Precautionary approach
In order to understand how exploitation of a fish stock
(that is, fishing mortality) can be managed sustainably, it is
useful to explain the ways in which catches and stock abundance
respond to different levels of fishing.
The figure below shows how catches from an unfished stock would
increase in line with exploitation, up to a point where the total
mortality on the stock causes so many fish to be caught at a
relatively small size (and discarded or landed) that the potential
of the stock to increase through growth is not realised. This
"growth overfishing" is common to most marine fish stocks
today.
However, providing sufficient fish survive to become adults and
spawn, they may still have the reproductive capacity to replace
themselves. Stock collapse can only occur when fishing mortality
reaches a level (Flim) such that removals from a stock are so high,
and its spawning capacity is so diminished, that fewer and fewer
juveniles are produced.
The impact of "recruit overfishing" is illustrated by the
history of the herring stock in the North Sea, which collapsed in
the 1970s. The figure below shows how the abundance of juvenile
herring has changed in relation to the spawning stock biomass
(weight of mature fish each year), such that at spawning-stock
levels below approximately 800 thousand tonnes (Blim), recruitment
is reduced. So, not only is the size of the stock being reduced by
too high a level of exploitation, but there are fewer juvenile fish
to replace those that are caught, and stock levels are likely to
fall even lower.
To avoid such stock collapses, fishing mortality needs to be
kept at levels which will ensure that stocks are sustained and
remain productive (i.e. well below Flim).
The precautionary approach, however, requires fisheries managers
to take account of uncertainties in managing stocks. This is done
by setting reference points, levels of fishing mortality or
spawning stock, at which action should be taken to avert potential
stock collapses due to overfishing.
The green zone in the figure above represents the situation in
which an exploited stock is within safe biological limits. That is,
the spawning biomass is above the biomass reference point (Bpa)
which is judged to give a reasonable certainty that, in spite of
year-to-year fluctuations, the stock will stay above Blim. The
other boundary of the "safe" zone is the level of fishing mortality
(Fpa) which is sufficiently below Flim that there is a low
probability of stock collapse.
A stock that lies within the amber zone, either because its
spawning biomass is approaching Blim too closely or it is being
exploited at a level above Fpa, or both, is considered to be
outside safe biological limits according to the precautionary
approach. In such cases, managers are warned not to allow levels of
exploitation that are likely to push the stock into the red zone,
but are encouraged to reduce fishing mortality so that more fish
survive and the stock returns to the safe (green) zone.
Scientific advice on the status of fish stocks in the northern
Atlantic is now being given on the basis of these fishery reference
points. They are consistent between stocks, and enable fishermen,
managers, consumers and environmentalists to judge whether these
renewable resources are being managed sustainably.