scholarly journals Estimating selectivity of experimental diamond (T0) and turned mesh (T90) codends in multi-species Mediterranean bottom trawl

2020 ◽  
Author(s):  
ANDREA PETETTA ◽  
BENT HERRMANN ◽  
MASSIMO VIRGILI ◽  
ROCCO De MARCO ◽  
GIOVANNI CANDUCI ◽  
...  
Keyword(s):  
Mesh Size ◽  
Bottom Trawl ◽  
Size Selectivity ◽  
Important Species ◽  
Trawl Fishery ◽  
The North ◽  
Common Squid ◽  
Commercially Important ◽  
The Mediterranean ◽  
The Difference

The effect of shifting from a diamond mesh codend (T0) to a 90° turned mesh codend (T90) on the size selectivity of seven commercially important species in the Mediterranean bottom trawl fishery was evaluated. During sea trials conducted in the north-western Adriatic, two experimental codends made of 54 mm nominal mesh size netting that differed only in mesh configuration were alternately mounted on the same trawl. Overall, the size selectivity was significantly improved for all the species analysed, shifting from T0 to T90. The difference in the average expected L50values between T90 and T0 was particularly marked in European hake (Merluccius merluccius, 21.26 vs 11.26 cm total length), common squid (Loligo vulgaris, 12.06 vs 7.88 cm mantle length) and mantis shrimp (Squilla mantis, 20.78 vs 13.35 mm carapace length). Both codends had an excessive size selectivity especially for red mullet (Mullus barbatus), Mediterranean horse mackerel (Trachurus mediterraneus) and whiting (Merlangius merlangus), thus involving a commercial loss. These findings demonstrate the efficiency of the T90 codend tested at excluding undersized specimens, especially of hake, whose average L50was above the minimum conservation reference size of 20 cm. The adoption of this practical and inexpensive solution could help improve the management of the demersal resources targeted by the Mediterranean bottom trawl fishery.

scholarly journals Effect of a lateral square-mesh panel on the catch pattern and catch efficiency in a Mediterranean bottom trawl fishery

2020 ◽  
Vol 21 (1) ◽  
pp. 105
Author(s):  
SARA BONANOMI ◽  
JURE BRČIĆ ◽  
BENT HERRMANN ◽  
EMILIO NOTTI ◽  
ALESSANDRO COLOMBELLI ◽  
...  
Keyword(s):  
Mesh Size ◽  
Lateral Position ◽  
Bottom Trawl ◽  
Important Species ◽  
Commercial Fish ◽  
Trawl Fishery ◽  
Red Mullet ◽  
Commercially Important ◽  
The Mediterranean ◽  
Trawl Fisheries

The current legal codends used in Mediterranean bottom trawl fisheries are at high risk of retaining undersized individuals of several commercial fish species. This entails that codends alone are unable to provide the desired exploitation pattern. A simple technological measure that potentially can provide higher release efficiency of undersized fish are Square-Mesh Panels (SMPs). SMPs are often applied in the upper section of the trawl belly, just ahead the codend. However, recent studies in the Mediterranean have demonstrated that SMPs mounted in this position provided limited release efficiency, because very few fish came into contact with their meshes. In attempt to improve SMPs efficiency in the Mediterranean bottom trawl fisheries, we applied them on the lateral sides of the last tapered section of the trawl belly, just ahead of the codend, and fitted two guiding panels in the trawl belly to enhance fish-SMP contact. We compared the catch performance of a standard commercial trawl with a 52 mm diamond-mesh codend and of a similar trawl fitted with lateral SMPs (70 mm mesh size) in the belly using a twin trawl. The study focused on red mullet (Mullus barbatus), a commercially important species, but data for gurnard (Chelidonichthys lucerna and Chelidonichthys cuculus)  were also obtained and analysed. In contrast to previous research on SMPs mounted in the top panel of the trawl, in this study SMPs induced a significant effect on catch performance for red mullet, demonstrating that their lateral position involved greater fish-SMP contact. However, since the test trawl lost a significant amount of legal-sized red mullet compared with the standard trawl, the effect was not wholly positive, possibly due to an excessively large mesh size. Therefore, future studies should be encouraged to test lateral SMPs with smaller mesh sizes.

Size Selectivity of Diamond and Square Mesh Codends in Pelagic Herring Trawls: Only Small Herring Will Notice the Difference

1992 ◽  
Vol 49 (10) ◽  
pp. 2104-2117 ◽  
Author(s):  
Petri Suuronen ◽  
Russell B. Millar
Keyword(s):  
Relative Size ◽  
Mesh Size ◽  
The Other ◽  
Size Selectivity ◽  
The Difference ◽  
Catch Rates ◽  
Two Sides ◽  
High Catch

A twin codend trawl was fished in the northern Baltic to study the size selectivity of square mesh and diamond mesh codends of 36-mm nominal mesh size. For each codend, 15 hauls were completed with a small mesh (20 mm) codend deployed on the other side of the trawl. The relative size of the catches in the two sides of the trawl varied considerably from haul to haul (the separator section was not operating properly) and selection curves were estimated from each individual haul using a method that incorporated the differences in catching efficiency of the two sides. The length of 50% retention decreased with increased catch for both the diamond and square mesh codends, although in neither case was this relationship statistically significant. Selection curves fitted to the combined haul data were asymmetric. The square mesh codend retained significantly less small herring than the diamond mesh codend, and for larger herring the two codends had similar selectivity. In both codends, most escapes occurred at the front of the catch bulge, from the upper side of the codend. At high catch rates, mesh blockage was observed for several metres ahead of the catch bulge during the later part of the tow.

Diets and trophic guilds of demersal fishes of the south-eastern Australian shelf

2001 ◽  
Vol 52 (4) ◽  
pp. 537 ◽  
Author(s):  
C. Bulman ◽  
F. Althaus ◽  
X. He ◽  
N. J. Bax ◽  
A. Williams
Keyword(s):  
Fish Species ◽  
Abundant Species ◽  
Trophic Guilds ◽  
The South ◽  
Important Species ◽  
Trawl Fishery ◽  
South Eastern ◽  
Total Fish ◽  
Wet Weight ◽  
Commercially Important

A total of 8200 stomach samples was collected from 102 fish species caught by trawl or gillnet during research surveys on the south-eastern Australian shelf from 1993 to 1996. Diet compositions were analysed based on percentages of wet weight of prey. Of the total fish examined, 70 species had sufficient stomach samples (i.e. >10) for further analysis. Ten trophic guilds were identified from cluster analysis. Benthic prey dominated the diets. However, analysis on a subset of 28 abundant species that were commercially and ecologically important, showed that pelagic prey was dominant, particularly for 12 quota species. This suggests that pelagic production contributes significantly to the trawl fishery production. Further analysis on the diets of these 28 species found that although fish was more important than invertebrate prey, there was no evidence of significant predation on commercially important species (quota species)by other fish species. A food web diagram was constructed, mostly based on the diet compositions, guild structure and relative abundance of commercially and ecologically important fish species, to show major trophic interactions of the shelf ecosystem.

scholarly journals A method of detecting patterns in mean lengths of samples of discarded fish, applied to the self-sampling programme of the Dutch bottom-trawl fishery

2011 ◽  
Vol 68 (8) ◽  
pp. 1712-1718 ◽  
Author(s):  
Sebastian S. Uhlmann ◽  
Stijn M. Bierman ◽  
Aloysius T. M. van Helmond
Keyword(s):  
Mixed Model ◽  
Linear Mixed Model ◽  
Abundant Species ◽  
The Self ◽  
Bottom Trawl ◽  
Trawl Fishery ◽  
The North ◽  
Grey Gurnard ◽  
Sampling Programme ◽  
Multiple Samples

Abstract Uhlmann, S. S., Bierman, S. M., and van Helmond, A. T. M. 2011. A method of detecting patterns in mean lengths of samples of discarded fish, applied to the self-sampling programme of the Dutch bottom-trawl fishery. – ICES Journal of Marine Science, 68: 1712–1718. In 2009, a self-sampling programme was organized in the Netherlands, fishers sampling ca. 80 kg of discards from randomly selected bottom trawls in the North Sea. A statistical procedure is proposed to highlight samples, trips (with multiple samples), or vessels (which may have multiple trips within a year) where extreme mean lengths of discarded fish were observed. Randomization methods were used to test for evidence of non-randomness in patterns of highlighted discard samples, e.g. repeated observations of extreme mean lengths for consecutive discard samples across trips from the same vessel. European plaice (Pleuronectes platessa), common dab (Limanda limanda), grey gurnard (Eutrigla gurnardus), and whiting (Merlangius merlangus) were considered because these were the most abundant species in most of the discard samples. A linear mixed model was used to estimate random-sample effects on the estimated mean lengths by species. These random effects were incorporated into uni- and bivariate procedures to identify extreme samples that were summed for each vessel, and the probability of observing such numbers was estimated. Excluding these samples from the dataset had marginal effects on estimated size distributions of fish.

scholarly journals Effect of a quality-improving cod end on size selectivity and catch patterns of cod in bottom trawl fishery

2019 ◽  
Vol 76 (11) ◽  
pp. 2110-2120 ◽  
Author(s):  
Jesse Brinkhof ◽  
Bent Herrmann ◽  
Roger B. Larsen ◽  
Tiago Veiga-Malta
Keyword(s):  
Gadus Morhua ◽  
Barents Sea ◽  
Relative Size ◽  
Bottom Trawl ◽  
Size Selectivity ◽  
Total Size ◽  
Trawl Fishery ◽  
The Barents Sea ◽  
Sea Bottom ◽  
Significant Difference

A new cod-end concept developed and tested exhibited significantly improved quality of caught cod (Gadus morhua) compared with that of the conventional cod end used in the Barents Sea bottom trawl fishery. However, the design of the new quality-improving cod end raised concerns about its size selectivity and the possibility that higher retention probability could negatively impact the catch pattern by increasing the proportion of undersized cod. Therefore, the goal of this study was to quantify and compare the size selectivity and catch pattern for cod when deploying, respectively, the conventional and new quality-improving cod end in the Barents Sea bottom trawl fishery. The new quality-improving cod end had significantly lower relative size selectivity than the conventional cod end, but no significant difference in the catch patterns was detected in the trawl. Further, estimation of the total size selectivity in the trawl revealed that the increased retention of small cod when using the quality-improving cod end was minor. Hence, despite the reduced selectivity, the quality-improving cod end can be used with low risk of retaining small cod.

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