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Over fishing: How can scientists tell?

Posted by | February 26, 2013 | Ocean Solutions | 3 Comments
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What is “MSY”?

In the news, we often hear that global fish populations are over-exploited, declining, or have collapsed. For example, the FAO (2010) reports that up to 85% of the global fisheries are being fished at the populations’ maximum sustainable yield, are over-exploited, depleted, or recovering from being depleted.

What is the threshold or benchmark that a scientist or fisheries manager uses to say “Yes, this population is being over-exploited!” Or, in the worst case scenario “The population has collapsed.”

Many organizations rely on the traditional benchmark of Maximum Sustainable Yield, otherwise known as MSY.

FAO_2010

Figure 19 – State of the World Fisheries and Aquaculture 2010

You can think of MSY like this: start with a fish population that is not exploited (no one fishes it). Left on their own the population will increase to a carrying capacity. The populations’ carrying capacity is exceeded when any more fish in the population would not survive because there are not enough resources to support any new individuals.

Now, suppose that a fish population is at its carrying capacity and every year some number of fish are caught in the fishery.  Fish caught and harvested (removed from the population) by the fishery would free up resources for young fish to enter the population, thereby allowing the population to return to its carrying capacity. The key here is that there has to be enough individual fish left, after fishing, to reproduce enough young fish and maintain population growth. So, MSY is the maximum sustainable catch (yield) of fish per year from a population by the fishery that will not result in a population decline.

This graph shows theMSY theory of maximum sustainable yield. The X-axis is fishing mortality, or effort expended to capture the fish, and the Y axis is the average number of fish caught, or yield. As long as the fishery does not take more fish than the recommended MSY, on the left hand side of the graph, the fish population will continue to sustain itself and the fishery can continue.

If more fish are caught annually than the recommended MSY, the right hand side of the graph, fishing effort may increase but the average catch would decline. This is because the remaining fish population cannot reproduce enough new individuals to compensate for the number of individuals being taken by the fishing industry. If fishermen continue to catch more than the MSY, for many years, they begin to deplete the population (sometimes recodferred to as stock size). When a population is severely depleted it is called ‘collapsed’. In Canada, the most famous fishery collapse is that of the cod.

To maintain populations that can sustain fishing, fisheries managers try to set fishing quotas at or below the MSY. Often, poor data quality leads to an overestimate of the MSY and fishing quotas that can result in population decline or collapse.

Now when you hear in the news that a fish population is over-exploited, over-harvested, or fished beyond its MSY – you’ll know exactly what that means!

This article by World Wildlife Fund gives more details on other sub-categories of MSY that you may find in fishery management reports (click here!)

There are many limitations of the MSY benchmark. MSY is usually a single fish species estimate limiting our ability to understand the influence of species interaction, it often does not account for year to year variability in climate or other environmental variables, and it takes A LOT of data to come up with a ‘good’ estimate of MSY, because it also requires estimating the populations’ carrying capacity.

Are there any other options for management benchmarks?

Multi-species and ecosystem-based management approaches have been making a big splash lately. Ecosystem-based management focuses on not just the abundance of one fish population, but the fish habitat, by-catch of a fishery, food-web effects, and genetic diversity. When we manage at the level of an ecosystem rather than individual species it is possible to set goals to sustain healthy ecosystems and implement policies that protect the structure and function of the whole ecosystem which will inevitably help sustain the target fish populations.

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3 Comments

  • Hector R. Leta says:

    In Fisheries Management is very important to know the prey-predator relationships between our target species and the species she feeds upon. This is the case of hake-anchovy or hake-squid in the Southwestern Atlantic. Both predated species were fishing available stocks too and if they were overfished hake avilable stock among other predator would diminish its number due to the lack of food. When something like this happens some compensatory phenoma appears like canibalism. I observed this many times on board when studying hake stomach contents. So it is very important to know the interrelatonships between fishing stocks and their balance in the ecosystem when managing fisheries.

  • [...] – of the non-certified fisheries are doing OK too. See our blog on maximum sustainable yield (Over fishing: How can Scientist Tell?) to help understand this [...]

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