319
Evaluation of Ecological Effects of the North Sea
weight relationships where needed from Coull et al. (1989). Consumption by quarter
between the key species is again shown in network diagrams (see Figs. 13a, b, c and
d).
The annual average biomass consumed by individual predators within the
1977-86 and 1987-94 periods is given in Table 4.
Ecopath Model
TRANSFER EFFICIENCIES:
Using an ECOPATH model (see Christensen and Pauly, 1992), the structure and
functioning of the North Sea ecosystem for the periods 1977-1986 and 1987-1994 were
compared. The ECOPATH model used in this analysis differs from that given in ICES
CM 1992/Assess:16 in that only the key species plus detritus and primary productivity
were defined rather than the 22 groups plus detritus of the earlier model. The category
“others” was primarily based on flatfish. Estimates of detritus and primary productivity
for the whole period were taken from Jones (1984), Franz and Gieskes (1994), Hannon
and Joiris (1987), ICES CM 1992/Assess:16 and the North Sea Study survey data
stored at the British Oceanographic Data Centre, NERC-Proudman Oceanographic
Laboratory. Input data were derived from MSVPA runs using the 1981 and 1991
stomach data for the two periods respectively; information about diet compositions for
the key species was taken from Daan (1989), Daan et al. (1990) and Greenstreet
(1996); consumption rates were derived as for Table 4. The values were then used to
derive an annual set of statistics for each period. The key results are given in Table 5.
Eight trophic levels were identified in the analysis. The allocation of species
to trophic level and relative flows by trophic level were: I phytoplankton and detritus;
II other species; III Norway pout, herring, sprat, sandeels, plaice and sole; IV cod
(52%), haddock, whiting (90%), saithe (78%); V cod (46%), haddock (2%), whiting
(9%) and saithe (21%); VI virtually no human consumption species at this level; VII
cod (1%), whiting (1%) and saithe (1%) and VIII cod (1%).
The transfer efficiencies of the North Sea altered significantly between the two
periods; in particular there was a large decline in efficiencies in trophic levels IV-VIII
in the latter period, together with a loss of several trophic interactions. It would appear
that these changes have been largely brought about by alterations in the top predators
and the numbers of species consumed by them. This would suggest that a trophic
“trickle” has appeared in the ecosystem, in which prey switching has diminished, and
the suitabilities of predatory species altered.
POTENTIAL FOR RECOVERY OF OVEREXPLOITED FISH STOCKS:
ACFM has adopted two precautionary biological criteria to ensure the sustainability of
stocks; one relates to growth, the other to reproduction. Whilst per capita growth is an
Evaluation of Ecological Effects of the North Sea
weight relationships where needed from Coull et al. (1989). Consumption by quarter
between the key species is again shown in network diagrams (see Figs. 13a, b, c and
d).
The annual average biomass consumed by individual predators within the
1977-86 and 1987-94 periods is given in Table 4.
Ecopath Model
TRANSFER EFFICIENCIES:
Using an ECOPATH model (see Christensen and Pauly, 1992), the structure and
functioning of the North Sea ecosystem for the periods 1977-1986 and 1987-1994 were
compared. The ECOPATH model used in this analysis differs from that given in ICES
CM 1992/Assess:16 in that only the key species plus detritus and primary productivity
were defined rather than the 22 groups plus detritus of the earlier model. The category
“others” was primarily based on flatfish. Estimates of detritus and primary productivity
for the whole period were taken from Jones (1984), Franz and Gieskes (1994), Hannon
and Joiris (1987), ICES CM 1992/Assess:16 and the North Sea Study survey data
stored at the British Oceanographic Data Centre, NERC-Proudman Oceanographic
Laboratory. Input data were derived from MSVPA runs using the 1981 and 1991
stomach data for the two periods respectively; information about diet compositions for
the key species was taken from Daan (1989), Daan et al. (1990) and Greenstreet
(1996); consumption rates were derived as for Table 4. The values were then used to
derive an annual set of statistics for each period. The key results are given in Table 5.
Eight trophic levels were identified in the analysis. The allocation of species
to trophic level and relative flows by trophic level were: I phytoplankton and detritus;
II other species; III Norway pout, herring, sprat, sandeels, plaice and sole; IV cod
(52%), haddock, whiting (90%), saithe (78%); V cod (46%), haddock (2%), whiting
(9%) and saithe (21%); VI virtually no human consumption species at this level; VII
cod (1%), whiting (1%) and saithe (1%) and VIII cod (1%).
The transfer efficiencies of the North Sea altered significantly between the two
periods; in particular there was a large decline in efficiencies in trophic levels IV-VIII
in the latter period, together with a loss of several trophic interactions. It would appear
that these changes have been largely brought about by alterations in the top predators
and the numbers of species consumed by them. This would suggest that a trophic
“trickle” has appeared in the ecosystem, in which prey switching has diminished, and
the suitabilities of predatory species altered.
POTENTIAL FOR RECOVERY OF OVEREXPLOITED FISH STOCKS:
ACFM has adopted two precautionary biological criteria to ensure the sustainability of
stocks; one relates to growth, the other to reproduction. Whilst per capita growth is an