scholarly journals Using vertical distribution to separate fish from crustaceans in a mixed species trawl fishery

2019 ◽  
Vol 76 (10) ◽  
pp. 1781-1794 ◽  
Author(s):  
Junita Diana Karlsen ◽  
Ludvig Ahm Krag ◽  
Bent Herrmann ◽  
Henrik Skaarup Lund
Keyword(s):  
Vertical Distribution ◽  
Separation Efficiency ◽  
Fishing Gear ◽  
Relative Height ◽  
Size Selectivity ◽  
Mixed Species ◽  
Nephrops Norvegicus ◽  
Trawl Fishery ◽  
Successful Separation ◽  
Commercial Species

A major challenge in mixed fisheries is achieving acceptable size selectivity for morphologically different species using the same fishing gear. Separator trawls can have different selective properties in the upper and lower compartments and provide successful separation of species. We used a horizontally divided codend with small square meshes (40 mm) and a simple frame to stimulate fish to swim into the upper compartment. The majority of the fish were separated successfully from Nephrops (Nephrops norvegicus), but their preference were uniform. Less than 10% of the Nephrops entered the upper compartment. Length-based analysis revealed three patterns of separation efficiency among nine commercial species: length-dependent separation and preference for the upper or lower compartments. The separation efficiency should be improved for small roundfish and flatfish. There was little diel effect on the separation efficiency. The preference of fish for a compartment, taking the relative height of that compartment into account, was established for this and similar previous studies to enable comparison of results. We recommend length-based analysis to account for the fished population when interpreting results.

scholarly journals Selectivity of diamond- and square-mesh codends in the deepwater crustacean trawl fishery off the Balearic Islands (western Mediterranean)

2006 ◽  
Vol 63 (1) ◽  
pp. 52-67 ◽  
Author(s):  
Beatriz Guijarro ◽  
Enric Massutí
Keyword(s):  
Economic Loss ◽  
Western Mediterranean ◽  
Balearic Islands ◽  
Size Selectivity ◽  
Trawl Fishery ◽  
Commercial Species ◽  
The One ◽  
Catch Composition ◽  
The Impact ◽  
Upper Slope

Abstract An analysis of 38 bottom trawl hauls at depths of 251–737 m off the Balearic Islands (western Mediterranean) during autumn 2002 and spring 2003 was used to compare the species and size selectivity of 40-mm diamond- and square-mesh codends under commercial conditions. There was no difference in the catch composition or the yield that could be attributable to mesh shape, although the percentage of total and commercial species discarded with a diamond mesh was higher than with a square mesh. At least in the short term, the escapement ratio and the economic loss with the diamond mesh were lower than with square mesh, but economic efficiency was no different between them. For all the main species compared, except one flatfish, size selectivity parameters were lower for the diamond-than for the square-mesh codend. Selectivity values for the one flatfish species were similar. From the results it is concluded that, within the context of precautionary management, introduction of a 40-mm square mesh in the codend could be an appropriate and plausible measure to improve the state of the resources exploited by the deepwater crustacean trawl fishery of the upper slope off the Balearic Islands, and to reduce the impact of the fishery on the ecosystem.

scholarly journals Towards sustainable demersal fisheries: NepCon image acquisition system for automatic Nephrops norvegicus detection

PLoS ONE ◽  
2021 ◽  
Vol 16 (6) ◽  
pp. e0252824
Author(s):  
Maria Sokolova ◽  
Fletcher Thompson ◽  
Patrizio Mariani ◽  
Ludvig Ahm Krag
Keyword(s):  
Learning Algorithm ◽  
Image Acquisition ◽  
Automatic Detection ◽  
Observation System ◽  
Fishing Gear ◽  
Learning Approaches ◽  
Nephrops Norvegicus ◽  
Trawl Fishery ◽  
Perception System ◽  
Catch Composition

Underwater video monitoring systems are being widely used in fisheries to investigate fish behavior in relation to fishing gear and fishing gear performance during fishing. Such systems can be useful to evaluate the catch composition as well. In demersal trawl fisheries, however, their applicability can be challenged by low light conditions, mobilized sediment and scattering in murky waters. In this study, we introduce a novel observation system (called NepCon) which aims at reducing current limitations by combining an optimized image acquisition setup and tailored image analyses software. The NepCon system includes a high-contrast background to enhance the visibility of the target objects, a compact camera and an artificial light source. The image analysis software includes a machine learning algorithm which is evaluated here to test automatic detection and count of Norway lobster (Nephrops norvegicus). NepCon is specifically designed for applications in demersal trawls and this first phase aims at increasing the accuracy of N. norvegicus detection at the data acquisition level. To find the best contrasting background for the purpose we compared the output of four image segmentation methods applied to static images of N. norvegicus fixed in front of four test background colors. The background color with the best performance was then used to evaluate computer vision and deep learning approaches for automatic detection, tracking and counting of N. norvegicus in the videos. In this initial phase we tested the system in an experimental setting to understand the feasibility of the system for future implementation in real demersal fishing conditions. The N. norvegicus directed trawl fishery typically has no assistance from underwater observation technology and therefore are largely conducted blindly. The demonstrated perception system achieves 76% accuracy (F-score) in automatic detection and count of N. norvegicus, which provides a significant elevation of the current benchmark.

Selectivity experiments with escape windows in the North Sea Nephrops (Nephrops norvegicus) trawl fishery

1999 ◽  
Vol 42 (1-2) ◽  
pp. 167-181 ◽  
Author(s):  
N Madsen ◽  
T Moth-Poulsen ◽  
R Holst ◽  
D Wileman
Keyword(s):  
North Sea ◽  
Nephrops Norvegicus ◽  
Trawl Fishery ◽  
The North ◽  
The North Sea

Development of a codend concept to improve size selectivity of Nephrops (Nephrops norvegicus) in a multi-species fishery

2011 ◽  
Vol 111 (1-2) ◽  
pp. 116-126 ◽  
Author(s):  
R.P. Frandsen ◽  
B. Herrmann ◽  
N. Madsen ◽  
L.A. Krag
Keyword(s):  
Size Selectivity ◽  
Nephrops Norvegicus

scholarly journals Separating species using a horizontal panel in the Scottish North Sea whitefish trawl fishery

2007 ◽  
Vol 64 (8) ◽  
pp. 1543-1550 ◽  
Author(s):  
R. S. T. Ferro ◽  
E. G. Jones ◽  
R. J. Kynoch ◽  
R. J. Fryer ◽  
B-E. Buckett
Keyword(s):  
North Sea ◽  
Water Clarity ◽  
Light Level ◽  
Separation Performance ◽  
Separate Species ◽  
Trawl Fishery ◽  
The North ◽  
Acoustic Methods ◽  
Commercial Species ◽  
The North Sea

Abstract Ferro, R. S. T., Jones, E. G., Kynoch, R. J., Fryer, R. J., and Buckett, B-E. 2007. Separating species using a horizontal panel in the Scottish North Sea whitefish trawl fishery. – ICES Journal of Marine Science, 64: 1543–1550. In the North Sea, Scottish vessels target haddock, cod, whiting, monkfish, saithe, and flatfish in a mixed whitefish trawl fishery. These species mature at different sizes and hence have a range of minimum landing sizes. Their different shapes and swimming capabilities imply different selection characteristics when escaping from trawl gear. However, they are often caught at the same time on the same grounds. Optimal exploitation can only be achieved by ensuring that the selection of each species varies appropriately with length during the fishing process. This paper describes one part of a large European project to develop species-selective trawl gear to improve the exploitation pattern of North Sea cod, while maintaining the catch of other important commercial species. A gear suitable to the Scottish mixed whitefish fishery was fitted with a horizontal panel in the tapered part of the net to separate species into an upper and lower compartment. Trials were conducted on research vessels to measure separation performance for nine species in different light conditions, at different towing speeds, and with different lengths of panel. Most haddock, whiting, and saithe pass above the panel, whereas most cod, flatfish, and monkfish pass below it. Towing speed and panel length had no significant effect on separation. At lower light levels during the night (April at latitude 58° to 61°N), fewer dab, sole, plaice, and cod pass below the panel. Observations and measurements of fish behaviour using acoustic methods are described. They suggest that the height at which fish enter the net mouth may be influenced by light level and water clarity.

scholarly journals Catch pattern and size selectivity for a gear designed to prevent fish injuries during the capture process in a North-East Atlantic demersal trawl fishery

2020 ◽  
Vol 40 ◽  
pp. 101525
Author(s):  
Bent Herrmann ◽  
Manu Sistiaga ◽  
Roger B. Larsen ◽  
Jesse Brinkhof ◽  
Svein Helge Gjøsund ◽  
...  
Keyword(s):  
Size Selectivity ◽  
East Atlantic ◽  
Capture Process ◽  
Trawl Fishery ◽  
North East

Reproductive biology of Polybius henslowii Leach, 1820 (Decapoda, Polybiidae): a discard species from the northwestern Portuguese beam trawl fishery

Crustaceana ◽  
2014 ◽  
Vol 87 (7) ◽  
pp. 784-800 ◽  
Author(s):  
L. Magalhães ◽  
M. Felício ◽  
M. B. Gaspar ◽  
V. Quintino ◽  
A. M. Rodrigues
Keyword(s):  
Reproductive Biology ◽  
Current Knowledge ◽  
Carapace Width ◽  
Fishing Gear ◽  
Northern Coast ◽  
Beam Trawl ◽  
Total Catch ◽  
Important Species ◽  
Trawl Fishery ◽  
Minimum Landing Size

Polybius henslowii Leach, 1820, a Portunid crab distributed from the British Isles to Morocco and the Mediterranean Sea is caught in high densities by many types of fishing gear and frequently thrown back to the sea. Along the northern coast of Portugal, P. henslowii is an important species discarded in beam trawl fishery. There are no quantitative estimations of discards and very little is known about its biology, information that is needed to value this species. The present work was conducted from March 2007 to December 2010 in order to improve the current knowledge of the reproductive biology of P. henslowii. By-catches were registered aboard commercial vessels that volunteered to participate in the study. In that period P. henslowii represented 57.3% of the total catch and was present in all hauls analysed. The target species, Palaemon serratus (Pennant, 1777), represented 1.2% of the total catch, showing the low selectivity of the fishing gear and the need for suppression of this gear to catch this shrimp species. Differences in size between sexes were found, with males being larger and heavier than females. Mature females occurred mainly in autumn and the carapace width at which 50% of the females reach maturity was estimated to be 37.8 mm, which could be set as the minimum landing size of this potential exploitable resource. A macroscopic ovarian maturity stages classification was proposed, allowing a rapid field assessment of female sexual maturity. This study fills a gap in the knowledge of a very important species discarded in beam trawl fishery that may be basis of further work.

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