- Email: [email protected]
Reduced bycatch of red king crab (Paralithodes camtschaticus) in the gillnet fishery for cod (Gadus morhua) in northern Norway
Fisheries Research 62 (2003) 377–384
Reduced bycatch of red king crab (Paralithodes camtschaticus) in the gillnet fishery for cod (Gadus morhua) in northern Norway Hallvard Godøy∗ , Dag Furevik, Svein Løkkeborg Fish Capture Division, Institute of Marine Research, P.O. Box 1870, N-5817 Bergen, Norway Received 15 February 2002; received in revised form 21 October 2002; accepted 1 November 2002
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. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Bycatch; Gillnet; Red king crab; Cod; Northern Norway
1. Introduction The red king crab (Paralithodes camtschaticus) is a new species in the Norwegian fauna. To establish a commercially exploitable king crab population in the Barents Sea the Russian authorities released juvenile and adult crabs off Murmansk in the 1960s (Orlov and ∗ Corresponding author. Tel.: +47-5-5236804; fax: +47-5-5236830. E-mail address: [email protected] (H. Godøy).
Ivanov, 1978; Kuzmin and Olsen, 1994). The stock of red king crabs has increased dramatically in recent years (Iversen, 2002), and the government’s (Russia and Norway) intention is to build up a sustainable resource for future exploitation. The king crab stock is therefore protected and may be fished only via a limited pot fishery with a total quota of 200,000 crabs in 2001. Fishing with traditional stationary gear in eastern Finnmark (northern Norway) has come into conflict with the king crab population, which has created a
0165-7836/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0165-7836(02)00281-3
378
H. Godøy et al. / Fisheries Research 62 (2003) 377–384
considerable bycatch problem, especially in the gillnet fisheries for cod (Gadus morhua; Sundet, 1998). Since king crab may be caught only by the pot fishery, crabs caught in gillnets must be discarded. The crabs are often crushed in the net hauling system, and are also often crushed by the fishermen to make them easier to disentangle from the net. In winter crabs may freeze to death on deck while large bycatches are disentangled; thus discarded crabs die or suffer considerable injuries, and the bycatch may therefore make an important contribution to king crab mortality. Disentangling large bycatches of crabs also means extra work for the fishermen, and often damages their gear. Entangled crabs reduce the net area for the target species, and crabs feeding on fish caught in the net reduce the value of the catch. Again, bycatches of king crab seriously reduce gear efficiency and profitability. Proper management of the king crab stock and a profitable commercial gillnet fishery for cod require the development of gears that reduce the bycatch. Bycatch may be reduced by use of norsel-mounted
nets floated off the bottom; this enables crabs to pass beneath the net without becoming entangled. Norsel-mounted cod nets were compared with standard cod nets in regard to reduce the bycatch of king crabs, while leaving the catch rates of target species unaffected.
2. Material and methods Experiments were done in three periods, two in 1999 (17 March–11 April and 9–28 May), one in 2001 (2–26 April). The experiments took place off Bugøynes in the Varangerfjord, northern Norway (Fig. 1). The fjord is deep (400 m) with steep slopes. The gillnets are traditionally set down slope, starting at about 60–90 m and ending at about 220–250 m for a typical gillnet fleet of 300–400 m length. The experiments were carried out from the 35 feet long “Eskil”, a fishing boat usually setting gillnets and pots. Standard gillnets used were monofilament
Fig. 1. Map of the Varangerfjord. The experiments took place off Bugøynes (shaded area).
H. Godøy et al. / Fisheries Research 62 (2003) 377–384
379
Fig. 2. Sketch of a norsel-mounted gillnet. The netting itself is raised 0.5 m above the seabed by the norsels.
(0.55 mm, transparent green) with 84 mm mesh size (bar mesh), hanging ratio (ET ) of 0.5 and 50 meshes deep. The nets were 27.5 m long and 7.3 m high. The “Mega float” headline had a buoyancy of 65 g/m and the solerope weighted 250 g/m. The experimental nets were standard nets mounted on norsels (Fig. 2). A “norsel” is defined as a “separate length of cord connecting at a distance the netting to the frame rope” (Bridger et al., 1981). The fishing trials in period 1 showed that the norsel-mounted nets needed extra floats to stand properly “stretched”. As extra floats there were used three rings (each with a buoyancy of 240 g) attached to each net, and norsel-mounted nets both with and without extra floats were tested in period 2. In period 3, floats were used on all norsel and standard nets. The norsel-mounted nets were compared with standard nets. The gillnet fleets (10–15 nets of similar type in each fleet) were set at similar depths and with a minimum distance between fleets of 0.5 nautical miles. Setting and hauling time depended on weather conditions, but the fleets were usually set at noon and hauled the following morning. Paired comparisons of catch per unit effort (CPUE) between fleets of norsel-mounted nets and standard nets were analysed statistically. The CPUE for fleet i was calculated as CPUEi =
ci n i si
where ci is the catch (number of individuals) in fleet i, ni the number of nets in fleet i and si the soaktime (days) for fleet i.
To record the vertical distribution of the fish caught in the net, the net panel was divided into three equal parts: upper, middle and lower, each approximately 2.4 m in height. Since the fish were length measured, not weighed, a simple equation describing the relationship between length (L) and weight (W) was used to calculate the weight: W = L3 In addition to the three research cruises, data were collected from three coastal boats (“Eskil”, “Oddson” and “Tommy André”) during winter 2001. Data were collected only on the number of cod and king crab caught in the two different types of net. The gillnet parameters varied somewhat from boat to boat; “Eskil” and “Tommy André” used nets with extra floats whilst “Oddson” did not use extra floats but used a 12 mm floatline as foot cord. “Eskil” set some fleets off Bugøynes but most were set in the inner part of the Varangerfjord at a depth of about 150 m. This part of the fjord is relatively shallow and flat. “Oddson” fished only in the inner part of the Varangerfjord at about 150 m, whereas “Tommy André” fished off Vardø at about 150 m, in an area that is flat and has a low density of crabs.
3. Results The catches during the research cruises were mainly cod, but several other species were also caught, the most frequent being haddock (Melanogrammus
380
H. Godøy et al. / Fisheries Research 62 (2003) 377–384
Table 1 Catch data from period 1 for standard and norsel nets Net type
Standard Norsel
Cod
Other
Crab
Number
Mean CPUE (±S.D.)
Mean length
Number
Mean CPUE
Number
Mean CPUE (±S.D.)
Mean length
217 203
2.1 ± 0.8 1.0 ± 0.5
66.4 62.6
77 180
0.6 0.7
14 80
0.1 ± 0.1 0.2 ± 0.2
14.8 13.5
aeglefinus), saithe (Pollachius virens), redfish (Sebastes marinus) and long rough dab (Hippoglossoides platessoides). These species are referred to as “other” in the catch data.
in the lower part of the panel whereas the predominance of cod were in the lower and middle panels of norsel nets. All parts of the standard net caught more cod than the norsel net, this difference being most pronounced for the lower part.
3.1. Period 1 3.2. Period 2 In period 1, 20 fleets with a total of 298 nets (103 standard and 195 norsel) were set. The mean soak times were 29 and 31 h for standard and norsel nets, respectively. A total of 94 king crabs and 677 fish were caught (Table 1). Norsel nets caught only half as many cod as standard nets (Wilcoxon paired test, P < 0.01). The mean length of cod caught on norsel nets was significantly lower than those of standard nets (Student’s t-test, P < 0.01). Only a few crabs were caught during this period. Norsel nets caught more crab than standard nets, but this was due to high crab catch in one norsel net fleet. The difference was not significant (Wilcoxon paired test). The vertical distribution of the cod catch is shown in Table 2. Most cod caught in standard nets were found
In period 2, 39 fleets with a total of 458 nets (130 standard, 129 norsel and 199 norsel with rings) were set. Mean soak times were 27 and 26 h for standard and norsel nets, respectively. A total of 1430 king crabs and 2591 fish were caught. Norsel nets caught 67% and norsel nets with rings caught 66% less cod (65 and 59% by weight) than standard nets (Table 3). The number of cod caught by both types of norsel nets was significantly lower than by standard nets (P < 0.01), and their mean length was significantly higher on norsel nets (P < 0.05) and norsel nets with rings (P < 0.001) versus standard nets. Norsel nets with rings caught fewer crabs than standard nets (P = 0.055) and norsel nets without rings (P < 0.01).
Table 2 Vertical distribution and mean CPUE of cod caught in the lower, middle and upper net panels of standard and norsel nets in period 1 Net type
Lower
Standard Norsel
Middle
Upper
Percent
Mean CPUE
Percent
Mean CPUE
Percent
Mean CPUE
60 44
1.3 0.4
24 38
0.5 0.4
16 18
0.3 0.2
Table 3 Catch data from period 2 for standard nets, norsel nets and norsel nets with rings (norsel R) Net type
Standard Norsel Norsel R
Cod
Other
Crab
Number
Mean CPUE (±S.D.)
Mean length
Number
Mean CPUE
Number
Mean CPUE (±S.D.)
Mean length
1263 493 631
9.6 ± 7.1 3.2 ± 2.6 3.3 ± 2.5
63.6 64.8 67.4
88 66 50
0.6 0.4 0.3
429 881 121
3.0 ± 4.1 6.2 ± 6.0 0.5 ± 0.5
10.0 10.0 10.1
H. Godøy et al. / Fisheries Research 62 (2003) 377–384
381
Table 4 Vertical distribution and mean CPUE of cod caught in the lower, middle and upper panels of the nets: standard, norsel and norsel with rings (norsel R) in period 2 Net type
Lower
Standard Norsel Norsel R
Middle
Upper
Percent
Mean CPUE
Percent
Mean CPUE
Percent
Mean CPUE
47 44 50
4.5 1.4 1.7
45 49 46
4.3 1.6 1.5
8 7 4
0.8 0.2 0.1
Table 5 Catch data from period 3 for standard nets with rings (standard R) and norsel nets with rings (norsel R) Net type
Standard R Norsel R
Cod
Other
Crab
Number
Mean CPUE (±S.D.)
Mean length
Number
Mean CPUE
Number
Mean CPUE (±S.D.)
Mean length
881 639
5.2 ± 4.0 3.6 ± 2.8
70.5 74.3
117 105
0.7 0.5
386 201
2.4 ± 1.9 0.7 ± 0.7
11.4 11.3
The vertical distribution of the cod catch in the net panels is shown in Table 4. The three different net types had virtually identical vertical catch distributions.
The vertical distribution of the cod catch in the net panel is shown in Table 6. The norsel nets caught a slightly higher percentage in lower parts of the net panel compared with the standard nets.
3.3. Period 3
3.4. Additional data collected during winter 2001
In period 3, 27 fleets with a total of 352 nets (156 standard and 196 norsel) were set, both types with rings. Mean soak times were 26 and 27 h for standard and norsel nets, respectively. A total of 587 king crabs and 1742 fish were caught. Norsel nets caught 31% fewer cod (19% by weight) than standard nets (Table 5), but this difference was not significant. The mean length of the cod was significantly higher on norsel nets than in standard nets (P < 0.001). Norsel nets caught significantly fewer crabs than standard nets (P < 0.001).
The three boats collected catch data from a total of 4237 nets (3112 standard and 1125 norsel) that caught 18,931 cods and 1100 king crabs (Table 7). “Eskil” caught similar number of cod per norsel net and 51% fewer king crabs than with standard nets. “Oddson” caught 77% more cod per norsel net than with standard nets. Bycatches of king crab were relatively small but the norsel nets caught 88% fewer crabs per net. “Tommy André” caught 11% fewer cod on norsel nets than with standard nets. This vessel’s bycatch of king crab was insignificant.
Table 6 Vertical distribution and mean CPUE of cod caught in the upper, middle and lower net panels of standard nets with rings (standard R) and norsel nets with rings (norsel R) in period 3 Net type
Standard R Norsel R
Lower
Middle
Upper
Percent
Mean CPUE
Percent
Mean CPUE
Percent
Mean CPUE
61 67
3.2 2.4
35 28
1.8 1.0
4 5
0.2 0.2
382
H. Godøy et al. / Fisheries Research 62 (2003) 377–384
Table 7 Number of nets, catch of cod and crab per net (and total) for data collected by three coastal fishing vessels using standard and norsel-mounted cod gillnets during winter 2001 Net type
Standard R Norsel R
Eskil
Oddson
Tommy Andr´e
Nets
Cod
Crab
Nets
Cod
Crab
Nets
Cod
Crab
376 341
3.5 (1328) 3.4 (1168)
0.8 (294) 0.4 (130)
1540 176
3.9 (5992) 6.9 (1210)
0.4 (631) <0.1 (9)
1196 608
5.3 (6366) 4.7 (2867)
<0.1 (25) <0.1 (11)
4. Discussion The king crab bycatch problem is the greatest in the cod gillnet fishery. In the spring cod fishery in 1999, king crab bycatches of up to 5000 individuals on one gillnet fleet (10–15 nets) were reported in the Varangerfjord, suggesting that bycatches of crabs are probably greater than the Norwegian research quota of 100,000 crabs (2001). As the crab population expands westward more fjord and coastal areas will face this problem. The aim of our study was to investigate the use of norsel-mounted nets to avoid king crabs becoming entangled in the net. If norsel-mounted nets are to avoid catching crabs, it is imperative that the norsels stand properly in the sea. This means that the netting must be above the reach of crabs, so that they do not become entangled as they move under the net. Several factors will influence the norsel height, e.g. the vertical distance between the foot cord and the sole rope; large fish catches, entanglement during setting and setting nets on steep/rough bottom will all reduce the norsel height. Currents affect the net and can produce significant differences in the height of the headline (Stewart and Ferro, 1985; Fridman, 1986; Stewart, 1988). It is therefore likely that currents may also affect the norsel height. Furthermore, UV observations have shown that crabs can pull down a norsel and get entangled in the net if the net has too little buoyancy (unpublished observations). Bycatches of king crab were reduced on the norsel nets (with sufficient flotation) compared with standard nets. It will probably be difficult to completely eliminate bycatches, but it should be possible to make further improvements in gear to reduce the bycatch. Using longer norsels (e.g. 1 m) might result in lower bycatches of king crabs, but not necessarily less fish than with 0.5 m norsels.
Crab bycatches varied widely; one net in a fleet might catch hundreds of crabs while the rest took only a few. The king crab has a “social” behaviour called “podding” whereby large number of crabs aggregate within a small area as a protection against predators (Powell and Nickerson, 1965; Bright, 1967). Aggregations of up to 500,000 crabs in a single “pod” have been reported in Alaska (Powell and Nickerson, 1965). Studies have shown a diurnal rhythm in this behaviour with periods of resting in “pods” during the day, and foraging as a group at night (Dew, 1990). This phenomenon may explain the large variations in size of the bycatches. Gillnet efficiency and selectivity are affected by many factors, directly or in interaction with other factors (e.g. Hamley, 1975; Pope et al., 1975; Baranov, 1976; Dickson, 1989; Engås and Løkkeborg, 1994; Machiels et al., 1994). It is therefore likely that rigging the net on norsels would affect various net characteristics. One of the most obvious “side effects” is loss of fish in the norsel area. Size selection might also be affected. Using norsels raised the net panel 0.5 m in the water column. The net area was theoretically the same and it is therefore somewhat surprising that the catches were reduced in all three vertical sections of the net. It is obvious that fish close to the bottom may swim under the norsel nets, but losses of fish in the two other sections must be due to less direct consequences of mounting the net on norsels. Vision is regarded as an important factor in determining whether fish are caught in a gillnet (Jester, 1973; Cui et al., 1991; Wardle et al., 1991). The catch rate thus depends on the net generating a minimum level of stimulation which might provoke avoidance responses (Engås and Løkkeborg, 1994). One reason that norsel-mounted nets caught fewer fish than standard nets might therefore be that the foot cord and the
H. Godøy et al. / Fisheries Research 62 (2003) 377–384
norsels increased net visibility (see Fig. 2), enabling the fish to “steer clear” of the net or swim under the foot cord. Also fish density as measured by the cruises was patchy. One fleet could have a good catch and an adjacent fleet a poor catch. During period 2, two standard fleets had especially good catches, with a CPUE of 22.4 and 21.3 cods. Such high catches were not seen in norsel fleets. Reduced catches on the norsel nets may therefore be partly due because of patchy fish distribution and not necessarily lower catching efficiency alone. The results of the research cruises showed that the norsel nets made lower catches of cod (significantly lower in two of the three cruises), while the additional data collected by fishermen showed similar catches of cod for two vessels and higher catches for the third boat. The “Oddson’s” higher catch rate might have been caused by patchy fish distribution. This boat’s use of a floatline as foot cord is also a possible explanation, since the floatline was of a different colour from the other foot cord, and might have been less visible to the fish. There are several possible explanations for the difference in results obtained by the research cruises and commercial boats. The two sets of data were col-
383
lected in different areas. The area in which the research cruises were conducted has relatively steep bottom conditions compared to the area sampled by the commercial vessels that fished in the inner part of the fjord and off Vardø with smoother and flatter bottom conditions. The type of bottom is capable of influencing both net- and fish-related parameters. The degree of steepness can affect the shape of the meshes as well as the shape and slackness of the webbing and thus the visibility of the net. These factors may influence the catching properties and selectivity of the net. The bottom type may also affect fish behaviour, e.g. vertical distribution, swimming speed and swimming patterns, which in turn may influence catch rates. Crab density also varied between the two areas, a factor that also might affect the behaviour of cod. Studies of different hanging ratios have shown that nets with low hanging ratio (loosely hung) catch fish over a wider size range than tightly hung nets (Angelsen et al., 1979; Engås and Løkkeborg, 1994). A low hanging ratio is likely to increase the proportion of entangled fish (Hamley, 1975). More flotation tightens up the net webbing, which may affect how the fish are caught or entangled, and thus selectivity. This may explain the higher mean length of fish caught on norsel nets with rings compared to standard nets in
Fig. 3. Cross-section of a standard net and a norsel net influenced by current.
384
H. Godøy et al. / Fisheries Research 62 (2003) 377–384
period 2. On the other hand, norsel nets with rings also caught larger cod during period 3 than standard nets with the same amount of flotation. A possible explanation is that mounting the net on norsels may affect size selectivity. It is likely that crabs also become entangled more easily in a loose than a tight net. Studies of entangled nets showed that small spanner crabs (Ranina ranina) were more easily entangled in loosely hung (ET = 0.5) than tightly hung nets (ET = 0.7) (Sumpton et al., 1995). During periods 1 and 2, norsel nets without extra floats made higher bycatches of crabs than the standard nets. One explanation of this may be that the webbing of a norsel net is mounted on the foot cord. This attaches the net more loosely (to the foot cord) than mounting the webbing directly to the sole rope, thus making it easier to entangle crabs. Another explanation may be a “hammock-effect” on norsel nets that have little flotation due to the influence of current (Fig. 3). The use of norsel nets significantly reduced the bycatch of king crabs but also reduced the catches of cod. It appears that area and bottom conditions may have influenced the catch efficiency of norsel nets more than that of standard nets. Improvements in gear might reduce bycatches of king crab further, and simultaneously maintain cod catch rates. Acknowledgements We are grateful to Leif Ingilæ and Bjarne Hansen onboard “Eskil” for their cooperation and help during the data collection. Thanks to Svein Floen, Roar Skeide and Jostein Saltskår for good assistance in the field, and to Terje Jørgensen for useful help with the statistical analysis. References Angelsen, K., Haugen, K., Floen, S., 1979. The catching efficiency of cod gillnets with different hanging ratio (E) and different floatline buoyancy. ICES CM 1979/B:19. Baranov, F.I., 1976. Selected Works on Fishing Gear. Commercial Fishing Techniques, vol. 1. Keter, Jerusalem, 631 pp. Bridger, J.P., Foster, J.J., Margetts, A.R., Strange, E.S., 1981. Glossary of United Kingdom Fishing Gear Terms. Fishing News Books Ltd., Surrey, 115 pp. Bright, D.B., 1967. Life histories of the king crab, Paralithodes camtschatica, and the tanner crab, Chionoecetes bairdi, in Cook
Inlet, Alaska. Ph.D. Thesis. University of Southern California, Los Angeles, 265 pp. Cui, G., Wardle, C.S., Glass, C.W., Johnstone, A.D.F., Mojsiewicz, W.R., 1991. Light level thresholds for visual reaction of mackerel, Scomber scombrus L., to coloured monofilament nylon gillnet materials. Fish. Res. 10, 255–263. Dew, C.B., 1990. Behavioral ecology of podding red king crab, Paralithodes camtschatica. Can. J. Fish. Aquat. Sci. 47, 1944– 1958. Dickson, W., 1989. Cod gillnet effectiveness related to local abundance, availability and fish movement. Fish. Res. 7, 127– 148. Engås, A., Løkkeborg, S., 1994. Abundance estimation using bottom gillnet and longline—the role of fish behaviour. In: Fernö, A., Olsen, S. (Eds.), Marine Fish Behaviour in Capture and Abundance Estimation. Fishing News Books, Oxford, pp. 134–165. Fridman, A.L., 1986. Calculations for Fishing Gear Designs. FAO Fish. Man. Fishing News Books Ltd., London, 241 pp. Hamley, J.M., 1975. Review of gillnet selectivity. J. Fish. Res. Board Can. 32, 1943–1969. Iversen, S.A., 2002. Havets ressurser 2002. Fisken og havet, saernr. 1-2002, 151 pp. (in Norwegian with English abstract). Jester, D.B., 1973. Variation in catchability of fishes with color of gillnets. Trans. Am. Fish. Soc. 1, 109–115. Kuzmin, S., Olsen S., 1994. Barents Sea king crab (Paralithodes camtschatica). The transplantation experiments were successful. Shellfish Committee. ICES CM 1994/K: 12, 12 pp. Machiels, M.A.M., Klinge, M., Lanterns, R., van Densen, W.L.T., 1994. Effect of snood length and hanging ratio on efficiency and selectivity of bottom-set gillnets for pikeperch, Stizostedion lucioperca L., and bream, Abramis brama. Fish. Res. 19, 231– 239. Orlov, Y.I., Ivanov, B.G., 1978. On the introduction of the Kamchatka King Crab Paralithodes camtschatica (Decapoda: Anomura: Lithodidae) into the Barents Sea. Mar. Biol. 48 (4), 373–375. Pope, J.A., Margetts, A.R., Hamley, J.M., Akyuiz, E.F., 1975. Manual of methods for fish stock assessment. Part III. Selectivity of fishing gear. FAO Fish. Tech. Pap. No. 41, 65 pp. Powell, G.C., Nickerson, R.B., 1965. Aggregations among juvenile king crabs (Paralithodes camtschatica), Tilesius. Kodiac Alaska Anim. Behav. 13 (2–3), 374–380. Stewart, P.A.M., 1988. Measurements of the effects of tidal flow on the headline heights of bottom-set gillnets. Fish. Res. 6, 181–189. Stewart, P.A.M., Ferro, R.S.T., 1985. Measurements on gillnets in a flume tank. Fish. Res. 3, 29–46. Sumpton, W.D., Brown, I.W., Kennelly, S.J., 1995. Fishing gears that minimise the damage incurred by discarded spanner crabs (Ranina ranina): laboratory and field experiments. Fish. Res. 22, 11–27. Sundet, J.H., 1998. Bifangst av kongekrabbe i det ordinære fisket. En kartlegging blant fiskere i Øst-Finnmark. Rapport Fiskeriforskning 1—1998, Tromsø, 15 pp. Wardle, C.S., Cui, G., Mojsiewicz, W.R., Glass, C.W., 1991. The effect of colour on the appearance of monofilament nylon under water. Fish. Res. 10, 243–253.
Reduced bycatch of red king crab (Paralithodes camtschaticus) in the gillnet fishery for cod (Gadus morhua) in northern Norway Hallvard Godøy∗ , Dag Furevik, Svein Løkkeborg Fish Capture Division, Institute of Marine Research, P.O. Box 1870, N-5817 Bergen, Norway Received 15 February 2002; received in revised form 21 October 2002; accepted 1 November 2002
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. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Bycatch; Gillnet; Red king crab; Cod; Northern Norway
1. Introduction The red king crab (Paralithodes camtschaticus) is a new species in the Norwegian fauna. To establish a commercially exploitable king crab population in the Barents Sea the Russian authorities released juvenile and adult crabs off Murmansk in the 1960s (Orlov and ∗ Corresponding author. Tel.: +47-5-5236804; fax: +47-5-5236830. E-mail address: [email protected] (H. Godøy).
Ivanov, 1978; Kuzmin and Olsen, 1994). The stock of red king crabs has increased dramatically in recent years (Iversen, 2002), and the government’s (Russia and Norway) intention is to build up a sustainable resource for future exploitation. The king crab stock is therefore protected and may be fished only via a limited pot fishery with a total quota of 200,000 crabs in 2001. Fishing with traditional stationary gear in eastern Finnmark (northern Norway) has come into conflict with the king crab population, which has created a
0165-7836/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0165-7836(02)00281-3
378
H. Godøy et al. / Fisheries Research 62 (2003) 377–384
considerable bycatch problem, especially in the gillnet fisheries for cod (Gadus morhua; Sundet, 1998). Since king crab may be caught only by the pot fishery, crabs caught in gillnets must be discarded. The crabs are often crushed in the net hauling system, and are also often crushed by the fishermen to make them easier to disentangle from the net. In winter crabs may freeze to death on deck while large bycatches are disentangled; thus discarded crabs die or suffer considerable injuries, and the bycatch may therefore make an important contribution to king crab mortality. Disentangling large bycatches of crabs also means extra work for the fishermen, and often damages their gear. Entangled crabs reduce the net area for the target species, and crabs feeding on fish caught in the net reduce the value of the catch. Again, bycatches of king crab seriously reduce gear efficiency and profitability. Proper management of the king crab stock and a profitable commercial gillnet fishery for cod require the development of gears that reduce the bycatch. Bycatch may be reduced by use of norsel-mounted
nets floated off the bottom; this enables crabs to pass beneath the net without becoming entangled. Norsel-mounted cod nets were compared with standard cod nets in regard to reduce the bycatch of king crabs, while leaving the catch rates of target species unaffected.
2. Material and methods Experiments were done in three periods, two in 1999 (17 March–11 April and 9–28 May), one in 2001 (2–26 April). The experiments took place off Bugøynes in the Varangerfjord, northern Norway (Fig. 1). The fjord is deep (400 m) with steep slopes. The gillnets are traditionally set down slope, starting at about 60–90 m and ending at about 220–250 m for a typical gillnet fleet of 300–400 m length. The experiments were carried out from the 35 feet long “Eskil”, a fishing boat usually setting gillnets and pots. Standard gillnets used were monofilament
Fig. 1. Map of the Varangerfjord. The experiments took place off Bugøynes (shaded area).
H. Godøy et al. / Fisheries Research 62 (2003) 377–384
379
Fig. 2. Sketch of a norsel-mounted gillnet. The netting itself is raised 0.5 m above the seabed by the norsels.
(0.55 mm, transparent green) with 84 mm mesh size (bar mesh), hanging ratio (ET ) of 0.5 and 50 meshes deep. The nets were 27.5 m long and 7.3 m high. The “Mega float” headline had a buoyancy of 65 g/m and the solerope weighted 250 g/m. The experimental nets were standard nets mounted on norsels (Fig. 2). A “norsel” is defined as a “separate length of cord connecting at a distance the netting to the frame rope” (Bridger et al., 1981). The fishing trials in period 1 showed that the norsel-mounted nets needed extra floats to stand properly “stretched”. As extra floats there were used three rings (each with a buoyancy of 240 g) attached to each net, and norsel-mounted nets both with and without extra floats were tested in period 2. In period 3, floats were used on all norsel and standard nets. The norsel-mounted nets were compared with standard nets. The gillnet fleets (10–15 nets of similar type in each fleet) were set at similar depths and with a minimum distance between fleets of 0.5 nautical miles. Setting and hauling time depended on weather conditions, but the fleets were usually set at noon and hauled the following morning. Paired comparisons of catch per unit effort (CPUE) between fleets of norsel-mounted nets and standard nets were analysed statistically. The CPUE for fleet i was calculated as CPUEi =
ci n i si
where ci is the catch (number of individuals) in fleet i, ni the number of nets in fleet i and si the soaktime (days) for fleet i.
To record the vertical distribution of the fish caught in the net, the net panel was divided into three equal parts: upper, middle and lower, each approximately 2.4 m in height. Since the fish were length measured, not weighed, a simple equation describing the relationship between length (L) and weight (W) was used to calculate the weight: W = L3 In addition to the three research cruises, data were collected from three coastal boats (“Eskil”, “Oddson” and “Tommy André”) during winter 2001. Data were collected only on the number of cod and king crab caught in the two different types of net. The gillnet parameters varied somewhat from boat to boat; “Eskil” and “Tommy André” used nets with extra floats whilst “Oddson” did not use extra floats but used a 12 mm floatline as foot cord. “Eskil” set some fleets off Bugøynes but most were set in the inner part of the Varangerfjord at a depth of about 150 m. This part of the fjord is relatively shallow and flat. “Oddson” fished only in the inner part of the Varangerfjord at about 150 m, whereas “Tommy André” fished off Vardø at about 150 m, in an area that is flat and has a low density of crabs.
3. Results The catches during the research cruises were mainly cod, but several other species were also caught, the most frequent being haddock (Melanogrammus
380
H. Godøy et al. / Fisheries Research 62 (2003) 377–384
Table 1 Catch data from period 1 for standard and norsel nets Net type
Standard Norsel
Cod
Other
Crab
Number
Mean CPUE (±S.D.)
Mean length
Number
Mean CPUE
Number
Mean CPUE (±S.D.)
Mean length
217 203
2.1 ± 0.8 1.0 ± 0.5
66.4 62.6
77 180
0.6 0.7
14 80
0.1 ± 0.1 0.2 ± 0.2
14.8 13.5
aeglefinus), saithe (Pollachius virens), redfish (Sebastes marinus) and long rough dab (Hippoglossoides platessoides). These species are referred to as “other” in the catch data.
in the lower part of the panel whereas the predominance of cod were in the lower and middle panels of norsel nets. All parts of the standard net caught more cod than the norsel net, this difference being most pronounced for the lower part.
3.1. Period 1 3.2. Period 2 In period 1, 20 fleets with a total of 298 nets (103 standard and 195 norsel) were set. The mean soak times were 29 and 31 h for standard and norsel nets, respectively. A total of 94 king crabs and 677 fish were caught (Table 1). Norsel nets caught only half as many cod as standard nets (Wilcoxon paired test, P < 0.01). The mean length of cod caught on norsel nets was significantly lower than those of standard nets (Student’s t-test, P < 0.01). Only a few crabs were caught during this period. Norsel nets caught more crab than standard nets, but this was due to high crab catch in one norsel net fleet. The difference was not significant (Wilcoxon paired test). The vertical distribution of the cod catch is shown in Table 2. Most cod caught in standard nets were found
In period 2, 39 fleets with a total of 458 nets (130 standard, 129 norsel and 199 norsel with rings) were set. Mean soak times were 27 and 26 h for standard and norsel nets, respectively. A total of 1430 king crabs and 2591 fish were caught. Norsel nets caught 67% and norsel nets with rings caught 66% less cod (65 and 59% by weight) than standard nets (Table 3). The number of cod caught by both types of norsel nets was significantly lower than by standard nets (P < 0.01), and their mean length was significantly higher on norsel nets (P < 0.05) and norsel nets with rings (P < 0.001) versus standard nets. Norsel nets with rings caught fewer crabs than standard nets (P = 0.055) and norsel nets without rings (P < 0.01).
Table 2 Vertical distribution and mean CPUE of cod caught in the lower, middle and upper net panels of standard and norsel nets in period 1 Net type
Lower
Standard Norsel
Middle
Upper
Percent
Mean CPUE
Percent
Mean CPUE
Percent
Mean CPUE
60 44
1.3 0.4
24 38
0.5 0.4
16 18
0.3 0.2
Table 3 Catch data from period 2 for standard nets, norsel nets and norsel nets with rings (norsel R) Net type
Standard Norsel Norsel R
Cod
Other
Crab
Number
Mean CPUE (±S.D.)
Mean length
Number
Mean CPUE
Number
Mean CPUE (±S.D.)
Mean length
1263 493 631
9.6 ± 7.1 3.2 ± 2.6 3.3 ± 2.5
63.6 64.8 67.4
88 66 50
0.6 0.4 0.3
429 881 121
3.0 ± 4.1 6.2 ± 6.0 0.5 ± 0.5
10.0 10.0 10.1
H. Godøy et al. / Fisheries Research 62 (2003) 377–384
381
Table 4 Vertical distribution and mean CPUE of cod caught in the lower, middle and upper panels of the nets: standard, norsel and norsel with rings (norsel R) in period 2 Net type
Lower
Standard Norsel Norsel R
Middle
Upper
Percent
Mean CPUE
Percent
Mean CPUE
Percent
Mean CPUE
47 44 50
4.5 1.4 1.7
45 49 46
4.3 1.6 1.5
8 7 4
0.8 0.2 0.1
Table 5 Catch data from period 3 for standard nets with rings (standard R) and norsel nets with rings (norsel R) Net type
Standard R Norsel R
Cod
Other
Crab
Number
Mean CPUE (±S.D.)
Mean length
Number
Mean CPUE
Number
Mean CPUE (±S.D.)
Mean length
881 639
5.2 ± 4.0 3.6 ± 2.8
70.5 74.3
117 105
0.7 0.5
386 201
2.4 ± 1.9 0.7 ± 0.7
11.4 11.3
The vertical distribution of the cod catch in the net panels is shown in Table 4. The three different net types had virtually identical vertical catch distributions.
The vertical distribution of the cod catch in the net panel is shown in Table 6. The norsel nets caught a slightly higher percentage in lower parts of the net panel compared with the standard nets.
3.3. Period 3
3.4. Additional data collected during winter 2001
In period 3, 27 fleets with a total of 352 nets (156 standard and 196 norsel) were set, both types with rings. Mean soak times were 26 and 27 h for standard and norsel nets, respectively. A total of 587 king crabs and 1742 fish were caught. Norsel nets caught 31% fewer cod (19% by weight) than standard nets (Table 5), but this difference was not significant. The mean length of the cod was significantly higher on norsel nets than in standard nets (P < 0.001). Norsel nets caught significantly fewer crabs than standard nets (P < 0.001).
The three boats collected catch data from a total of 4237 nets (3112 standard and 1125 norsel) that caught 18,931 cods and 1100 king crabs (Table 7). “Eskil” caught similar number of cod per norsel net and 51% fewer king crabs than with standard nets. “Oddson” caught 77% more cod per norsel net than with standard nets. Bycatches of king crab were relatively small but the norsel nets caught 88% fewer crabs per net. “Tommy André” caught 11% fewer cod on norsel nets than with standard nets. This vessel’s bycatch of king crab was insignificant.
Table 6 Vertical distribution and mean CPUE of cod caught in the upper, middle and lower net panels of standard nets with rings (standard R) and norsel nets with rings (norsel R) in period 3 Net type
Standard R Norsel R
Lower
Middle
Upper
Percent
Mean CPUE
Percent
Mean CPUE
Percent
Mean CPUE
61 67
3.2 2.4
35 28
1.8 1.0
4 5
0.2 0.2
382
H. Godøy et al. / Fisheries Research 62 (2003) 377–384
Table 7 Number of nets, catch of cod and crab per net (and total) for data collected by three coastal fishing vessels using standard and norsel-mounted cod gillnets during winter 2001 Net type
Standard R Norsel R
Eskil
Oddson
Tommy Andr´e
Nets
Cod
Crab
Nets
Cod
Crab
Nets
Cod
Crab
376 341
3.5 (1328) 3.4 (1168)
0.8 (294) 0.4 (130)
1540 176
3.9 (5992) 6.9 (1210)
0.4 (631) <0.1 (9)
1196 608
5.3 (6366) 4.7 (2867)
<0.1 (25) <0.1 (11)
4. Discussion The king crab bycatch problem is the greatest in the cod gillnet fishery. In the spring cod fishery in 1999, king crab bycatches of up to 5000 individuals on one gillnet fleet (10–15 nets) were reported in the Varangerfjord, suggesting that bycatches of crabs are probably greater than the Norwegian research quota of 100,000 crabs (2001). As the crab population expands westward more fjord and coastal areas will face this problem. The aim of our study was to investigate the use of norsel-mounted nets to avoid king crabs becoming entangled in the net. If norsel-mounted nets are to avoid catching crabs, it is imperative that the norsels stand properly in the sea. This means that the netting must be above the reach of crabs, so that they do not become entangled as they move under the net. Several factors will influence the norsel height, e.g. the vertical distance between the foot cord and the sole rope; large fish catches, entanglement during setting and setting nets on steep/rough bottom will all reduce the norsel height. Currents affect the net and can produce significant differences in the height of the headline (Stewart and Ferro, 1985; Fridman, 1986; Stewart, 1988). It is therefore likely that currents may also affect the norsel height. Furthermore, UV observations have shown that crabs can pull down a norsel and get entangled in the net if the net has too little buoyancy (unpublished observations). Bycatches of king crab were reduced on the norsel nets (with sufficient flotation) compared with standard nets. It will probably be difficult to completely eliminate bycatches, but it should be possible to make further improvements in gear to reduce the bycatch. Using longer norsels (e.g. 1 m) might result in lower bycatches of king crabs, but not necessarily less fish than with 0.5 m norsels.
Crab bycatches varied widely; one net in a fleet might catch hundreds of crabs while the rest took only a few. The king crab has a “social” behaviour called “podding” whereby large number of crabs aggregate within a small area as a protection against predators (Powell and Nickerson, 1965; Bright, 1967). Aggregations of up to 500,000 crabs in a single “pod” have been reported in Alaska (Powell and Nickerson, 1965). Studies have shown a diurnal rhythm in this behaviour with periods of resting in “pods” during the day, and foraging as a group at night (Dew, 1990). This phenomenon may explain the large variations in size of the bycatches. Gillnet efficiency and selectivity are affected by many factors, directly or in interaction with other factors (e.g. Hamley, 1975; Pope et al., 1975; Baranov, 1976; Dickson, 1989; Engås and Løkkeborg, 1994; Machiels et al., 1994). It is therefore likely that rigging the net on norsels would affect various net characteristics. One of the most obvious “side effects” is loss of fish in the norsel area. Size selection might also be affected. Using norsels raised the net panel 0.5 m in the water column. The net area was theoretically the same and it is therefore somewhat surprising that the catches were reduced in all three vertical sections of the net. It is obvious that fish close to the bottom may swim under the norsel nets, but losses of fish in the two other sections must be due to less direct consequences of mounting the net on norsels. Vision is regarded as an important factor in determining whether fish are caught in a gillnet (Jester, 1973; Cui et al., 1991; Wardle et al., 1991). The catch rate thus depends on the net generating a minimum level of stimulation which might provoke avoidance responses (Engås and Løkkeborg, 1994). One reason that norsel-mounted nets caught fewer fish than standard nets might therefore be that the foot cord and the
H. Godøy et al. / Fisheries Research 62 (2003) 377–384
norsels increased net visibility (see Fig. 2), enabling the fish to “steer clear” of the net or swim under the foot cord. Also fish density as measured by the cruises was patchy. One fleet could have a good catch and an adjacent fleet a poor catch. During period 2, two standard fleets had especially good catches, with a CPUE of 22.4 and 21.3 cods. Such high catches were not seen in norsel fleets. Reduced catches on the norsel nets may therefore be partly due because of patchy fish distribution and not necessarily lower catching efficiency alone. The results of the research cruises showed that the norsel nets made lower catches of cod (significantly lower in two of the three cruises), while the additional data collected by fishermen showed similar catches of cod for two vessels and higher catches for the third boat. The “Oddson’s” higher catch rate might have been caused by patchy fish distribution. This boat’s use of a floatline as foot cord is also a possible explanation, since the floatline was of a different colour from the other foot cord, and might have been less visible to the fish. There are several possible explanations for the difference in results obtained by the research cruises and commercial boats. The two sets of data were col-
383
lected in different areas. The area in which the research cruises were conducted has relatively steep bottom conditions compared to the area sampled by the commercial vessels that fished in the inner part of the fjord and off Vardø with smoother and flatter bottom conditions. The type of bottom is capable of influencing both net- and fish-related parameters. The degree of steepness can affect the shape of the meshes as well as the shape and slackness of the webbing and thus the visibility of the net. These factors may influence the catching properties and selectivity of the net. The bottom type may also affect fish behaviour, e.g. vertical distribution, swimming speed and swimming patterns, which in turn may influence catch rates. Crab density also varied between the two areas, a factor that also might affect the behaviour of cod. Studies of different hanging ratios have shown that nets with low hanging ratio (loosely hung) catch fish over a wider size range than tightly hung nets (Angelsen et al., 1979; Engås and Løkkeborg, 1994). A low hanging ratio is likely to increase the proportion of entangled fish (Hamley, 1975). More flotation tightens up the net webbing, which may affect how the fish are caught or entangled, and thus selectivity. This may explain the higher mean length of fish caught on norsel nets with rings compared to standard nets in
Fig. 3. Cross-section of a standard net and a norsel net influenced by current.
384
H. Godøy et al. / Fisheries Research 62 (2003) 377–384
period 2. On the other hand, norsel nets with rings also caught larger cod during period 3 than standard nets with the same amount of flotation. A possible explanation is that mounting the net on norsels may affect size selectivity. It is likely that crabs also become entangled more easily in a loose than a tight net. Studies of entangled nets showed that small spanner crabs (Ranina ranina) were more easily entangled in loosely hung (ET = 0.5) than tightly hung nets (ET = 0.7) (Sumpton et al., 1995). During periods 1 and 2, norsel nets without extra floats made higher bycatches of crabs than the standard nets. One explanation of this may be that the webbing of a norsel net is mounted on the foot cord. This attaches the net more loosely (to the foot cord) than mounting the webbing directly to the sole rope, thus making it easier to entangle crabs. Another explanation may be a “hammock-effect” on norsel nets that have little flotation due to the influence of current (Fig. 3). The use of norsel nets significantly reduced the bycatch of king crabs but also reduced the catches of cod. It appears that area and bottom conditions may have influenced the catch efficiency of norsel nets more than that of standard nets. Improvements in gear might reduce bycatches of king crab further, and simultaneously maintain cod catch rates. Acknowledgements We are grateful to Leif Ingilæ and Bjarne Hansen onboard “Eskil” for their cooperation and help during the data collection. Thanks to Svein Floen, Roar Skeide and Jostein Saltskår for good assistance in the field, and to Terje Jørgensen for useful help with the statistical analysis. References Angelsen, K., Haugen, K., Floen, S., 1979. The catching efficiency of cod gillnets with different hanging ratio (E) and different floatline buoyancy. ICES CM 1979/B:19. Baranov, F.I., 1976. Selected Works on Fishing Gear. Commercial Fishing Techniques, vol. 1. Keter, Jerusalem, 631 pp. Bridger, J.P., Foster, J.J., Margetts, A.R., Strange, E.S., 1981. Glossary of United Kingdom Fishing Gear Terms. Fishing News Books Ltd., Surrey, 115 pp. Bright, D.B., 1967. Life histories of the king crab, Paralithodes camtschatica, and the tanner crab, Chionoecetes bairdi, in Cook
Inlet, Alaska. Ph.D. Thesis. University of Southern California, Los Angeles, 265 pp. Cui, G., Wardle, C.S., Glass, C.W., Johnstone, A.D.F., Mojsiewicz, W.R., 1991. Light level thresholds for visual reaction of mackerel, Scomber scombrus L., to coloured monofilament nylon gillnet materials. Fish. Res. 10, 255–263. Dew, C.B., 1990. Behavioral ecology of podding red king crab, Paralithodes camtschatica. Can. J. Fish. Aquat. Sci. 47, 1944– 1958. Dickson, W., 1989. Cod gillnet effectiveness related to local abundance, availability and fish movement. Fish. Res. 7, 127– 148. Engås, A., Løkkeborg, S., 1994. Abundance estimation using bottom gillnet and longline—the role of fish behaviour. In: Fernö, A., Olsen, S. (Eds.), Marine Fish Behaviour in Capture and Abundance Estimation. Fishing News Books, Oxford, pp. 134–165. Fridman, A.L., 1986. Calculations for Fishing Gear Designs. FAO Fish. Man. Fishing News Books Ltd., London, 241 pp. Hamley, J.M., 1975. Review of gillnet selectivity. J. Fish. Res. Board Can. 32, 1943–1969. Iversen, S.A., 2002. Havets ressurser 2002. Fisken og havet, saernr. 1-2002, 151 pp. (in Norwegian with English abstract). Jester, D.B., 1973. Variation in catchability of fishes with color of gillnets. Trans. Am. Fish. Soc. 1, 109–115. Kuzmin, S., Olsen S., 1994. Barents Sea king crab (Paralithodes camtschatica). The transplantation experiments were successful. Shellfish Committee. ICES CM 1994/K: 12, 12 pp. Machiels, M.A.M., Klinge, M., Lanterns, R., van Densen, W.L.T., 1994. Effect of snood length and hanging ratio on efficiency and selectivity of bottom-set gillnets for pikeperch, Stizostedion lucioperca L., and bream, Abramis brama. Fish. Res. 19, 231– 239. Orlov, Y.I., Ivanov, B.G., 1978. On the introduction of the Kamchatka King Crab Paralithodes camtschatica (Decapoda: Anomura: Lithodidae) into the Barents Sea. Mar. Biol. 48 (4), 373–375. Pope, J.A., Margetts, A.R., Hamley, J.M., Akyuiz, E.F., 1975. Manual of methods for fish stock assessment. Part III. Selectivity of fishing gear. FAO Fish. Tech. Pap. No. 41, 65 pp. Powell, G.C., Nickerson, R.B., 1965. Aggregations among juvenile king crabs (Paralithodes camtschatica), Tilesius. Kodiac Alaska Anim. Behav. 13 (2–3), 374–380. Stewart, P.A.M., 1988. Measurements of the effects of tidal flow on the headline heights of bottom-set gillnets. Fish. Res. 6, 181–189. Stewart, P.A.M., Ferro, R.S.T., 1985. Measurements on gillnets in a flume tank. Fish. Res. 3, 29–46. Sumpton, W.D., Brown, I.W., Kennelly, S.J., 1995. Fishing gears that minimise the damage incurred by discarded spanner crabs (Ranina ranina): laboratory and field experiments. Fish. Res. 22, 11–27. Sundet, J.H., 1998. Bifangst av kongekrabbe i det ordinære fisket. En kartlegging blant fiskere i Øst-Finnmark. Rapport Fiskeriforskning 1—1998, Tromsø, 15 pp. Wardle, C.S., Cui, G., Mojsiewicz, W.R., Glass, C.W., 1991. The effect of colour on the appearance of monofilament nylon under water. Fish. Res. 10, 243–253.