Fish Ecology and Fisheries - Freshwater Ecology: Concepts and Environmental Applications - page 443

managed for a high yield of usable fish. The uncertainties in predicting re-
cruitment and growth make absolute prediction of the effects of exploita-
tion difficult. This uncertainty has led to numerous cases of overexploita-
tion of fish (Hilborn, 1996). Thus, assessments of major fisheries will
increasingly include risk analyses. If risk assessment is used, a manager will
set limits based in part on the potential risk of long-term harm to produc-
tion of the fishery.
One of the most common regulations in sports fisheries is creel limits,
or the numbers of fish that can be taken per day. Such limits rarely are ef-
fective in managing fish populations because most anglers do not catch
their limit. However, reaching the limit does provide a degree of satisfac-
tion for the angler.
Setting size limits is another method that is used to ensure that fish
reach a size that is acceptable to anglers. Generally, these size limits are set
so that several year classes of reproductive fish are less than the catchable
size. Thus, in a poor year for recruitment or reproductive failure in a spe-
cific age class, the fishery will still have some recruits entering the catch-
able size range. Given a single length limit and heavy fishing pressure, the
size distribution can consist of many fish just below the allowable size. This
can lead to intraspecific competition and low recruitment to fish of allow-
able size (Noble and Jones, 1999).
Slot limits that do not allow harvest of fishes of intermediate size are
also used. In this case, surplus young fish can be taken. If the small fish are
taken, intraspecific competition and recruitment into intermediate size
lengths are lower, and growth rates of these fish are high. This creates high
production of large fish that are desirable for anglers.
Seasonal limits are often imposed. The most common of these is not
allowing any taking of fishes while they are spawning. Fish can be partic-
ularly vulnerable to angling pressure at this point in their life cycle. Fish
that aggregate to spawn in highly predictable parts of the environment are
most susceptible.
STOCKING FISH FOR FISHERIES
Many fisheries need additions of human-produced stock to allow ex-
ploitation to occur at desirable rates (Heidinger, 1999). Stocking is done for
many reasons, including (i) introduction of new exploitable species,
(ii) introduction of new prey species, (iii) introduction of biological control
agents, (iv) provision of fish to be caught immediately or after they grow,
(v) satisfying public pressure to enhance fisheries, (vi) reestablishing species
where they have been extirpated, and (vii) manipulating the size or age class
structure of existing fish populations (Heidinger, 1999). These programs are
expensive. For example, approximately 2.5 billion sports fish are stocked
each year in United States and Canada. (Heidinger, 1999). Unfortunately,
cost-benefit analyses on stocked fish are rare.
The stocking of fish can be a useful management tool. In cases in which
reservoirs and ponds are constructed, native stream and river fishes may not
survive. Thus, there is little reason not to introduce sport fish (assuming
they will not move into rivers and streams and cause problems there).
Stocking may also be crucial to the recovery of some endangered fishes.