Abstract
The size selectivity and usability of three codends were quantified and compared for the first time in the inshore Northern shrimp (Pandalus borealis) trawl fishery of Iceland using the covered codend method: a conventional diamond-mesh codend (T0), conventional square-mesh codend (T45), and a 90° turned mesh codend (T90) constructed of four panels and with shortened lastridge ropes. Fishers, wanting to increase the average-individual size of captured shrimp, had requested the T90 codend to be compared with conventional codends for consideration in the fishery. Results showed that, on average, the T45 and T90 codends had better size selectivity than the T0 codend in terms of releasing individuals smaller than 13 mm carapace length (Minimum References Size; MRS). The T90 codend retained significantly less Northern shrimps between 9 and 19 mm than the T0 codend and between 15 and 19 mm than the T45 codend. No significant difference of size selectivity between T45 and T0 codends was observed. All three codends presented high retention ratios of Northern shrimps above MRS (>63%) for the population encountered. However, the T0 codend was not effective at sorting out small Northern shrimps; at least 86% of Northern shrimps smaller than 13 mm were retained in the T0 codend if encountered. Catches from T45 and T90 codends had a lower proportion of shrimp below MRS. Since discarding of undersized Northern shrimps is prohibited in Iceland and fishers wanted to catch on average larger shrimp, using the novel T90 codend would enable fishers to use their quotas more efficiently.
Highlights
Discards refer to all biomass that is disposed of at sea off a fishing vessel, and bycatch is the capture of non-targeted species whether disposed of or retained (Alverson, Freeberg, Murawski, & Pope, 1994)
Compared to the the netting is rotated 45◦ (T45) and T0 codends, the T90 codend presented better size selection performance in terms of releasing undersized catch; the difference was significant for catch at certain length ranges between the T90 and T0 (9–19 mm) but not between the T45 and T0
The meshes of the T90 codend were able to remain more open than the conventional codends, which led to the reduced capture of small shrimp
Summary
Discards refer to all biomass that is disposed of at sea off a fishing vessel, and bycatch is the capture of non-targeted species whether disposed of or retained (Alverson, Freeberg, Murawski, & Pope, 1994). Shrimp trawling is associated with high amounts of both bycatch and discards due to their use of small-mesh codends to retain small-bodied target species (Alverson et al, 1994; Howell & Langan, 1992). In Iceland, Northern shrimp mainly occur off the north coast of the country and are highly abundant in shallow inshore coastal fjords (Marine and Freshwater Research Institute (MFRI), 2018a, b). In Iceland, the minimum reference size (MRS) of Northern shrimp is 13 mm carapace length (CL) in the inshore fishery.
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