Abstract
In order to improve the capacity of bottom trawl fishing gears to reduce catches of young fish and discards in a highly exploited demersal trawl fisheries in the Mediterranean, the size-selection performance of a 36-mm square-mesh codend and two sorting grids with 20 and 15 mm bar spacing was assessed. Alternate hauls were used to assess the selectivity of 36-mm square-mesh codend. Selectivity of sorting grids was assessed using a double codend in which fish that escaped through the grid were captured in the lower codend while other fish were guided into the upper codend. The mean selection length (L50) for European hake was 18.5 cm and that for the Atlantic horse mackerel was 14.0 cm with the 36-mm square-mesh codend . These values are close to their current minimum landing sizes (20 cm for hake and 12 cm for horse mackerel). The sorting grid with 20-mm bar spacing showed L50 value of 13.3 cm for hake, suggesting that a larger grid-spacing would be needed to obtain sufficient sorting performance. Similarly, for Atlantic mackerel the estimated L50 of 14.3 cm indicates that larger grid spacing is required to attain an L50 that would be close to the current MLS (18 cm). For Atlantic horse mackerel and red mullet, the L50 obtained with the sorting grid with 20-mm bar spacing was close to the MLS of these species (the MLS of red mullet is 11 cm). The selectivity of the sorting grid with 15-mm bar spacing was generally very poor. Size-selection performance of sorting grids was assessed for the first time in the local fisheries. In order to effectively improve size-selection, seasonal and depth-dependent differences between target and by-catch species must be taken into account, which is a good indicator of the difficulty of implementing a single mesh size or grid spacing in the Mediterranean demersal trawl fishery.
Highlights
Fishing pressure generated by the multispecies trawl fishery in the Mediterranean Sea has increased during the last two decades and many commercial species are overexploited
Selectivity of sorting grids was assessed using a double codend in which fish that escaped through the grid were captured in the lower codend while other fish were guided into the upper codend
For Atlantic mackerel the estimated L50 of 14.3 cm indicates that larger grid spacing is required to attain an L50 that would be close to the current minimum landing size (MLS) (18 cm)
Summary
Fishing pressure generated by the multispecies trawl fishery in the Mediterranean Sea has increased during the last two decades and many commercial species are overexploited. An indication of this is the high share of juvenile fish in the catches (Larrañeta et al, 1969; Stergiou et al, 1997; Lleonart, 2001; Sánchez et al, 2004). Poor selectivity of bottom trawls using the legislated 40-mm diamond mesh codend greatly contributes to the unfavourable catch composition and relatively high discard rates (Martín et al, 2001; Sánchez et al, 2004). Many other commercially important species caught in large quantities are undersized and immature, mainly due to the use of a 40 mm diamond-shaped mesh that is poorly selective, preventing the small individuals from escaping the nets (e.g. Sardá et al, 2004)
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