Trawling for Science
by S.W. Ross (UNCW, 7 May 2013)
On this cruise the ROV Jason II is our main science sampling tool; however, underwater vehicles cannot stay on the bottom indefinitely nor can they collect every type of sample needed. We use a variety of gear specific to different needs, and many types of nets are included in our arsenal of sampling gear. On this cruise we are using a bottom otter trawl to collect fishes and invertebrates. This is a small net (16 ft wide) that is held open by water pressure on otter doors (rectangular wood boards weighted with steel). Most of this sampling takes place at night after ROV operations, but if the weather is too rough for the ROV, we will trawl during the day, sometimes in very deep water (1670 + m). All of our tows are only 30 min, and this matches our data over the last 12 years, yielding a consistent data set for us to compare samples from the Gulf of Mexico to the Mid-Atlantic. While the tows are short, the actual time to conduct deep-water tows can be long because the amount of wire we must let out is 2.5-3 times the water depth. For deep trawls we have out 4,000 m of wire; thus, one tow will require 5 hours of vessel time.
Despite this long time commitment, trawling can be very productive and can give us data not available from other methods. For example, this project has many objectives related to deep-sea corals. Our last two trawl samples from water depths around 1,600 m yielded a number of solitary corals, and most of these came up alive. These would have been hard to find with the ROV or other gear, and they will be used by our genetics team, the coral biologist, for our trophic objectives, and for distributional data. The trawl also gave us several species of invertebrates and fishes not yet seen on the cruise. These are used for several purposes (feeding studies, reproduction biology, ecology). All data are useful and fit together to give us a better understanding of this poorly known ecosystem.
While trawls produce a lot of our seafood, and as noted above can be valuable for science, they can also cause damage to bottom habitats. We have seen lost trawl nets in these canyons wrapped around rocks and corals. In addition, large commercial trawls catch a lot of animals that are not used, are discarded dead, and are thus wasted. Fishermen do not want to lose their gear and many are very concerned about conservation. Technological improvements (better bottom maps, better navigation, sensors on nets) are allowing trawling to become more efficient and less damaging; future improvements will continue to help these valuable fisheries.