Food Webs and Fisheries

OCN 201 Biology Lecture 10Steward

A series of different species of organisms at different trophic levels in an arrangement such that each species feeds only on organisms one trophic level below and serves as food only for the next level above.

Food Chain

Primary Producer

Primary Consumer

Secondary Consumer

Tertiary Consumer

Trophic Levels

• Much of the biomass consumed by an organism is respired (released as CO2 and heat) or released as waste products

• Typically only 10% is used to make biomass

• This results in the trophic pyramid (or biomass pyramid)

Trophic Efficiency

Trophic Pyramid

10,000 kg

1,000 kg

100 kg10 kg

1 kg

Phyto-plankton

Zoo-plankton

Sm. Fish

Med. Fish

Top Consumer

The complex feeding arrangements among all the organisms in a community that takes into account that any organisms may feed on more than one species of prey and on more than one trophic level.

Food Web

Simplified Marine Food Web

BiomassPyramid

• There are may interdependent connections in the marine food web!

• Disturbance of one component of a food web can have unexpected consequences at many other levels

Trophic Cascades

North Pacific Whale Catch

Springer et al 2003 PNAS

Trophic Cascade

Whaling Moratorium

Urchin BarrensHealthy Kelp Forest

Ocean Fisheries

NASA

Global Fishing Watch

Patterns of Phytoplankton(from satellite-derived

chlorophyll)

Fishing Activity

• Seaweed

• Invertebrates

• Fish

• Whales

Food from the Sea

Commercial whaling ceased in 1987 (IWC moratorium)

Resumed by Norway in 1993, Japan never stopped: Minke whales taken for ”scientific purposes”; meat and blubber sold to market. In 2000, extended to blue and Bryde’s whales

• Harpoon - whales, swordfish, bluefin tuna

• Pole and Line - tuna, mahi-mahi

• Longlining - swordfish, tuna (pelagic); cod, halibut (bottom)

• Trolling - salmon, albacore, mahi-mahi

• Drift (gill) netting - various pelagic fish

• Trawling - anchovies (pelagic); cod, halibut (bottom)

• Purse Seine - Sardines, herring, mackerel, tuna

• Traps and Pots - Crabs, lobster, rock fish

Commercial Fishing Methods

Mariculture

Kona-BlueKona Kampachi

(yellowtail)

• Many farmed fish fed fish meal - inefficient!

• High concentrations of fish - lots of waste water, diseases, antibiotic resistance

• Shrimp farms often built at the expense of mangroves - destroy important fish habitat

Some Problems

• Farm vegetarian fish (Carp, Tilapia)

• Farm shellfish (filter feeders--clean water)

• Better nutrition and better engineering to minimize waste

Some Solutions

• Maximum Sustainable Yield: maximum number of fish that can be harvested without depleting stocks in the future

• Present harvests remain < 100 mmt; cannot sustainably increase

• Estimated 45% of fisheries overfished at present

Fisheries Production

Fisheries Model

CostCatch Value

Amount of Effort

Mon

ey

MSY

Max Profit

• Overcapitalization: when fishing is good, more boats built, more people employed

• When yields decline, much more difficult to scale back (protect capital investments, jobs)

Ratchet Effect

Trend in Fisheries Collapses

http://news.bbc.co.uk/2/hi/science/nature/6108414.stm

Earlier Fears: Current Assessments:

Marine Fish Harvest

Marine fish catch has declined since

mid-nineties

Appears to be at or above maximum sustainable yield

Total Fish SupplyFigure from Food and Agriculture Organization

Marine + Inland

Caught

Farmed

Aquaculture is becoming increasingly important

How do we avoid the tragedy?

• Lessons learned from previous collapses

• Innovative economic incentives

• Improving monitoring

• An informed public making better choices

Economic Incentives

• Limited Access Privilege Programs

• Fishers own share of total allowable catch

• Can buy and sell shares

• Can lead to more responsible and more efficient use of resources

Catch Shares Programs

Improve Monitoring

Informing Public

Global Fishing Watch