World Fisheries: Declines, Potential and Human Reliance

When we can drain the Ocean into mill-ponds,and bottle up the 
Force of Gravity, to be sold by retail, in gas jars; then may we hope to comprehend the infinitudes of man's soul under formulas of Profit and Loss; and rule over this too, as over a patent engine, by checks, and valves, and balances 

- Thomas Carlyle (1795-1881), Scottish essayist, historian


Format for printing

In this lecture period, we wish to learn:

  • 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? 

Jump to: [Fish Stocks and Fish Harvests] [Importance of Fish] [Principles and Terms] [Evidence of Over-Exploitation] [New Methods, New Targets, and Overcapacity] [Maximum Sustained Yield of the World's Oceans] [Solutions] [What Can you Do]

1.  Fish Stocks and Fish Harvests

We can group economically important marine organisms in to five major families:

Demersal fish. These are bottom-living fish such as cod and haddock. These species tend to concentrate on broad continental shelves, especially of the North Atlantic. 
Pelagic fish. Pelagic fishes are species that inhabit the water column, such as herring, mackerel, anchovy, and tuna. The most spectacular fish catches are made of surface-shoaling pelagic species. Demersal fishes and Pelagic fishes combines make up the majority of the fish catch--about 72 million tons per year.

Crustaceans. This group consists of bottom-dwelling species (crabs and lobsters) as well as swimming invertebrates (krill, shrimp)Crustacean fisheries are important to many countries and regions, such as the Chesapeake Bay of the U.S. About 4 million tons of this group are harvested each year.
Molluscs and Cephalopods. These include various species of squid, cuttlefish, and octopus. More cephalopod stocks are harvested by the Japanese than by any other nation. They also serve as an important source of protein for many Mediterranean and developing countries. About 2.5 million tons of cephalopods are harvested each year.

Marine mammals. This group has been heavily exploited for oil and meat, although they make a relatively small portion of the global fish catch. Following the commercial extinction of the large baleen whales such as the blue, humpback, and fin, smaller species such as the minke and sei are being taken. Dolphins and porpoises are hunted locally, particularly in some tropical archipelagos. 
Figure 1: Families of economically important fish


2. The Importance of Fish

Why are we concerned about the status of our global fisheries? In addition to more lofty environmental reasons, such as the preservation of biodiversity, humans have stock in the status of our world's fisheries. Here are some statistics to give you an idea of the scope of human dependence on marine life:

  • Over 90% of the world's living biomass is contained in the oceans, which cover 71% of the Earth's surface. At present, we harvest about 0.2% of marine production. (You might think that there is room for growth).

  • Marine sources provide about 20% of the animal protein eaten by humans. Another 5% is provided indirectly via livestock fed with fish.
    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.

3. Principles and Terms

First, we need to become familiar with some terms used when discussing fish populations and the fishing industry.

  • Stock. A stock is the portion of a species or population that is harvestable.

  • Stock Assessment is the estimation of abundance of a resource, rate at which it is being removed, and reference rates for sustainable yields.

  • Fishing Mortality Rate is a function of the fishing effort (amount, types of gear, etc.)

  • Harvest Rate The harvest rate is the fraction or amount of stock harvested per year.

  • Production Rate. The production rate is the sum of growth in weight of individual fish, plus the addition of biomass from new recruits, minus loss in biomass to natural mortality.

  • Production Function shows the relationship between production rate and fishing effort. As effort increases, the biomass drops and the production function typically goes through a fairly stable maximum. 

To aid in fish management, we can assess stocks by using a combination of three methods: 

  • biological sampling

  • annual catch statistics

  • catch per unit effort statistics 

Stock assessment and management is becoming increasingly important, as is illustrated by these statistics on the global harvest history:

  • Total catch has climbed fairly steadily since the 1950's. 

  • Now, about 100 million metric tons/year are taken from the sea. This figure seems to be stabilizing.

  • However, the harvest per capita has grown little (see Figure 2). This implies that if the current limit can not be increased, seafood availability per person will shrink as population expands. This will lead to rising prices. 

Figure 2: Global absolute and per capita fish catch, 1950-2000

4.  Evidence of Over-Exploitation

We can assemble a large amount of evidence that points to the fact that our marine resources have been over-exploited. First, there is a long list of over-utilized resources. These are some species which have been overfished:

  • New England groundfish and flounder

  • Southeast Spiny Lobster

  • Atlantic Bluefin Tuna and Swordfish

  • Main Hawaiian Island Bottomfish and Pelagic Armorhead

  • Large Coastal Sharks

  • Gulf of Mexico King Mackerel and Pink Shrimp

  • Atlantic/Gulf of Mexico/Caribbean Reef Fish Complex

  • Pacific Ocean Perch

  • North Pacific Albacore

  • Oysters, Hard Clams, and Abalones in many location

Secondly, the dates at which over-fishing began for various North Atlantic fisheries are alarming. From the table below, we can see that as we overfished one species, we simply moved to another and overfished that as well.

Table 2: Peak catch year of some fish species
Species Peak Year Peak Catch 1992 Catch Decline
(in millions of tons)
Percent Change
Pacific herring 1964 0.7 0.20 0.5 -71%
Atlantic herring 1966 4.1 1.50 2.6 -63%
Atlantic cod 1968 3.9 1.20 2.7 -69%
South African Pilchard 1968 1.7 0.10 1.6 -94%
Haddock 1969 1.0 0.20 0.8 -80%
Peruvian anchovy 1970 13.1 5.50 7.6 -58%
Polar cod 1972 0.35 0.02 0.33 -94%
Cape hake 1972 1.1 0.20 0.9 -82%
Silver hake 1973 0.43 0.05 0.38 -88%
Greater yellow croaker 1974 0.20 0.04 0.16 -80%
Atlantic redfish 1976 0.7 0.30 0.4 -57%
Cape horse mackerel 1977 0.7 0.40 0.3 -46%
Chub mackerel 1978 3.4 0.90 2.5 -74%
Blue whiting 1980 1.1 0.50 1.8 -26%
South American Pilchard 1985 6.5 3.10 3.4 -52%
Alaska pollock 1986 6.8 0.50 1.8 -26%
North Pacific hake 1987 0.30 0.06 0.24 -80%
Japanese pilchard 1988 5.4 2,5 2.9 -54%
TOTALS ---  51.48 21.77 29.71 -58%
Source: FAO

Finally, specific examples of fishery declines highlight the over-consumption problem. 

5.  New Methods, New Targets, and Over-Capacity

Numerous statistics point to over-capacity:

Despite warnings of a slowdown in the marine catch in the 1970's and 80's, the fishing industry increased fishing efforts. Over the past 40 years, the technology used in fishing has improved. Now, boats are more powerful, fish are located electronically through sonar, larger nets are used, and there are just more fishing operations.

Today, the industry is twice as large as necessary. It could go back to the smaller, fewer boats of 1970 and still produce the same yield. This overcapacity is global: Norway is 60% over, while the European Union is 40% over. In the U.S., there are ten times the number of boats needed for the surf clam industry.



Figure 3: Drift-net in use.









How did this overcapacity develop? Competition led to an all-comers welcome approach. More competition for declining resources leads to overcapitalization in ever larger boats and nets.

Drift nets (see Figure 3) are a spectacular example of the new more efficient fishing methods. These monster nets (50 feet by up to 65 km) kill all that they encounter. They are banned by every fishing country within its own territorial waters. The combination of Japanese, Korean, and Taiwanese drift nets cast every night in international waters reaches about 48,000 km--enough to encircle the globe.

Another piece of evidence suggesting that we are overharvesting our seas is that we have been relegated to fishing for previously unfished stocks. We are now eating species heretofore thought of as "bait". 

The Peruvian Anchovy Fishery

To illustrate how overcapacity works, we will study the example of the Peruvian anchovy, which in boom years was the largest new fishery in the world. 

Figure 4: Annual catch of the Peruvian
Anchovy Fishery from 1960-1990

Before 1950, fish in Peru were harvested mainly for human consumption. The total annual catch was 86,000 tons. In 1953, the first fish meal plants were developed. Within 9 years, Peru became the number one fishing nation in the world by volume. This lead to a period of boom years in Peru. 1,700 purse seiners exploited a 7-month fishing season.

Fearing a crash, in 1970, a group of scientists in the Peruvian government issued a warning. They estimated that the sustainable yield was around 9.5 million tons, a number that was currently being surpassed (see Figure 4). The government turned a deaf ear toward its own scientists. Due to the collapse of the Norwegian and Icelandic herring fisheries the previous year, Peru was more poised than ever to earn yet more hard currency. Therefore, in 1970, the government allowed a harvest of 12.4 million tons. The following year, 10.5 million tons were harvested. In 1972, the combination of an El Nino year and the prolonged overfishing led to a complete collapse of the fishery. It has not recovered.

6.  Maximum Sustained Yield of the World's Oceans

Figure 5: Relative productivity of ocean zones















To fish our waters more sustainably, we need to know what the sustained maximum yield is. One theoretical estimate puts the estimated annual production at 240 million metric tons. The estimated annual harvest is half of this: 100-120 mmt. The current annual harvest is about 100 mmt.

Not all areas of the ocean are equally productive (Figure 5). As you can see from this figure, the coastal margins such as mangroves and saltmarshes are much more productive relative to their volume than the open ocean. Therefore, to accurately estimate the maximum yield of the ocean, we must look at the zones separately. The estimate used above was obtained by dividing the ocean into three zones: open ocean, coastal areas, and upwelling areas. 

The estimate for the productivity of each of the three zones was estimated based on three values: primary plant production, food chain length, and food chain efficiency. To further understand the relationship these values and productivity, you may want to review some lectures from last semester:

Figure 6: High phytoplankton production zones










Figure 6 shows the areas of highest phytoplankton production. It is these areas upon which we most rely for our fish. Blooming across large regions, phytoplankton form large fields that sustain the marine food web. A high proportion of these productive zones are found where the ocean is rich in minerals. 99% of the worldwide annual commercial ocean catch comes from coastal waters, within 200 nautical miles of the coastline. These narrow coastal fringes of the world's oceans are at once its most productive and most vulnerable zones.

Food chains tend to be short in highly productive areas, such as upwelling zones, and longer in the open ocean (see Figure 8 below).  As a consequence, not only is there less primary production in the open ocean, but it must be transferred through many more levels of the food chain before reaching the trophic level that we harvest.  Because each transfer of energy from trophic level to trophic level has an average efficiency of about 10%, much less energy is available to humans if we are consumers at the end of a long food chain.


Figure 8:  Food chains of the open ocean, continental shelves, and upwelling zones

We can now combine all of this information (Table 2).  Nutrient availability, primary production, food chain length and food chain efficiency all differ across these three zones.  We suspect that food chain efficiency is highest in the upwelling zone because organisms are concentrated within a small area.  Note that coastal and upwelling zones contribute about equally to harvestable fish production, while the open ocean's contribution is relatively small.

Table 2: Estimated Production of Harvestable Fish
Open Ocean Coastal Waters Upwelling Areas
Nutrient concentration low intermediate high
Primary Production low intermediate high
Food Chain Length long intermediate short
Ecological Efficiency lower ------------> higher
Fish Production negligible about half of total about half of total

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 shifted from long-lived, high-trophic level fish (e.g. cod and haddock) to low-trophic-level fish and invertebrates such as anchovy and krill.

  • Paradoxically, catches stagnated or declines, as competitors (such as inedible jellyfish) fill the void. "If things go unchecked, we might end up with a marine junkyard dominated by plankton." 

7.  Solutions

The increasing trend toward aquaculture may take some of the pressure off our overfished seas. Between 1984 and 1994, aquaculture was the fastest growing supplier of fish worldwide. Fish farms now account for more than 1/8th of the worlds catch. In China, India, and Japan, aquaculture accounts for half of the total fish eaten. Aquaculture has already eased some of the pressure on shrimp. Also, aquaculture allows for more optimal use of feedgrains than the poultry or beef industury (this means less grain per pound is needed for fish than those sources). However, there is some question about the long-term sustainability of aquaculture. Negative impacts of aquaculture include disease, genetic weakening of stocks, and coastal habitat destruction.

Downsize the existing fishing fleet. It is likely that a reduction of 30-50% will be required.

Reduce subsidies to the fishing industry. As seen in the following figure, currently the cost of fishing outweighs the revenues 80 billion to 75 billion.


International agreements of fish catch limits. International agreements are crucial to prevent commercial extinction, as fish do not respect international borders. This creates a "Tragedy of the Commons" situation, where any fish that you do not take go into your neighbors mouths. Although they rarely make the evening news, there have been more fishery conflicts in the 1990's than in the whole 19th century. Some examples are:

  • The Cod Wars between Norway and Iceland

  • Turbot Wars between Canada/Spain, Argentina/Taiwan, and China/Marshall Islands

  • The Tuna Wars of the Northeast Atlantic

  • Crab Wars of the Southwest Atlantic

  • Squid Wars

  • Pollock Wars in the Sea of Okhotsk

  • The Salmon Wars of the Northern Pacific 

What can you do?

The above solutions need to be carried out by consensus among and within governments. But you, by yourself, can make a difference by being an informed consumer. Don't buy species of fish that are over-exploited, such as Atlantic Cod, Atlantic Sea Scallops, Black Sea Bass, Farm Raised Shrimp, Gulf Shrimp, Monkfish, Redfish, Swordfish, Shark, Red Snapper, Sturgeon, and Winter Flounder. Instead, order species like Alaska Salmon, Pacific Coast Dungeness Crab, and trapped shrimp that are not currently overfished. See the Monterey Bay Aquarium website for more information.

Fisheries Self Test

Suggested Readings:

  • World Resources Institute. 1994. World Resources 1994-95: A guide to the global environment. Oxford.

  • Norman Myers, ed. 1993. Gaia: An Atlas of Planet Management, Anchor Books.

  • U.S. Department of Commerce, 1995. Fisheries of the United States, 1994, NOAA Current Fisheries Statistics No. 9400.

  • Gurney, R.J, J.L. Foster, and C. L. Parkinson. 1993. Atlas of Satellite Observations related to Global Change, Cambridge Press.


All materials © the regents of the University of Michigan unless otherwise noted.