Benthic Trawl | U.S. Fish & Wildlife Service

Benthic Trawl

Description

Benthic Trawl Example

Benthic trawls in freshwater are primarily Otter trawls. Beam trawls have limited use compared to marine applications. Push trawls (Otter trawl type) are considered surface trawls but can be modified for application in smaller streams to capture benthic fishes. See Introduction to Trawls and Push Trawls modules for more materials. Accordingly, this module will concentrate on Otter trawls. Benthic Otter trawls have few floats which are intended to keep the trawl mouth open. Additional weights are added to the Groundline or Footrope to increase contact with the bottom. The Groundline may have a forward-running chain as well for bottom contact and to protect against obstructions. Benthic trawls for freshwater are bow trawled with the water current. Trawling may be accomplished by hand or mechanically assisted. Electricity is applied in some situations. See Electrified Benthic Otter Trawl, Electrified Dozer Trawl, and Electrified Side (Abeam) Trawl for electric trawls.

Habitat deployed	Lakes, Mid- to large rivers, flooded tributaries. Water depth greater than 1m (3 ft.).
Target species	Many species, often small to medium bodied fishes and size classes- gar (Lepiosteidae), minnows and shiners (Cyprinidae), darters (Percidae), Suckers (Catostomidae), Sturgeon (Acipenseridae)

Benthic Otter Trawl Anatomy

Benthic Trawl Anatomy

A benthic Otter trawl dorsal view.

A Benthic trawl used for capturing sturgeon in the Missouri River

A Benthic trawl used for capturing sturgeon in the Missouri River. The instructor holds the cod end which itself is weighted with a chain and is tied to a buoy line. The Footrope has additional weights added. There are very few floats on the Headrope, intended just to keep the trawl mouth open as it runs along the bottom of a lake or river.

A Benthic trawl closeup of the Footrope and Forward-running chain

A Benthic trawl closeup of the Footrope and Forward-running chain. Note the weighting on the Footrope.

Closeup of the Forward-running chain showing a “Breakaway” link that gives way to help retrieval of trawls snagged on obstructions.

Closeup of the Forward-running chain showing a “Breakaway” link that gives way to help retrieval of trawls snagged on obstructions.

Benthic Trawl Anatomy

Benthic Trawl Anatomy

Overview of the parts of a benthic trawl for sturgeon

VIDEO LINK: https://youtu.be/vg5VEAQHKYk

Benthic Trawl for Small Fish

Benthic Trawl for Small Fish

Demo of a small-mesh benthic trawl with significant chains for running on the river bottomand capturing Young-of-Year or small bodied individuals

VIDEO LINK: https://youtu.be/SLMMtCQfVFA

Benthic Skate Trawl for Diverse Sturgeon Sizes

Benthic SKT Trawl for Diverse Sturgeon Sizes

Demo of a benthic trawl for sturgeon of various sizes

VIDEO LINK: https://youtu.be/imrFMjDrKZ8

Three wooden Otter boards showing size differences

Three wooden Otter boards showing size differences. All have metal strips or runners on the board bottom. (Note the metal otter boards in the background which are surface trawl Otter boards).

Otter Board Inspection Techniques

Otter Board Inspection Techniques

Overview and inspection of Otter boards for proper sampling

VIDEO LINK: https://youtu.be/asEPeXmPKCE

Bow (Benthic) Trawling Overview

Benthic trawling in freshwaters is accomplished by bow trawling. Bow trawling typically and historically was manually, but mechanized systems are being developed to improve sampling.

Introduction to Bow Trawling

Mechanized Bow Trawling Part 1

Mechanized Bow Trawling Part 1

VIDEO LINK: https://youtu.be/WV4HKSwdb_8

Mechanized Bow Trawling Part 2

Mechanized Bow Trawling Part 2

VIDEO LINK: https://youtu.be/fDYVXLUoRnc

Manual Bow Trawling Intro and Snag Retrieval

Manual Bow Trawling Intro and Snag Retrieval

VIDEO LINK: https://youtu.be/nRWsszOs1rc

Mechanized vs Manual Bow Trawling

Mechanized vs Manual Bow Trawling

VIDEO LINK: https://youtu.be/HU6pDMxUsww

For additional information on net design, Otter boards, cod end function, fykes, and tying, lines, rigging, maintenance, storage, and some technical aspects, see Introduction to Trawls module.

Bow (Benthic) Trawling Field Methods

Manual Bow Trawling

Manual Bow Trawling and Snag Retrieval

Manual Bow Trawling and Snag Retrieval

Two trawling runs on the Missouri River, the first ending with a snag and net retrieval and the second trawl being completed.

VIDEO LINK: https://youtu.be/k20kNXHvURU

Manual Bow Trawling with Fish

Manual Bow Trawling with Fish

One trawling run on the Potomac River that includes some fish workup

VIDEO LINK: https://youtu.be/W9LVeKW4dlQ

Otter trawl is being prepared for sampling

The benthic Otter trawl is being prepared for sampling. The buoy, net, warps, and Otter boards are visible.

The trawl deployment begins with tossing out the buoy (which is tied to the cod end).

The trawl deployment begins with tossing out the buoy (which is tied to the cod end).

The cod end is tossed out after the buoy

The cod end is tossed out after the buoy

The Otter boards are held in position for proper deployment

The Otter boards are held in position for proper deployment. The trawl body is nearly submerged.

The Otter boards are carefully played-out into the water to maintain the correct positioning of the trawl net

The Otter boards are carefully played-out into the water to maintain the correct positioning of the trawl net

The trawl is being let out to the desired sampling position

The trawl is being let out to the desired sampling position. Note that the ends of the line are attached to the side of the boat near the rear of the bow deck.

At the completion of the sampling run, the tow lines or Warps are pulled in to retrieve the trawl

At the completion of the sampling run, the tow lines or Warps are pulled in to retrieve the trawl. Note again the tie of the Warp to the hull.

The trawl is being hauled up on the deck

The trawl is being hauled up on the deck.

The crew is grabbing the trawl to bring it onboard

The crew is grabbing the trawl to bring it onboard. Note the Forward-running chain near the Groundrope.

The trawl has been successfully retrieved

The trawl has been successfully retrieved. Next, the cod end will be emptied of the catch.

Mechanized Bow Trawling

Bow Trawling on the Missouri River using powered winches and booms

Mechanized Bow Trawling

Bow Trawling on the Missouri River using powered winches and booms

VIDEO LINK: https://youtu.be/7sC8s4rLG04

Fish workup of the sample taken by mechanized trawling

Trawl Missouri River Fish Workup

Fish workup of the sample taken by mechanized trawling

VIDEO LINK: https://youtu.be/16GvP0RvH_w

The mechanized bow trawl is positioning to deploy the benthic trawl.

The mechanized bow trawl is positioning to deploy the benthic trawl. The Otter boards can be seen near the top of the booms and the trawl net is near the boom base.

The trawl is being lowered into the water as the boom extends downward.

The trawl is being lowered into the water as the boom extends downward.

Bow trawling in progress.

Bow trawling in progress. The boat is reversing, going with the current. The Warps are fully extended.

A closer sideview of the crew positioning during sampling.

A closer sideview of the crew positioning during sampling.

At the end of the sampling transect, the mechanical booms are being raised thereby lifting the trawl.

At the end of the sampling transect, the mechanical booms are being raised thereby lifting the trawl.

The trawl has been retrieved and the cod end is being emptied into a 5-gallon bucket.

The trawl has been retrieved and the cod end is being emptied into a 5-gallon bucket.