Fisheries in the seas
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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

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

“young-of-year” YOY


Migratory Circuit

“larval drift”


Reproductive isolation
Reproductive Isolation

  • Location of spawning

  • Timing of spawning


What controls population size
What controls population size?

Possibilities:

  • No. eggs/spawning success

  • Mortality in the young stages (egg, larvae, juvenile)

  • Mortality among adults (food limitation, competition)


Mortality in young stages
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


Most marine fish populations are maintained by irregular, strong year classes.

What does this

mean for

management?


Fisheries management
Fisheries Management strong year classes.

  • 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


Fishing down food webs
“Fishing down food webs” strong year classes.

  • 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)


Why has marine fisheries management failed
Why has marine fisheries management failed? strong year classes.

  • 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


Why has marine fisheries management failed1
Why has marine fisheries management failed? strong year classes.

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


Initial response
Initial Response? strong year classes.

Exclusive economic zone – 200 mi limit


Initial response1
Initial Response? strong year classes.

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


Problems with msy model
Problems with MSY model strong year classes.

  • 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


Changes in size strong year classes.


Problems with msy model1
Problems with MSY model strong year classes.

  • 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


Problem of by catch non target organisms also caught
Problem of By-catch – strong year classes.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


Impacts of removing by catch
Impacts of removing by-catch strong year classes.

  • 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


Habitat destruction by bottom trawling
Habitat Destruction by Bottom Trawling strong year classes.

  • Tears up benthic habitats and species

  • Has been compared to clear-cutting the forest


Potential solutions
Potential Solutions strong year classes.

  • 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


Potential solutions1
Potential Solutions strong year classes.

  • Marine reserves?

    • Habitat fragmentation in the sea

    • How to place them, police them

  • Precautionary principle, burden of proof


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