What’s a bottom trawl survey?

By Rachel Berman

The ocean is full of a nearly uncountable number of fish, crabs, sharks, and other sea life; the true number may never be truly known. As John Shepherd once said, “counting fish is like counting trees&emdash;except they are invisible and they keep moving.” To help meet this challenge, bottom trawl surveys provide a way for scientists to gather information about what lives in the ocean. As you may have noticed, the OceanAdapt website is built using data from bottom trawl surveys.
In a trawl survey, boats go out to areas that have been pre-selected in the ocean and gather a sample of the aquatic life. These surveys are an important investment because they offer society an unbiased survey of fish and other animals from an area (fishing boats, in contrast, specifically search out certain species).

Demersal trawl diagram

Source: GoodFishBadFish.com.au

Data collection in a bottom trawl survey involves dragging a large net (see cartoon) along the ocean floor for a set time or distance. The net is then pulled up into the boat. The trawl “haul” is sorted into species and the composition of species is recorded. Because these surveys are conducted over multiple years, changes in the abundance of animals like cod, flounder, rockfish, crabs, and other species can be detected. In addition, the loads from the trawl nets allow scientists to measure length, age, weight, genetics, stomach contents and other factors. The resulting information is then used to figure out how many fish are in the ocean, which guides fisheries management decisions about how many fish can be caught each year. In addition, we use these data on the OceanAdapt site to figure out whether fish and other marine animals have shifted deeper or to different latitudes.
In the U.S., NOAA has some of its own boats to conduct trawl surveys (including some of the boats shown here). NOAA and other agencies sometimes contract with commercial fishing boats as well. Some of the first NOAA survey trawling vessels in the northeast U.S. were in use as early as 1948 (Albatross ΙΙΙ) and in 1950 (Delaware).
For more information about bottom trawl surveys, check out some of these sources: