Many seabirds are killed in the course of human fishing operations, during both trawling and longlining. This mortality is having a major detrimental impact on the numbers of larger seabirds such as albatrosses, and governments, conservation groups and fishing industries around the world are trying to find solutions to the problem.
Because many trawl vessels discharge offal, heads, tails, and unwanted fish as a result of onboard processing at sea, they have been likened to floating cafeterias for seabirds. In their frenzy to get at the food, seabirds unwittingly fly into or are struck by the large cables (termed warps) that are used to tow the trawl nets. In trials in the New Zealand southern squid fishery in 2006, bird strikes ranged from zero when there was no offal discharge up to five bird strikes per hour with continuous discharge. As most fishermen, observers and scientists would agree, get rid of the offal and you get rid of a big part of the problem. But that is not always feasible and other solutions are being sought.
In February, the results of the of the southern squid fishery trial of mitigation devices—a cooperative fishing industry, Ministry of Fisheries, DOC and WWF-NZ project—were released in a report, The Efficacy of Warp Strike Mitigation Devices: Trials in the 2006 Squid Fishery.
The report looked at the performance of three mitigation devices currently used within the New Zealand trawl fishery to reduce seabird by-catch. The trials took place in the southern squid fishery (Auckland Islands and Stewart-Snares Shelf), which in recent years has had consistently higher seabird by-catch rates than most other New Zealand trawl fisheries. A report published in 2007 by NIWA scientists Susan Jane Baird and Murray Smith for the Ministry of Fisheries estimated seabird captures—seabirds brought on-board (dead or alive including with injuries) after colliding with fishing gear—for this fishery in 2003-04 at 827 birds. In 2004-05 the equivalent figure was 1291 birds. Researchers note that seabird captures represent only birds that are recovered—many more may be killed or injured.
In the Falkland Islands/Malvinas trawl fleet mitigation measures have also been on trial. “Offal is the problem, but there are logistic, safety and economic reasons behind when offal is discharged from a trawler,” says Ben Sullivan, Faulklands Conservation/BirdLife Seabirds-at-Sea Team. “Changing protocol around offal management is a tall order—although one many are working to fill—but for the sake of expediency in 2003 we decided to look more closely at mitigation measures. There was a lot of anecdotal information about which mitigation measures worked best, but no one had ever undertaken a scientific study to compare them.”
The Falkland Islands’ finfish fleet consists of 12 to 16 similar vessels that fish year round and primarily target hoki, kingclip (ling), hake and southern blue whiting. During spring when black-browed albatrosses are returning to the area to breed and rough seas cause the fleet to increase the frequency of their offal discharge, the number of black-browed albatrosses killed used to rise significantly. It was during that high-risk season that Ben and his team chose to compare three warp-strike mitigation measures: tori lines, warp scarers and bafflers along with a no-mitigation control.
Over the course of the three-month trial the number of confirmed mortalities for the control group was 14, for the baffler was three, for the warp scarer was one, and for twin tori lines was nil. As total actual mortalities were low, warp contacts were used as the primary measure of seabird mishaps and it was shown that total warp contact rates per hour of observation were significantly lower for tori lines and warp scarers than for the control.
With this information in hand, the team felt they had a strong case for the use of tori lines within the fleet. The government agreed and called for the mandatory deployment of tori lines for all finfish trawlers in Falkland Island waters beginning in August 2004.
During the 2002-03 fishing season 1500 birds were estimated to have been killed in the Falklands/Malvinas trawl fleet, 1400 of them blackbrowed albatrosses. With the introduction of tori lines across the fleet in 2004-05 there was an 89 per cent reduction in seabird by-catch with an estimated 169 birds killed.
In late 2005, the use of twin tori lines by large trawlers was introduced as part of a package of interim measures aimed at reducing seabird by-catch in New Zealand’s trawl fisheries. Tori lines were chosen in large part because of the findings of the Falklands/Malvinas study. However, over the course of the next few months, reports trickled in about the practical difficulties crews were finding in using tori lines in the trawl fleet. Questions also arose about the effectiveness of this particular mitigation measure, especially in high winds and on some types of vessels. There were also reports of seabird strikes on the tori lines themselves.
In response, the Ministry of Fisheries altered the regulations in 2006 to allow the use of any of three mitigation devices (bird baffler, twin tori lines or warp scarer). That same year, the research study referred to earlier was set up to trial warp-strike mitigation devices in the southern squid fishery from January to May 2006. Of particular interest was whether the Falklands results were generally applicable to the more diverse New Zealand trawl fleet.
The trials compared the three mitigation measures along with a control group. Over the course of the trials, trained observers logged about 600 hours of total observations on 18 different vessels. Like the Falklands study, the New Zealand trial used seabird strikes as the primary measurement. Observers recorded heavy contacts of both small birds (i.e. petrels and shearwaters) and large birds (i.e. albatrosses and giant petrels) on both the mitigation devices and the warps during the 15 minute observation periods.
Overall, as the study’s authors note, “Heavy strikes by seabirds on the warps or mitigation devices were rare.” Of the 1581 warp observation periods, 1381 or 87 per cent had no heavy seabird contacts recorded. For the 989 mitigation device observation periods, 875 or 89 per cent had no recorded heavy seabird strikes. Strike rates were clearly related to offal discharge by vessels. Only four bird contacts were recorded during 786 observation periods when nothing was being discharged and rates were higher with offal discharges than liquid discharges.
Because of the paucity of data and the clear influence of variables such as the discharge of offal, the researchers found it useful to employ statistical modelling to compare the mitigation devices and the control. The New Zealand trials produced similar results as the Falkland trials: Tori lines were the most effective at reducing warp strikes by seabirds—reducing the frequency of strikes to between 5 and 20 per cent of the control group. Warp scarers and bird bafflers reduced large bird strikes 35–90 per cent compared to the control group, but both were less effective for small birds.
Middleton adds, “During the trials, warp strike rates showed considerable variation between vessels. We also found that certain devices proved more effective on some vessel types. For example, there were indications that bird bafflers—a device that was largely ineffective in the Falklands study—was more effective on some smaller vessels in the New Zealand’s fleet.”
Although no dead birds were hauled in with any of the mitigation devices, in the New Zealand study a large number of bird strikes were noted on the tori lines themselves. In fact, for large birds, the number of strikes observed on vessels with tori lines was similar to the number of warp strikes observed in the control group. In contrast, heavy seabird strikes on bird bafflers and warp scarers were rare.
“Only the bird baffler treatment showed a reduction in overall strikes in the New Zealand study,” Middleton notes. “On some vessels seabird strikes were apparently displaced from the warps to the tori lines. On others, strikes were observed on tori lines even though the warps had low strike rates in the absence of mitigation devices.”
Observers involved in the trials had varying opinions about the impact of strikes on tori lines compared to strikes on warps. Some felt that tori line strikes were relatively benign for the birds, but others gave examples of seeing birds becoming entangled in the lines or dragged under the water by the lines.”
Ben Sullivan comments, “Based on our research and observations in the Falkland Islands’ finfish fleet, it appears that the severity of heavy strikes on tori lines is much less a threat to seabirds than heavy strikes on warps. This could be because tori lines are light and give when they are struck. When a bird strikes a warp it’s coming into contact with a solid, unyielding object. After the first year of tori lines becoming mandatory in the finfish fleet there were three recorded bird mortalities associated with tori lines, but given the dramatic reduction in estimated mortality across the fleet, we believe that the benefit of tori lines greatly outweighs the risk.”
The New Zealand study is more cautious, stating that “There is no information on whether seabird strikes on tori lines are as dangerous as those on trawl warps.”
“Although tori lines are of lighter materials than trawl warps, observer comments suggest that some collisions with tori lines can be dangerous to seabirds,” says Middleton. “Based on our findings it’s hard to ignore the prevalence of recorded tori line strikes. It would be desirable to quantify the impact these strikes are having on seabirds.
“From a practical view we’ve needed to break down the seabird by-catch issue bird type by bird type, fishery by fishery and vessel by vessel to come up with a game plan to stop killing birds. One of our approaches has been to require vessel management plans that recognise characteristics peculiar to each particular vessel or vessel class, and focus on ways to manage offal so that seabirds aren’t coming into harms way,” says Richard Wells, fisheries advocate, Clement and Associates. “For industry the results from both studies provide us with more information about how, when and where mitigation devices work most effectively.”
Perhaps the most robust finding in the New Zealand trials was the further confirmation of the primary role offal discharge plays in determining seabird interactions with trawlers. As the study’s authors note, “For observations where no discharge was occurring, almost no seabird strikes were recorded.”
Although seabird strikes were used as the primary index of seabird interactions, 59 seabirds were killed on warps during trips that were part of the trials. Most occurred on trawls where no mitigation was used or bird bafflers were used. Over half were killed on a single trip.
Captures of seabirds in nets can also be a problem: observers counted 227 birds caught in trawl nets—165 dead, 65 alive. As yet no winning mitigation strategies have been developed to address net captures, though offal management would reduce the attractiveness of fishing vessels to seabirds generally.
The New Zealand trawl fleet are back at sea this year. According to early fishing industry reports, it has been a good season in terms of reducing seabird by-catch. Mitigation devices are required on all vessels and fishermen are learning which mitigation devices work best on different vessels and in various conditions. Fishermen are also implementing offal management techniques that appear to reduce the number of seabirds attracted to fishing vessels. Again this year observers are assisting with additional research aimed at reducing seabird by-catch—this time by comparing offal management practices.