to a reduction in the industrial fisheries
under reported 1994 conditions. The removal of 50% of the industrial fishery has the
effect of reducing the transfer efficiency of the trophic level where these species exist,
suggesting that many of the gains would be dissipated rather than passed on up
through the ecosystem. However, any changes that have been observed in the
ecosystem over the past two decades appear to be attributed not only to shifts in
predator-prey interactions but also to the higher levels of extraction of top predators.
The overall conclusion of this analysis is that the high levels of over quota
landings and discards from the roundfish fishery will mitigate against the full effect of