Issues in fisheries sustainability

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Issues in fisheries sustainability

  1. 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. 2. What is a “fishery” • A linked dynamic relationship between a set of valued fish and a set of fishermen who pursue those fish Fish stock(s) Fishing “fleet” Catch Mortality
  3. 3. The global status of fisheries
  4. 4. The global status of fisheries, revisited From Branch et al. 2010. The trophic fingerprint of marine fisheries. Nature, doi:10.1038/nature09528
  5. 5. Many (25-30%) of the world’s fisheries have “collapsed” to catches less than 10% of historical peak Plateau 0 0.5 1 0246810 Years before collapse Relativecatch 20 Smooth 0 0.5 1 0246810 Years before collapse Relativecatch 31 12 16 38 Erratic 0 0.5 1 0246810 Years before collapse Relativecatch 5 17 8 16From Mullon et al. 2005. The dynamics of collapse in world fisheries. Fish and Fisheries 6: 111-120. (an examination of 1500 catch time series)
  6. 6. Where are fisheries collapsing? Newfoundland North sea California current India China Indonesia Australia Benguela
  7. 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. 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 populations • High fishing mortality rate can result in sustainability, but at low biomass and catch 0 5000 10000 15000 20000 25000 30000 0 0.2 0.4 0.6 0.8 1 Fishing mortality rate F Biomass Biomass Production Catch x 10 Food consumption
  9. 9. What causes surplus production to occur when fishing reduces stock size? • “Compensatory” improvement in juvenile survival rates and/or growth rates • These compensatory improvements result from – Reduction 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. 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 harvested • Regulate the fishing mortality rate F – Input control: control fishing activity, area swept by fishing – Output 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. 11. Most fisheries impact multiple stocks, create tradeoffs where not all stocks can be harvested at best rates • Fishing may “target” particular stocks/species, but fishing activity typically causes catch of other species • Discarding non-target stocks is typically wasteful • “collateral damage” reduces biological diversity and threatens ecological basis for sustainability
  12. 12. Fraser sockeye salmon have returned to near historical peak levels, but there has been a worrisome decline Total Fraser River sockeye run size, Pacific Salmon Commission estimates 0 10,000,000 20,000,000 30,000,000 40,000,000 50,000,000 1890 1910 1930 1950 1970 1990 2010 Totalrunsize Total Run Spawners
  13. 13. Productive fisheries often depend on diverse mixtures of individual spawning stocks, most obvious with Pacific salmon 0 2,000,000 4,000,000 6,000,000 8,000,000 10,000,000 12,000,000 1952 1956 1960 1964 1968 1972 1976 1980 1984 1988 1992 1996 2000 2004 2008 latemisc harrison cultus portage weaver early shuswap adams birkenhead stellako late stuart quesnel chilko upi gates nadina estuart seymour scotch raft fennell bowron Hilborn showed a similar pattern of shifting contributions for major Bristol Bay stocks Fraser sockeye abundance by stock
  14. 14. There is a severe tradeoff between harvesting and maintenance of stock structure (biodiversity) 0 5000000 10000000 15000000 20000000 25000000 0 0.2 0.4 0.6 0.8 1 Overall exploitation rate Catch,Escapement 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Proportionofstocksoverfishedorextinct Total Harvest Total Escapement Pextinct Poverfished Tradeoff between catch and stock "health" Fraser River sockeye 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 2000000 4000000 6000000 8000000 10000000 12000000 14000000 16000000 Average long term catch Proportionofstocksnotoverfished 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 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?

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