Abstract
Mediterranean trawl fisheries are characterized by a critical combination of high fishing effort and low size at first capture for most commercial species. In this study we tested the use of sorting grids installed on bottom trawl nets to minimize the catch of undersized deep-water rose shrimp (Parapenaeus longirostris, DPS) and European hake (Merluccius merluccius, HKE). The catch of a traditional trawl net was compared with that obtained with the same net equipped with ad hoc designed juveniles and trash excluder devices (JTEDs) during an experimental survey. Three different JTEDs were tested: the first (G1-SM40) was built with a net of 40-mm square mesh; the second (G2-ST20) and third (G3-ST25) with vertical steel bars spaced 20 and 25 mm, respectively. The probability of retaining DPS and HKE by length class in trawl nets with and without JTED was analysed using generalized additive mixed models. With G1-SM40, the reduction of undersized individuals in the cod-end was about 60% and 44% for DPS and HKE, respectively. With G2-ST20, a 34% catch decrease of HKE individuals smaller than 20 cm total length was observed. A loss of marketable fractions of DPS was recorded with G1-SM40 (–25%) and G2-ST20 (30%). Finally, JTD G3-ST25 was efficient at reducing the catch of undersized specimens of DPS and HKE, but showed a higher loss of marketable fractions than the other JTEDs. Although further fishing trials would be required to maximize the sorting efficiency of the grids, the results obtained clearly indicated that grids can substantially reduce unwanted catches of undersized DPS and HKE in Mediterranean bottom trawl fisheries.
Highlights
Mediterranean demersal fisheries are characterized by a critical combination of high fishing effort and low size at first capture of commercial species (Lleonart and Maynou 2003, Vasilakopoulos et al 2014, Colloca et al 2017)
In the case of deep-water rose shrimp (DPS), mainly G1-SM40 led to a reduction in the catch of shrimps of less than 20 mm carapace length (CL) (–6%), while all three grids produced less catch of HKE
The results of our catch comparison experiment showed that the use of juveniles and trash excluder devices (JTEDs) mounted on traditional trawl nets of Mediterranean trawlers can be beneficial in reducing the by-catch of undersized individuals of two important commercial species, the deep-water rose shrimp and the European hake
Summary
Mediterranean demersal fisheries are characterized by a critical combination of high fishing effort and low size at first capture of commercial species (Lleonart and Maynou 2003, Vasilakopoulos et al 2014, Colloca et al 2017). This is mainly due to the progressive growth in engine power of trawlers, the rapid technological advances in fish finding and fishing technology, and the traditional use of fine mesh size in the cod-end (Fiorentini et al 2004). Minimum conservation reference sizes (MCRS) were defined for the most important species, in addition to a minimum legal mesh size in the trawl net cod-end of 40-mm square mesh or 50-mm diamond mesh
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