Basking shark sub-surface behaviour revealed by animal-towed cameras

Abstract

While biologging tags have answered a wealth of ecological questions, the drivers and consequences of movement and activity often remain difficult to ascertain, particularly marine vertebrates which are difficult to observe directly. Basking sharks, the second largest shark species in the world, aggregate in the summer in key foraging sites but despite advances in biologging technologies, little is known about their breeding ecology and sub-surface behaviour. Advances in camera technologies holds potential for filling in these knowledge gaps by providing environmental context and validating behaviours recorded with conventional telemetry. Six basking sharks were tagged at their feeding site in the Sea of Hebrides, Scotland, with towed cameras combined with time-depth recorders and satellite telemetry. Cameras recorded a cumulative 123 hours of video data over an average 64-hour deployment and confirmed the position of the sharks within the water column. Feeding events only occurred within a metre depth and made up ¾ of the time spent swimming near the surface. Sharks maintained similar tail beat frequencies regardless of whether feeding, swimming near the surface or the seabed, where they spent surprisingly up to 88% of daylight hours. This study reported the first complete breaching event and the first sub-surface putative courtship display, with nose-to-tail chasing, parallel swimming as well as the first observation of grouping behaviour near the seabed. Social groups of sharks are thought to be very short term and sporadic, and may play a role in finding breeding partners, particularly in solitary sharks which may use aggregations as an opportunity to breed. In situ observation of basking sharks at their seasonal aggregation site through animal borne cameras revealed unprecedented insight into the social and environmental context of basking shark behaviour which were previously limited to surface observations.

Fig 1. Study site and towed camera tags.

(A) Study site and tagging location of the six basking sharks (orange circles) within the Sea of the Hebrides Marine Protected Area (blue polygon) (B). (C) Towed camera technical drawing (not to scale) with a schematic of the rear and forward field of view (FOV) of the camera attached to 7m shark by a 1.8m tether in relation to the shark’s swimming speed. The FOV of a camera towed by shark swimming at 0.5 m.s^-1 is represented in blue while the FOV when swimming at 2 m.s^-1 is highlighted in red.

Fig 2. Response to tagging.

(A) Tail beat frequency in response to tagging calculated from 30 second segments of video data taken every minute for first 30 minutes of video data (uses continuously recorded data gathered in 2018 only). (B) Tail beat frequency calculated for each 30 second recording period every 5 minutes (for 30 minutes) for duty cycled camera tags deployed in 2019. Local order regression smoothing (colour lines; 0.5 span).

Fig 3. Depth use behaviour of basking sharks gathered from towed camera tags.

(A) Shark 1, (B) shark 2, and (C) shark 3 tagged in 2018, and (D) shark 4, (E) shark 5 and (F) shark 6 tagged in 2019. Periods of seabed (black bars) or surface swimming (red bars) when ambient light permitted habitat characterisation, where the top bars in A-C represent depth use inferred from duty-cycled video data, and lower bars continuous data. Tracking duration (h) shown in parentheses.

Fig 4. Habitat association by basking sharks.

Proportion of time sharks 1–6 (A to F) spent swimming over different habitat types characterised by their European Nature Information System (EUNIS) code (A3.125 mixed kelp with opportunistic red seaweed on sand-covered infralittoral rock, A3.212 Laminaria hyperborea on tide-swept infralittoral rock, A3.215 Dense foliose red seaweeds on silty moderately exposed infralittoral rock, A4.21 Echinoderms and crustose communities on circalittoral rock, A5.13 Infralittoral coarse sediment, A5.23 Infralittoral fine sand, A5.52 Kelp and seaweed communities on sublittoral sediment). Includes proportion of time sharks spent in the water column (No Vis. Seabed), or when habitat could not be characterised owing to poor light conditions from deep diving (Blackout Day) or recording at night (Blackout Night).

Fig 5. Basking shark satellite tracking during camera tag attachment.

(A) Satellite tracking locations for shark 4 (green), shark 5 (orange) and shark 6 (red) during camera tag attachment periods in 2019. Location of feeding events for each shark (squares in corresponding colours). Selected behaviours are highlighted with given locations (B-G). In (B) to (E) shark 4 is seen feeding at the surface following a second male shark. Shark 5 camera captures a second shark swimming slowly over the seabed in (F) as it swims towards the surface. (G) Shark 6 feeding at the surface.

Fig 6. Observed behaviours from towed camera tags.

(A) Shark 3 moving over sandy seabed, (B) shark 4 feeding while following a second feeding shark at the surface, (C) shark 2 moving over mixed ground with seaweeds (D) shark 5 trailed closely by a second shark, (E) shark 3 engaged in early morning group behaviour—three sharks visible; instrumented shark (centre) and two other sharks (far left and top right), (F) shark 4 rearward camera obscured by plastic sheet, (G) shark 5 rearward camera revealing defecation, and (H) shark 5 rearward camera revealing small horse mackerel following the tagged shark.

Fig 7. Basking shark breaching.

Breach recorded by a towed camera tag. (A) The shark starts to ascend from 72 m depth at approximately 0.94 m s-1 of vertical gain, reaching the surface (in view, B) in 77 seconds The shark can be seen completely out of the water (C), before descending (D) to depth again. The timing and depth associated with each image (A-D) are identified on the breaching depth profile (E).