Overfishing, Overfished, and rebuilding stocks

Since peak exploitation in the 1980s when many stocks all over the globe were fished beyond their limits, management has been getting better—a lot better. A conservative estimate says at least 82% of wild-caught fish come from fisheries with healthy populations. We all want to eat fish from MSY-maintained populations, but can fish from a depleted, or overfished, population ever be sustainable? The answer is actually, yes, so long as overfishing is not occurring. In this post we explain overfished, overfishing, and rebuilding stocks.

Overfishing & overfished

In general, we have called fisheries “sustainable” or “unsustainable” according to the United Nations FAO’s conclusions on which fisheries are overexploited and which are not. Overfishing, if persistent, generally leads to a stock being overfished and thus, overexploited. That may sound obvious, but “overfishing” and “overfished” are two different occurrences measured by two different units.

Overfishing refers to fishing mortality (F), or the rate of fish killed by catching them (just think of this as the proportion of fish caught). There is an ideal proportion of fish to catch that will produce MSY—this is called FMSY. If the proportion of fish caught (F) is greater than FMSY, overfishing is happening. If F is less than FMSY, underfishing is happening. Fishing mortality is usually given as a ratio of F/FMSY; a ratio over 1 means overfishing.

Overfished refers to the biomass (B) of a population, or stock, of fish. This is the amount of fish in the water. There is some amount of biomass, B, that will produce MSY—this is BMSY. If the biomass of fish in the water is well below BMSY, the stock is overfished, or depleted. If the amount of fish in the water is more than would produce MSY it is underfished. The ratio of B/BMSY is commonly used, though the number of demarcation varies by governing body (B may represent spawning biomass, vulnerable biomass, or total stock biomass). According to the UN FAO, a ratio under 0.8 signifies an overfished fishery; however the USA typically defines an overfished fishery as any with B/BMSY below 0.5. The reason for this is to make governance easier—fisheries have natural population cycles, some may naturally stay below 0.8 for a few years then be over 0.8 for a few years. If the government is legally mandated to assist overfished fisheries (as the USA is), it makes sense to help fisheries that are significantly depleted instead of those that may be on the downswing of a natural cycle. We typically refer to overfished as anything with a B/Bmsy below 0.8, but you should know that not all of these fisheries are in dire shape.

We can plot the F/Fmsy ratio against the B/Bmsy ratio to get a sense of the sustainability of a fishery. Below is one of these “Kobe plots” of most of the world’s fisheries. Each dot represents a fishery. The red dots represent data from the RAM Legacy Stock Assessment Database, and the gray dots represent estimates for unassessed fisheries from Costello et al. 2016. Dot size scales to fishery MSY. The triangle is the median and the square is MSY-weighted mean. Try to guess which quadrants are sustainable and which are not. Mouse over to see the answers.

The intersection of the two ratios is theoretical MSY. The goal of management is to get a fishery as close as possible to those ratios. Fisheries with B/Bmsy below 1 (overfished), but with F/Fmsy also below 1 (underfishing), are rebuilding. The fishery is not at the right population level, but fishing pressure is low enough that B will continue to increase.

Closing a fishery

If a fishery is overfished, shouldn’t we stop fishing (F/Fmsy =0) it to let the population recover? In some cases of significant depletion, fishery managers have closed a fishery to rebuild, but remember—fishery management has multiple goals, one of which is to ensure well-being for people who depend on a fishery being open, e.g. fishermen and women can’t simply not work for a year.

r vs K

Fortunately, fish stocks can rebuild quickly even without closing fisheries because they evolved to reproduce quickly. Animals want their offspring to pass on genes through further generations—some animals take great care in raising their offspring to be successful in this endeavor. They invest time, energy, and patience in their young to guide them into adulthood successfully. For example, orangutan mothers spend 9 years raising their babies before they are ready to set off on their own. Investing time and energy into offspring is a K-selected reproductive strategy.


Other animals take a different approach to babies: instead of investing time and energy into raising a few offspring, they produce orders of magnitude more offspring and invest little to no time, strategizing that with so many offspring, a few are bound to be successful. Nearly every fish caught for commercial fishing reproduces this way, sending hundreds of millions of gametes into the water column. This r-selected reproductive strategy bodes well for rebuilding populations if population numbers are below an environment’s carrying capacity. With the right management, fisheries around the world can rebuild in just a short amount of time—we are already seeing it with so many stocks rebuilt from the depleted years of the 1980s & 1990s.

Fisheries are a renewable resource.

A natural resource is taken from the Earth and used by humans. We rely on natural resources, like freshwater, for survival. We cut down trees to build our homes, we dig up coal to power them, and claim the land our homes sit on. The raw components of nearly all our possessions come from natural sources; the device you are reading this on is reliant on precious metals mined from Earth.

Some natural resources, like coal, land, and metals are finite—there is only so much on Earth and we have to make do with what we have or find alternatives. The best kinds of natural resources are renewable—they can be used by people continually (so long as they are managed well). Of the world’s fisheries, 69% are sustainably managed. We are working to ensure that number eventually reaches 100%.


This post is part of Sustainable Seafood 101

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