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The estimation and improvement of the selectivity in crustacean and fish trawls
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Abstract(s)
Scientific evidence points to the overfishing of some of the most important commercial stocks exploited in Portuguese
waters (ICES sub-area IXa) by the bottom trawling fleets targeting fish and crustaceans. While temporary
fishing interdiction has been implemented in specific areas off the south-west coast, in an attempt to reduce the
fishing effort upon juvenile fish, there has been considerable resistance to the introduction of gear modifications,
including the increase in cod end mesh sizes. The small cod end mesh sizes currently in use, particularly in crustacean
trawling, largely contribute to catch discarding or misreporting, causing increasing conflicts between trawl
fishermen and fishermen from other métiers competing for common resources.
Recent studies have shown that discard rates can attain very high levels, up to 70% on board of both crustacean
and fish trawlers off the Portuguese south coast, with a high number of low-valued species, especially small
pelagics, being discarded in large amounts.
A number of gear modifications were tested in crustacean and fish trawls, aiming at reducing the amount of undersized
fish from the target species and allowing for the escapement of a significant fraction of non-commercial
by-catch. Their usefulness is discussed in a review, six papers and an overview of the thesis.
The improvement of size-selectivity, by increasing cod end mesh size and changing mesh configuration was
addressed for the deep groundfish assemblage off the south coast exploited by crustacean trawlers (Papers I and
II), and for the shallow and deep groundfish assemblages off the south west coast, where a number of fish trawlers
usually operate (Papers III and IV). Cod end selectivity parameters for three different mesh sizes and two
mesh configurations, diamond and square mesh, were estimated for a large number of target and by-catch species.
In a number of cases, the data structure allowed for the analysis of between-haul variation, and selectivity models
were proposed which relate the estimated parameters to the variables under test and also to external variables such
as cod end catch and trawling depth, giving a first insight into the mechanisms involved in cod end size selectivity.
By-catch reducing devices (BRD’s) placed in the rear part of the trawls or in the cod end, comprising different
combinations of oblique separator panels in association with square mesh windows, and square mesh windows
alone, were tested in crustacean fishing grounds off the south coast (Papers V and VI), with the purpose of excluding
the non-commercial by-catch. The effectiveness of the different BRD’s was separately evaluated for the
most captured species. Between-species differences in behaviour towards the sorting devices are discussed. Sizedependence
in escapement through the square mesh windows was recorded for a number of species, and the window
selectivity could then be separately estimated.
The cod end selectivity experiments carried out for crustacean trawling suggested that an increase in cod end
mesh size from the current 55 mm to 70 mm diamond mesh, without changing cod end design or material, would
be advisable in order to reduce the amount of undersized catch. Such an increase would simultaneously allow for
the exclusion of a large fraction of non-commercial by-catch. The results obtained for fish trawling provide, on
the other hand, evidence of the difficulty in managing a number of target species of different shapes and sizes
based only on mesh size or mesh configuration regulations.
The use of BRD’s greatly contributed to the exclusion of non-commercial by-catch. While evidence of active
escape behaviour through the square mesh windows was found for some by-catch species, for others the exclusion
from the trawl relied on previous guidance to upper trawl areas by the separator panels used. Overall, the results
suggest a significant potential for the use of by-catch reducing devices in this fishery.
Description
Tese de dout., Ciências e Tecnologias das Pescas, Faculdade de Ciências do Mar e do Ambiente, Univ. do Algarve, 2003
Keywords
Pescas Peixe Crustáceos Redes de arrasto Costa Algarvia