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Size selection of Antarctic krill (Euphausia superba) in trawls.

Krag LA, Herrmann B, Iversen SA, Engås A, Nordrum S, Krafft BA - PLoS ONE (2014)

Bottom Line: However, our results indicated that size selectivity of krill is a well-defined process in which individuals encounter meshes at an optimal orientation for escapement.The simulation-based results were supported by data from experimental trawl hauls and underwater video images of the mesh geometry during fishing.The methods developed and results described are important tools for selecting optimal trawl designs for krill fishing.

View Article: PubMed Central - PubMed

Affiliation: DTU Aqua, Technical University of Denmark, Hirtshals, Denmark.

ABSTRACT
Trawlers involved in the Antarctic krill (Euphausia superba) fishery use different trawl designs, and very little is known about the size selectivity of the various gears. Size selectivity quantifies a given trawl's ability to catch different sizes of a harvested entity, and this information is crucial for the management of a sustainable fishery. We established a morphological description of krill and used it in a mathematical model (FISHSELECT) to predict the selective potential of diamond meshes measuring 5-40 mm with mesh opening angles (oa) ranging from 10 to 90°. We expected the majority of krill to encounter the trawl netting in random orientations due to high towing speeds and the assumed swimming capabilities of krill. However, our results indicated that size selectivity of krill is a well-defined process in which individuals encounter meshes at an optimal orientation for escapement. The simulation-based results were supported by data from experimental trawl hauls and underwater video images of the mesh geometry during fishing. Herein we present predictions for the size selectivity of a range of netting configurations relevant to the krill fishery. The methods developed and results described are important tools for selecting optimal trawl designs for krill fishing.

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Related in: MedlinePlus

Image of the netting wall (mesh size15.4 mm) of the krill trawl captured during fishing operations (A).Digitizition of selected meshes to establish realistic values for mesh opening angles (oa) (B). The camera is located 10 m from the codline-end, pointing backwards. The intire 15.4 mm trawl was covered with 200 mm double 5 mm PE diamond netting for protection.
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pone-0102168-g004: Image of the netting wall (mesh size15.4 mm) of the krill trawl captured during fishing operations (A).Digitizition of selected meshes to establish realistic values for mesh opening angles (oa) (B). The camera is located 10 m from the codline-end, pointing backwards. The intire 15.4 mm trawl was covered with 200 mm double 5 mm PE diamond netting for protection.

Mentions: In addition to a morphological description of krill, we needed a precise description of sizes and shapes of the meshes used in commercial trawls for which experimental size selectivity data were collected. Weaved diamond mesh polyamide (PA) netting with a nominal mesh size of 16 mm (stretched inside measure) is commonly used in the commercial krill fishery. A small sample of this netting was placed on a flatbed scanner with no tension in the netting together with a measuring unit to determine the precise mesh size. Individual meshes in the picture were analyzed in FISHSELECT using the built-in image analysis function, and mesh size was assessed following the procedures described in Sistiaga et al. [11]. Standard mesh measuring methods (e.g., the OMEGA measuring gauge [22]), which are applied for larger mesh sizes, could not be used in this study because the measuring jaws are too large for the small mesh sizes used in the krill fishery. We used underwater video recordings made during commercial fishing operations onboard the Norwegian vessel Antarctic Sea during the 2013 season to assess the shapes of the meshes during fishing. Following Sistiaga et al. [11], the digitized images were used to extract the mesh opening angle (oa) to identify the best shape description of the meshes (e.g., diamond, hexagonal, square) (Fig. 4).


Size selection of Antarctic krill (Euphausia superba) in trawls.

Krag LA, Herrmann B, Iversen SA, Engås A, Nordrum S, Krafft BA - PLoS ONE (2014)

Image of the netting wall (mesh size15.4 mm) of the krill trawl captured during fishing operations (A).Digitizition of selected meshes to establish realistic values for mesh opening angles (oa) (B). The camera is located 10 m from the codline-end, pointing backwards. The intire 15.4 mm trawl was covered with 200 mm double 5 mm PE diamond netting for protection.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4126659&req=5

pone-0102168-g004: Image of the netting wall (mesh size15.4 mm) of the krill trawl captured during fishing operations (A).Digitizition of selected meshes to establish realistic values for mesh opening angles (oa) (B). The camera is located 10 m from the codline-end, pointing backwards. The intire 15.4 mm trawl was covered with 200 mm double 5 mm PE diamond netting for protection.
Mentions: In addition to a morphological description of krill, we needed a precise description of sizes and shapes of the meshes used in commercial trawls for which experimental size selectivity data were collected. Weaved diamond mesh polyamide (PA) netting with a nominal mesh size of 16 mm (stretched inside measure) is commonly used in the commercial krill fishery. A small sample of this netting was placed on a flatbed scanner with no tension in the netting together with a measuring unit to determine the precise mesh size. Individual meshes in the picture were analyzed in FISHSELECT using the built-in image analysis function, and mesh size was assessed following the procedures described in Sistiaga et al. [11]. Standard mesh measuring methods (e.g., the OMEGA measuring gauge [22]), which are applied for larger mesh sizes, could not be used in this study because the measuring jaws are too large for the small mesh sizes used in the krill fishery. We used underwater video recordings made during commercial fishing operations onboard the Norwegian vessel Antarctic Sea during the 2013 season to assess the shapes of the meshes during fishing. Following Sistiaga et al. [11], the digitized images were used to extract the mesh opening angle (oa) to identify the best shape description of the meshes (e.g., diamond, hexagonal, square) (Fig. 4).

Bottom Line: However, our results indicated that size selectivity of krill is a well-defined process in which individuals encounter meshes at an optimal orientation for escapement.The simulation-based results were supported by data from experimental trawl hauls and underwater video images of the mesh geometry during fishing.The methods developed and results described are important tools for selecting optimal trawl designs for krill fishing.

View Article: PubMed Central - PubMed

Affiliation: DTU Aqua, Technical University of Denmark, Hirtshals, Denmark.

ABSTRACT
Trawlers involved in the Antarctic krill (Euphausia superba) fishery use different trawl designs, and very little is known about the size selectivity of the various gears. Size selectivity quantifies a given trawl's ability to catch different sizes of a harvested entity, and this information is crucial for the management of a sustainable fishery. We established a morphological description of krill and used it in a mathematical model (FISHSELECT) to predict the selective potential of diamond meshes measuring 5-40 mm with mesh opening angles (oa) ranging from 10 to 90°. We expected the majority of krill to encounter the trawl netting in random orientations due to high towing speeds and the assumed swimming capabilities of krill. However, our results indicated that size selectivity of krill is a well-defined process in which individuals encounter meshes at an optimal orientation for escapement. The simulation-based results were supported by data from experimental trawl hauls and underwater video images of the mesh geometry during fishing. Herein we present predictions for the size selectivity of a range of netting configurations relevant to the krill fishery. The methods developed and results described are important tools for selecting optimal trawl designs for krill fishing.

Show MeSH
Related in: MedlinePlus