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Fisheries in the Seas

Fisheries in the Seas. Fish life cycles: Egg/sperm pelagic larvae juvenile (first non-feeding – critical period – then feeding). Indeterminant growth Growth rates vary Age determination – otoliths, cohorts, --- find very large variations in size of year classes

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Fisheries in the Seas

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  1. Fisheries in the Seas Fish life cycles: Egg/sperm pelagic larvae juvenile (first non-feeding – critical period – then feeding)

  2. Indeterminant growth Growth rates vary Age determination – otoliths, cohorts, --- find very large variations in size of year classes “young-of-year” YOY

  3. Migratory Circuit “larval drift”

  4. Reproductive Isolation • Location of spawning • Timing of spawning

  5. What controls population size? Possibilities: • No. eggs/spawning success • Mortality in the young stages (egg, larvae, juvenile) • Mortality among adults (food limitation, competition)

  6. Mortality in young stages • Critical period – what determines if larvae find food or not? • Survival of feeding larvae • Juvenile survival Successful recruitment – many stocks seem to be maintained by sporadic strong year classes

  7. Most marine fish populations are maintained by irregular, strong year classes. What does this mean for management?

  8. Fisheries Management • Oceans provide ~20% of the animal protein consumed by humans worldwide (FAO 1993) • Over half of the world’s fish stocks are fully exploited, at least 25 - 35% are overexploited

  9. “Fishing down food webs” • Globally, fisheries first target higher-order predators • As these decline, move to species in the next trophic level down, where abundances have increased due to release from predation • Today, only 10% of all large fish populations present in 1950, including cod, tuna, swordfish, grouper, marlin, halibut, and flounder, remain (Myers and Worm 2003)

  10. Why has marine fisheries management failed? • Must be based on a good understanding of the population biology of the fish • Sampling problem – independent sample – use harvest data (landings) CPUE – Catch per unit effort • Variations in successful year classes

  11. Why has marine fisheries management failed? 2. Harvest methods have become much more efficient • Early fisheries – hook and line (until 1920s) – trawling – took off in 1930s – gill nets, purse seines, long lines • Refrigeration – large factory ships

  12. Initial Response? Exclusive economic zone – 200 mi limit

  13. Initial Response? Exclusive economic zone – 200 mi limit Underlying cause of the problem – the way we manage – Fisheries Councils that balance economics with catches, but at mismatched time scales; base catch limits on MSY

  14. Problems with MSY model • MSY model assumes spatial and temporal uniformity of the population • Temporally – know not true – year class phenomenon • Spatially – suspect that there are favorable and less favorable sites – source and sink populations • Fish populations change rapidly • Are there warning signs? Change in size distribution – smaller average size

  15. Changes in size

  16. Problems with MSY model • MSY model assumes spatial and temporal uniformity of the population • Temporally – know not true – year class phenomenon • Spatially – suspect that there are favorable and less favorable sites – source and sink populations • Fish populations change rapidly • Are there warning signs? Change in size distribution – smaller average size • Ignores interspecies interactions – predator/prey dynamics, competition

  17. Problem of By-catch – non-target organisms also caught • Shrimp trawl fishery – in south Atlantic and Gulf of Mexico, 90% of what is caught is not shrimp • Bottom trawling – barn door skate in coastal New England • Purse seine fishery for yellow fin tuna – high dolphin mortality • Long-lines – tangle diving birds, marine mammals, turtles

  18. Impacts of removing by-catch • Juvenile fishes never grow up (redfish in Gulf of Mexico) • Removing “baitfish,” invertebrate prey for other species • Food subsidy for aggressive bird predators – gulls and other nuisances; blue crabs and sharks can sometimes benefit

  19. Habitat Destruction by Bottom Trawling • Tears up benthic habitats and species • Has been compared to clear-cutting the forest

  20. Potential Solutions • Ecosystem management – looking at fish as part of larger ecosystem; ecologically sustainable yield • Food web models • Coupled physical and biological models • Managing species in complexes rather than individually

  21. Potential Solutions • Marine reserves? • Habitat fragmentation in the sea • How to place them, police them • Precautionary principle, burden of proof

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