Download The FAO (Food and Agriculture Organization) on the light of a

Abstract
The Food and Agriculture Organization (FAO), in the light of a rational management of the marine
resources strongly recommends that “Selective and environmentally safe fishing gear and practices
should be further developed and applied, to the extent practicable, in order to maintain biodiversity
and to conserve the population structure and aquatic ecosystems and protect fish quality”. It is
therefore important to take measures aimed at reducing fishing impacts through technical
modifications of gear designs, towards the preservation of benthic ecosystems and the sustainable
exploitation of fisheries resources. In this context, technical and biological-ecological issues are
strongly linked. Bycatch (living creatures caught unintentionally by fishing gear) occurs because
most fishing gears and practices are not perfectly selective for the species and sizes targeted.
Improving the selectivity of a bottom trawl and reducing the amount of bycatch and discards
released at sea will contribute to a rational management of fisheries resources. The present study
was built on recent advances by providing solutions to reduce the adverse impacts of fisheries on
the marine environment. This was achieved through two complimentary strategies: the transfer of
existing gear innovations and technology between fisheries, the modification of current gear
designs. This study is aimed at describing the basic principles that can be used to reduce the impact
of bottom trawling. In particular, the possibility of mitigating adverse effects of a fishery through
technical solutions is assessed, not only in terms of bycatch/discard reduction and selectivity in
bottom trawl nets, but also by comparing different fishing gears. The following parameters, which
can influence the selectivity performance of a bottom trawl net, were investigated: twine thickness,
mesh opening, mesh configuration (square and diamond), speed and catch size.
The present study provides information on the different performance of 40 mm diamond mesh and
40 mm square mesh codends in the Adriatic Sea waters. It highlights the existing problem regarding
the very poor selection of the traditional diamond mesh used by bottom trawlers in multi-species
fisheries of the Mediterranean Sea. The results of the trawling trials carried out off Ancona showed
that a considerable fraction of the catch was composed of species of no commercial value, either
because they were undersized or damaged or because they were unmarketable. The square mesh
codend resulted more selective than the traditional one giving juveniles of most species investigated
a good possibility of escapement. The square meshes also allowed a reduction of the bycatch and of
the fraction discarded at sea. Nevertheless the different mesh configuration seemed not to influence
the commercial fraction of the catch, which often represented a similar percentage in weight in both
codends. Finally, the square mesh codend also allowed the escape of a wider range of species.
Underwater video-camera observations demonstrated that fishes (red mullet in our case) in the
codend are able to see the net (at least at the end of the haul) and to swim: thus placing a square
mesh netting panel in the codend should provide juveniles with an escapement.
The study allows the conclusion that the square mesh codend is not a definitive solution of the low
selection properties of the traditional bottom trawl net but it could represent a reasonable, simple
and inexpensive solution towards a more sustainable management of the resources in the Adriatic,
at least in certain multispecies fishing grounds and seasons.
The study also shows that other parameters of netting panels should be considered for the
reasonable management of resources: in particular the increase in twine netting thickness reduces
the selectivity.
Certain important mitigation measures such as the Turtle Excluder Devices (TEDs) were also
tested. The TEDs tried in Adriatic Sea acted practically in the same way as JTED (Juveniles and
Trash Excluder Device) used in other countries (mainly south-east Asia). In fact they allowed a
reduction of the fraction discarded at sea and the exclusion of anthropic debris. The TEDs seemed
to be effective in releasing immediately the turtles from the net, reducing their level of stress; good
results were obtained both with artificial turtle simulators and with a specimen of Caretta caretta,
that was alive and in healthy conditions.
Alternative fishing gears were also tested in order to reduce the impact of traditional gears on the
seabed. The rapido trawl is considered as one of the most impacting fishing gears (its teeth are able
to penetrate the sea sediments for few centimetres and the pressure on the bottom increases
proportionally with vessel speed). The technical parameters registered onboard in the present study
showed that beam trawls require low power to be towed, thus their pressure on the bottom is less
than that recorded for rapido trawls; this implied a reduction of the fraction rejected at sea (mainly
Gastropods, Bivalves, Echinoderms) with beam trawls. Rapido trawls were more efficient with
commercial species but their fuel consumption was really high.
In the light of previous considerations it is clear that more research is needed to quantify various
impacts of different fishing gears used in different fisheries under different fishing ground
conditions. There is little or no escape survival data available for most Mediterranean species even
if the present study shows that most red mullets are still alive at the end of the haul. The effect of
discards on food-web functioning is still widely unknown. Therefore, the overall effect of improved
selectivity on commercial species remains unclear unless the study is performed on an ecosystem
context progressing towards an Ecosystem Approach to Fisheries (EAF).
Coordinatore del corso: Prof.ssa Oliana Carnevali
Tutor: Prof. Roberto Danovaro
Co-tutor: Dott. Antonello Sala