Over-Exploitation
1957
Reed Noss
UCF
Early 1980s
Trophy fish caught on Key West
charter boats
(McClenachan 2009
Conserv. Biol.)
2007
8.8 Density dependence stems from relationships between population density and per capita rates
of birth and death
8.9 (A) Logistic population growth of a population up to a maximum population size, Nmax
8.9 (B) Sustainable yield, Y against population size for the logistic case shown in (A)
8.10 Equilibria and population stability under constant quota exploitation
8.11 Equilibria and population stability under proportional exploitation
8.12 Probability of overexploitation in relation to mean yield of marten in commercial trapping
Fryxell et al. (2010, Science)
“…in reality many management agencies have
limited ability to control the number of resource
users or harvest effort…As a result of the openaccess philosophy typically underlying wildlife
harvest management (that is, employing few
restrictions on participation in hunting, instead
changing quotas on an annual basis), large-scale
fluctuations in abundance are to be expected.”
Predicted time series for dynamic harvest-effort-quota system (A) without
environmental stochasticity, and (B) with SD in environmental stochasticity
From: Fryxell et al. (2010), Science
Predicted probability of population collapse (to 10% of equilibrium density)
in a model system with constant effort (open circles) vs. weak compensatory
changes in effort (solid circles).
From: Fryxell et al. (2010), Science
Over-Exploitation:
Greatest Threat to Marine Biodiversity?
• The sea is the largest source of animal protein
•
•
used by humans, with most of it from wild
stocks
Unlike the land, most animals harvested are
from higher trophic levels, which tends to
destabilize ecosystems
Many fisheries have collapsed worldwide, e.g.,
Atlantic cod, Atlantic bluefin tuna, swordfish,
orange roughy, several salmon stocks, several
shark species, and many more
Worldwide depletion of predatory fish
biomass – 80% within 15 years of
exploitation (Myers and Worm 2003)
Cascading Effects of Collapse of Cod on Scotian Shelf (Scheffer et al. 2005)
Mean size and SE of:
(a) trophy fish in 1956-60, 1965-79, 1980-85, and 2007,
(b) Trophy fish excluding species whose capture is currently prohibited, and
(c) sharks.
From McClenachan (2009), Conserv. Biol.
Marine Reserves – Most Important
Tool for Marine Conservation
• Documented to be critical population source
•
•
•
areas for many species
Result in large, rapid, and long-lasting increases
in population sizes, numbers of species, and
reproductive output
A network of reserves in St. Lucia increased
adjacent catches 46-90%; reserve zones in
Merritt Island NWR have supplied increasing
numbers of world record-sized fishes since 1970
Reserves need not be huge, but no-take
regulations must be rigorously enforced
Case 8.3 Table B
Case 8.3 Table B