Selectivity of conventional and alternative codend design for five fish species in the Aegean Sea

Abstract

Currently, traditional Mediterranean trawls are generally made with non-selective netting and the fishing boats are involved in multi-species fisheries. As a result, most near-shore stocks are over-exploited. Weather permitting, the demersal trawl fleet tends to fish in relatively deeper, international waters of the Aegean Sea, where the catch is usually higher. Therefore, the need for evaluation of the codends used in this fishery and the potential improvements to their selectivity are of prime importance. In the present study, selectivity data were collected for hake (Merluccius merluccius), blue whiting (Micromesistius poutassou), greater forkbeard (Phycis blennoides), blackbelly rosefish (Helicolenus dactylopterus dactylopterus) and fourspotted megrim (Lepidorhombus boscii) in commercial (300 MC) and square mesh top panel (SMTPC) codends. Trawling was carried out at depths of 274-426 m onboard a commercial vessel chartered for a 15-day sea trial in August 2004. Selection parameters were obtained by fitting a logistic equation using a maximum likelihood method. Results of the selectivity analysis indicated that the commercially used 40 mm nominal mesh size PE codend was rather unselective for the species investigated in this study. In general, the square mesh top panel codend has relatively higher L 50 values than the commercial codend. However, except for blue whiting, even this codend is rather unselective when 50% maturity lengths (L M50 ) are considered.

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“…Stewart (2002) and Sala et al (2008) highlighted that square mesh codend gave higher 50% retention length than diamond mesh at the same mesh size for round fish. Many studies detected that using square mesh (40 mm) when considering diamond mesh (40 mm) increased selectivity for hake (Özbilgin et al, 2005 and2012;Bahamon et al, 2006;Guijarro and Massuti, 2006;Ordines et al, 2006;Luchetti, 2008;Sala et al, 2008;Sala and Luchetti, 2010;Tokaç et al, 2010). Turned mesh (T90) gave higher L50 values than both D44 (despite 10% higher mesh size) and same mesh size of square mesh (S40) for red mullet and Atlantic horse mackerel.…”

The effects of subsampling and between haul variation on the size selectivity estimation of Chilean hake (Merluccius gayi gayi)

“…The correction for the effects of subsampling was performed according to Millar (1994) who showed that for subsampled hauls Tokaç et al (2010). Argentine hake: Rojo & Silvosa (1970); Verazay et al (1992).…”

“…Haddock: Sakhno & Sadokhin (1982); Robertson & Stewart (1988); ; Reeves et al (1992); Sangster & Lehmann (1994); Halliday et al (1999); Halliday (2002); Graham et al (2004); He (2007); Grimaldo et al (2008). Blue whiting: Campos et al (2003a); Campos et al (2003b); Sala & Lucchetti (2010); Tokaç et al (2010). Pollock (saithe): Smolowitz (1983); Dahm (1998) and Graham et al (2004).…”

Effects of square-mesh codend on the benthic by-catch for the beam trawl fisheries of veined rapa whelk, Rapana venosa

“…The studies carried out to increase the selectivity of bottom trawl net in the Mediterranean Sea showed that square-mesh codend or panels give more chance to juveniles of most species to escape when compared with diamond-mesh codends (Sala et al 2008, Ateş et al 2010, Tokaç et al 2010. Underwater observations carried out by Robertson and Stewart (1988) for bottom trawl codends showed that the mesh has a tendency to close, depending upon increasing catch amount in diamond-mesh codend but in squaremesh codend, the mesh stayed open during the tow.…”