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J. Robertson et al.
obvious criterion to monitor and manage, perhaps the most fundamental issue in
fisheries management is the relationship between spawner abundance and recruitment.
The importance of this relationship has been further underscored by the phenomenon,
otherwise known as depensation, inverse density-dependence or the Allee effect, in
which reduced reproductive success coincides with low population size. Populations
with multiple equilibria may suddenly shift to a regime in which a reduction in fishing
mortality is insufficient to induce stock recovery.
To date, a lack of conclusive evidence has led some scientists to state that
recruitment overfishing is impossible. But, in two recent studies, Myers and
Cowcadens (Myers et al., 1994) (pers. comm. R. Myers, DFO, Newfoundland Canada)
have examined 200 populations and 129 species respectively, including the most
commercially important North Sea species, to see a) whether fish recruitment is related
to spawner abundance and b) whether depensatory effects are present. They concluded
that when there is a sufficient range in spawner abundance the answer to the question
is yes at all levels but that depensatory effects could only be detected in Icelandic
herring and Pacific salmon. In an overall sense then this should not be an issue in
current discussions about the North Sea.
From a practical point of view it is important that if fishing mortality
thresholds are used in management then they should hold true with observations on
spawner abundance and reproductive success. If as Mace (1994) has suggested poor
recruitment occurs at one half of the maximum of the stock recruitment (S-R) curve,
then the shape of the S-R curve is very important. Only in this way will it be possible
to tell whether the threshold estimate is sufficiently conservative without needlessly
restricting the harvest. The S-R curves for the key North Sea species were thus
examined following Myers et al. (1994) to determine the degree of risk associated with
current fishing levels. Where possible the threshold values used were MBAL otherwise
50% of maximal recruitment was used. The S-R data were divided into those above
and below the threshold, and then regressions performed on the log-transformed data.
The S-R plots for the key species are shown in Fig. 14 (for cod, haddock, saithe,
Norway pout, whiting, plaice and sole respectively). From the Myers approach cod,
haddock, plaice, whiting and sole all appear to be in a category where the threshold
currently set is reasonable; for saithe, where there is a negative relationship between
stock and recruitment, the threshold value is difficult to apply.
Calculating the ratio of recruitment above and below such thresholds is
important in the argument that reducing a stock below the estimated level will have an
impact on productivity. In the absence of extensive data, the threshold must be seen as
a precautionary approach. However, it does suggest that within the current
management regime, biomass thresholds to protect recruitment could be used to define
both the biological and economic risks associated with higher levels of fishing
(McGlade and Shepherd, 1992). In the current regime however, as most of the stocks
are above the threshold values, the argument has to be made that not only are there
biological losses accruing but also long-term economic ones as a result of the fishing
practices in place.
J. Robertson et al.
obvious criterion to monitor and manage, perhaps the most fundamental issue in
fisheries management is the relationship between spawner abundance and recruitment.
The importance of this relationship has been further underscored by the phenomenon,
otherwise known as depensation, inverse density-dependence or the Allee effect, in
which reduced reproductive success coincides with low population size. Populations
with multiple equilibria may suddenly shift to a regime in which a reduction in fishing
mortality is insufficient to induce stock recovery.
To date, a lack of conclusive evidence has led some scientists to state that
recruitment overfishing is impossible. But, in two recent studies, Myers and
Cowcadens (Myers et al., 1994) (pers. comm. R. Myers, DFO, Newfoundland Canada)
have examined 200 populations and 129 species respectively, including the most
commercially important North Sea species, to see a) whether fish recruitment is related
to spawner abundance and b) whether depensatory effects are present. They concluded
that when there is a sufficient range in spawner abundance the answer to the question
is yes at all levels but that depensatory effects could only be detected in Icelandic
herring and Pacific salmon. In an overall sense then this should not be an issue in
current discussions about the North Sea.
From a practical point of view it is important that if fishing mortality
thresholds are used in management then they should hold true with observations on
spawner abundance and reproductive success. If as Mace (1994) has suggested poor
recruitment occurs at one half of the maximum of the stock recruitment (S-R) curve,
then the shape of the S-R curve is very important. Only in this way will it be possible
to tell whether the threshold estimate is sufficiently conservative without needlessly
restricting the harvest. The S-R curves for the key North Sea species were thus
examined following Myers et al. (1994) to determine the degree of risk associated with
current fishing levels. Where possible the threshold values used were MBAL otherwise
50% of maximal recruitment was used. The S-R data were divided into those above
and below the threshold, and then regressions performed on the log-transformed data.
The S-R plots for the key species are shown in Fig. 14 (for cod, haddock, saithe,
Norway pout, whiting, plaice and sole respectively). From the Myers approach cod,
haddock, plaice, whiting and sole all appear to be in a category where the threshold
currently set is reasonable; for saithe, where there is a negative relationship between
stock and recruitment, the threshold value is difficult to apply.
Calculating the ratio of recruitment above and below such thresholds is
important in the argument that reducing a stock below the estimated level will have an
impact on productivity. In the absence of extensive data, the threshold must be seen as
a precautionary approach. However, it does suggest that within the current
management regime, biomass thresholds to protect recruitment could be used to define
both the biological and economic risks associated with higher levels of fishing
(McGlade and Shepherd, 1992). In the current regime however, as most of the stocks
are above the threshold values, the argument has to be made that not only are there
biological losses accruing but also long-term economic ones as a result of the fishing
practices in place.