TACtful or TACtless:
Conventional Methods of Control*
In
order to maintain high, yet sustainable, yields, conventional fisheries
management tries to control fishing mortality rates by regulating catch
based on physical attributes of the fish or restricting fishing on a temporal
or spatial basis. These approaches are either effort-based, where the
amount or efficiency of fishing effort is limited, or quota-based, where
the total amount of fish harvested is regulated through quotas and allocations.
Effort-based fisheries management ranges from restrictions on gear, vessels,
and number of fishers to temporal and spatial closures related to fish
behavior. Gear and vessel restrictions target the design and efficiency
of fishing instrumentation. The type and amount of fishing gear may be
modified, as well as its dimensions regarding hook spacing on longlines,
mesh size of fish nets, or fish trap structure. Design, length, or engine
horsepower may act as further restrictions on fishing vessels. Human efficiency
is also restricted through license allocations to fishers on a fisher
or vessel basis.
Temporal and spatial closures help to protect fish stocks when they are particularly vulnerable to fishing, such as reproductive periods when fish aggregate on spawning grounds. Temporal closures also help to raise the value of the fish if, for example, harvesting is only allowed during certain parts of the year when the fish are larger and are in better conditioned. Spatial closures may range from small-scale to large, and may or may not be combined with a temporal closure, which may range from short-term to long. These combined closures allow fish stocks the time and area to rebuild after a period of overharvesting. Rotating closures can provide an even broader range of both habitat and fish stock renewal. Permanent spatial closures are becoming a very popular means of protecting fish stocks and portions of their critical habitat and related biological communities. These protected areas may be designated through top-down restrictions, such as the United States' National Marine Sanctuaries system, or through bottom-up establishment as marine protected areas (MPAs).
Quota-based
fisheries management, commonly used to regulate the amount of fish caught,
relies on accurate estimates of exploitable stock sizes and trends in
stock abundance over time. These estimates help formulate the Total Allowable
Catch (TAC) for each industry. TAC, as well as all other management tools,
is further limited by a seasonal estimate of how much an industry may
extract so as to maintain a stock size that allows maximum sustainable
yield (MSY) for following harvests.
MSY is a fixed exploitation rate that limits how much each fisheries
may extract in a season based on assumptions made on adult stock size
and average stock productivity. Once this exploitation rate has been reached
for each fisheries, that particular industry is closed down until the
following season. However, fish stocks do not maintain fixed populations
and thus a fixed exploitation rate does not necessitate sustainable harvests.
Considerable variability in annual reproductive success and recruitment
may result in prosperous harvests when fishing at MSY levels on favorable
years, but may prove damaging to stocks when harvesting during years of
poor productivity and larval settlement.
In general, a mixture of all these methods are used to help best sustain
fishery stocks. For example, both temporal and spatial restrictions may
be used to regulate harvests, with gear, catch, and fish size restrictions
applying during the harvesting season. Unfortunately, these methods have
only mildly helped to sustain fishery stocks. For, conventional fisheries
management is based on three simplifying assumptions:
1) Only single-species stocks are targeted by fishing fleets,
2) Targeted stocks are temporally and spatially isolated from other stocks,
3) Effects of fishing are spread over the entire stock since individuals are perfectly mixed.
Although these assumptions have helped define some form of fisheries management, they do not incorporate enough practical information to properly protect fish stocks from overharvesting, small and immature fish from capture, bycatch, and environmental degradation.
*Commission on Geosciences,
Environment and Resources. 2001. Marine
Protected Areas: Tools for Sustaining Ocean Ecosystem. http://www.fao.org/DOCREP/003/X8002E/X8002E00
retrieved 4/8/02.