Large trawl vessel warp mitigation

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MIT2022-05 Warp strike mitigation in large-vessel trawl fisheries in New Zealand (PDF, 9,189K)

Summary 

Efforts to prevent seabird interactions with commercial fisheries include the use of bycatch mitigation devices during active fishing operations. In New Zealand, mandatory mitigation requirements for trawl fishing by large vessels (≥28 m length) include the use of at least one of three types of mitigation device, bird scaring (tori, streamer) lines, bird bafflers, or warp scarers (warp deflectors). During fishing operations, fisheries observers record information pertaining to the use of these mitigation devices, including seabird interactions, such as incidental captures. These data were used in the current study in an initial exploration aimed at characterising the use of mitigation devices in large-vessel trawl fisheries. This data exploration also included an assessment of seabird captures in relation to mitigation device type. The analysis was limited to warp captures, omitting records of net captures. The characterisation also considered the configuration of mitigation devices used across different large-vessel trawl fisheries.

The present assessment included statistical modelling to examine whether the use of mitigation devices influenced seabird capture rates. This part of the analysis used generalised linear mixed models to estimate capture rates from fisheries observer data. Covariates included in the models were fishing year, fishery, area and vessel as random effects, with mitigation gear included as a categorical fixed effect (at levels baffler-only, tori-line-only, baffler-and-tori-line).

The characterisation of warp mitigation devices showed that bird bafflers were the most prevalent mitigation gear used in large-vessel trawl fisheries. In addition, there was an increase in the use of this mitigation gear type over time. Bird bafflers in combination with tori lines were also used on a comparatively high number of tows, followed by the use of tori lines. There were only limited observer records of warp strike mitigation using warp scarers or other combinations of mitigation gear. These data were insufficient to be included in the statistical modelling.

Comparing the effectiveness of mitigation devices with bird bafflers as the reference indicated that tori lines were more effective than the former mitigation device type. The combination of bird bafflers and tori lines was slightly less effective than the use of bird bafflers only, but this finding was inconsistent across models. It may also be confounded by varying mitigation requirements; e.g., the use of multiple mitigation types required during “high risk” periods when there are more birds foraging such as during the peak squid trawl season. In addition, the modelling suggested that the effectiveness of bird bafflers has improved, especially after 2015. The inclusion of data that describes mitigation device gear changes (in both design and configuration) over time.

For the analysis of the configuration of mitigation gear, there was considerable variation in the amount of information available in the observer records. In addition, a high number of unique configurations of mitigation devices used during fishing prevented the analysis of particular configurations in relation to seabird captures.

Highlighting these limitations and challenges, recommendations from the present study include the exploration of different analysis techniques to assess the diversity of gear configurations. These techniques include clustering algorithms and dimensionality reduction, to group configurations of mitigation gear into consistent sets. The configuration groups can then be used to assess differences in bycatch rates, while taking into account the relative abundance of seabirds. The latter may also include the use of seabird count data that are regularly recorded at the back of fishing vessels. In addition, in-depth analysis of mitigation gear data may be augmented by including data on vessel waste management.

To provide context for the analysis, the current project also included a comparison of the three mitigation devices, based on existing information. Although there has been ongoing research into the use of different mitigation devices for a number of years, relatively few studies provide sufficient information to support formal analyses of their efficacy. Most research has focused on assessing the effectiveness of bird scaring lines, with findings consistently indicating a reduction in warp strike associated with their use across different trawl fisheries. Based on this evidence, and consistent with the model results in this study, bird scaring lines are the only mitigation device type included in the recommendations of best-practice mitigation measures by the Agreement on the Conservation of Albatrosses and Petrels.

Publication information 

Large, K. and Berkenbusch, K. and Richard, Y. and Neubauer, P. 2024. Warp strike mitigation in large-vessel trawl fisheries in New Zealand. Final report prepared for Department of Conservation. 33 p.