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Summary
This study aimed to assess the fate of live released spinetail devilrays (Mobula japanica) captured in commercial purse seine fisheries, and to describe their spatial and vertical behaviour. Six rays were tagged with popup archival satellite tags by an observer off northeastern North Island, New Zealand, in January−February 2013. Only four of the six tags reported data, and three of the four rays that provided data died within 2−4 days of release.The fourth ray (tag 115490) provided 82 days of track data before the tag pulled free, washed ashore, and was recovered, enabling the archived data to be downloaded.
Ray 115490 did a large loop to the north of New Zealand soon after release, before returning to near North Cape. It then spent six weeks off the east Northland coast beyond the edge of the continental shelf. The ray apparently stayed within the New Zealand Exclusive Economic Zone for the whole track, i.e. January to March. Spinetail devilrays could potentially travel much greater distances than indicated by the single short track obtained here. Longer tag deployments and a larger sample size are necessary to determine the habitat of spinetail devilrays in the New Zealand region.
Ray 115490 spent most of its time shallower than 100 m, with more time in shallow water during the night (2000–0500 hours NZST) than during the day/twilight (0500–2000 hours): the proportions of time spent in 0–25 m, 25–50 m, and greater than 50 m were 60.8%, 28.4% and 10.9% respectively by night, and 43.5%, 34.3% and 22.2% respectively by day. The median night time depth was 18.5 m and the median day/twilight depth was 28.5 m. The distribution of temperatures experienced was similar by night and day. The ray made periodic, short deep dives. There was no diel pattern in the timing of dives deeper than 75 m, but dives deeper than 100 m were most common at dusk (1600−1900 NZST), whereas dives deeper than 200 m were nearly all made during the day or twilight, with peaks at dawn (0600–0700) and dusk (1700–1900). The ray made three dives deeper than 500 m, and reached a maximum depth of 649 m, which is the deepest dive so far reported for this species. The ray was very active, making almost continuous vertical movements of at least 30–40 m amplitude. However, its vertical behaviour varied considerably through a diel cycleand over the full deployment period. Most days showed movement between the surface and about 40 m, but deeper dives were irregular with some periods having few dives deeper than 200 m (e.g. mid January) and others having many dives deeper than 300 m (e.g. mid March).
The three deaths observed out of four rays released indicate that mortality may be significant. Until further devilrays can be tagged, and a reasonable estimate of mortality of released animals obtained, fishers should be encouraged to release animals from the purse seine net while it is still in the water, rather than after they have been lifted on to the deck, to maximise their chances of survival.
Publication information
Francis, M.P. 2014. Survival and depth distribution of spinetail devilrays (Mobula japanica) released from purse-seine catches. Report prepared by the National Institute of Water and Atmosphere for the Department of Conservation, Wellington. 23p.