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Download Maximum sustainable yield in fisheries
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Review Logistic growth curve. dN N r0 N (1 ) dt K 1631 was the first notion that fisheries could be exhausted (King Charles I). However, 200 yrs later scientists were still stating that fisheries show to be inexhaustible. Main part of MSY, that excess fish are produced So, where is there evidence that these populations are exhaustive?: the existence of Management Most fisheries management, at least in the western world, focuses on Q2: World is limited, so how many can we take? Other areas, such as Pacific islands focus on other management venues such as area limitations of where to fish (MPAs) MSY: the largest catch that can be taken from a species’ stock over an infinite period. MSY is WIDELY used in fisheries management. And we have seen some success but also many issues and failures. With the main concept of taken the maximum catch possible What are some potential concerns with MSY? - Taking the maximum is risky, with no room for marginal error Severe declines in marine megafauna (by catch) Overharvesting (shark fact for this, 11,000 per hour or 200 mil yr) THIS IS A PROBLEM. Many communities are dependent on fish in their diet. Ecosystem/ Food Web effects Assumptions Fish population grows according to logistic model (and is restrained by K) - density dependent population growth If catch rate is less than fish population growth rate, the fishery is sustainable Density-dependence is merely that population growth rate depends on the density of that population Why is this concept so accepted? - Easy concept: remove potential competitors of survivors which should help the population - Related other terrestrial examples (such as pine tree) and assuming that works in the oceans But WHERE DO YOU THINK this “excess production” of a fishery goes? -competition, predation, abiotic factors Why do we use it? You have AT LEAST some estimate to set “bag” limits and quite frankly, MSY has worked better than expected “it’s better than nothing” Other tibdits to cover here: Fishery isn’t considered depleted until we fish population down to 1/3 of its original estimate (and on the other hand once it reaches 1/3 we consider its recovered MSY is working reasonably well in the US, Iceland, New Zealand, and Australia, but not without issues or continuous reform of management. Why is that? – proper resources allocated to research as well as enforcement. Not possible everywhere (e.g., Bahamas) Must (and isn’t often) considered: Food web interactions, life history characteristics, Allee effect some populations present a form of positive density-dependenceknown as the Allee Effect Allee Effect: positive correlation between population size or density and the mean individual fitness) of a population or species Cod example: Key point: fishermen harvested the amount recommended by fisheries biologists, and the fishery still crashed. Take Home Messages • Because of differences in incentives for people at individual and community levels, common resources like fisheries are overexploited • The maximum sustainable yield is below the maximum fish population density • Because of environmental variability, the optimal harvest is less than the maximum productivity • It is important to consider food web ecology, and characteristics of populations and species of interest.