Fishery Science

In this part of Seafood 101, we simplify the fundamentals of commercial fishing in hopes of explaining its social and biological functions worldwide.

Catching Fish Sustainably

Commercial fishing is vital to global food production. Wild-caught fish contain every essential amino acid, require no land or freshwater, and are a renewable resource when managed sustainably. In addition to providing access to healthy, low-impact protein, the seafood industry is worth over a trillion dollars annually and employs 40 million people—ensuring its sustainability is vital to economies all over the world. We explain seafood industry regulations in our section on fishery management—but first, the fundamental key to understanding sustainable seafood is grasping the science of catching fish.


Fisheries are composed of fish stocks and the fishing fleet that catches them. A fish stock is simply a harvested population. It refers to one specific species in one particular place, like Gulf of Maine cod. A fishery is the intersection of a stock (or group of stocks) and the means of harvest. Fishing fleets can use several different methods to capture fish, each method describes the fishery and guides management.

A fishery is sustainable when the amount harvested does not compromise future harvests.

But how do we know how much fish can be harvested?

Fishery science is the process that answers that question, primarily through stock assessments. A stock assessment uses several different kinds of data to understand the health of a stock and determine how much can be fished. You can think of the data as the A,B,Cs of stock assessments – abundance, biology, and catch. 

  • Abundance is how many fish are in the population; estimates of abundance are made based on samples that are gathered using various methods.
  • Sampling can also collect biological data such as: age and length from which we can estimate levels of natural mortality and fishing mortality. Together, these data help estimate the reproductive rate of a population, which in turn allows us to predict how many fish will be around next year.
    • During sampling, environmental data like temperature, salinity, dissolved oxygen and other ecological variables are also collected.
  • Catch data are our historical records of how many or what weight of fish was caught during a calendar year or a fishing season.

This video does an excellent job of explaining stock assessments.

Using stock assessment data, fishery scientists create mathematical models that predict how a population will respond to different levels of fishing. With this information, fishery managers can aim to maintain fish stocks near their Maximum Sustainable Yield (MSY), the most amount of fish that can be sustainably harvested each year.

A stock being fished at maximum sustainable yield is maximally sustainably fished, (previously called fully-exploited or fully-fished), meaning the potential for food has been realized without jeopardizing the future abundance of the stock.

This is the goal for nearly every fishery in the world. Currently, the Food and Agriculture Organization (FAO) of the United Nations estimates that 60% of world fisheries are fully-fished. Fisheries that take more than a sustainable amount of fish are over-exploited (“overfished”). The latest data show that 33% of fisheries are overfished. Under-exploited (“under-fished”) fisheries, or those where potential catch is not being met, compose the remaining 7%. Under-exploited fisheries are sustainable, but from a food security perspective (and perhaps, a carbon perspective), they are not ideal as they leave nutritious, low-carbon food in the ocean.

However, it is not as simple as 67% of fish eaten are sustainable (fully or under-exploited) while 33% are not, because fisheries vary in size. For example, say a sustainable fishery is taking in 80 tons of fish while an unsustainable fishery takes 20 tons. In this scenario, 80% of consumed fish is sustainable, but only 50% of the fisheries are. Generally, management resources, like regulation and enforcement are concentrated in larger fisheries to ensure that a larger percentage of consumed fish is sustainable. With over 5,000 individual stocks and fisheries to track, it is difficult to come up with a set percentage of consumed fish that is sustainable. A recent estimate showed that about 82% of consumed fish are sustainable while 18% come from unsustainable fisheries.

What not to do:

Though this headline is technically correct, it is extremely misleading as fully exploited and overfished fisheries drastically differ in sustainability

Other sustainability factors

There are different ways to think about “sustainability” in fisheries. There is biological sustainability, where a stock of fish is caught at a rate that ensures future catches, as we’ve just discussed—but social welfare is also matter of sustainability. Social and environmental justice are issues in nearly every facet of society; fisheries are no different. We discuss these issues in a full-post here.

Another consideration in assessing the sustainability of commercial fishing is bycatch and further ecological damage caused by fishing. When fishermen and women go out to catch fish, they have one or more target species. They try to catch their target as efficiently as possible, but sometimes other species are pulled on board—this is bycatch. Sometimes bycatch is brought back to port and sold even though it may be less valuable than target species. Other times, bycatch is discarded overboard as an unfortunate environmental cost—comparable to spoiled crops that never make it to the store (though some bycatch survives being caught and thrown back). However, bycatch data is considered in stock assessments and plays a role in determining sustainable fishing levels. Too much bycatch is unsustainable. A 2011 assessment by the National Resources Defense Council (NRDC) found that about 11% of fish were thrown back. The graphic below is from that assessment and offers a summary of where in the supply chain different kinds of food are wasted. 

To reduce waste, many organizations are working to create new markets for bycatch species; this is perfectly sustainable as fish that would otherwise be thrown away are eaten. It seems as though progress is being made: a paper out last year showed only 10% of fish caught in 2016 were thrown back—this equates to hundreds of thousands of tons of fish no longer being wasted.

Potential for environmental damage caused by commercial fishing gear is also regulated through fishery management. We get into the details of fishery management and fishing gear later in Seafood 101, but understanding who is involved in fishing is crucial as well: here we layout all the stakeholders in fisheries.

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This post is part of Sustainable Seafood 101

Continue reading below:  

Seafood in a Global Context

Commercial Fishing

People & Fish