Fisheries in the SeasFisheries in the Seas

Fish life cycles:Fish life cycles:

Egg/sperm pelagic larvaeEgg/sperm pelagic larvaejuvenilejuvenile

(first non-feeding – (first non-feeding – critical periodcritical period – then – then feeding)feeding)

Indeterminant growthIndeterminant growth

Growth rates varyGrowth rates vary

Age determination – otoliths, cohorts,Age determination – otoliths, cohorts,--- find very large variations in size --- find very large variations in size

of year classesof year classes

““young-of-yearyoung-of-year” YOY” YOY

Migratory CircuitMigratory Circuit

“larval drift”

Reproductive IsolationReproductive Isolation

• Location of spawningLocation of spawning

• Timing of spawningTiming of spawning

What controls population What controls population size?size?

Possibilities:Possibilities:

• No. eggs/spawning successNo. eggs/spawning success

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

• Mortality among adults (food Mortality among adults (food limitation, competition)limitation, competition)

Mortality in young stagesMortality in young stages

• Critical period – what determines if Critical period – what determines if larvae find food or not?larvae find food or not?

• Survival of feeding larvaeSurvival of feeding larvae• Juvenile survivalJuvenile survival

Successful recruitment – many stocks Successful recruitment – many stocks seem to be maintained by sporadic seem to be maintained by sporadic strong year classesstrong year classes

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

What does thisWhat does thismean for mean for management?management?

Fisheries ManagementFisheries Management

• Oceans provide ~20% of the animal Oceans provide ~20% of the animal protein consumed by humans protein consumed by humans worldwide (FAO 1993)worldwide (FAO 1993)

• Over half of the world’s fish stocks Over half of the world’s fish stocks are fully exploited, at least 25 - 35% are fully exploited, at least 25 - 35% are overexploitedare overexploited

““Fishing down food webs”Fishing down food webs”

• Globally, fisheries first target higher-order Globally, fisheries first target higher-order predators predators

• As these decline, move to species in the next As these decline, move to species in the next trophic level down, where abundances have trophic level down, where abundances have increased due to release from predationincreased due to release from predation

• Today, only 10% of all large fish populations Today, only 10% of all large fish populations present in 1950, including cod, tuna, present in 1950, including cod, tuna, swordfish, grouper, marlin, halibut, and swordfish, grouper, marlin, halibut, and flounder, remain (Myers and Worm 2003)flounder, remain (Myers and Worm 2003)

Why has marine fisheries Why has marine fisheries management failed?management failed?

1.1. Must be based on a good Must be based on a good understanding of the population biology understanding of the population biology of the fishof the fish

• Sampling problem Sampling problem

– – independent sampleindependent sample

– – use harvest data (landings)use harvest data (landings)

CPUE – Catch per unit effortCPUE – Catch per unit effort

• Variations in successful year classesVariations in successful year classes

Why has marine fisheries Why has marine fisheries management failed?management failed?

2.2. Harvest methods have become Harvest methods have become much more efficientmuch more efficient

• Early fisheries – hook and line (until Early fisheries – hook and line (until 1920s) 1920s)

– – trawling – took off in 1930strawling – took off in 1930s

– – gill nets, purse seines, long linesgill nets, purse seines, long lines

• Refrigeration – large factory ships Refrigeration – large factory ships

Initial Response?Initial Response?

Exclusive economic zone – 200 mi limitExclusive economic zone – 200 mi limit

Initial Response?Initial Response?

Exclusive economic zone – 200 mi limitExclusive economic zone – 200 mi limit

Underlying cause of the problem Underlying cause of the problem – the – the way we manage – Fisheries Councils way we manage – Fisheries Councils that that balance economics with balance economics with catches, but at mismatched time catches, but at mismatched time scales; scales; base catch limits on MSYbase catch limits on MSY

Problems with MSY modelProblems with MSY model

1.1. MSY model assumes spatial and MSY model assumes spatial and temporal uniformity of the populationtemporal uniformity of the population

• Temporally – know not true – year class Temporally – know not true – year class phenomenonphenomenon

• Spatially – suspect that there are favorable Spatially – suspect that there are favorable and less favorable sites – source and sink and less favorable sites – source and sink populationspopulations

2.2. Fish populations change rapidlyFish populations change rapidly• Are there warning signs?Are there warning signs?

Change in size distribution – smaller average sizeChange in size distribution – smaller average size

Changes in sizeChanges in size

Problems with MSY modelProblems with MSY model1.1. MSY model assumes spatial and MSY model assumes spatial and

temporal uniformity of the populationtemporal uniformity of the population• Temporally – know not true – year class Temporally – know not true – year class

phenomenonphenomenon• Spatially – suspect that there are favorable Spatially – suspect that there are favorable

and less favorable sites – source and sink and less favorable sites – source and sink populationspopulations

2.2. Fish populations change rapidlyFish populations change rapidly• Are there warning signs?Are there warning signs?

Change in size distribution – smaller average sizeChange in size distribution – smaller average size

3.3. Ignores interspecies interactions – Ignores interspecies interactions – predator/prey dynamics, competitionpredator/prey dynamics, competition

Problem of By-catch – Problem of By-catch – non-non-target organisms also caughttarget organisms also caught

• Shrimp trawl fishery – in south Shrimp trawl fishery – in south Atlantic and Gulf of Mexico, 90% of Atlantic and Gulf of Mexico, 90% of what is caught is not shrimpwhat is caught is not shrimp

• Bottom trawling – barn door skate in Bottom trawling – barn door skate in coastal New Englandcoastal New England

• Purse seine fishery for yellow fin Purse seine fishery for yellow fin tuna – high dolphin mortalitytuna – high dolphin mortality

• Long-lines – tangle diving birds, Long-lines – tangle diving birds, marine mammals, turtlesmarine mammals, turtles

Impacts of removing by-Impacts of removing by-catchcatch

• Juvenile fishes never grow up (redfish Juvenile fishes never grow up (redfish in Gulf of Mexico)in Gulf of Mexico)

• Removing “baitfish,” invertebrate Removing “baitfish,” invertebrate prey for other speciesprey for other species

• Food subsidy for aggressive bird Food subsidy for aggressive bird predators – gulls and other predators – gulls and other nuisances; blue crabs and sharks can nuisances; blue crabs and sharks can sometimes benefitsometimes benefit

Habitat Destruction by Bottom Habitat Destruction by Bottom TrawlingTrawling

• Tears up benthic habitats and Tears up benthic habitats and speciesspecies

• Has been compared to clear-cutting Has been compared to clear-cutting the forestthe forest

Potential SolutionsPotential Solutions

• Ecosystem management – looking at Ecosystem management – looking at fish as part of larger ecosystem; fish as part of larger ecosystem; ecologically sustainable yield ecologically sustainable yield – Food web modelsFood web models– Coupled physical and biological modelsCoupled physical and biological models– Managing species in complexes rather Managing species in complexes rather

than individuallythan individually

Potential SolutionsPotential Solutions

• Marine reserves?Marine reserves?– Habitat fragmentation in the seaHabitat fragmentation in the sea– How to place them, police themHow to place them, police them

• Precautionary principle, burden of Precautionary principle, burden of proofproof