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K.h Nedreaas - One of the best experts on this subject based on the ideXlab platform.
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Relative selectivity of Greenland halibut (Reinhardtius hippoglossoides, Walbaum) by trawls, longlines and Gillnets
1999Co-Authors: Irene Huse, A.c Gundersen, K.h NedreaasAbstract:Abstract Selectivity parameters for Greenland halibut ( Reinhardtius hippoglossoides , Walbaum) were calculated from trawl and Gillnet catches in the Norwegian scientific fisheries for greenland halibut. Within the chosen individual areas and seasons, our data show no trend in length distribution with depth. A trouser-trawl selectivity experiment gave an L 50 at 42 cm in a 135 mm codends. A selectivity analysis of the Gillnets using a log normal model showed maximum probability of retention for lengths of 46.8–73.5 cm for 140–220 mm stretched meshes. By using expected catch proportions and life history parameters, the selectivity in longline and trawl was shown to be bell-shaped, both gears losing both the smallest and largest fish. Calculated estimated life-history parameters were biased by sampling gears, giving slower growth and larger L ∞ for females caught in Gillnets. The maturation length ( M 50 ) of female Greenland halibut was largest in trawl catches, smallest for Gillnet catches. The sex ratio in the Gillnet catches was shown to be a direct function of mesh size. These analyses will provide a better understanding of possible sampling bias when sampling a stock by means of only one type of gear.
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Performance and Biological Implications of a Multi-gear Fishery for Greenland Halibut (Reinhardtius hippoglossoides)
1996Co-Authors: K.h Nedreaas, A V Soldal, A BjordalAbstract:Simultaneous full-scale fishing operations using bottom trawl, Gillnet and longline for Greenland halibut (Reinhardtius hippoglossoides, Walbaum) were performed in the Barents Sea (ICES Divisions IIa and IIb) for two weeks in October 1992. The mesh size of the trawl codend was 135 mm, and that of the Gillnets 220 mm. Additionally, seven Gillnet fleets of 180 mm mesh size were set for selectivity comparison. The catch rates and the length distributions of the Greenland halibut in the catches taken by trawl, longline and Gillnet were different. Gillnets almost exclusively caught fish between 60 and 70 cm, mostly mature females (about 90%). The size distribution taken by longline was wider than that taken by Gillnet. Two thirds of the longline catches were females, and the proportion of immature individuals was larger (14–30%) than in the Gillnet catches. The trawls, however, caught large amounts of fish between 40 and 60 cm, consisting of almost equal amounts of males and females, and 30–40% immature fish. Gear-specific selection properties were the main reasons for the observed differences. Yield-per-recruit and spawning-stock-biomass-per-recruit analyses showed that fixed gears captured proportionally fewer immature fish, and thus indicated they would provide greater spawning biomass and yield-per-recruit than would trawl gear, for an equivalent reference fishing mortality rate.
Dag M. Furevik - One of the best experts on this subject based on the ideXlab platform.
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Catches of Greenland halibut (Reinhardtius hippoglossoides) in deepwater ghost-fishing Gillnets on the Norwegian continental slope
2003Co-Authors: Odd-børre Humborstad, Svein Løkkeborg, Nils-roar Hareide, Dag M. FurevikAbstract:Abstract Fishing gear may continue to fish after it has been lost. Large catches have been observed during cruises to retrieve lost Gillnets in Norwegian waters, especially in the fishery for Greenland halibut (Reinhardtius hippoglossoides). The Norwegian Greenland halibut is overexploited, and there is serious concern about the effect of lost nets on this stock. Catches in deliberately lost Gillnets were studied in the fishery for Greenland halibut off the coast of mid-Norway in July 2000 and June 2001. Gillnet fleets were deployed at depths of between 537 and 851 m, and the soak time ranged from 1 to 68 days. Most of the catch consisted of the target species, and the proportions of different species did not change with soak time. All individuals caught were categorized in terms of seven condition states. A gradual shift from fresh to decomposed individuals over time was evident. The catching efficiency of Gillnets decreased with soak time, presumably due to the weight of the catch causing the headline height to decrease, and after 45 days was only about 20–30% of that of nets used in the commercial fishery. Catch rates were estimated after stabilization at 67–100 and 28–43 kg per day per Gillnet fleet in 2000 and 2001, respectively. The results indicated that Gillnets lost in this area continue to fish for long periods of time. Annual losses of nets need to be quantified in order to estimate the effects of ghost fishing on stock levels, a figure that is currently lacking.
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reduced bycatch of red king crab paralithodes camtschaticus in the Gillnet fishery for cod gadus morhua in northern norway
2003Co-Authors: Hallvard Godoy, Dag M. Furevik, Svein LøkkeborgAbstract:Abstract Bycatches of red king crab (Paralithodes camtschaticus) in stationary fishing gears, especially Gillnets, are a growing problem for inshore fishermen in northern Norway. Large bycatches of king crabs cause extra work for the fishermen and damage their gear and catch. In the cod (Gadus morhua) Gillnet fisheries, the problem could be solved by designing Gillnets that float above the seabed (“norsel-mounted” nets). Norsel-mounted nets floating 0.5 m above the seabed were compared with standard nets in the Varangerfjord (eastern Finnmark) in three periods. In the first period crab catches were generally small and the norsel nets caught 52% fewer cod than standard nets; it was also shown that norsel nets needed more floats than standard nets to stand properly in the sea. By using extra flotation (rings) on certain of the norsel nets during the second period, bycatches of king crab were reduced to an acceptable level, with a catch per unit effort (CPUE) of 0.5 crabs compared with 3.0 crabs with standard placement. Norsel nets with extra floats caught 66% fewer cod than standard nets. During the third period, extra floats were used on both norsels and standard nets. Norsel nets had a CPUE of 0.7 crabs, standard nets 2.4 crabs; norsel nets caught 31% fewer cod than standard nets. In the second and third period norsel nets caught larger cod, thus the catch differences between net types were less in terms of weight. Additional data collected by three coastal fishing vessels operating under different conditions (area and bottom topography), showed that norsel nets fished about the same number of cod and 58% fewer king crab than standard nets. This study indicates that the gear configuration was capable of reducing the bycatch of red king crab. Reduction in fish catches was the largest in periods with poor fishery, and area and bottom conditions may have influenced the catchability of norsel nets to a greater degree than standard nets.
Reidar Borgstrom - One of the best experts on this subject based on the ideXlab platform.
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Gillnet catchability of brown trout Salmo trutta is highly dependent on fish size and capture site
2019Co-Authors: Reidar Borgstrom, Knut Bergum, Trond Erik Børresen, Martin A. SvenningAbstract:Use of experimental Gillnet fleets is common both in scientific studies of fish populations and in fish sampling for management purposes. Fish catchability may vary considerably with fish and Gillnet mesh size, and catches obtained by Gillnet fleets composed of nets with different mesh sizes may give length and age distributions that deviate considerably from the length and age structure of the population. We have estimated the absolute catchability of allopatric brown trout (Salmo trutta) in the littoral and pelagic habitat of a small lake based on a mark-recapture experiment. The brown trout catchability varied considerably both with fish size and habitat type, probably due to a size-related variation in swimming distance per time unit and a size-related use of the different lentic habitats. The sampling bias in experimental Gillnet fishing may be reduced by operating the Gillnet fleets in all possible lentic habitats and most fundamentally, by use of catchability data obtained from populations with ‘known’ length and age structures. By reducing this sampling bias, more realistic estimations of the age and length distribution for a given population will be possible
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effect of population density on Gillnet catchability in four allopatric populations of brown trout salmo trutta
1992Co-Authors: Reidar BorgstromAbstract:The Gillnet catchability of allopatric brown trout (Salmo trutta) populations was studied in four Norwegian high-mountain lakes, with large differences in population densities. The number of fish and the length composition of the populations were estimated by mark–recapture. The catchability coefficient and the catch per unit effort were described by power functions, and the catchability was inversely related to number of fish present. An important fisheries management implication is that brown trout populations with low densities are more vulnerable to Gillnets than populations with high densities. Furthermore, the use of Gillnet catches as an estimator of brown trout population density will be biased.
Jon Helge Volstad - One of the best experts on this subject based on the ideXlab platform.
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spatial and temporal variations in seabird bycatch incidental bycatch in the norwegian coastal Gillnet fishery
2019Co-Authors: Kim Magnus Baerum, Tycho Ankernilssen, Signe Christensendalsgaard, Kirstin Fangel, T Williams, Jon Helge VolstadAbstract:The general decline of seabird populations worldwide raises large concerns. Although multiple factors are interacting to cause the observed trends, increased mortality from incidental bycatch in fisheries has proven to be important for many species. However, the bulk of published knowledge is derived from longline fisheries, whereas bycatch in Gillnet fisheries is less studied and even overlooked in some areas. We present seabird bycatch data from a 10-year time-series of fishery data from the large fleet of small-vessels fishing with Gillnets along the Norwegian coast—a large area and fishery with no prior estimates of seabird bycatch. In general, we document high rates of incidental bycatch (averaging 0.0023 seabirds/net, or approximately 0.08 seabirds/fishing trip). This results in an estimated annual bycatch between 1580 and 11500 (95% CI) birds in this fishery. There was a surprisingly high percentage (43%) of surface-feeding seabirds in the bycatch, with northern fulmar being the most common species. Among the diving seabirds caught, common guillemot was most numerous. Our findings suggest that coastal Gillnet fisheries represent a more general threat to a wider range of seabird populations, as opposed to longline fisheries where surface-feeding seabird species seem to dominate the bycatch. The bycatch estimates for the Norwegian Gillnet-fishery varied in time, between areas, and with fishing depth and distance from the coast, but we found no clear trends in relation to the type of Gillnets used. The results enabled us to identify important spatio-temporal trends in the seabird bycatch, which can allow for the development and implementation of more specific mitigation measures. While specific time closures might be an efficient option to reduce bycatch for diving seabirds, measures such as gear modification and reduction in release of wastewater during fishing operation are probably a more effective mitigation approach for reducing bycatch of surface-feeding seabirds.
Svein Løkkeborg - One of the best experts on this subject based on the ideXlab platform.
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Catches of Greenland halibut (Reinhardtius hippoglossoides) in deepwater ghost-fishing Gillnets on the Norwegian continental slope
2003Co-Authors: Odd-børre Humborstad, Svein Løkkeborg, Nils-roar Hareide, Dag M. FurevikAbstract:Abstract Fishing gear may continue to fish after it has been lost. Large catches have been observed during cruises to retrieve lost Gillnets in Norwegian waters, especially in the fishery for Greenland halibut (Reinhardtius hippoglossoides). The Norwegian Greenland halibut is overexploited, and there is serious concern about the effect of lost nets on this stock. Catches in deliberately lost Gillnets were studied in the fishery for Greenland halibut off the coast of mid-Norway in July 2000 and June 2001. Gillnet fleets were deployed at depths of between 537 and 851 m, and the soak time ranged from 1 to 68 days. Most of the catch consisted of the target species, and the proportions of different species did not change with soak time. All individuals caught were categorized in terms of seven condition states. A gradual shift from fresh to decomposed individuals over time was evident. The catching efficiency of Gillnets decreased with soak time, presumably due to the weight of the catch causing the headline height to decrease, and after 45 days was only about 20–30% of that of nets used in the commercial fishery. Catch rates were estimated after stabilization at 67–100 and 28–43 kg per day per Gillnet fleet in 2000 and 2001, respectively. The results indicated that Gillnets lost in this area continue to fish for long periods of time. Annual losses of nets need to be quantified in order to estimate the effects of ghost fishing on stock levels, a figure that is currently lacking.
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reduced bycatch of red king crab paralithodes camtschaticus in the Gillnet fishery for cod gadus morhua in northern norway
2003Co-Authors: Hallvard Godoy, Dag M. Furevik, Svein LøkkeborgAbstract:Abstract Bycatches of red king crab (Paralithodes camtschaticus) in stationary fishing gears, especially Gillnets, are a growing problem for inshore fishermen in northern Norway. Large bycatches of king crabs cause extra work for the fishermen and damage their gear and catch. In the cod (Gadus morhua) Gillnet fisheries, the problem could be solved by designing Gillnets that float above the seabed (“norsel-mounted” nets). Norsel-mounted nets floating 0.5 m above the seabed were compared with standard nets in the Varangerfjord (eastern Finnmark) in three periods. In the first period crab catches were generally small and the norsel nets caught 52% fewer cod than standard nets; it was also shown that norsel nets needed more floats than standard nets to stand properly in the sea. By using extra flotation (rings) on certain of the norsel nets during the second period, bycatches of king crab were reduced to an acceptable level, with a catch per unit effort (CPUE) of 0.5 crabs compared with 3.0 crabs with standard placement. Norsel nets with extra floats caught 66% fewer cod than standard nets. During the third period, extra floats were used on both norsels and standard nets. Norsel nets had a CPUE of 0.7 crabs, standard nets 2.4 crabs; norsel nets caught 31% fewer cod than standard nets. In the second and third period norsel nets caught larger cod, thus the catch differences between net types were less in terms of weight. Additional data collected by three coastal fishing vessels operating under different conditions (area and bottom topography), showed that norsel nets fished about the same number of cod and 58% fewer king crab than standard nets. This study indicates that the gear configuration was capable of reducing the bycatch of red king crab. Reduction in fish catches was the largest in periods with poor fishery, and area and bottom conditions may have influenced the catchability of norsel nets to a greater degree than standard nets.
