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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

The experimentally obtained L50 value is indicated with the solid line (exp L50).L50 predictions for CS1_CS2 and for CS3 are indicated for the commercial mesh size (15.4 mm). The realistic mesh opening angles during commercial fishing is indicated with the vertical gray interval. The horizontal interval indicated the selective range for the 15.4 mm commercial mesh size.
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pone-0102168-g009: The experimentally obtained L50 value is indicated with the solid line (exp L50).L50 predictions for CS1_CS2 and for CS3 are indicated for the commercial mesh size (15.4 mm). The realistic mesh opening angles during commercial fishing is indicated with the vertical gray interval. The horizontal interval indicated the selective range for the 15.4 mm commercial mesh size.

Mentions: Compared to the contact mode CS1_CS2, little escapement occurs for the larger CS3 cross section, even for rather large meshes (Fig. 7). When the meshes of the commercially used trawl (15.4 mm) are open optimally, krill smaller than 18 mm length can potentially escape (Fig. 7). The experimentally obtained results show that L5 (5% retention likelihood) is above 26 mm (Table 6), which means that this type of contact plays no role in defining the size selection for krill in this type of gear. In reality, we retained only 50% of the individuals with a body length of 33 mm (L50) (Table 6). The experimental L50 of krill with the 15.4 mm commercial trawl was estimated to be 32.72 mm with an SR (L75–L25) value of 4.85 mm (Table 7). The selectivity curve shown in Figure 8 demonstrates that size selectivity occurs for individuals smaller than 40 mm in the commercial trawl. Based on the fit statistics in Table 7 it is demonstrated that the applied model (2) in Fig. 8 is able to describe the experimental data sufficiently (p-value>0.05). The experimental selectivity results also show that fewer than 5% of the krill smaller than 26 mm length that enters the trawl will be retained (Table 6). Based on this result, any contribution to the size selectivity of krill from contact modes with L50 less than 26 mm will be very limited. Furthermore, the majority of individuals seem to be able to meet the meshes with a far more optimal body orientation (CS1_C2), at least for their decisive (last) contact with the netting. This is clarified in Figure 9, in which the potential selectivity based on CS1_CS2 and CS3 is compared to the observed selective range in the commercial netting (15.4 mm), the observed oa range during commercial fishing, and the experimental L50 value. Selectivity based on CS3 does not reach the experimentally observed selective range; in contrast, the estimated selectivity based on CS1_CS2 has reasonable overlap with both the expected experimental oa range and the selective range for the 15.4 mm mesh size (Fig. 9). However, Figure 9 shows that results could be slightly biased towards smaller oa values. Such difference may result from effect of attack angle with netting or none optimal rotation for krill during contact with the netting. This potential effect is investigated in detail in the next section.


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

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

The experimentally obtained L50 value is indicated with the solid line (exp L50).L50 predictions for CS1_CS2 and for CS3 are indicated for the commercial mesh size (15.4 mm). The realistic mesh opening angles during commercial fishing is indicated with the vertical gray interval. The horizontal interval indicated the selective range for the 15.4 mm commercial mesh size.
© Copyright Policy
Related In: Results  -  Collection

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

pone-0102168-g009: The experimentally obtained L50 value is indicated with the solid line (exp L50).L50 predictions for CS1_CS2 and for CS3 are indicated for the commercial mesh size (15.4 mm). The realistic mesh opening angles during commercial fishing is indicated with the vertical gray interval. The horizontal interval indicated the selective range for the 15.4 mm commercial mesh size.
Mentions: Compared to the contact mode CS1_CS2, little escapement occurs for the larger CS3 cross section, even for rather large meshes (Fig. 7). When the meshes of the commercially used trawl (15.4 mm) are open optimally, krill smaller than 18 mm length can potentially escape (Fig. 7). The experimentally obtained results show that L5 (5% retention likelihood) is above 26 mm (Table 6), which means that this type of contact plays no role in defining the size selection for krill in this type of gear. In reality, we retained only 50% of the individuals with a body length of 33 mm (L50) (Table 6). The experimental L50 of krill with the 15.4 mm commercial trawl was estimated to be 32.72 mm with an SR (L75–L25) value of 4.85 mm (Table 7). The selectivity curve shown in Figure 8 demonstrates that size selectivity occurs for individuals smaller than 40 mm in the commercial trawl. Based on the fit statistics in Table 7 it is demonstrated that the applied model (2) in Fig. 8 is able to describe the experimental data sufficiently (p-value>0.05). The experimental selectivity results also show that fewer than 5% of the krill smaller than 26 mm length that enters the trawl will be retained (Table 6). Based on this result, any contribution to the size selectivity of krill from contact modes with L50 less than 26 mm will be very limited. Furthermore, the majority of individuals seem to be able to meet the meshes with a far more optimal body orientation (CS1_C2), at least for their decisive (last) contact with the netting. This is clarified in Figure 9, in which the potential selectivity based on CS1_CS2 and CS3 is compared to the observed selective range in the commercial netting (15.4 mm), the observed oa range during commercial fishing, and the experimental L50 value. Selectivity based on CS3 does not reach the experimentally observed selective range; in contrast, the estimated selectivity based on CS1_CS2 has reasonable overlap with both the expected experimental oa range and the selective range for the 15.4 mm mesh size (Fig. 9). However, Figure 9 shows that results could be slightly biased towards smaller oa values. Such difference may result from effect of attack angle with netting or none optimal rotation for krill during contact with the netting. This potential effect is investigated in detail in the next section.

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