Gear technology in Nephrops trawl fisheries

Abstract

Nets with a small mesh size are required to catch Nephrops norvegicus, consequently large quantities of small whitefish are also caught, and much of this bycatch is undersized and is discarded dead. The main bycatch species are whiting (Merlangius merlangus), haddock (Melanogrammus aeglefinus) and cod (Gadus morhua). Here we summarize the known behavioural reactions of these species towards conventional trawls and review the results of using different trawl modifications to increase selectivity of Nephrops trawls. The trawl modifications are categorised as separator grids, separator and guiding panels, square-mesh panels, capture avoidance designs and codend modification. Finally, the extent to which these developments have been legislated for is discussed including the conditions under which new gear regulations have been introduced.Haddock and whiting rise during the trawling process facilitating their separation from Nephrops and escape, however the behaviour of small fish of these species is less consistent. Cod and Nephrops remain on bottom of the trawl, so to separate these species requires some physical filtering process. Overall, there is currently sufficient technical ability to improve selectivity in Nephrops trawls. The design of choice is dependent on the objectives of managers; for reducing discards but retaining marketable fish, square-mesh panels offer the most useful tool; to eliminate all bycatch and create a single-species fishery, grids and traditional Nephrops trawls show most potential. Whatever the objectives of the new measures, it is likely that a short-term economic impact will follow, and some form of incentive may be required to implement effective measures. A voluntary uptake of new measures by industry is preferable, however, to date, restrictions on fishing opportunities have been necessary to introduce innovative gear designs.

Comparing selectivity of a standard and turned mesh T90 codend during towing and haul-back

“…The use of a 120 mm square mesh panel does not provide any major improvement of the selectivity for these three species ), and additional means are clearly needed to improve it. Considering the high release of Norway lobster and cod during haul-back, the tendency towards a general loss of Norway lobster above MLS, and the lack of effect on plaice, the T 90 codend does not appear to be an optimal solution for the Norway lobster-directed fishery in the KattegatSkagerrak area or the many other areas where comparable problems exist (Catchpole and Revill 2008). Increasing retention of Norway lobster above MLS by reducing the T 90 mesh size would also cause increased catches of cod and plaice below MLS.…”

“…However, the relatively small mesh sizes used to retain Norway lobster cause high bycatch rates of juvenile individuals in many areas. Due to regulations on minimum landing size (MLS) and size-related value of the fish, this results in generally high discard rates in European Norway lobster fisheries (Catchpole and Revill 2008), which could be reduced by improved selectivity patterns. The Kattegat-Skagerrak area is characterized by high discard rates Valentinsson and Ulmestrand 2008;Frandsen et al 2009;Feekings et al 2012), the selectivity of the standard 90 mm diamond mesh codend used in this area is poor Frandsen et al 2009), and the cod stock in Kattegat is in a critical state (Madsen and Valentinsson 2010).…”

Testing Nordmøre grids on the target and by-catch species of the commercial bottom trawl fishery in the Gulf of Cadiz

“…If fishers cannot be convinced of the medium and long-term conservation and economic benefits of more selective fishing, and be persuaded to voluntarily adopt new measures, then direct economic incentives would be required (Catchpole and Revill 2007). This type of approach, adopting different measures and agreements for each type of licence, could be acceptable to the commercial trawling fleet in the Gulf of Cadiz, reducing the overharvesting of demersal resources and contributing to a more sustainable multispecies fishery.…”

Fleet dynamics in a changing policy environment

“…towing speed or tow duration) (Dahm et al, 2002, Catchpole andGray, 2010). Gear modifications may consist of changing the configuration of the net by increasing mesh sizes , van Marlen, 2003, Erzini et al, 2006 or by inserting square-or diamond-mesh escape panels (Stewart and Ferrell, 2002, Suuronen et al, 2005, Catchpole and Revill, 2008, Briggs, 2010 and separator and sorting grids (Polet, 2002, Graham andFryer, 2006), but can also lead to the development of alternative fishing techniques such as the pulse trawl in the Dutch demersal fisheries (chapter 4) (van Marlen et al, 2014. In mixed fisheries the development of gear modifications can be challenging as improving the selectivity for a given species or size class can coincide with losses or additional retention of others (Polet, 2002, Stewart andFerrell, 2002).…”

“…This may create an incentive for the fishing industry to use more selective fishing gears to avoid unwanted catch (Condie et al, 2013, Condie et al, 2014. Gear modifications such as an increase in cod-end mesh size or the deployment of escape panels and sieve nets have proven to reduce the discards of unwanted and undersized fish Revill and Holst, 2004;Catchpole and Revill, 2008).…”