Presentation on theme: "Issues in fisheries sustainability"— Presentation transcript:
1 Issues in fisheries sustainability What is a “fishery”?The global status of fisheries: will we soon be eating only jellyfish?What does “sustainability” mean, and what is the ecological basis for it?What does it mean to “manage” a fishery?
2 What is a “fishery”A linked dynamic relationship between a set of valued fish and a set of fishermen who pursue those fishFish stock(s)CatchFishing “fleet”Mortality
4 The global status of fisheries, revisited From Branch et al The trophic fingerprint of marine fisheries. Nature, doi: /nature09528
5 Many (25-30%) of the world’s fisheries have “collapsed” to catches less than 10% of historical peak From Mullon et al The dynamics of collapse in world fisheries. Fish and Fisheries 6:(an examination of 1500 catch time series)
6 Where are fisheries collapsing? IndiaNewfoundlandChinaNorth seaCalifornia currentIndonesiaAustraliaBenguela
7 What does “sustainability” mean? Lack of collapse?Capable of recovery after collapse, especially for collapses not caused by fishing?Harvested at near maximum sustainable yield?Harvested at near maximum sustainable harvest rate?
8 Sustainable fisheries depend on creation of “surplus production” Surplus production is biological production (growth) that can be translated either into catch or into population growth.On average, surplus production is zero in unharvested natural populationsHigh fishing mortality rate can result in sustainability, but at low biomass and catch
9 What causes surplus production to occur when fishing reduces stock size? “Compensatory” improvement in juvenile survival rates and/or growth ratesThese compensatory improvements result fromReduction in predator abundances (uncommon)Increase in food abundance (more common)Increase in available food abundance leading to better growth and/or reduced predation risk (very common)Reduction in juvenile mortality due to cannibalism (common)
10 What does it mean to “manage” a fishery? Protect the ecological basis for production (biophysical habitat, forage base)Control the quality (size, age) of fish harvestedRegulate the fishing mortality rate FInput control: control fishing activity, area swept by fishingOutput control: control the catch, given estimate of biomass (since F=catch/biomass)Seek balance in situations where fishing impacts multiple stocks so as to create tradeoffs
11 Most fisheries impact multiple stocks, create tradeoffs where not all stocks can be harvested at best ratesFishing may “target” particular stocks/species, but fishing activity typically causes catch of other speciesDiscarding non-target stocks is typically wasteful“collateral damage” reduces biological diversity and threatens ecological basis for sustainability
12 Fraser sockeye salmon have returned to near historical peak levels, but there has been a worrisome decline
13 Productive fisheries often depend on diverse mixtures of individual spawning stocks, most obvious with Pacific salmonFraser sockeye abundance by stockHilborn showed a similar pattern of shifting contributions for major Bristol Bay stocks
14 There is a severe tradeoff between harvesting and maintenance of stock structure (biodiversity) Is it wise or just for people who will not pay the bill to demand that fishers give up 50% of their income as an insurance policy for biodiversity?At the harvest rate expected to produce maximum average yield, about 50% of the (mostly small) stocks would be overharvested, and about 10% would be threatened with extinction. The tradeoff will be even worse if diverge in productivity continues