Updated analysis of spine-tailed devil ray post-release survival

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INT2018-05: Updated analysis of spine-tailed devil ray post-release survival (PDF, 1,300K)

Summary 

Spine-tailed devil rays (Mobula mobular) are frequently taken as bycatch in purse-seine fisheries targeting skipjack tuna (Katsuwonus pelamis) around the northern North Island. Devil rays are a protected species, and they are returned to the water following capture. In previous research projects carried out for the Department of Conservation, we estimated the survival of devil rays returned to the sea, determined the factors that influence the capture and post-release survival of devil rays, and made recommendations on ways to reduce and mitigate devil ray captures. In this report, we update some of our previous analyses using observer and tagging data to the end of the 2018–19 fishing year, and also update the recommendations.

Sixteen devil rays were tagged with ‘pop-up’ tags during 2013 to 2018, and 14 of the tags transmitted data, allowing a determination of whether the devil rays had survived 30 days following tag and release. Four of the first seven devil rays tagged in 2013−2015 (57%) died. However, only one of seven devil rays tagged in 2016−2018 (14%) died. The overall mortality rate for tagged rays in 2013–2018 was 36%. All mortalities of tagged rays resulted from skunked sets followed by lifting of devil rays aboard in the net. In contrast, all devil rays that were tagged from successful sets were brailed aboard, and all of them survived. The number of devil rays tagged is too small to draw strong conclusions, but a reduction in the mortality rate of released devil rays is consistent with observed improvements in handling and releasing methods used by purse seine crews.

Recommendations are made for avoiding devil ray captures, reducing mortality of rays that are caught, and improving data collection and analyses.

Publication details

Francis, M. & Jones, E. 2019. Updated analysis of spine-tailed devil ray post-release survival. Final report prepared by NIWA for the Conservation Services Programme, Department of Conservation. 21 p.