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Seabird and longline interactions: effects of a bird-scaring streamer line and line shooter on the incidental capture of northern fulmars Fulmarus glacialis
Biological Conservation 106 (2002) 359–364 www.elsevier.com/locate/biocon
Seabird and longline interactions: effects of a bird-scaring streamer line and line shooter on the incidental capture of northern fulmars Fulmarus glacialis Svein Løkkeborga,*, Graham Robertsonb a
Institute of Marine Research, Fish Capture Division, PO Box 1870 Nordnes, N-5817 Bergen, Norway b Australian Antarctic Division, Channel Highway, Kingston, Tasmania 7050, Australia
Received 23 February 2001; received in revised form 9 October 2001; accepted 21 November 2001
Abstract Interactions between seabirds and longline fishing can cause incidental bird mortality and reduced gear efficiency. The potential for solving these problems by using a bird-scaring streamer line and a line shooter was investigated in commercial longlining in the northern Atlantic off the coast of Norway. We compared the bycatch rate of northern fulmars Fulmarus glacialis, the loss rate of baits to fulmars and the catch rates of fish target species among lines set with either of these mitigation measures, a combination of both and no mitigation measure. A total of 58,420 hooks were set in each of the four treatments. No birds were caught using the bird-scaring line alone and a single fulmar was caught when the bird-scaring line was used in combination with the line shooter. In contrast, 32 fulmars were caught in sets with no mitigation device and thirteen in sets with the line shooter alone. Losses of mackerel bait were reduced when the bird-scaring line was used, but not by using the line shooter alone. Longlines set with the line shooter reached 3 m depth 15% faster then lines set without the line shooter; beyond this depth sinking rates were similar (about 15 cm s 1). Fish catch did not vary significantly among setting methods. These results should also be applicable to the bycatch of Fulmarus spp. in demersal longline fisheries worldwide. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Longlining; Incidental mortality; Mitigation measures; Mortality reduction; Bycatch
1. Introduction Over the last decade increasing concern has been aroused on ecosystem effects of fishing activities. The bycatch of seabirds in longline fisheries poses a threat to seabird population worldwide, especially in the Southern Ocean, where longline fisheries have been linked to the declines of several albatross populations (Croxall et al., 1990; Moloney et al., 1994; Weimerskirch et al., 1997). In addition, the seabird depredation of baits from the hooks as they are deployed can have significant effect on fishing efficiency (Løkkeborg, 2001). In the north Atlantic, northern fulmars Fulmarus glacialis interact strongly with longline fishing and limit fishing success; but to date, there is no evidence of negative, population level effects on the northern fulmar populations (Lloyd et al., 1991; * Corresponding author. Tel.: +47-5523-8500; fax: +47-55238531. E-mail address: [email protected] (S. Løkkeborg).
Løkkeborg, 1998). Even so we believe that it is not consistent with the principles of ecologically sustainable management for fisheries to take large numbers of seabirds, even very abundant species like fulmars, particularly when simple and easy-to-use measures exist to minimise seabird mortality. Several mitigation measures capable of reducing the likelihood of interactions between seabirds and longline fisheries have been suggested and described (Brothers et al., 1999). The greatest potential for solving this problem in the north Atlantic fishery lies in modifications that either make the baited hooks less available to seabirds or devices that deter birds from the area astern the vessel where the baited hooks submerge (Løkkeborg, 2001). Thus a fishing experiment to test a line shooter and a bird-scaring line was carried out in commercial longlining off the Norwegian coast. A bird-scaring line has proved to be a very effective mitigation measure in this fishery (Løkkeborg, 1998, 2001), whereas the performance of a line shooter, which sets the line without
0006-3207/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0006-3207(01)00262-2
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tension and is believed to increase the sinking rate and thus make the baits less available, has never been tested. As well as determining the effects of these modifications on the numbers of fulmars and fish caught and on bait losses, we measured the sinking rate of the longline to determine the effect of line tension on sinking rate by comparing sinking rates of longlines set with and without using the line shooter (line set loosely versus line set with tension). Furthermore, from the sinking rates we could determine the distance behind the vessel that the baited longline was accessible to seabirds, and thus the optimum length of the bird-scaring line. Although we conducted our experiment in the north Atlantic where northern fulmar comprises the great majority of seabirds caught (Løkkeborg, 1998), the results should have relevance to longline fisheries worldwide where interactions with Fulmarus spp. occur.
during line setting. It comprised an end segment with three gillnet float rings and a punctured buoy, a 21-m intermediate segment, and a segment with twelve 8 cm wide streamers of yellow tarpaulin attached at intervals of 5.4 m and increasing in length from 0.5 m at the after end to 2.0 m at the end secured to the stern of the vessel. A line shooter, which consists of opposing rubber and metal sheaves through which the line is pulled at a constant speed, is designed to set lines at a speed slightly faster than the vessel speed during line setting. The speed of the line shooter was manually adjusted during setting to ensure that the line was set slack (no tension). Line tension astern is believed to delay line sinking and keep baits available to birds for longer. All marketable species and seabirds were counted for each set during the haul. 2.3. Measuring longline sinking rate
2. Methods 2.1. Fishing ground, vessel and fishing gear The experiment was conducted from August 9–20 1999 on a commercial autoliner (M/S Nyvoll Senior) operating at 174–512 m depth on fishing grounds off the coast of A˚lesund (mid-Norway). The main fish species targeted were torsk Brosme brosme and ling Molva molva. The vessel was equipped with the Mustad autoline system and line shooter. She used 9 mm longlines (specific weight: 1.10) rigged with EZ-baiter hooks (size: 13/0) on 40 cm long snoods, and baited with a combination of mackerel and squid baits. Vessel setting speed varied from 6.5–8.0 knots and the longline was deployed from 2 m above the water and about 1.5 m to starboard of the propeller, which was about 4.5 m beneath the waters’ surface. During line setting the longline landed on the down-wash side of the propeller. Longlines were set in fleets consisting of 2, 3, 4 or 5 magazines of line, each magazine being 1820 m of longline and having 1270 hooks. The length of each fleet varied according to the requirements of the fishing operation. 2.2. Experimental design and mitigation devices Every day, four fleets of experimental lines were set in randomised order using different mitigation measures. Lines were set in the morning and retrieved during the day and night. Number of hooks of fleets set in the same day was similar but varied among days (2540– 6350 hooks/fleet). A total of 58,420 hooks were set in each treatment. One of the four fleets was set as a control; the other three were set using each of three types of mitigation measures, that is a bird-scaring streamer line, a line shooter and the combination of the two. The birdscaring line was 90 m long and was deployed astern
Longline sinking rates were determined with Mk7 time-depth sensors (Wildlife Computers, USA) taped to the longline immediately behind the knots that joined each magazine of line: this allowed the devices to pass through the line shooter without being damaged. The sensors recorded depth to the nearest 0.5 m every second. To minimise the confounding effects of fleet length and proximity to the anchor, sensors were deployed at magazine junctions throughout a fleet. Differences in longline sinking characteristics (see later) due to position in the fleet were examined by comparing data collected from instruments deployed along the length of the 5-magazine fleets, the commonest fleet length used in the fishing operation. The sensors were deployed in pairs and kept at the same temperature as seawater until 30 s before deployment to minimise the incidence of anomalous depth readings. The exact time of water entry of the sensors was recorded with a hand-held digital clock. Upon retrieval the sensors were downloaded into a computer to determine line sinking rates. Time-depth profiles were assessed visually for anomalous readings, particularly in the linear phase of the line sinking rate. Data from the paired sensors were averaged, except where readings looked spurious, in which case data from one recorder only were analysed. Recordings made where the longline was yanked tight due to hook-ups in the baiting machine or by the knot joining two magazines jamming between the sheaves of the line shooter, were not used in the analysis. This reduced the useful sample sizes to 20 (tense line) and 13 (loose line) for the five magazine fleets, 3 and 6 for the four magazine fleets, 4 and 3 for the three magazine fleets and 6 and 5 for the two magazine fleets. Data from the time-depth sensors were used in two ways: to determine time taken for the longline to reach 3 m depth and the sinking rate of the line from 3–20 m
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depth (the linear phase of sink profiles). The upper few metres or so of the water column is likely to include the depths in which most seabird attacks occur. However, due to the limited sensitivity (0.5 m depth increments) of the instruments, and the effects of wave height and propeller turbulence on the accuracy of the readings, time–depth relationships near the surface are likely to be inaccurate. Therefore sinking time (as against sinking rate) to 3 m depth (approximate maximum height of the swell) was chosen as one variable in the analysis. Sinking rate of the longline from 3–20 m depth was chosen as another variable because this depth range included the linear phase of the line sinking rate, since by 3 m depth the longline was beneath the wave zone and was sufficiently astern the vessel to be unaffected by propeller upwellings. Given the diving abilities of fulmars (see later) sinking below 3 m may not be relevant to them but it may be relevant to deeper diving species.
3. Results 3.1. Seabird capture All seabirds caught in the course of the experiment were northern fulmars. When no mitigation measure was used, 32 fulmars (0.55 birds/1000 hooks) were caught, whereas 13 fulmars (0.22 birds/1000 hooks) were caught when the line shooter was used, and zero and one fulmar respectively with the bird line and the combination of the two mitigation measures (Table 1). The effect of the bird line and the line shooter on reducing seabird bycatch was tested statistically by comparing number of days on which the numbers of birds caught were different. For the bird line, there was a significant difference both between the bird line and the control and between the bird line/line shooter and the line shooter (one-tailed binomial test, P < 0.01 and P < 0.05, respectively). The line shooter had no significant effect, either alone or in combination with the bird line.
2.4. Measuring bait loss Bait loss caused by seabirds was determined by setting lines without anchors and retrieving them immediately in order to prevent fish and scavengers at the seabed from taking baits. Empty hooks were counted during retrieval. This test was conducted four times (i.e. on four days), and in each test, one magazine of line (i.e. 1270 hooks) was set for each of the four treatments. In three of the tests, the lines were baited with mackerel and in the fourth test with squid. Because line setting with an autoline system results in a proportion of the hooks being set without bait (Løkkeborg, 1998), the lines were video recorded during setting, so we could determine the proportion of hooks set with bait and thereby calculate the actual bait loss caused by seabirds.
3.2. Bait loss There were differences in bait loss between the setting methods (chi-square, X23=536.7 for mackerel bait and 161.6 for squid bait, P < 0.001). Few mackerel baits were taken by seabirds when lines were set using the bird line alone or in combination with the line shooter, whereas 13–14% of the baits were taken from lines that were set without any device or with the line shooter (Table 2). Two fulmars were caught when no mitigation measure was used. With the exception of lines set with the combination of the bird line and the line shooter, the bait losses were low for squid bait, squid being more difficult for birds to remove from hooks than mackerel.
Table 1 Catches (number) of northern fulmars, torsk and total catches of target fish species for longlines set with no mitigation measure, bird-scaring line, line shooter, and combination of bird-scaring line and line shooter Day No.
No measure Fulmar
1 2 3 4 5 6 7 8 9 10 11 12 Total a
Bird line Torsk
Total
Fulmar
0 5 0 2 8 0 1 0 5 3 1 7
131 140 138 147 131 254 114 110 68 112 122 217
236 360 175 354 260 312 163 127 83 119 133 258
0 0 0 0 0 0 0 0 0 0 0 0
32
1572
2461
0
Line shooter Torsk
Total
255 184 153 142 123 112 264 139 43 –a 250 270
399 225 313 252 178 156 419 146 76 –a 323 318
1935
2805
Fulmar
Bird line and line shooter
Torsk
Total
2 1 0 0 6 0 0 1 0 0 2 1
119 164 65 160 129 377 220 148 32 117 160 116
258 268 187 308 203 603 315 172 57 138 174 167
13
1690
2712
Fulmar
Torsk
Total
0 1 0 0 0 0 0 0 0 0 0 0
202 147 200 99 180 157 181 263 60 115 63 223
241 265 392 301 223 189 270 294 77 135 63 397
1
1775
2712
– The catch was not recorded for lines set with the bird-scaring line and the catches of Day 10 are therefore not included in the total catch for any of the treatments.
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Table 2 Bait losses (percentage of hooks without bait) to northern fulmars of mackerel and squid baits for longlines set with no mitigation measure, bird-scaring line, line shooter, and combination of bird-scaring line and line shootera Mitigation measure
Mackerel
Squid
No measure Bird line Line shooter Bird line and line shooter
14.5 2.1 12.7 4.2
1.6 0.9 3.7 10.6
a
(3363) (4491) (3302) (2223)
(1105) (1114) (1108) (1103)
Total numbers of baited hooks set are given in parentheses.
3.3. Longline sinking time and sinking rate Neither time to reach 3 m depth (ANOVA, F3,39= 0.30, P=0.82) nor sinking rate from 3–20 m depth (F3,32=0.65, P=0.98) varied as a function of position within the fleet. This is not surprising because at 7 knots (3.6 m s 1) setting speed, the end of the first magazine (1820 m; the location of the first pair of sensors) would have entered the water 506 s after the first anchor, which would have already been on the seabed and would have stopped pulling the longline down. Although the statistical power of both analyses was low, the small range in the means of the estimates (25.5–28.0 s, 14.5–14.8 cm s 1) suggests that the sinking characteristics of magazines within a fleet were similar. Therefore the timedepth records for fleets of the same length were pooled. With the time taken for longlines to reach 3 m depth, the interaction between line tension and fleet length was not statistically significant (ANOVA: F3,59=1.16, P=0.33, Fig. 1 a), suggesting that sinking time to this depth varied independent of fleet length. Overall, lines set with the line shooter reached 3 m depth in 22.6 4.1 s whereas lines set without the line shooter took 26.6 7.3 s to reach 3 m; this 15% difference just reached conventional levels of statistical significance (P=0.046). With the sinking rate of longlines from 3–20 m depth there was a significant interaction between line tension and fleet length (F3,59=4.82, P< 0.005) principally because in the three-magazine fleets, longlines set with the line shooter sank much faster than lines set without the line shooter (Tukey test: q=5.05, P < 0.001; Fig. 1b). We have no logical explanation for this and suspect the result could be an artefact of the small sample size (n=7) for the three-magazine fleets. For fleets of other lengths, line tension had no effect on longline sinking rate from 3– 20 m depth. Overall, the sinking rate of longlines in the linear phase of the descent was about 15 cm s 1. 3.4. Fish catch success The catches consisted mainly of torsk, but ling, cod Gadus morhua and redfish Sebastes marinus were also taken. There were large variations in catch rates
Fig. 1. Relationships between fleet length and (a) time taken for longlines set with and without a line shooter to reach 3 m depth, (b) sinking rate from 3–20 m depth of longlines set with and without a line shooter. Shown are means1 standard deviations.
between days, and although the results indicate higher catches for lines set when one or other of the mitigation measures were used, there was no significant difference in catch rates between the setting methods (ANOVA, F3,30=0.53, P=0.66 for torsk and F3,30=0.20, P=0.90 for total catch; Table 1).
4. Discussion The use of a bird-scaring streamer line when setting baited longlines was shown to eliminate bycatch of fulmars in this experiment conducted in the north Atlantic longline fishery. Similar studies have also demonstrated the efficiency of this device in deterring fulmars from taking baits and becoming hooked (Løkkeborg, 1998, 2001). A bird line with streamers works both as a visual and physical deterrent because the narrowly spaced streamers move with the speed of the vessel and will hit birds as they approach the baited line. Although birds may habituate to visual deterrents, habituation to a bird
S. Løkkeborg, G. Robertson / Biological Conservation 106 (2002) 359–364
line is unlikely because of the unpredictable movements of the streamers. Furthermore, bird lines have been used at Norwegian fishing grounds since 1992 when the first experiment was conducted (Løkkeborg and Bjordal, 1992), and this study showed that the bird line was still efficient at the end of the 12-day period tested (see also Løkkeborg, 1998, 2001). During line setting operations the vessel was followed by an estimated 70–600 northern fulmars. When longlines were set without protection of the bird line, birds would swarm over the area of the water where the longline was sinking and attempt to take baits either by surface seizure or, less commonly, by ‘duck’ diving. Dives appeared to be to depths equivalent to about 1–2 bird body lengths (i.e. < 0.5 m depth), suggesting that fulmars mostly take baits at or very near to the surface. This behaviour when taking bait has implications for the interpretations that can be drawn from the sinking characteristics of longlines set with the line shooter. Lines set with the line shooter alone caught more birds than lines set with the protection of a bird line, but fewer birds (although not significant) than lines set without any mitigation, indicating that lines set loosely (with line shooter) may reduce the availability of baited hooks to fulmars. This is supported by the fact that slack lines reached 3 m depth 15% (4 s) faster than lines set under tension (Fig. 1a). However, we cannot be certain about the effect of this on the availability of baits to fulmars because the sensors were unable to measure accurately sink dynamics in the top 0.5 m of the water column, and measurements to 3 m depth probably masked effects that occurred in the first few seconds after the longlines entered the water. Clearly, other means (e.g. underwater video recording techniques) must be found to record accurately the interactions between seabirds and longlines at or just beneath the surface. The faster sinking time to 3 m depth by lines set with the line shooter may be explained, in part, by differences in the distance behind the vessel that longlines entered the water. Lines set with tension astern entered the water about 8 m behind the vessel, whereas lines set with the line shooter fell in loose loops near-vertically about 0.5 m behind the vessel. Thus lines set with the line shooter could have, potentially, commenced sinking 2.1 s sooner at a setting speed of 7 knots (3.6 m s 1). While 2.1 s seems a trivial difference in time it is possible that lines set loosely at the stern of the vessel into propeller downwash cleared surface depths faster than lines set under tension astern, and this might have reduced the availability of baited hooks to fulmars. The sinking time to 3 m depth can be used to determine the aerial extent of the bird scaring line. At a setting speed of 7 knots, 25 s would have elapsed for a baited hook to clear the protection zone of the 90 mlong bird line, by which time longlines set under tension
363
astern would have reached 3 m depth (see earlier). Hence for seabird species capable of diving to 3 m depth, bird lines 90 m long should be of sufficient length. Since fulmars mostly take baits at or very near to the surface, it is likely that the baited hooks will be well clear of their reach by the time the hooks pass behind the area protected by a bird line of this length. Except for the three magazine fleets there was no difference in the sinking rate from 3–20 m depth between longlines set with and without the line shooter, suggesting that a line shooter does not increase line sinking rates over the greater part of the depth range. This is intuitive, because once a longline clears the area influenced by wave action and propeller turbulence, sinking rate would vary as a function of specific weight, not whether or not it lay loose or tight in the water column. Losses of mackerel bait were insignificant for lines set with the bird line, whereas 13–14% of the baits were taken by birds when lines were set without any device or with the line shooter. Loss of squid bait was lower than that of mackerel bait due to its greater physical strength. The increased bait loss of squid for lines set with the combination of the bird line and the line shooter compared with the other setting methods is difficult to explain, but the lower sample size for squid bait should be noticed. There were no significant differences in catches rates of target species between the setting methods although the result indicated higher catches for lines set when one or other of the mitigation measures were used. In a similar experiment where the catch rate of birds for the control lines was twice as high as in this experiment (1.06 versus 0.55 birds/1000 hooks), the use of mitigation measures (bird line and setting funnel) were shown to give increased catches of target species (Løkkeborg, 2001). Relatively low bait losses to birds and large variations in catch rates due to factors such as setting time and sites may explain the lack of differences in catch rates in the present study. Finally, although the experiment was conducted on northern fulmars, the results reported should have relevance to the conservation of Fulmarus spp. worldwide because of the morphological similarities and similar feeding behaviour (i.e. mostly by surface seizure) of these species. Thus it is likely that the bird-scaring streamer line is efficient in reducing incidental catch of fulmars also in other longline fisheries, e.g. in the Antarctic where the southern fulmar F. glacialoides breeds.
Acknowledgements We are grateful to Louise Wynen for assistance in reducing the data from the time–depth recorders. We are also grateful to Edward F. Melvin for his helpful comments on the manuscript.
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References Brothers, N.P., Cooper, J., Løkkeborg, S., 1999. The Incidental Catch of Seabirds by Longline Fisheries: Worldwide Review and Technical Guidelines for Mitigation. FAO Fisheries Circular No. 937, Rome. Croxall, J.P., Rothery, P., Pickering, S., Prince, P.A., 1990. Reproductive performance, recruitment and survival of Wandering Albatrosses Diomedea exulans at Bird Island, South Georgia. Journal of Animal Ecology 59, 775–796. Lloyd, C., Tasker, M.L., Partridge, K., 1991. The Status of Seabirds in Britain and Ireland. T. and A.D. Poyser, London. Løkkeborg, S., 1998. Seabird by-catch and bait loss in long-lining using different setting methods. ICES Journal of Marine Science 55, 145–149. Løkkeborg, S., 2001. Reducing seabird bycatch in longline fisheries by means of bird-scaring lines and underwater setting. In: Melvin, E.F.,
Parrish, J.K. (Eds.), Seabird Bycatch: Trends, Roadblocks, and Solutions. University of Alaska Sea Grant, AK-SG-01-01, Fairbanks, pp. 33–41. Løkkeborg, S., Bjordal, A˚., 1992. Reduced Bait loss and Bycatch of Seabirds in Longlining by using a Seabird Scarer. Document to Working Group FSA-92 CCAMLR, Hobart, Australia. Moloney, C.L., Cooper, J., Ryan, P.G., Siegfried, W.R., 1994. Use of a population model to assess the impact of longline fishing on Wandering Albatross Diomedea exulans populations. Biological Conservation 70, 195–203. Weimerskirch, H., Brothers, N.P., Jouventin, P., 1997. Population dynamics of Wandering Albatross Diomedea exulans and the Amsterdam Albatross D. amsterdamensis in the Indian Ocean and their relationships with long-line fisheries: conservation implications. Biological Conservation 79, 257–270.
Seabird and longline interactions: effects of a bird-scaring streamer line and line shooter on the incidental capture of northern fulmars Fulmarus glacialis Svein Løkkeborga,*, Graham Robertsonb a
Institute of Marine Research, Fish Capture Division, PO Box 1870 Nordnes, N-5817 Bergen, Norway b Australian Antarctic Division, Channel Highway, Kingston, Tasmania 7050, Australia
Received 23 February 2001; received in revised form 9 October 2001; accepted 21 November 2001
Abstract Interactions between seabirds and longline fishing can cause incidental bird mortality and reduced gear efficiency. The potential for solving these problems by using a bird-scaring streamer line and a line shooter was investigated in commercial longlining in the northern Atlantic off the coast of Norway. We compared the bycatch rate of northern fulmars Fulmarus glacialis, the loss rate of baits to fulmars and the catch rates of fish target species among lines set with either of these mitigation measures, a combination of both and no mitigation measure. A total of 58,420 hooks were set in each of the four treatments. No birds were caught using the bird-scaring line alone and a single fulmar was caught when the bird-scaring line was used in combination with the line shooter. In contrast, 32 fulmars were caught in sets with no mitigation device and thirteen in sets with the line shooter alone. Losses of mackerel bait were reduced when the bird-scaring line was used, but not by using the line shooter alone. Longlines set with the line shooter reached 3 m depth 15% faster then lines set without the line shooter; beyond this depth sinking rates were similar (about 15 cm s 1). Fish catch did not vary significantly among setting methods. These results should also be applicable to the bycatch of Fulmarus spp. in demersal longline fisheries worldwide. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Longlining; Incidental mortality; Mitigation measures; Mortality reduction; Bycatch
1. Introduction Over the last decade increasing concern has been aroused on ecosystem effects of fishing activities. The bycatch of seabirds in longline fisheries poses a threat to seabird population worldwide, especially in the Southern Ocean, where longline fisheries have been linked to the declines of several albatross populations (Croxall et al., 1990; Moloney et al., 1994; Weimerskirch et al., 1997). In addition, the seabird depredation of baits from the hooks as they are deployed can have significant effect on fishing efficiency (Løkkeborg, 2001). In the north Atlantic, northern fulmars Fulmarus glacialis interact strongly with longline fishing and limit fishing success; but to date, there is no evidence of negative, population level effects on the northern fulmar populations (Lloyd et al., 1991; * Corresponding author. Tel.: +47-5523-8500; fax: +47-55238531. E-mail address: [email protected] (S. Løkkeborg).
Løkkeborg, 1998). Even so we believe that it is not consistent with the principles of ecologically sustainable management for fisheries to take large numbers of seabirds, even very abundant species like fulmars, particularly when simple and easy-to-use measures exist to minimise seabird mortality. Several mitigation measures capable of reducing the likelihood of interactions between seabirds and longline fisheries have been suggested and described (Brothers et al., 1999). The greatest potential for solving this problem in the north Atlantic fishery lies in modifications that either make the baited hooks less available to seabirds or devices that deter birds from the area astern the vessel where the baited hooks submerge (Løkkeborg, 2001). Thus a fishing experiment to test a line shooter and a bird-scaring line was carried out in commercial longlining off the Norwegian coast. A bird-scaring line has proved to be a very effective mitigation measure in this fishery (Løkkeborg, 1998, 2001), whereas the performance of a line shooter, which sets the line without
0006-3207/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0006-3207(01)00262-2
360
S. Løkkeborg, G. Robertson / Biological Conservation 106 (2002) 359–364
tension and is believed to increase the sinking rate and thus make the baits less available, has never been tested. As well as determining the effects of these modifications on the numbers of fulmars and fish caught and on bait losses, we measured the sinking rate of the longline to determine the effect of line tension on sinking rate by comparing sinking rates of longlines set with and without using the line shooter (line set loosely versus line set with tension). Furthermore, from the sinking rates we could determine the distance behind the vessel that the baited longline was accessible to seabirds, and thus the optimum length of the bird-scaring line. Although we conducted our experiment in the north Atlantic where northern fulmar comprises the great majority of seabirds caught (Løkkeborg, 1998), the results should have relevance to longline fisheries worldwide where interactions with Fulmarus spp. occur.
during line setting. It comprised an end segment with three gillnet float rings and a punctured buoy, a 21-m intermediate segment, and a segment with twelve 8 cm wide streamers of yellow tarpaulin attached at intervals of 5.4 m and increasing in length from 0.5 m at the after end to 2.0 m at the end secured to the stern of the vessel. A line shooter, which consists of opposing rubber and metal sheaves through which the line is pulled at a constant speed, is designed to set lines at a speed slightly faster than the vessel speed during line setting. The speed of the line shooter was manually adjusted during setting to ensure that the line was set slack (no tension). Line tension astern is believed to delay line sinking and keep baits available to birds for longer. All marketable species and seabirds were counted for each set during the haul. 2.3. Measuring longline sinking rate
2. Methods 2.1. Fishing ground, vessel and fishing gear The experiment was conducted from August 9–20 1999 on a commercial autoliner (M/S Nyvoll Senior) operating at 174–512 m depth on fishing grounds off the coast of A˚lesund (mid-Norway). The main fish species targeted were torsk Brosme brosme and ling Molva molva. The vessel was equipped with the Mustad autoline system and line shooter. She used 9 mm longlines (specific weight: 1.10) rigged with EZ-baiter hooks (size: 13/0) on 40 cm long snoods, and baited with a combination of mackerel and squid baits. Vessel setting speed varied from 6.5–8.0 knots and the longline was deployed from 2 m above the water and about 1.5 m to starboard of the propeller, which was about 4.5 m beneath the waters’ surface. During line setting the longline landed on the down-wash side of the propeller. Longlines were set in fleets consisting of 2, 3, 4 or 5 magazines of line, each magazine being 1820 m of longline and having 1270 hooks. The length of each fleet varied according to the requirements of the fishing operation. 2.2. Experimental design and mitigation devices Every day, four fleets of experimental lines were set in randomised order using different mitigation measures. Lines were set in the morning and retrieved during the day and night. Number of hooks of fleets set in the same day was similar but varied among days (2540– 6350 hooks/fleet). A total of 58,420 hooks were set in each treatment. One of the four fleets was set as a control; the other three were set using each of three types of mitigation measures, that is a bird-scaring streamer line, a line shooter and the combination of the two. The birdscaring line was 90 m long and was deployed astern
Longline sinking rates were determined with Mk7 time-depth sensors (Wildlife Computers, USA) taped to the longline immediately behind the knots that joined each magazine of line: this allowed the devices to pass through the line shooter without being damaged. The sensors recorded depth to the nearest 0.5 m every second. To minimise the confounding effects of fleet length and proximity to the anchor, sensors were deployed at magazine junctions throughout a fleet. Differences in longline sinking characteristics (see later) due to position in the fleet were examined by comparing data collected from instruments deployed along the length of the 5-magazine fleets, the commonest fleet length used in the fishing operation. The sensors were deployed in pairs and kept at the same temperature as seawater until 30 s before deployment to minimise the incidence of anomalous depth readings. The exact time of water entry of the sensors was recorded with a hand-held digital clock. Upon retrieval the sensors were downloaded into a computer to determine line sinking rates. Time-depth profiles were assessed visually for anomalous readings, particularly in the linear phase of the line sinking rate. Data from the paired sensors were averaged, except where readings looked spurious, in which case data from one recorder only were analysed. Recordings made where the longline was yanked tight due to hook-ups in the baiting machine or by the knot joining two magazines jamming between the sheaves of the line shooter, were not used in the analysis. This reduced the useful sample sizes to 20 (tense line) and 13 (loose line) for the five magazine fleets, 3 and 6 for the four magazine fleets, 4 and 3 for the three magazine fleets and 6 and 5 for the two magazine fleets. Data from the time-depth sensors were used in two ways: to determine time taken for the longline to reach 3 m depth and the sinking rate of the line from 3–20 m
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depth (the linear phase of sink profiles). The upper few metres or so of the water column is likely to include the depths in which most seabird attacks occur. However, due to the limited sensitivity (0.5 m depth increments) of the instruments, and the effects of wave height and propeller turbulence on the accuracy of the readings, time–depth relationships near the surface are likely to be inaccurate. Therefore sinking time (as against sinking rate) to 3 m depth (approximate maximum height of the swell) was chosen as one variable in the analysis. Sinking rate of the longline from 3–20 m depth was chosen as another variable because this depth range included the linear phase of the line sinking rate, since by 3 m depth the longline was beneath the wave zone and was sufficiently astern the vessel to be unaffected by propeller upwellings. Given the diving abilities of fulmars (see later) sinking below 3 m may not be relevant to them but it may be relevant to deeper diving species.
3. Results 3.1. Seabird capture All seabirds caught in the course of the experiment were northern fulmars. When no mitigation measure was used, 32 fulmars (0.55 birds/1000 hooks) were caught, whereas 13 fulmars (0.22 birds/1000 hooks) were caught when the line shooter was used, and zero and one fulmar respectively with the bird line and the combination of the two mitigation measures (Table 1). The effect of the bird line and the line shooter on reducing seabird bycatch was tested statistically by comparing number of days on which the numbers of birds caught were different. For the bird line, there was a significant difference both between the bird line and the control and between the bird line/line shooter and the line shooter (one-tailed binomial test, P < 0.01 and P < 0.05, respectively). The line shooter had no significant effect, either alone or in combination with the bird line.
2.4. Measuring bait loss Bait loss caused by seabirds was determined by setting lines without anchors and retrieving them immediately in order to prevent fish and scavengers at the seabed from taking baits. Empty hooks were counted during retrieval. This test was conducted four times (i.e. on four days), and in each test, one magazine of line (i.e. 1270 hooks) was set for each of the four treatments. In three of the tests, the lines were baited with mackerel and in the fourth test with squid. Because line setting with an autoline system results in a proportion of the hooks being set without bait (Løkkeborg, 1998), the lines were video recorded during setting, so we could determine the proportion of hooks set with bait and thereby calculate the actual bait loss caused by seabirds.
3.2. Bait loss There were differences in bait loss between the setting methods (chi-square, X23=536.7 for mackerel bait and 161.6 for squid bait, P < 0.001). Few mackerel baits were taken by seabirds when lines were set using the bird line alone or in combination with the line shooter, whereas 13–14% of the baits were taken from lines that were set without any device or with the line shooter (Table 2). Two fulmars were caught when no mitigation measure was used. With the exception of lines set with the combination of the bird line and the line shooter, the bait losses were low for squid bait, squid being more difficult for birds to remove from hooks than mackerel.
Table 1 Catches (number) of northern fulmars, torsk and total catches of target fish species for longlines set with no mitigation measure, bird-scaring line, line shooter, and combination of bird-scaring line and line shooter Day No.
No measure Fulmar
1 2 3 4 5 6 7 8 9 10 11 12 Total a
Bird line Torsk
Total
Fulmar
0 5 0 2 8 0 1 0 5 3 1 7
131 140 138 147 131 254 114 110 68 112 122 217
236 360 175 354 260 312 163 127 83 119 133 258
0 0 0 0 0 0 0 0 0 0 0 0
32
1572
2461
0
Line shooter Torsk
Total
255 184 153 142 123 112 264 139 43 –a 250 270
399 225 313 252 178 156 419 146 76 –a 323 318
1935
2805
Fulmar
Bird line and line shooter
Torsk
Total
2 1 0 0 6 0 0 1 0 0 2 1
119 164 65 160 129 377 220 148 32 117 160 116
258 268 187 308 203 603 315 172 57 138 174 167
13
1690
2712
Fulmar
Torsk
Total
0 1 0 0 0 0 0 0 0 0 0 0
202 147 200 99 180 157 181 263 60 115 63 223
241 265 392 301 223 189 270 294 77 135 63 397
1
1775
2712
– The catch was not recorded for lines set with the bird-scaring line and the catches of Day 10 are therefore not included in the total catch for any of the treatments.
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Table 2 Bait losses (percentage of hooks without bait) to northern fulmars of mackerel and squid baits for longlines set with no mitigation measure, bird-scaring line, line shooter, and combination of bird-scaring line and line shootera Mitigation measure
Mackerel
Squid
No measure Bird line Line shooter Bird line and line shooter
14.5 2.1 12.7 4.2
1.6 0.9 3.7 10.6
a
(3363) (4491) (3302) (2223)
(1105) (1114) (1108) (1103)
Total numbers of baited hooks set are given in parentheses.
3.3. Longline sinking time and sinking rate Neither time to reach 3 m depth (ANOVA, F3,39= 0.30, P=0.82) nor sinking rate from 3–20 m depth (F3,32=0.65, P=0.98) varied as a function of position within the fleet. This is not surprising because at 7 knots (3.6 m s 1) setting speed, the end of the first magazine (1820 m; the location of the first pair of sensors) would have entered the water 506 s after the first anchor, which would have already been on the seabed and would have stopped pulling the longline down. Although the statistical power of both analyses was low, the small range in the means of the estimates (25.5–28.0 s, 14.5–14.8 cm s 1) suggests that the sinking characteristics of magazines within a fleet were similar. Therefore the timedepth records for fleets of the same length were pooled. With the time taken for longlines to reach 3 m depth, the interaction between line tension and fleet length was not statistically significant (ANOVA: F3,59=1.16, P=0.33, Fig. 1 a), suggesting that sinking time to this depth varied independent of fleet length. Overall, lines set with the line shooter reached 3 m depth in 22.6 4.1 s whereas lines set without the line shooter took 26.6 7.3 s to reach 3 m; this 15% difference just reached conventional levels of statistical significance (P=0.046). With the sinking rate of longlines from 3–20 m depth there was a significant interaction between line tension and fleet length (F3,59=4.82, P< 0.005) principally because in the three-magazine fleets, longlines set with the line shooter sank much faster than lines set without the line shooter (Tukey test: q=5.05, P < 0.001; Fig. 1b). We have no logical explanation for this and suspect the result could be an artefact of the small sample size (n=7) for the three-magazine fleets. For fleets of other lengths, line tension had no effect on longline sinking rate from 3– 20 m depth. Overall, the sinking rate of longlines in the linear phase of the descent was about 15 cm s 1. 3.4. Fish catch success The catches consisted mainly of torsk, but ling, cod Gadus morhua and redfish Sebastes marinus were also taken. There were large variations in catch rates
Fig. 1. Relationships between fleet length and (a) time taken for longlines set with and without a line shooter to reach 3 m depth, (b) sinking rate from 3–20 m depth of longlines set with and without a line shooter. Shown are means1 standard deviations.
between days, and although the results indicate higher catches for lines set when one or other of the mitigation measures were used, there was no significant difference in catch rates between the setting methods (ANOVA, F3,30=0.53, P=0.66 for torsk and F3,30=0.20, P=0.90 for total catch; Table 1).
4. Discussion The use of a bird-scaring streamer line when setting baited longlines was shown to eliminate bycatch of fulmars in this experiment conducted in the north Atlantic longline fishery. Similar studies have also demonstrated the efficiency of this device in deterring fulmars from taking baits and becoming hooked (Løkkeborg, 1998, 2001). A bird line with streamers works both as a visual and physical deterrent because the narrowly spaced streamers move with the speed of the vessel and will hit birds as they approach the baited line. Although birds may habituate to visual deterrents, habituation to a bird
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line is unlikely because of the unpredictable movements of the streamers. Furthermore, bird lines have been used at Norwegian fishing grounds since 1992 when the first experiment was conducted (Løkkeborg and Bjordal, 1992), and this study showed that the bird line was still efficient at the end of the 12-day period tested (see also Løkkeborg, 1998, 2001). During line setting operations the vessel was followed by an estimated 70–600 northern fulmars. When longlines were set without protection of the bird line, birds would swarm over the area of the water where the longline was sinking and attempt to take baits either by surface seizure or, less commonly, by ‘duck’ diving. Dives appeared to be to depths equivalent to about 1–2 bird body lengths (i.e. < 0.5 m depth), suggesting that fulmars mostly take baits at or very near to the surface. This behaviour when taking bait has implications for the interpretations that can be drawn from the sinking characteristics of longlines set with the line shooter. Lines set with the line shooter alone caught more birds than lines set with the protection of a bird line, but fewer birds (although not significant) than lines set without any mitigation, indicating that lines set loosely (with line shooter) may reduce the availability of baited hooks to fulmars. This is supported by the fact that slack lines reached 3 m depth 15% (4 s) faster than lines set under tension (Fig. 1a). However, we cannot be certain about the effect of this on the availability of baits to fulmars because the sensors were unable to measure accurately sink dynamics in the top 0.5 m of the water column, and measurements to 3 m depth probably masked effects that occurred in the first few seconds after the longlines entered the water. Clearly, other means (e.g. underwater video recording techniques) must be found to record accurately the interactions between seabirds and longlines at or just beneath the surface. The faster sinking time to 3 m depth by lines set with the line shooter may be explained, in part, by differences in the distance behind the vessel that longlines entered the water. Lines set with tension astern entered the water about 8 m behind the vessel, whereas lines set with the line shooter fell in loose loops near-vertically about 0.5 m behind the vessel. Thus lines set with the line shooter could have, potentially, commenced sinking 2.1 s sooner at a setting speed of 7 knots (3.6 m s 1). While 2.1 s seems a trivial difference in time it is possible that lines set loosely at the stern of the vessel into propeller downwash cleared surface depths faster than lines set under tension astern, and this might have reduced the availability of baited hooks to fulmars. The sinking time to 3 m depth can be used to determine the aerial extent of the bird scaring line. At a setting speed of 7 knots, 25 s would have elapsed for a baited hook to clear the protection zone of the 90 mlong bird line, by which time longlines set under tension
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astern would have reached 3 m depth (see earlier). Hence for seabird species capable of diving to 3 m depth, bird lines 90 m long should be of sufficient length. Since fulmars mostly take baits at or very near to the surface, it is likely that the baited hooks will be well clear of their reach by the time the hooks pass behind the area protected by a bird line of this length. Except for the three magazine fleets there was no difference in the sinking rate from 3–20 m depth between longlines set with and without the line shooter, suggesting that a line shooter does not increase line sinking rates over the greater part of the depth range. This is intuitive, because once a longline clears the area influenced by wave action and propeller turbulence, sinking rate would vary as a function of specific weight, not whether or not it lay loose or tight in the water column. Losses of mackerel bait were insignificant for lines set with the bird line, whereas 13–14% of the baits were taken by birds when lines were set without any device or with the line shooter. Loss of squid bait was lower than that of mackerel bait due to its greater physical strength. The increased bait loss of squid for lines set with the combination of the bird line and the line shooter compared with the other setting methods is difficult to explain, but the lower sample size for squid bait should be noticed. There were no significant differences in catches rates of target species between the setting methods although the result indicated higher catches for lines set when one or other of the mitigation measures were used. In a similar experiment where the catch rate of birds for the control lines was twice as high as in this experiment (1.06 versus 0.55 birds/1000 hooks), the use of mitigation measures (bird line and setting funnel) were shown to give increased catches of target species (Løkkeborg, 2001). Relatively low bait losses to birds and large variations in catch rates due to factors such as setting time and sites may explain the lack of differences in catch rates in the present study. Finally, although the experiment was conducted on northern fulmars, the results reported should have relevance to the conservation of Fulmarus spp. worldwide because of the morphological similarities and similar feeding behaviour (i.e. mostly by surface seizure) of these species. Thus it is likely that the bird-scaring streamer line is efficient in reducing incidental catch of fulmars also in other longline fisheries, e.g. in the Antarctic where the southern fulmar F. glacialoides breeds.
Acknowledgements We are grateful to Louise Wynen for assistance in reducing the data from the time–depth recorders. We are also grateful to Edward F. Melvin for his helpful comments on the manuscript.
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