Evaluation of Ecological Effects of the North Sea
303
• a ban on processing operations which may be used to control the levels of
apparent by-catch on board the vessel (Article l0a of R[EEC] 3094/86
• a prohibition on fishing for Norway pout with small mesh nets in a particular
area of the North Sea called the “Norway pout box” (Annexe 1 of R[EEC] 3094/86
• Total Allowable Catches for the Norway pout, sprat, blue whiting and horse
mackerel in Area IV.
Apart from the EU regulations each EU member country may have separate and
additional measures to control the fishery. For example, Denmark in 1996
implemented a loss of fishing licence scheme for over by-catch limits within the sprat
fishery. Some other measures are in force but are not listed here.
Feeding Interactions and Stomach Content Data
In describing the various interactions which will ultimately have an effect on the
production and yield of fish from the North Sea, it is important to look at variability in
feeding patterns. One method is to obtain stomach samples from as wide a range of
areas, and time as possible, to see if any general patterns emerge which can be used to
establish and/or validate different models. These data can be augmented with
information from feeding studies.
To date, two "Year of the Stomach" surveys have been undertaken, one in
1981 and 1991; additional sampling has also been undertaken on research surveys in
various quarters of the year in 1980, 1982, 1985 and 1987. In this analysis the results
from both surveys have been used in the calculations of total biomass consumed by
predators; the 1981 data were used for the period 1983-1985 and for the 1977-1986
period and the 1991 data, as revised by the 1995 Multispecies Assessment Working
Group (ICES CM 1996/ Assess:3) for the period 1987-1994. Reference was also made
to the studies on cod feeding (Daan, 1973), whiting (Hislop et al., 1983), and mackerel
(Mehl and Westergard, 1983).
In the MSVPA model, the food composition of a particular predator age group
is predicted from the relative abundance of the various prey age groups weighted by
their relative suitability as prey. A basic assumption is that suitabilities are constant for
each predator age prey combination over time. The suitabilities or preferences are
proportional to the probability of encounter between the prey and predator multiplied
by the probability of the predator eating the prey once encountered.
For suitabilities to remain constant, the product of these two probabilities
needs to stay the same. Over the past two decades there have been large changes both
in absolute abundance in species such as herring and sandeel, as well as their spatial
occurrence. For example, 28% of the total sandeel biomass was distributed in the
northern area in 1981, while in 1985, 86 and 87 it went from 15% to 20% to 35%
respectively. Thus there are very likely to have been changes in suitabilities due to
alterations in spatial overlap.
303
• a ban on processing operations which may be used to control the levels of
apparent by-catch on board the vessel (Article l0a of R[EEC] 3094/86
• a prohibition on fishing for Norway pout with small mesh nets in a particular
area of the North Sea called the “Norway pout box” (Annexe 1 of R[EEC] 3094/86
• Total Allowable Catches for the Norway pout, sprat, blue whiting and horse
mackerel in Area IV.
Apart from the EU regulations each EU member country may have separate and
additional measures to control the fishery. For example, Denmark in 1996
implemented a loss of fishing licence scheme for over by-catch limits within the sprat
fishery. Some other measures are in force but are not listed here.
Feeding Interactions and Stomach Content Data
In describing the various interactions which will ultimately have an effect on the
production and yield of fish from the North Sea, it is important to look at variability in
feeding patterns. One method is to obtain stomach samples from as wide a range of
areas, and time as possible, to see if any general patterns emerge which can be used to
establish and/or validate different models. These data can be augmented with
information from feeding studies.
To date, two "Year of the Stomach" surveys have been undertaken, one in
1981 and 1991; additional sampling has also been undertaken on research surveys in
various quarters of the year in 1980, 1982, 1985 and 1987. In this analysis the results
from both surveys have been used in the calculations of total biomass consumed by
predators; the 1981 data were used for the period 1983-1985 and for the 1977-1986
period and the 1991 data, as revised by the 1995 Multispecies Assessment Working
Group (ICES CM 1996/ Assess:3) for the period 1987-1994. Reference was also made
to the studies on cod feeding (Daan, 1973), whiting (Hislop et al., 1983), and mackerel
(Mehl and Westergard, 1983).
In the MSVPA model, the food composition of a particular predator age group
is predicted from the relative abundance of the various prey age groups weighted by
their relative suitability as prey. A basic assumption is that suitabilities are constant for
each predator age prey combination over time. The suitabilities or preferences are
proportional to the probability of encounter between the prey and predator multiplied
by the probability of the predator eating the prey once encountered.
For suitabilities to remain constant, the product of these two probabilities
needs to stay the same. Over the past two decades there have been large changes both
in absolute abundance in species such as herring and sandeel, as well as their spatial
occurrence. For example, 28% of the total sandeel biomass was distributed in the
northern area in 1981, while in 1985, 86 and 87 it went from 15% to 20% to 35%
respectively. Thus there are very likely to have been changes in suitabilities due to
alterations in spatial overlap.