How a Trawl Works

So a month ago (yes, I am extremely punctual) Brian sent me a link to this google earth photo asking, “what are those trails behind [the boats]? Is that engine exhaust? I’m assuming these are trawlers? Could this be something being kicked up from the nets?”

He suggested I blog the questions, should I ever get around to answering them. So here we go. This is a generic sketch of an otter trawl:

The key component is the doors, or “otter boards” which drag along the bottom in order to scare and herd target species into the net. These Louisiana boats were undoubtedly fishing for shrimp on muddy bottom, in shallow coastal waters. Those doors kick up the mud, leaving a “mud cloud” in their wake which is easily visible in a few fathoms of water. The large, offshore bottom trawlers used for groundfish that I am used to here in New England fish much the same way, but you will not likely see mud clouds behind them in an aerial photo because they are fishing at considerable depth (or on hard bottom substrate).

Obviously there is a large body of scientific literature on damage to the ocean floor from trawling. Vehement Greenpeace-types demand ocean-wide bans on the gear on a regular basis. [I came across this link while doing a quick web search which illustrates the point.] Certainly sensitive coral habitats should be protected, and MPAs have been established in many of these fragile zones which prohibit bottom trawling. It does get my blood pumping a bit when people suggest trawls be outlawed. What would you have the Lousiana shrimp trawler or the Maine groundfisherman do? Pay to overhaul their entire boat (or buy a new one) that can fish gillnets? You cannot catch shrimp in a gillnet, so purportedly all our shrimp would then come from farms, which are not exactly eco-friendly. Gillnets are known for high incidence of marine mammal bycatch, and their tendency to fish forever as “ghost gear” when lost at sea. It strikes me as shocking how narrow-minded these “extreme conservationists” can be. I hoped to leave that to the right-wing evangelicals…but I digress. I believe in technological advances to minimize trawl impacts in combination with MPAs and temporal closures as a way to avoid excessive damage to critical habitats.

So there is a brief lesson on otter trawls. Your fisheries education for the day.

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This is an article that was posted on Oct 2, 03:27 PM.

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