Selectivity studies usually describe the effects on target
species, whereas information on by-catch and discards is scarce.
Nevertheless, large quantities of undersized individuals and invertebrates
are discarded in the Mediterranean multi-species bottom trawl fishery. The
present work analyses the data from two surveys carried out on the shallow
and deep continental shelf (50–78 m, and 147–189 m, respectively) off the
Balearic Islands (western Mediterranean). In these surveys, the
traditionally used 40 mm diamond mesh codend and an experimental square mesh
codend were used under commercial conditions. Catch composition, yields,
size selectivity of both target and by-catch species, and discards were
compared between the two mesh types. The mean selection length (L50)
clearly increased for most species when using square mesh, escaping many
more individuals under their minimum landing size. Yield of Spicara smaris was
significantly lower by using the square mesh, changing the composition of
the retained catch. Escapement ratio and economic loss were significantly
higher with square mesh, although economic loss was almost negligible for
both meshes on the deep continental shelf. The use of square mesh significantly reduced the discards of
algae in the shallow waters and fish on the deep continental shelf. The
results confirmed that square mesh codend reduces the fishing pressure on
small specimens as well as the impact of trawling on the ecosystem. These
benefits would not lead to a reduction of the yields neither of the main
target species, the fishes Merluccius merluccius, Mullus surmuletus, Zeus faber, and the cephalopods Loligo vulgaris and Octopus vulgaris, nor of the rest
of commercial categories, except for Spicara smaris.