Download 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
• Location of spawning
• Timing of spawning
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
• 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
• 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”
• 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?
1. 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
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
Initial Response?
Exclusive economic zone – 200 mi limit
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
Problems with MSY model
1. 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
2. Fish populations change rapidly
•
Are there warning signs?
Change in size distribution – smaller average size
Changes in size
Problems with MSY model
1. 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
2. Fish populations change rapidly
•
Are there warning signs?
Change in size distribution – smaller average size
3. Ignores interspecies interactions –
predator/prey dynamics, competition
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
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
Habitat Destruction by Bottom Trawling
• Tears up benthic habitats and species
• Has been compared to clear-cutting the
forest
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
Potential Solutions
• Marine reserves?
– Habitat fragmentation in the sea
– How to place them, police them
• Precautionary principle, burden of proof