Abstract
The effects of an increase in cod end mesh size from 65 to 70 and 80 mm and a change of mesh configuration from 65 mm diamond to 65 mm square mesh on the size selectivity of horse mackerel (Trachurus trachurus), hake (Merluccius merluccius) and axillary seabream (Pagellus acarne) of the shallow groundfish assemblage off the Portuguese southwest coast were evaluated. The increase in mesh size had a small but significant effect on size selectivity for the three species, while the change in mesh configuration led to a much more pronounced increase in the selectivity parameters. For horse mackerel, the L50 estimates ranged from 14.4 to 16.0 cm in the diamond mesh cod ends-values that are close to the minimum landing size of 15 cm. For hake, L50 of 17.0 and 18.3 cm were estimated for the 70 and 80 mm diamond cod ends respectively, while for the axillary seabream the L50 estimated was 13.9 cm for the 80 mm diamond mesh cod end. These values are well below the minimum landing sizes of 27 and 18 cm for these species. The corresponding estimates in the square mesh cod end were 21.9, 32.4 and 19.6 cm, with the loss of a high percentage (76%) of horse mackerel above the minimum landing size. For all the cod ends tested, the observed retention was presented as a function of maximum girth/mesh perimeter, which allowed a better understanding of the selection process for the species in study. Selectivity estimates for horse mackerel and hake were also compared to those obtained by Campos et al. (2003) for the same species in 1993 off the south coast, where they are captured as a by-catch in the crustacean fishery.
Highlights
The horse mackerel Trachurus trachurus, the axillary seabream Pagellus acarne and the European hake Merluccius merluccius are three commercially important species of the shallow ground fish assemblage of southern Portuguese waters (Gomes et al., 2001)
Horse mackerel was captured in large numbers within a length range of 6 to 34 cm approximately, with the mode at 17 cm, the vast majority of the catches were found to be included in a much narrower range of 15 to 20 cm, for all mesh sizes (Fig. 3)
In the 70D codend mesh size, where a higher proportion of larger individuals was caught, a second mode can be noticed at 22 cm
Summary
The horse mackerel Trachurus trachurus, the axillary seabream Pagellus acarne and the European hake Merluccius merluccius are three commercially important species of the shallow ground fish assemblage of southern Portuguese waters (Gomes et al., 2001). A number of other commercially valuable species, including seabreams and cephalopods, as well as some species with no commercial value and discarded, are included in this group. This assemblage extends over the southwest and southern continental coasts of Portugal at depths to approximately 120 m, and is exploited by the coastal bottom finfish trawling fleet. There are no studies quantifying the discard rates for the finfish trawlers fishing off the south west coast.
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