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Dynamics of fish populations and fishing
Over-exloited fisheries
Potential harvest of fish
Increasing fish production
Recent history of marine food production:
4-fold increase in past four decades.
Increase is leveling out, despite more technology
and greater intensity of fishing.
Declines in many traditional and valuable fisheries:
Important questions to address:
Controls on size of fish stocks?
Effect of fishing on the population of fish stocks?
Effects of over-fishing?
Steady-state biomass of a virgin stock of fish: Gains = Losses
growth of individuals + reproduction (new members)
= deaths + natural predation
Effects of fishing: add human predation
--> decrease steady-state population
Fishing (over-fishing) decreases the age and size of individual
fish.
This leads to decrease in yield (harvest) [mass of fish harvested
/ year]
1. Older, larger fish taken first --> increased growth of younger
fish
--> net growth of population.
2. To maintain yield, smaller & younger fish (breeding population)
are taken.
3. Reproduction decreases, yield decreases; population may not
rebound even if fishing stops -- Over-fishing!
Important question: What is the "maximum sustainable
yield?"
Not known for most commercial fish stocks.
Estimates range from 1/3 to 2/3 of total production
Peruvian anchovies -- Overfishing and environmental change.
Most important fishery of the l960's -- 12 million metric tons
per year.
Over-fished at peak harvest in l970.
Devastating l972 El Nino.
Since then, yield has been 2 million metric tons per year.
Pacific Salmon -- Overfishing and environmental degradation.
Degradation of spawning "home" streams:
... dams
... alter stream banks (reduce shade)
... water quality.
Baleen Whales -- Overfishing.
Harvest and populations of many species
have declined markedly since l960's
| Province | Net Primary Prod. [million metric tons (live weight) / year] | Trophic level harvested | Trophic efficiency(%) | Maximum fish production [million metric tons / year] | |
| Open Ocean | 209,000 | 5 | 10 | 2 | |
| Coastal | 68,000 | 3 | 15 | 230 | |
| Upwelling | 1,000 | 1.5 | 20 | 120 | |
| Total | 278,000 | 352 |
The recent history of marine food production has some troubling aspects. True, the world fish catch has increased 4-fold over the past four decades. But the rate of increase is leveling out despite advancements in technology and increased intensity of fishing. Concurrent with the general increase are declines in many traditional and valuable fisheries. Important questions to address include (1) What controls fish stocks? (2) What is the effect of fishing on the population of fish stocks? (3) What happens when we over-fish?
We can be pretty confident that the total biomass of a "virgin" stock (population) of fish is maintained at a steady-state balance between gains and losses. In other words,
Growth of individuals + reproduction (new members) = deaths + natural predation
The effect of fishing should be obvious: Human predation is added to the "loss" side, and the steady-state population decreases. As harvesting of the stock continues (and intensifies), there are significant effects on the age and size of individuals. These effects lead to a decrease in the yield (harvest) a fish stock (i.e., mass of fish harvested per year).
1. Initially, the older, larger fish are taken first. This
results in a net growth of the population, because the older,
larger individuals compete successfully for food with the younger,
rapidly growing fish.
2. Then, more of the smaller and younger fish must be taken to
maintain the yield.
3. Eventually, enough of the breeding population is taken so that
reproduction decreases. The yield (harvest) decreases despite
increased intensity of fishing. The stock is over-exploited, and
the population may not increase even if fishing is stopped or
severely regulated.
An obviously important questions: What is the maximum sustainable yield that can be harvested without causing a catastrophic decrease in population? We do not know this for most commercial fish stocks. Estimates range from 1/3 to 2/3 of total natural production of the stock.
Peruvian Anchovy fishery has declined as a combined result
of overfishing and natural environmental change. This was the
most important fishery of the 1960's, with yields of 12 million
metric tons per year. Fishing (actually over-fishing) reached
its peak harvest in 1970. There was a particular severe El Nino
in 1972. Thereafter, the yield has been 2 million metric tons
per year.
Pacific Salmon fishery has declined as a combined result of overfishing
and environmental degradation of the "home" streams
in which salmon spawn. Degradation includes poor water quality
(i.e., pollution), building dams, and removing shade cover from
stream banks.
Baleen Whale fishery has declined solely as a result of over-harvesting.
Both the harvest and the populaltion have declined markedly since
l960's.
Potential Harvest of Fish from the Oceans. J. A. Ryther (l969)
conducted a careful evaluation of primary productivity and trophic
efficiency of food chains/webs. Ryther's calculations have been
revised subsequently (See Sumich, Table15.2, p. 402):
The actual world fish production is about 4 times the maximum production. Are we approaching the maximum sustainable yield for many commerical fish species? Probably so.
Several approaches have been suggested; a few are under way:
1. Mariculture / Aquaculture. Marine and fresh-water fish and
shellfish raised under controlled conditions will account for
20% of fish consumed by year 2000. Examples of current practices
include (a) salmon raised in near-shore pens, and (2) molluscs
and crustaceans raised in ponds. In addition to "traditional"
aquaculture practices, two other approaches show some promise:
Open-ocean "ranching." The basic strategy is to release
eggs, larva, and fry to the oceans, and then harvest the adults.
Molluscs and crustaceans are best stuited for "ranching"
because (a) they are low on the food chain/web, (2) their life
cycles are well studied, (3) they "stay put" as adults,
and (4) they have high consumer appeal. Finfish show less promise
(although it is being done): Fish are more difficult to feed and
raise to maturity, and there is the question of jurisdiction and
ownership of the ranched stock in the open ocean.
Genetic "engineering" to improve or increase (a) rate
of growth, (b) survivability in harsh environmental conditions,
and (c) the quality of the product.
2. Development of new and underexploited fisheries. An example here are pollock and whiting populations of the northeast Pacific. Although these fish are abundant, they are rather oily and therefore not very appealing. The smelly oils can be removed by processing, and the flesh processed into more marketable products like artificial crab and shrimp meat. Another example would be to keep the incidentally caught "trash fish" and deep-water fish that are now simply dumped (dead) back into the sea. Fish-protein concentrate could produced from these otherwise undesirable species.
3. Harvest lower on the food chain. In principle, this seems like a "can't-miss" proposition. If humans could harvest lower on the marine food chain, we would have access to a much higher biomass. But there are pitfalls. For example, it was estimated in the l960's that if we harvested krill from high-latitude waters (mostly in the Southern Ocean), we could double the the present annual world fish catch. This spurred the development of krill fisheries in several Eastern European countries. However, the maximum krill catch has been only a fraction of the earlier estimate, about 300,000 metric tons per year. In addition, we must keep in mind that krill are vital components in the polar food web. If the krill population declined, so would the population of animals further up the food web, e.g., whales. Why not harvest phytoplankton directly? This would substantially shorten the food chain to humans and phytoplankton have a high protein content. But there are disadvantages to this approach: Because they are microscopic and widely dispersed, the cost in collecting and concentrating phytoplankton would be enormous. In addition, if humans became primary herbivores in the oceans, there would be serious and detrimental effects on other consumer organisms higher in the marine food chain.
4. Proper regulation and management of currently fished stocks. An obvious approach that is continually broght to the public's attention by marine scientists and environmentalists.
A concluding remark. Even if we were able to double the current yield of food from the oceans, we would not add substantially to total global food production. "Food from the sea" is not the answer to the nutritional needs of an expanding world population.
-In general terms, how has the world's fish catch changed over
the past 40 years? Does that trend seem to be leveling off? Does
that seem surprising given that the investment in fishing vessels
and modern technology continues to increase?
- What is the importance of fish to the global human diet? Is
it just a matter of calories, or do fish supply an important nutrient
to humans?
- What fraction of the world's fish catch is consumed directly
by humans? Would you consider this to be an efficient use of that
food resource?
- Identify the two factors that account for the occurrence of
commercially important fishing areas. That is, what two things
are needed to produced a reasonably high concentration of desirable
fish in a small area? Explain how those factors work.
- Tuna and other commercially important fin fish (like some mackerel)
are harvested from upwelling zones in open-ocean areas. Although
primary productivity in those areas is high, the likelihood of
substantially increasing the catch of tuna is not high. Why?
- Explain how the combination of trophic structure and high primary
productivity make coastal regions an attractive region for commercial
fisheries.
- Enormous numbers of anchovies flourish (or at least did flourish)
in coastal upwelling zones off South America. What aspects of
primary productivity and trophic structure are responsible for
that?
- The biomass of a virgin fish stock probably is maintained at
a steady-state level governed by the overall gains and losses
to the stock. Identify those gains and losses. When humans intervene
by fishing that stock, how does the balance change?
- As a fish stock is harvested over a number of years, both the
size and the age of individuals changes. After years of (over-)
harvesting, the yield of fish decreases even as the intensity
of the fishing effort increases. Describe briefly how that dynamic
works.
- What is meant by the "maximum sustainable yield" for
a fish stock? What is the range of estimates for this yield?
- Describe briefly the current state of the Peruvian anchovy fishery.
What factors contributed to its decline?
- According to current estimates of maximum fish production, which
area of the world ocean has the highest theoretical yield? How
does the total maximum fish production compare to current world
fish catch? Is this a matter of concern?
- What strategies have been proposed, or are being developed,
to increase food production from the world's oceans?
- How important is mariculture as a food resource at present?
- Why are benthic organisms, like oysters, a better prospect for
open-ocean "ranching" than fish?
- How could the new technology of genetic engineering help to
improve the quality and quantity of food from the seas?
- A number of species of incidentally caught "trash"
fish are currently being dumped back into the sea (usually dead).
How could we make use of this potential food resource?
- A concerted attempt to harvest the euphasiid "krill"
could double our harvest of food from the oceans. Why wouldn't
this be a good idea?
- Likewise, harvesting phytoplankton directly would seem to make
sense because we would shorten the food chain -- we would become
"primary consumers!" What are the problems with this
approach?
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