Overfishing : The Plight of our Oceans

All figures from 2010. Source

All figures from 2010. Source

When was the last time you ate sushi? Fish and chips? Clam chowder? Fish tacos? Fish is delicious, and we know it. Fish are an excellent source of protein, and millions of tonnes are consumed per year. Plus, the ocean is such a convenient food source-- in 1998, 3.2 billion people lived and worked in a coastal strip just 200 kilometers wide (120 miles), while a full two-thirds, or 4 billion, were found within 400 kilometers of a coast.

All figures from the Fisheries and Agriculture Organization.

All figures from the Fisheries and Agriculture Organization.

Economic Terms

Rivalrous Good

A good you cannot consume at the same time as another consumer (such as food or clothes).

Non-excludable Good

A good you can access even if you have not paid for it (such as televsion)

Technological advances, such as larger nets and flash freezers, coupled with larger fishing fleets, have made us more effective at fishing. Almost too effective: in 2011, we caught 93.7 million tonnes in 2011, as compared to the 20 million we caught back in 1950. No one stopped to think if this was okay, since the fish technically belonged to everybody- they were a rivalrous, non-excludable resource. And since fishing is, by its very nature, an extractive industry, and we were taking way more than we should have been, our exploitation eventually resulted in a phenomenon known as the Tragedy of the Commons.

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Soon enough, fisheries started collapsing. In the 1970s, a particularly bad El Nino decimated anchovy populations off the coast of Peru, which lead to about 35,000 fishermen losing their jobs. They went from harvesting 10.2 million metric tonnes of anchovies in 1971 to only about four million for the next five years. Similarly, in 1992, the Northwestern cod fisheries collapsed. Their populations were down to 1% of their biomass from calculated original levels, and the companies simply couldn’t turn a profit. Today, the cod have recovered to about a third of their original levels, but they still have a way to go before they can be considered healthy again. Today, we have new species to worry about: Pacific bluefin tuna are currently at 4 or 5% of their original levels, with about 40,000 wild adults left. Thankfully, we have started taking steps to remedy this!

Hopefully not too late though: as of 2009, only about 13 percent of fish stocks were in the "non-fully exploited" category, down from 40 percent in 1974. Some 57 percent are fully exploited, and 30 percent is overexploited (like the tuna and cod). We are clearly taking too much. But how much should we be taking then, you wonder? Here is one theory: on the graph below, the maximum population (N) is at it’s carrying capacity. This basically means the death rates equal the birth rates, and the population is neither increasing or decreasing. The slope of the graph is the rate of growth: that’s why, at a and b, the rate of population growth is the same, although population size is drastically different. Fisheries, using this, decided that, at half the maximum population, when growth rate is at its maximum, they could remove the most fish without affecting their populations: the maximum sustainable yield. They would be making less money off the fish now, but at least they wouldn't be depleting them too quickly, right?

*can is perhaps the wrong word, since clearly we CAN take all the fish. Source

*can is perhaps the wrong word, since clearly we CAN take all the fish. Source


Wrong. This was still overfishing, although we were doing marginally better. We failed to take into account predation, ecosystem interactions, seasonal changes, migration and a host of other factors. The assumption was that overfishing was simply taking too many fish. At its essence, that is basically what it is, but that is just the basic definition. Overfishing can occur in more than one way:

Optimum Sustainable Yield

The Magnuson-Stevens Fishery Conservation and Management Act defines optimum yield as:

  • the amount of fish which will provide the greatest overall benefit to the Nation, particularly with respect to food production and recreational opportunities, and taking into account the protection of marine ecosystems;
  • is prescribed as such on the basis of the maximum sustainable yield from the fishery, as reduced by any relevant economic, social, or ecological factor
  • in the case of an overfished fishery, provides for rebuilding to a level consistent with producing the maximum sustainable yield in such fishery.

ECONOMIC OVERFISHING occurs when an individual removes more fish than he needs to maximize his monetary gains. This is what we had originally been doing.

RECRUITMENT OVERFISHING occurs when too many adults are harvested, depleting overall reproductive capacity. Since they can’t spawn, the population can’t replenish itself.

GROWTH OVERFISHING is when fish are harvested as adults, but not at their maximum size. This can occur during sportfishing: the fish caught isn’t considered a juvenile, but it still has time to grow bigger and reproduce.

ECOSYSTEM OVERFISHING is when smaller fishes from a lower trophic level (we’ll get back to that later) dominate an ecosystem because the large predatory fish are all fished out, shifting the ecosystem balance and preventing the predatory fishes reintroduction.

We actually need to fish at Optimum Sustainable Yield, or OSY. At this level, which is lower than MSY, fish stocks can be rebuilt with ease. US fisheries are trying, but a lot of their stocks still aren’t doing well.

What are Trophic Levels?

The term trophic level refers to an organism’s position in the food chain. The first trophic level always contains the primary producers, since they convert inorganic substances into energy. On land, these are plants, and in marine webs, these are phytoplankton. They are eaten by primary consumers, or herbivorous zooplankton. All the consumers from this point forth are carnivorous: these guys are eaten by secondary consumers, generally carnivorous zooplankton, which are in turn consumed by tertiary consumers, usually planktivorous fish. Finally these guys are eaten by top predators- the carnivorous fish. Food chains are a simplified version of the complex webs that surround us in nature, but they’re still pretty useful. As example of a marine food web, with the trophic levels in parentheses, could be:

Diatoms [TL1] ---> Rotifers [TL2] ---> Jellyfish [TL3] ---> Herring [TL4] ---> Tuna [TL5]

These fish are all on overfishing lists, and all of the fish marked with an asterisk in the above gallery (*) are also on overfished lists. The New England fishes (marked with a cross) are so overfished that all the species couldn't fit on the image! A lot of these fish occupy the highest trophic level-- the fifth one, where they are the top predators. 

As we remove them, the secondary consumers right under them flourish. This (as previously mentioned) can shift the balance of the ecosystem but this doesn’t always happen. Sometimes, when fishermen notice the increase in abundance of these fish, they start fishing them instead. As they remove them, they start the cycle all over it: with the four trophic level depleted, the third one can proliferate. This is known as fishing down the food web. A very real side effect of this is the explosion in jellyfish populations, also known as a bloom. Their lack of predators, as well as other factors such as change is sea surface temperatures,  is causing a noticeable ecosystem shift, and they are flourishing.

 Of course, now people are now trying to figure out how to eat jellyfish. Just Google ‘jellyfish recipes’ and look at what shows up: everything from salad to a burger. Even if you aren’t a particularly adventurous eater, this should be a clear sign that we need to change the way we eat. This doesn't mean we should stop eating fish entirely; in fact, the cool thing about the ocean is that it can repair itself if we let it. So, if you’re an avid seafood lover, the next time you eat out or buy fish, be careful what you order; just make sure it's sustainable!a

a. Monterey Bay Aquarium has a great website where you can figure out if the fish you're eating is sustainable. In fact, you can even print it out, or get the app!