Modify design or arrangement of tickler chains/chain mats in a bottom trawl
Overall effectiveness category Awaiting assessment
Number of studies: 2
Background information and definitions
Trawl nets are towed through the water by fishing vessels. Some types of bottom trawl, particularly beam trawls, are rigged with ‘tickler’ chains or a chain mat, that spread from one side of the trawl mouth to the other, contacting the seabed. Tickler chains and chain mats physically disturb the seabed to increase the likelihood of animals entering the mouth of the trawl. However, they often cause unwanted species to enter the trawl as well as those targeted commercially. Modifying tickler chain arrangement (such as reducing the size of chain links, or attaching only one end of the chain to the beam; Bergman & Van Santbrink 2000; Broadhurst et al. 2015), or even removing them altogether, may reduce the seabed disturbance and therefore result in fewer unwanted fish species/sizes being retained by the gear.
For related interventions describing other modifications to the overall design of different types or parts of trawl gear is summarized under ‘Fishing gear modification - Modify the design or configuration of trawl gear (mixed measures)’, ‘Change the size of the main body of a trawl net’, ‘Decrease the circumference or diameter of the codend of a trawl net’, ‘Modify the design or configuration of trawl doors’ and ‘Modify a bottom trawl to raise parts of the gear off the seabed during fishing’.
Bergman M.J.N. & Van Santbrink J.W. (2000) Mortality in megafaunal benthic populations caused by trawl fisheries on the Dutch continental shelf in the North Sea in 1994. ICES Journal of Marine Science, 57, 1321–1331.
Broadhurst M.K., Sterling D.J. & Millar R.B. (2015) Traditional vs. novel ground gears: Maximising the environmental performance of penaeid trawls. Fisheries Research, 167, 199–206.
Supporting evidence from individual studies
A replicated, paired, controlled study in 1999 on bottom fishing grounds in the North Sea between the Netherlands and UK (van Marlen et al. 2005) found that two different modifications to the way tickler chains were attached to a beam trawl (hanging parallel from, or in loops along, the beam) did not reduce the discarded fish catch, compared to a conventional tickler chain arrangement. For unwanted fish (undersized commercial target species and all other non-target species), there were no differences in catches between the modified and conventional tickler chain arrangements, except for small whiting Merlangius merlangus, of which fewer were caught by the modified gear (data not reported). Data were collected from a series of beam trawl deployments along six parallel strips on the seabed (2,000 m × 30 m) using two modified and one conventional tickler chain arrangement. In March-April 1999, a total of 72 deployments were carried out with three different configurations of parallel chains (numbers and spacing, connected pairs) hung along the beam. In October 1999, a total of 35 deployments were undertaken with three configurations of chains hung in loops from the beam. In addition, a standard trawl with conventional tickler chain arrangement (attached to the shoe plates on either end of the beam) was towed simultaneously during each deployment. All catch was weighed.Study and other actions tested
A replicated, paired, controlled study in 2005 in an area of seabed in deep water in the Atlantic Ocean off the Isle of Skye, Scotland, UK (Kynoch et al. 2015) found that removal of the tickler chain from a bottom trawl reduced the capture of unwanted skates/rays (Batoidea) and unwanted sharks (Chondrichthyes) compared to a standard trawl with a tickler chain. Overall, trawl gear without a tickler chain decreased the catch of skates/rays (four species) and sharks (three species) compared to a trawl with a tickler chain (skates/rays, without: 198 individuals, with: 625 individuals; sharks, without: 993 individuals, with: 1,357 individuals). For the more commercially valuable species, overall catch rates of three flatfish species (Pleuronectidae) and two cod-like species (Gadidae) were similar between trawls, however, catch rates of two anglerfish Lophius sp. decreased with a tickler chain (see paper for data). Trials took place onboard a commercial fishing vessel in October 2005. A total of 17 paired deployments of standard bottom fish trawls, one with the tickler chain removed, were made, parallel to one another, at 120–170 m depth.Study and other actions tested