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A REVIEW OF STUDIES OF FISHING GEAR SELECTIVITY IN THE MEDITERRANEAN : fishing gear selectivity studies in the Mediterranean ; a review and a summary of the 2001 COPEMED workshop report
Stewart, Peter A. M.
Weitere Beteiligte (Hrsg. etc.): Robles, Rafael
|BK - Klassifikation:
||21.3 Küsten- und Hochseefischerei
||Bericht / Forschungsbericht / Abhandlung
||Informes y Estudios // COPEMED
|Kurzfassung auf Deutsch:
||There is considerable interest throughout the world in methods of improving the size and species selectivity of commercial fishing gears, to reduce fishin g mortality and to conserve fish stocks. Recent work in several Mediterranean fisheries suggests that the techniques developed elsewhere might be useful, even in multi-species fisheries, to reduce discarding,particularly of non-target species. This study was commissioned to review past and present
work on gear selectivity in the Mediterranean, to assess the relevance of work done elsewhere and to advise on experimental methods.
By restricting the search to marine fisheries within the western and eastern Mediterranean,116 relevant papers were found. A bibliography was compiled with nine categories: reviews,demersal fish selectivity, shellfish selectivity, novel cod-end selectivity selectivity of set gear and artisanal fisheries, stock assessment surveys, gear performance and catch composition,techniques of measurement and analysis and by-catches of large sea creatures.
The scarcity of review papers demonstrated the need for this review. Selectivity parameters for a relatively small number of commercially important species are reported. The aim of most of the earlier work on gear selectivity was to determine the length and age at first capture, for stock assessment. The effects of increasing mesh size were often investigated.
Small mesh cod-end covers were used. Small catches per haul of the species of interest, meant catches had to be pooled to estimate selection parameters and prevented estimation of variance. Recent work testing more selective cod-ends (square mesh, roped, narrow, etc) was carried out in Greece and Turkey. The complex effects on catch composition of altering codend mesh shape are also being studied. Artisanal fisheries are large, varied and important but
most reported studies concentrated on the selectivity of gill and trammel nets.
The selectivity of survey trawls is an important issue for stock assessment and was studied for both survey and commercial trawls in international programmes, indicating large differences between gears. The catch composition of gears differs with their design and high opening nets were compared with traditional low headline nets in several studies.
Underwater observation of fish behaviour and gear operation was described in a few papers.
Improved small mesh covers are now in use for measuring cod-end selectivity and studies of the survival of escaping fish are noted. There is some progress in data analysis. Trials of acoustic scaring methods for dolphins and Turtle Excluder Devices are described.
Key papers, describing the work done elsewhere to investigate and improve gear selectivity,are listed in the final section of the Bibliography. These identify the main aspects of gear selectivity research to be considered by COPEMED and emphasise the importance of underwater observation in making progress in this area. The main findings of the fish behaviour studies, and the various techniques devised to improve the size selectivity of cod-ends and species separation are covered. Considerable effort went into refining and
comparing methods for estimating cod-end selectivity, including hooped covers and various types of divided net. Data analysis methods developed in parallel now permit more comprehensive comparison of gear selectivity. Modelling the selectivity of nets is progressing, despite the number of technical, environmental and biological factors influencing selectivity.
Priorities for future work in the Mediterranean are considered in the review. Continuing and
more precise work on the current types of commercial trawls is needed to determine age at
first capture for assessment purposes. More selective fishing gears may not have general value
in multi-species fisheries but may be effective in particular cases. It is worth trying to identify
these fisheries and develop suitable gears. Progress will be made more quickly if the gears
and fish can be observed underwater and the relative merits of RCVs, divers, hydrofoils and
fixed cameras need to be assessed. Selection of fish by a gear is not confined to the cod-end,
other components play a part. Catch composition depends on gear design and more study in
this area is warranted to minimise the capture of non-target species. Varying cod-end mesh
shape also affects catch composition and this needs more investigation. The species
separating abilities of trawls with horizontal panels and of selection grids could be studied.
Data analysis techniques need to be adapted to deal with small catches in an optimal manner
to obtain parameter estimates with confidence limits.
There are management and economic considerations related to gear selectivity. The fisheries
of the Mediterranean are very diverse and studies of comparative selectivity between metiers
might reveal significant differences in selectivity. This would help to identify the fisheries
where reducing by-catch and discarding would most benefit the stocks. All reductions in
fishing mortality are achieved at the cost of a loss of marketable catch and income to the
fishermen. These points need to be examined.
To assist the scientific committees responsible for the management of fish stocks in the
Mediterranean, a workshop was held to review present and past studies on fishing gear
selectivity and to identify priorities for future work. Delegates from member states of
COPEMED and invited experts met on Kerkennah Island, Tunisia in October 2001. The
delegates described current progress with their selectivity projects and the review of
selectivity in this document was presented. Techniques developed elsewhere were considered
and the need for selectivity data for stock assessment was explained. Data analysis methods,
gear interactions with cetaceans and techniques for underwater observation where presented
and discussed. There was a vigorous discussion on future priorities, concluding that work
should concentrate on obtaining basic selectivity data and applying new analysis methods;
studying the selectivity of coastal/artisanal fisheries; investigating the selectivity of survey
gears; separating species in towed gears; reducing the by-catch of cetaceans. To aid this new
work it was recommended that formal consultation with the fishing industry should take
place; the capacity to observe gear and fish underwater should be developed; training in data
analysis and fish behaviour should be arranged.