Transcript Lecture 7 Overexploitation of Marine Fisheries and Shifting Baselines

Marine Fisheries Terms to Know
 Fishery – Refers to aspects of harvesting and managing
aquatic organisms. Can refer specifically to a species
being harvested, the methods of harvesting, or the
ecosystem from which the organisms are harvested.
 Term is not limited to animals classified as fish
 Stocks – Groups of organisms considered to be distinct
units, typically distinct populations.
From: Beckman 2013
Marine Fisheries Terms to Know
 Fisheries management – The regulation and
monitoring of a fishery, including setting harvest
limits, developing management plans, and protecting
fish habitat.
 Fisheries landings – Common measure to monitor
fisheries. Is the amount of fish caught, measured as
the biomass or number of individuals caught and
brought to shore.
Types of Fishers
 Recreational (Sport)
 Subsistence
 Artisanal
 Commercial
 Small-scale
 Mid-sized
 Industrial
Methods of Fishing
 Spears/harpoons
 Dredging
 Hook-and-line
 Includes long-lining
 Traps and weirs
 Gill and drift nets
 Seines
 Trawlers (bottom and mid-water)
Sustainability in Marine Fisheries
 Sustainability = harvesting at levels that allow
continued harvest indefinitely
 Can be achieved if the population remaining after
harvest is able to replenish itself by reproduction
 Have to determine the surplus that can be taken
without adversely affecting production
Overfishing affects Marine Ecology
 Major top-down force that has led to large depletions of
top predators
 By removing species which exert control over lower
consumers, there are severe consequences to marine
ecosystems through trophic cascades
 Pauly et al. (1998) that chronicled fishing down marine
food webs
Overfishing: Continued
 There is great difficulty in sustaining global fisheries
production. In response to declines of big, slower
growing species, fishers have begun "fishing down
the food chain", targeting smaller species of less
value, but which can play critical roles in food webs.
 Over-capitalization of the industry has led to the buildup
of excessive fishing fleets, particularly of the larger-scale
vessels, catching too many fish.
Pauly, D. et al. 1998. Fishing down marine food
webs. Science 279: 860-863
Trophic Level 4
Trophic Level 3
Trophic Level 2
Trophic Level 1
Overfishing: Impacts on
Biodiversity
 Fishing can be an agent of selection, affecting age
distribution, age and size at maturity, and growth
 Fishing can alter species composition and interactions
among fished species and their prey.
 Fisheries often begin on large predators but their
reduced numbers may lead to increased numbers of
prey species, which may themselves become fished.
 Intense fishing can lead to dominance by r-selected
species, which often become major parts of mature
fisheries. Other species can also be affected; e.g.,
fishery discards have caused long-term changes in
seabird species composition.
Overfishing (= Intense Predation)
 The global marine catch has increased more than 4x in the
past 40 years.
 Overfishing has pushed many fish populations into steep
declines. Catches are falling, even though fleets are
fishing harder than ever before
 The U.N. Food and Agriculture Organization (FAO)
reports that ~70% of commercially targeted fish stocks are
at least heavily exploited
Exploitation of global fisheries
Ocean Fisheries
 > 109 people (mostly developing nations) depend
upon marine fish for primary source of protein
 40 y ‘fishing boom’ has now ended: catch increasing
steadily since 1950, but since 1989 world catch has
stayed the same
 Catch of ‘high value’ fish decreasing; catch of low
value fish increasing; Also catching smaller sized fish
Essington, T. et al. 2006. Fishing through marine
food webs. PNAS 103: 3171-3175
 Alternative view is fishing through food webs where
continue to fish for upper trophic levels with
sequential addition of lower trophic level fisheries.
 E.g. Continue to fish for snapper and grouper while
concurrently targeting parrotfish and other herbivores
A.) Sequential collapse/replacement
mode where mean trophic level
declines over time and old fisheries
are replaced with new ones at lower
trophic levels
B.) Sequential addition mode of
fishing down the food web where
continue to fish at all trophic levels
and introduce new fisheries as well
Overfishing: A few success stories
 One success story,
Spanish mackerel in the
Gulf of Mexico are no
longer overfished and,
in fact, have become a
sustainable fishery.
 Pompano are also
recovering in the Gulf
Jackson, J.B.C. et al. 2001. Historical Overfishing
and the Recent Collapse of Coastal Ecosystems.
Science 293: 629-638
 Historical overfishing has altered coastal ecosystem
structure through targeting of top consumers:
 Resulted in losses of previously abundant members of
the food web and increases in other members who were
once less abundant
 Analyzing historical and paleontological data allowed
for reconstruction of historical ecosystem structure
prior to the impacts of overfishing
Shifting Baselines
 Diversity & ecosystem structure today may be strongly
altered relative to a few human generations ago
 We might mistakenly take today’s situation as the
baseline for conservation
 The baseline for a natural community has shifted over
generations because we have forgotten the original
natural state