Movement, depth distribution and survival of spinetail devilrays (Mobula japanica) tagged and released from purse‐seine catches in New Zealand

Abstract

1. Mobulid rays are protected in New Zealand, but the spinetail devilray Mobula japanica is caught as bycatch in skipjack tuna purse seine fisheries.2. Between 2005 and 2014, rays were recorded in 8.2% of observed purse seine sets. Rays were caught during summer, with a 'hotspot' (24.3% of sets) near the shelf edge off North Island over seabed depths of 150-350 m. Rays were usually brailed aboard with the tuna catch from successful sets, but were often entangled in the bunt of the net during unsuccessful sets.3. Observers tagged nine rays with popup archival tags to obtain preliminary information on their post-release survival, and spatial and vertical movements. Seven of the nine tags reported data, and four of those rays died within 2-4 days of release. All four rays that died had been brought aboard entangled in the bunt. The three surviving rays were all brailed aboard with the tuna catch. 4. One surviving ray remained near New Zealand for 2.7 months during summer, and the other two migrated 1400-1800 km northward to tropical waters near Vanuatu and Fiji at minimum speeds of 47 and 63 km day À1 at the end of summer. 5. Archive data from one ray showed that it made regular vertical movements of 25-100 m amplitude, but spent most of its time shallower than 50 m, more so during the night (89.6%) than the day (76.6%), and mainly experienced temperatures of 18À22°C. Dives deeper than 200 m were usually made during the day or twilight.6. All three surviving rays typically moved between the surface and 200-300 m daily, and reached greatest depths of 649 m, 1000 m and 1112 m, respectively, substantially exceeding the previous depth record for this species of 445 m. 7. Recommendations are made for reducing purse seine mortality of mobulid rays by avoiding areas of high ray abundance, avoiding setting on ray-associated tuna schools, and adopting best-practice methods of returning rays to the sea from the net or vessel.

Double tagging clarifies post-release fate of great hammerheads (Sphyrna mokarran)

“…Raw data are analyzed in-house by Wildlife Computers, who provide a report indicating pop-off date, location, daily values for temperature and depth, an indication of daily changes in light, and fate. Fate (i.e., the reason for tag release) is used to infer survivorship and is classified into one of four categories: (1) completed deployment, (2) sinker, a tag attached to a sinking animal that releases at 1700 m, (3) floater, a tag at the surface, and (4) sitter, a constant depth reading shallower than 1700 m. Completed deployment (category 1) indicates survival, sinker and sitter (categories 2 and 4) indicate mortality, and floater (category 3) could be interpreted as either survival or mortality, based on inspection of depth data [15].…”

Catch and post-release mortalities of deep-water sharks caught by bottom longlines in the Cantabrian Sea (NE Atlantic)

“…A recent study carried out with electronic tags on the spine-tailed devil ray Mobula japanica in New Zealand waters showed that individuals reported to have swum vigorously when released died after two days (Francis and Jones, 2016). Thus, post-release behaviour might not necessarily be a good indicator of survival, or could be species-specific.…”

Trophic overlap in mobulid rays: insights from stable isotope analysis

“…1). We selected these bounding boxes based on the expected distribution of mobulids in each region, which was supported by bycatch data from the eastern equatorial Pacific (Croll et al 2016), interviews with fishermen regarding capture locations in Sri Lanka, and records of long-distance movements in several Mobula species (Thorrold et al 2014, Francis & Jones 2017. We averaged chl a values over the entire bounding box for a given region and smoothed monthly averages into seasonal (3 mo) averages for plotting purposes.…”

“…The first possibility is that M. japanica is more piscivorous during the rainy season, and the landings and stomach content sampling during the dry season did not reflect the overall diet. While telemetry data suggest that M. japanica is restricted mainly to near-surface waters (Croll et al 2012), at least some individuals make regular dives to depths of 200 to 300 m (Francis & Jones 2017 (Heinrichs 2009), and it is possible that M. japanica engages in the same behavior in the Philippines. An alternative ex planation is that our trophic discrimination factors are incorrect, given the variability in published discrimination factors for elasmobranchs and the lack of experimentally derived discrimination factors for mobulids.…”

“…M. japanica and closely related Mobula mobular appear to spend the majority of their time in near-surface habitats, shallower than 50 m depth (Canese et al 2011, Croll et al 2012, Francis & Jones 2017. M. birostris makes deeper foraging dives and appears to spend substantial amounts of time below 100 m (Stewart et al 2016b).…”