Rapp. Comm. int. Mer Médit., 36,2001
296
The total annual catch in Catalonia (Spain) is approximately
50,000t, 70 % of which is caught by bottom trawl fishery. Catches
have been decreasing since 1994 and now mainly consist of under-
sized fish. The multi-specific character of this fishery makes it difficult
to manage, but some size selectivity experiments with diamond and
square mesh codends suggest that changing the shape of the mesh
codend could aid the escape of under-sized fish and (also very impor-
tant) reduce the number of discards (1,2).
The aims of this study were :
1) to evaluate the number and weight of discarded, escaped and land-
ed fish, and
2) to compare the selectivity of diamond and square mesh codends.
Sampling took place in the Gulf of Lions between July and August
2000 in a chartered commercial trawler.We made eight hauls with
every mesh codend at depths between 50 and 350 metres. The hauls
lasted between 3 and 4 hours and the towing speed was 3.8 knots.
Codend selectivity was calculated through the covered codend method
(3).
Catches were sorted for every mesh codend and separated into three
categories : landed, discarded and escaped fish. Each category was
sorted by species. The total number and weight per species and indi-
vidual lengths (to the nearest mm) were recorded. In this paper, we
describe the results for twelve commercially important species :
Merluccius merluccius, Micromesistius poutassou, Trisopterus minu-
tus capelanus, Mullus barbatus, Sardina pilchardus, Engraulis encra-
sicolus, Scomber scombrus, Trachurus trachurus, Trigla lyra, Eutrigla
gurnardus, Phycis blennoidesandLepidorhombus boscii.
The analysis of every category suggested that, with diamond mesh,
discards were between 20.0 and 75.8% of the total number retained,
and between 4.2 and 47.3% of the total weight retained. Square mesh
reduced the number of discards in both total number and weight
retained to 0-13.3% and 0-7.3 %, respectively.
In other words, the number of escapes with square mesh was over
50 % more than with diamond mesh.
Total number and weight landed with diamond mesh were 12.0-
72.7% and 39.2-96.1%, respectively; and with square mesh, 5.3-
63.6% and 15.7-94.0%. We observed that less fish were landed with
square mesh, but if we took away the under-sized fish landed with dia-
mond mesh, and then compared the diamond-mesh fish again with the
fish landed with square mesh, the difference was only about 5% for the
total weight landed.
Comparison of the under-sized fish landed with every type of mesh
showed big differences for the most commercial species like hake
(Merluccius merluccius) and red mullet (Mullus barbatus). The total
number of under-sized hake landed was 60.2% with diamond mesh
and 13.6% with square mesh, whereas the weight of under-sized hake
landed was 11.0% and 5.3%, respectively.The total number of under-
sized red mullet landed was 54.0% with diamond mesh and 10% with
square mesh; and the weight of under-sized red mullet landed was
11.4% with diamond mesh and 5.2% with square mesh.
The results of the present study show that :
(1) square mesh allows significantly more individuals to escape
through the mesh;
(2) consequently, the number of individuals discarded decreases with
a square mesh codend;
(3) the weight of fish landed is similar for the two types of mesh when
the under-sized fish landed are excluded; and
(4) the under-sized fish sold decrease if we use the square mesh
codend.
Acknowledgements
The authors wish to express their gratitude to the crew of the vessel
D. Borja for their assistance in the field work. The authors also wish
to thank Dr. G. Petrakis for fruitful discussions on trawl codend selec-
tivity.This study was financed by the Comissionat per a Universitats i
Recerca de la Generalitat de Catalunya (Contract Number
ACES99/00011).
References
1 - Robertson J.H.B. and Stewart P.A.M., 1988. A comparison of size
selection of haddock and whiting by square and diamond mesh cod-end.
Journal du Conseil International pour l'Exploration de la Mer, 44 : 148-
161.
2 - Petrakis G. and Stergiou K.I., 1997. Size selectivity of diamond and
square mesh codends for four commercial Mediterranean fish species.
ICES Journal of Marine Science, 54 : 13-23.
3 - Pope J.A., Margetts A.R., Hamley J.M., and Akyuz E.F., 1975.
Manual of methods for fish stock assessment. Part 3. Selectivity of
fishing gear.FAO Fisheries Technical Paper, 41 (Rev. 1) : 46 pp.
COMPARISON OF DISCARDED, ESCAPED AND LANDED FISH USING DIAMOND AND
SQUARE MESH CODENDS
Sandra Mallol*, Margarida Casadevall and Emili García-Berthou
Departament de Ciències Ambientals, Universitat de Girona, Catalonia, Spain - sandra.mallol@udg.es
Abstract
We compared 20 mm diamond-shaped codend with 20 mm square-shaped codend. Sampling was conducted with a chartered commercial
trawler in the Gulf of Lion during July and August 2000. The codend covered method was used. Number and weight landed, discarded
and escaped of twelve commercially important species were studied. Our results suggested great differences between the two codends in
the number and weight of fish escaped and discarded. Landed weight only differed between 0.01 and 8.0 % for all species studied,
excluding under-sized fish sold.
Keywords : trawl codend selectivity, square mesh, diamond mesh, fisheries, Gulf of Lions
