Opportunistic acoustic recordings of (potential) orangeback flying squid Sthenoteuthis pteropus in the Central Eastern Atlantic
Introduction
Oceanic squids have wide geographical ranges, from polar regions to the tropics Jereb and Roper, 2010, Arkhipkin et al., 2015. Squids are ecologically important as major predators, but are also vital as the prey of valuable commercial fish species and many endangered marine animals like whales, seals, and sharks (Boyle and Rodhouse, 2005). They serve as an important trophic link between small mesopelagic organisms and top vertebrate predators Gilly et al., 2006, Coll et al., 2013 Despite their importance in marine food webs, there is a lack of knowledge regarding their behaviour and daily rhythms (Watanabe et al., 2006). Squids are difficult to catch with nets (Clarke, 2006), which may be related to the considerable differences between the composition of trawl catches and what is found in the stomach contents of predators (Hoving et al., 2014). They are rarely detected acoustically, except for large individuals or aggregations in shallow waters or enclosed bays Benoit-Bird and Gilly, 2012, Benoit-Bird et al., 2008, Jefferts et al., 1987, Vaughan and Recksiek, 1979. In addition, several studies have been carried out in the Pacific Ocean to estimate their scattering properties Chen et al., 2013, Jefferts et al., 1987, Jones et al., 2009, Kang et al., 2005, Lee, 2013, Lee et al., 2012, Zhang et al., 2015.
The orangeback flying squid Sthenoteuthis pteropus (Fig. 1) is an opportunistic short-lived carnivore, among the fastest growing squids (Merten et al., 2016) that inhabits the Central Eastern Atlantic Ocean between 42°N ° N and 36°S ° S and surface water temperatures ranging from 16 to 32°C ° C, usually above 20 to 22°C ° C (Jereb and Roper, 2010). In the current study, we present an analysis of opportunistic field data of a rare and difficult-to-observe event. Visual detections of flying squids coincided with new acoustic echotraces detected on echograms carried out during a survey crossing the Central Eastern Atlantic. Several indications suggested that squids were producing those acoustic echotraces. This work tests this hypothesis based on acoustic properties and swimming behaviour. To our knowledge, this is the first recording of acoustic data showing the natural behaviour of flying squid aggregations in the open ocean.
Section snippets
Material and methods
Acoustic data were recorded in April 2015 during the “Migrants and Active Flux In the Atlantic Ocean” (MAFIA) survey from the Brazilian coast to the Canary Islands on board the RV Hesperides, along the track shown in Fig. 2, alternating 24-hour 24-h navigation with 24-h stations. Mesopelagic fishes were caught by means a midwater trawl, the “Mesopelagos” net, with a mean mouth opening of 5 ×7 5 × 7 m and a total length of 58 m (more information in Olivar et al., 2017). Fishing lines were deployed
Results
Hundreds of individual orangeback squid were observed at station 8 from the vessel deck at night, jumping and flying above the water’s surface, and four individuals of 24, 25, 26 and 30 cm mantle length (ML) were caught with fishing lines and identified at the species level. The station at which squids were detected showed a clear oceanographic front, with surface temperatures descending from 26° C ° C at station 7 to 22° C ° C at station 9 (Fig. 2), and a maximum surface salinity (38.8 psu), 3.1
Discussion
An opportunistic visual detection of the squid Sthenoteuthis pteropus took place at an oceanic station in the Central Eastern Atlantic close to Senegal and Cape Verde. The visual detection and identification of squid in subsurface waters concurred with previously unobserved echotraces detected on the echograms recorded at 38 and 120 kHz. A few individual squid were caught with a fishing line, having lengths between 24 and 30 cm, sizes that fall into the lower size range of mature female
Conclusion
An opportunistic visual detection of orangeback squid flying out of the water at night in the Central Eastern Atlantic coincided with previously unobserved acoustic echotraces detected on echograms. Acoustic properties, behaviour, swimming speeds and hydrography in the area are consistent with these echotraces being produced by these squid species. This manuscript provides more insight into the acoustic properties (which allow their identification on echograms) and natural behaviour of oceanic
Acknowledgments
The authors are grateful to the crew and technicians on board the R/V Hesperides for their help during the survey, and to all colleagues who participated in the survey. This research was funded by Spanish Ministerio de Economía y Competitividad (MINECO) through project CTM2012-39587-C04-03. RV was financed by the MINECO project AGL2012-39077.
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