Human Impacts on the Ocean: Commercial Fisheries

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Human Impacts on the OceanCommercial Fisheries

• Lecture 17
• Marine Biology

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Review
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Left Human population growth on Jamaica from
1870-2000 Right coral coverage decline from
1970s (hatched) to 1990s (black)
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Changes in algal net production and herbivore
grazing intensity before and after the Diadema
mass mortality. Result D. antillarum
important in structuring producer and
consumer components.
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Map showing the path of proposed Diadema pathogen
by charting mortality and mapping ocean currents
and gyres over one year.
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Data from Hughes (1994)
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Belize, Glovers Reef Pre-bleaching
Belize, Glovers Reef Post-bleaching, phase
shifted
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One of numerous hypotheses sought to explain the
outbreaks of Acanthaster by terrestrial runoff
(following heavy rains)
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Ocean Productivity

• Oceans are not uniform in their productivity.
  There are many more
• areas that support only low productivity, rather
  than high
• productivity. These areas are predominant in the
  major gyres.
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• The areas in the ocean that support the
  productivity vary considerably
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• overall the average productivity is about 8, of
  the total potentially
• available
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• - upwelling sites provide about 25 of the
  productivity
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• - temperature continental shorelines provide
  about 35 of productivity

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Fisheries
Landing of marine organisms in 1990's - 100
million tons/yr This has recently leveled
off.   Fishing industry employs 200 million
people worldwide with 70 billion generated at
the first level (fisherman to vendor)   Most
intense fisheries are on fish (vs. other seafood)
Largest tonnage from sardines and anchovies
(low on food chain). In the 1970's anchovies
taken off coast of Peru accounted for ½ world
catch (for fertilizer). This persisted through
the 1970's, then collapsed.  
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1992 - total of 98.1 million metric tons
harvested of that 82 tons were fishes 80 of
the marine catch.   This value represents 10 of
the protein intake of the human population.
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Other major fisheries include menhaden, herring,
etc.
2nd largest fishery Gadoids, includes cod,
haddock, pollock (N. Atl. and N. Pac) - all
benthic dwelling fishes.   3rd largest fishery
Scombroids - mackeral, tuna. These are open
ocean surface-dwelling fishes usually caught
with large purse-seines - relatively low
tonnage but high value fishes   Major overfishing
on all the above plus local benthic fish
populations, including salmon (recent dramatic
decline in Chinook salmon population due to
overfishing), halibut (no longer an Alaskan
fishery), sole, flounder, etc., etc.
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(No Transcript)
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Many fisheries focus on top predator(s) in the
ecosystem, but others on lower level trophic
species. Overexploitation of either can
influence integrity of the ecosystem.
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Direct Effects 1- decline of sardines off
California coast in 1940's led to an increase
in anchovies - competitive interactions
altered?   2 - Sardines increased after reduction
of anchovies off the coast of Peru in the
1970's. Here both high and lower trophic
levels were influences sea birds and marine
mammals.   3- Cod fishery in Canada/US mid 1990's
overfished this top carnivore
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Case study Anchovy fishery off Peru in 1970's
For this fishery, biologists calculated the MSY
(Maximum Sustained Yield) as 9.5 million
tons/yr and recommended 2 million go to the sea
birds and 7.5 million to man. The revenues
generated from this fishery accounted for 1/3
of Perus foreign exchange currency After
1972-73 ENSO, politicians did not act on
biologists recommendations to stop fishing to
allow populations to recover. Now there are no
anchovies - at least in sufficient numbers to
regain their former abundance.
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Extent of Overfishing Fishing mortality vs.
biomass
Variation in stocks of 3 impt. commercial
fisheries
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Peru Fishing case study of overfishing and
natural variation
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Indirect Effects Habitat destruction or
modification - dredging disturbance of oyster
beds resulted in appearance of diseases on
oysters - habitat stressed.   - Modification of
mangroves in tropics to form ponds for culture
of shrimp -loss of mangroves leads to erosion of
shoreline.   - Hunting of sea otters and
overfishing of lobsters led to increase in sea
urchin populations (food of otters and lobsters)
leading to overgrazing of algae by urchins.
- Reduction in kelps lead to formation of
barrens (areas with little or no algae).
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Other Indirect Effects   Over the past 20 years
- algal blooms (referred to as red tides)
have increased sharply world-wide resulting in
massive fish kills and toxic shell fish
poisoning. Causative agent - dinoflagellates.  
E.g. Chesapeake Bay - 1997 authorities
attempted to suppress findings of extemely
toxic dinoflagellate outbreak. Killed fish and
could debilitate humans. Why suppress? -
Concerns about economic impact in area. -
Sport and commercial fishing major industries in
area.
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Pfesteria
Micropredator dinoflagellate harmful algae
blooms
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Lack of an ecosystem approach to management.
Problems include unaccounted-for effects due
to physical processes. Marine systems have
inherent variability driven by short and long
term changes in climate. - e.g. ENSO events
affect areas of high productivity leading to
low production. Usually high fish stocks
decline - even moderate fishing causes collapse
of industry.
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MSY (Maximum Sustained Yield) - based on
catch information on 1) age-length structure
of population (not always correlated) 2)
biomass 3) independent surveys from fisheries
agencies - Stock estimation   size of stock is
determined by assessing quantity of landings
(not how much of the stock is left, but how much
is caught), which is a function of the
population size, spatial variability of the
abundance of the fish and the amount of effort to
catch them.  
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Basis of Maximum Sustained Yield Concept
Optimum harvest is at ½ the maximum
density Point where maximum rate of population
growth occurs
N population size R rate of increase K
carrying capacity
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Factors affecting slope of population growth
curve and response to overfishing
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Landings Fishing effort catch per unit
effort. Low effort/high catch ---gt large,
stable population.   High effort/low catch -----gt
population in decline. Idea is to maintain a
stable large population. As effort goes up,
fishing should go down. However, if price of
fish goes up, the catch can be small relative to
effort and still be profitable. Problem -
declining fish population eventually goes extinct
as a result of increasing value.
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Stock Health - assessed by production in
terms of previous year classes and recruitment
(young of the year)   ?W W t-1 - MW t-1 Rw
t-1 GW t-1   where, ?W change in mass of
total fish population W t-1 mass of
population at same time in previous year M
mortality (fraction of weight of fish that was
lost by death ) R recruitment (fraction
of weight added by recruitment) G growth
(fraction of weight added by growth in
previous year to present)  
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These results are evaluated to establish catch
limits in the next year. Politics weigh the
cost to the industry based on fisherman demands
to catch more fish. This decision is also
affected by economic considerations (what will
it to the economy of the fishing industry)
Result is continuous pressure to increase
fishing effort.  
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Vulnerability of species to overfishing
Reproductive equivalent 1 large 12.5 kg
fish produces an equivalent biomass of 212
fish totalling 233kg.
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Additional Problem There exists a lack of
understanding of all the factors controlling
population size of organisms in the ocean.
Solution holistic models that examine effects
both upstream and downstream in the trophic web
plus environmental variability must be factored
in stochastic physical factors. Most
importantly - reducing fishing pressure is
required to increase catch later.
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Population growth a factor in the exploitation
of fisheries
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Is it the fishing industries last wave
?????