Fisheries We wish to know: What is the importance of fish in the diet of humans? What are the...

Fisheries

We wish to know:• What is the importance of fish in the diet of

humans?• What are the important marine resources, and are

they harvested sustainably?• What is the sustainable yield of the oceans?• What possible solutions might allow humans to

more sensibly obtain food from the seas?

Fish Stocks and Fish Harvests

What are the important marine resources?• Demersal fish: bottom-living fish such as cod and

haddock• Pelagic fish: water-column fishes from herring to

tuna• Crustaceans: bottom-dwelling (crabs, lobsters) and

swimming (krill, shrimp) invertebrates• Molluscs and Cephalopods (squid and octopus)• Marine mammals: whales, walruses

What is the Importance of Fish?

• Marine sources provide about 20% of the animal protein eaten by humans.

• 60% of fish consumption is by the developing world.

• In Asia, about 1 billion people rely on fish as their primary source of protein.

• Estimates suggest that seafood production from wild fish stocks will be insufficient to meet growing U.S. and global demand for seafood products in the next century.

• The fishing enterprise employs some 200 million people worldwide

Principles and Terms

• Stock -- a harvestable species or population

• Harvest rate -- fraction or amount of stock harvested per year

• Production rate -- sum of growth in weight of individual fish plus the addition of biomass from new recruits, minus loss in biomass to natural mortality

Stock Management

• Stocks are assessed by: – biological sampling– annual catch statistics– catch per unit effort statistics

• Global harvest history:– catch has climbed fairly steadily since 1950s– now ~ 100 million metric tons/year– harvest per capita has grown little

Global Harvest History

Total catch (million metric tons) appears to have leveled off before 1990.

Per capita catch has been level since about 1970.

Evidence of Over-Exploitation

• Declines in catch per unit effort

• Declines in total catch

• Numerous case studies of collapse of fisheries– sardines of California (“Cannery Row”)– Peruvian anchovy– George’s Bank

New Methods, New Targets, and Over-Capacity

• Numerous statistics point to over-capacity

• drift-fishing is a spectacular example of more efficient fishing methods

• many previously unfished stocks are new targets (“we are eating bait”)

• collapse of Peruvian anchovy, which in boom years was largest new fishery in the world

Peruvian AnchovyIllustration by Chris Van Dusen

Anchovies (anchovetta) increased in 1986 as ocean water cooled, but declined again after 1994 and dropped sharply during the 1998 En nino. Shifts in fish distributions affect national economies.

Climate Variability and Marine Fisheries

Episodes of ocean warming seem to favor catches of Pacific Basin sardines.

Note collapse of California sardines and Peruvian anchovies may have involved a combination of over-fishing and climate fluctuations

Georges Bank

http://sciencebulletins.amnh.org/biobulletin/biobulletin/story1249.html

Georges Bank is an oval-shaped shoal (shallow ocean areas, 240 km long by 120 km wide, that lies at the southwestern end of the Grand Banks, which extend from Newfoundland to southern New England. Located 120 km off the coast, it is larger than the state of Massachusetts.

This rich fishing ground is famous for its demersal fish, especially cod.

Maximum Sustained Yield of the World’s Oceans

• A theoretical estimate puts estimated annual production at 240 million metric tons– estimated annual harvest is half: 100-120 mmt– current annual harvest is about 100 mmt

• Estimate is obtained by dividing ocean into three zones: open ocean, coastal areas, upwelling areas

• Estimate is based on three values:– primary (plant) production– food chain length– food chain efficiency

Food Chains and Efficiency

• Two lectures from GC-I may be useful– Lecture on primary production explains how

primary production is estimated in marine waters

– Lecture on food chains discusses ecological efficiency and loss of energy with each trophic transfer (“10% rule”)

While the open ocean comprises most of the area of the aquatic world, it contributes proportionately less to total productivity, because production per unit area is low (note log scale). Even so, most of the world’s aquatic primary production occurs in the open oceans

The open ocean is less productive than coastal zones and upwelling zones.

Estimated Production of Harvestable Fish

open coastal upwelling

ocean waters areas

nutrient concentr.low intermediate high

primary production low intermediate high

Food chain length long intermediate short

ecological efficiency lower --------------------> higher

fish production negligible ~ half ~half

of total of total

A theoretical estimate, based on this logic, puts the total fish production of the world’s oceans at 200 m tonnes

A Paradox: “Fishing Down” Reduces Yields

• Pauly, Dalsgaard and colleagues analyzed diet of 220 key species to assign each species of catch to a trophic level (Science 279:860, 6 Feb 1998)

• from 1950 to 1994, catch has gradually shfted from long-lived, high-trophic-level fish (eg, cod, haddock) to low-trophic-level fish and invertebrates such as anchovy and krill). (“We are eating bait”)

• paradoxically, catches stagnated or declined, as competitors (eg, jellyfish) fill the void. “If things go unchecked, we might end up with a marine junkyard dominated by

plankton”

Solutions• Aquaculture

– Fish farms now account for more than 1/8th of the world’s catch

• Downsize the existing fishing fleet

– likely that a reduction of 30-50% will be required

• Reduce the subsidies

– annual cost of fishing 80 billion, revenues 75 billion

• Forge international agreements on fish catch limits

• Establish Marine Reserves

SUMMARY

• Harvest of fishery resources is an important source of animal protein for many, as well as a source of jobs and income

• The global fish harvest, now ~ 100 million tonnes, is near its upper limit

• Meanwhile many fisheries have experienced over-harvest and collapse (Peruvian anchovy, George’s Bank)

• Over-capacity and improved technology threaten further damage, including “fishing down” the food chain

• solutions include greater reliance on aquaculture, fleet downsizing via regulation and reduced subsidies, establishment of marine reserves, and effective international agreements