Issues in fisheries sustainability

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
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
The global status of fisheries
4
The global status of fisheries, revisited
From Branch et al. 2010. The trophic fingerprint
of marine fisheries. Nature, doi10.1038/nature09
528
5
Many (25-30) of the worlds fisheries have
collapsedto catches less than 10 of
historical peak
From Mullon et al. 2005. The dynamics of collapse
in world fisheries. Fish and Fisheries 6
111-120. (an examination of 1500 catch time
series)
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Where are fisheries collapsing?
Australia
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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
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

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
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 Fcatch/biomass)
• Seek balance in situations where fishing impacts
  multiple stocks so as to create tradeoffs

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
Fraser sockeye salmon have returned to near
historical peak levels, but there has been a
worrisome decline
13
Productive fisheries often depend on diverse
mixtures of individual spawning stocks, most
obvious with Pacific salmon
Hilborn showed a similar pattern of shifting
contributions for major Bristol Bay stocks
Fraser sockeye abundance by stock
14
There is a severe tradeoff between harvesting and
maintenance of stock structure (biodiversity)
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?
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