Pj/Bj is thc production/biomass ratio (equal to instantaneous rate of total
mortalitv Z in stcady-statc svstcms) of (i);
is thc biomass of predator j ;
Q j /B j is the consumption/biomass ratio of prcdator j;
DC ji is the fraction of prey (i) by weight in the average diet of predator j;
EE j is thc ecotrophic efficiency of (i), cxpressing the fraction of total
production consumed by predators or caught b\
r
a fishery and usually
assumed to range from 0.7 to 0.9;
EXj is cxport of (i): sum of fishery catches plus emigration to adjacent
ecosystems.
Estimations of biomass, mortality, consumption and ecotrophic efficicncy of each trophic group by different methods are used in the
model. To homogenize the information, all the input data are those of
the year 1994. Thc available data from ICES Assessment Working
Groups and ICCAT for apex pelagic (tuna, sharks) havc been used to estimate thc biomass and mortality (P/B) of specics of commercial interest: hake, bluc whiting, horse mackerel, mackerel, monkfish, anchovy 7 ,
pilchard, megrims and tuna. Biomass of some groups have becn estimated from bottom trawl sunevs carried out in the area during the
same year and applying thc swept arca method (Sânchez et al., 1995).
The groups arc paramcterizcd top-down so that the flows at thc lower
levels arc calculated to match the food demands of the upper levels. A
classiflcation of specics according to thcir preys was realized previously.
On this basis, and to construct the mass-balance model, we havc sclected 25 trophic groups, 14 ot which are fish, 6 invertebrates, 3 arc
groups of small plankton, and thc rcmaining a detritus group. In each
group, we havc considcred spccics of similar size, habitat, diets, consumption ratcs, mortality and production ratcs. The main commercial
specics have bccn considered scparateh , sincc a better information
about thcir different input paramctcrs îs available. All the available data
on biomass, landings and discards have bccn convcrtcd into thc same
unit (tons/km
2 ) and are expressed in wet weight.
The links between groups are their feeding habits. The information
needed to set the diet matrix was taken from different sources. Quantitative analyses (tf 10 000 stomach contcnts of 36 species of fish in thc
study area have been carried out in 1994 (Velasco et al, 1996). Dvc species
selected constitutc a significant perccntage (90%) of the demersal fish
biomass. An intensivc literature research was carried out to find the diet
composition of the other fishcs and invertebrates.
The fisheries landings statistical data were provided by the ICKS Asscssment Working Cîroups and by the Fishery Data Base Projcct of IEO. We
havc summarised and combined thc data by trophic group. To adjust
the model for pelagic fishcrics, we have only taken into account thc
percentage of catchcs on the shclf and closc oceanic waters (neritic area).
Discards (20% of total catchcs in the Cantabrian Sea) arc consumed
189
mortalitv Z in stcady-statc svstcms) of (i);
is thc biomass of predator j ;
Q j /B j is the consumption/biomass ratio of prcdator j;
DC ji is the fraction of prey (i) by weight in the average diet of predator j;
EE j is thc ecotrophic efficiency of (i), cxpressing the fraction of total
production consumed by predators or caught b\
r
a fishery and usually
assumed to range from 0.7 to 0.9;
EXj is cxport of (i): sum of fishery catches plus emigration to adjacent
ecosystems.
Estimations of biomass, mortality, consumption and ecotrophic efficicncy of each trophic group by different methods are used in the
model. To homogenize the information, all the input data are those of
the year 1994. Thc available data from ICES Assessment Working
Groups and ICCAT for apex pelagic (tuna, sharks) havc been used to estimate thc biomass and mortality (P/B) of specics of commercial interest: hake, bluc whiting, horse mackerel, mackerel, monkfish, anchovy 7 ,
pilchard, megrims and tuna. Biomass of some groups have becn estimated from bottom trawl sunevs carried out in the area during the
same year and applying thc swept arca method (Sânchez et al., 1995).
The groups arc paramcterizcd top-down so that the flows at thc lower
levels arc calculated to match the food demands of the upper levels. A
classiflcation of specics according to thcir preys was realized previously.
On this basis, and to construct the mass-balance model, we havc sclected 25 trophic groups, 14 ot which are fish, 6 invertebrates, 3 arc
groups of small plankton, and thc rcmaining a detritus group. In each
group, we havc considcred spccics of similar size, habitat, diets, consumption ratcs, mortality and production ratcs. The main commercial
specics have bccn considered scparateh , sincc a better information
about thcir different input paramctcrs îs available. All the available data
on biomass, landings and discards have bccn convcrtcd into thc same
unit (tons/km
2 ) and are expressed in wet weight.
The links between groups are their feeding habits. The information
needed to set the diet matrix was taken from different sources. Quantitative analyses (tf 10 000 stomach contcnts of 36 species of fish in thc
study area have been carried out in 1994 (Velasco et al, 1996). Dvc species
selected constitutc a significant perccntage (90%) of the demersal fish
biomass. An intensivc literature research was carried out to find the diet
composition of the other fishcs and invertebrates.
The fisheries landings statistical data were provided by the ICKS Asscssment Working Cîroups and by the Fishery Data Base Projcct of IEO. We
havc summarised and combined thc data by trophic group. To adjust
the model for pelagic fishcrics, we have only taken into account thc
percentage of catchcs on the shclf and closc oceanic waters (neritic area).
Discards (20% of total catchcs in the Cantabrian Sea) arc consumed
189