Minimum and maximum size limit / Size window / Lopt
The goals of ecosystem-based fisheries management can be achieved using management. Fishing has less impact on stocks if fish are caught after they have reached maturity and reproduced at least once. It is therefore certain management practices, also historically, include a minimum size limit for allowed catch. This is comparatively easy to implement, setting rules for minimum allowed mesh sizes for nets etc. However, large females are considered to be most valuable for the recruitment of young-of-the-year, as large females both have large numbers of eggs and also slightly larger eggs, giving the fry a good start of their first season. To preserve large females in the population, a maximum size limit can be used as a management practice. A maximum size limit may be even more profitable for a sustainable fishing yield than a minimum size limit. These two size-based practices combined result in a size window for allowed catch. One example is the presently used allowed size window 40-75 cm for catching pike along the Swedish east coast.
The growth of individual fish is more or less variable, depending on the environment, food availability and genetic traits. Even though fish have become sexually mature they are able to continue to grow. For fish individuals, there is a trade-off between transforming energy into somatic (body) growth and producing gonads for reproduction. Individuals growing fast in the first period of life will usually mature later, while individuals growing slower will reach maturity earlier. There may a drawback in having a minimum size limit, because this can eventually induce changes in the distribution of growth patterns within the population, as has been shown to have taken place in some marine fish stocks. Fast-growing individuals will risk being fished as soon as they reach the minimum size limit, usually before they have reproduced. The result of using a minimum size limit may then after some generations result in a larger proportion of slow-growing individuals in the population, maturing early and remaining longer in the system, and therefore allowed to reproduce once or several times before being caught in the fishery.
One cohort (year-class) of a fish stock will increase in biomass as long as individual fish grow. There will also be some loss in the cohort biomass, due to mortality, either fishing mortality or natural mortality. If a maximum yield in terms of biomass is aimed for, it is optimal to start fishing at the size (Lopt) where the growth rate and biomass of the cohort are at maximum. Using such a management practice would result in a larger proportion of the stock biomass consisting of individuals of medium or large sizes, compared to starting fishing at a smaller individual size. This would allow both juveniles and adults to better fulfill their ecological roles, which in turn would benefit an ecosystem-based fisheries management. Minimum size limits are today usually set well below Lopt.
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Mullon, C., Field, J.G., Thébaud, O., Cury, P., Chaboud, C. 2011. Keeping the big fish: Economic and ecological tradeoffs in size-based fisheries management. Journal of Bioeconomics (22 November 2011), pp. 1–19.
Ulrika Beier, Off duty, Institute of freshwater research