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October 17, 2014

Oceana Magazine, Dr. Pauly Column: How Do We Know How Many Fish There Are in The Sea?


To set sustainable fishing quotas, fisheries scientists must first understand how big populations are so that species can continue to reproduce and build their populations while being fished. In this column, Oceana board member and fisheries professor Dr. Daniel Pauly discusses the methodology in determining stock assessments. This article first appeared in the summer 2014 issue of Oceana magazine, and you can read previous Dr. Pauly columns in past issues here.

Ask Dr. Pauly: How do we know how many fish there are in the sea?

Fishing is meant to remove fish from the sea, and so it is no wonder that there are fewer fish in the sea, given all that we do to catch them. However, we do not want to leave so few fish in the sea that they can’t maintain their population, and this begs the question in the title: How do we know how many fish there are in the sea?

Fisheries scientists answer this question by performing stock assessments to estimate the biomass, or weight, of fish in the sea. These assessments can involve a wide array of methods, as determined by the data that are available. One method is to divide the catch of a fishery (the weight of the fish that are  caught in a given year) by the effort needed to generate the catch (the number of fishing hours or days deployed to catch the fish in that year). The result of this calculation, called the catch per unit effort (CPUE), is going to be higher when the stock is abundant and lower when the stock is depleted. Thus, if CPUE estimates are available for a number of years, their trend will be roughly parallel to the trend of the (still unknown) biomass of a fish population.

Another technique is called the swept-area method, used for fish living on or near the sea floor that can be caught by bottom trawlers. Research trawlers drag a net of known width for say one hour at a known speed to cover an area of the sea floor that can be easily calculated. Thus, their catch during that hour can be multiplied by the number of times that area fits in the entire fishing ground, and voila! In reality, analyzing the results of bottom-trawl surveys is more complicated than that, but the basic idea remains simple.

Another technique is to use sound, which we already use to locate schooling fish (like dolphins and whales also do) to estimate the size of a school of fish. Thus, if a sound wave of known energy level is sent from a fishing boat, the fraction of this wave that is reflected as an echo by a fish school will tend to be proportional to the size of that school — so a small school will reflect less sound than a big school. Echosounding — or hydroacoustic methods in general — can then be calibrated using schools that have been caught and weighed, and the biomass in the water thus estimated. This method works best with small schooling fishes, including herring, sardine, and anchovies.

Still other methods involve tagging, where a certain number of fish are given a tag or mark and then released into the population. (Tags can range from a clipped fin to electronic devices that provide information on movements and information of the environments that the fish encounter.) Subsequent catches will contain both tagged and un-tagged fish, and using some simple arithmetic one can then calculate the size of the population, along with the exploitation rate.

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