Are close-following and breaching behaviours by basking sharks at aggregation sites related to courtship?

Abstract

Basking sharks Cetorhinus maximus tend to aggregate in summer at favoured locations along Britain and Ireland's west coast. Sharks have been described approaching and close-following one another, often to one side. This has been interpreted as putative pre-mating behaviour. At aggregation sites around the Inner Hebrides we used boat-based observation and in-water and overhead drone video-photography to document behaviour and to determine the sex of individuals. It was confirmed that a shark will frequently move purposely towards another from a distance and swim to maintain a position either directly behind or closely to one side of a conspecific for short periods. Contrary to expectation, we found no relationship between the sex of a shark or its size and close-following. This suggests that following behaviours are not mainly related to courtship. Further, abrasions on the nose suspected to be related to male behaviour were found to occur on both sexes, although abrasions on pectoral fins, similarly suggestive of mating-related behaviour, were predominantly on females. Breaching by basking sharks has also been proposed as a means of attracting the opposite sex. We observed breaching by solitary sharks but commonly by sharks within aggregations, and at other times by more than one shark on the same day at the same time; but there was not any clear evidence to indicate that breaching is primarily related to mating. More likely individuals show close following chiefly for feeding-related hydrodynamic advantage. It remains plausible however that mature sharks make use of feeding aggregations to initiate pre-courtship behaviour.

Operational Protocols for the Use of Drones in Marine Animal Research

“…Published research on sharks using drone-based methods has become commonplace in the scientific literature over the last five years. These studies have largely focused on detection and abundance estimates [19,29], and tracking and behavioural observations [25,[30][31][32][33][34][35]. Drones typically survey comparatively smaller spatial scales compared to manned aircraft [36,37], and while sharks may be rare in abundance [38], they are often easy to detect from drones if conditions are ideal [39,40].…”

“…Drones have also proven useful for behavioural observations and fine-scale tracking of various shark species [30,31,42,47]. Typically, obtaining data on shark movement and behaviour has largely been conducted using boat-based observations [33,48,49], underwater video [50], or more commonly, satellite and/or acoustic tracking methods [51,52]. However, in comparison to satellite tracking methods, drones provide highly detailed information at much finer scales.…”

Serendipitous re‐sighting of a basking shark Cetorhinus maximus reveals inter‐annual connectivity between American and European coastal hotspots

“…However, the timing of the tag deployment in Ireland (August 2014) and re‐sighting in the US (June 2017) illustrates that association with summer (April–September) hotspots in the North Atlantic Ocean might be more nuanced than previously thought. Certainly, individual sharks do return on an inter‐annual basis to the same coastal hotspot (Doherty et al ., ; Gore et al ., ) but this re‐sighting event shows that inter‐annual fidelity to a single summer hotspot is not obligate or necessarily exhibited in all individuals.…”

“…As large‐bodied planktivores, basking shark are typically associated with hydrodynamic features that contain or aggregate high densities of crustacean zooplankton (Sims, ). This foraging pattern often manifests itself as latitudinal movements between heterogeneous aggregations of prey over seasonal scales (Roshier et al ., ); compounded in turn by reproductive and physiological needs; e.g ., social behaviour (Gore et al ., ; Sims et al ., ) or thermal ecology (Braun et al ., ; Carrier et al ., ; Priede & Miller, ). However, the timing of the tag deployment in Ireland (August 2014) and re‐sighting in the US (June 2017) illustrates that association with summer (April–September) hotspots in the North Atlantic Ocean might be more nuanced than previously thought.…”

Adult blacktip sharks (Carcharhinus limbatus) use shallow water as a refuge from great hammerheads (Sphyrna mokarran)

“…The use of UAVs permits unobtrusive observation and allows natural behaviours to be documented in the wild, providing insight into seldom‐seen predator–prey interactions (Lea et al ., 2019; Raoult et al ., 2018). Other studies have also used aerial drone footage to document various behaviours exhibited by different shark species in the wild, including feeding, social and aggregative behaviour (Gore et al ., 2019; Ho et al ., 2017; Rieucau et al ., 2018). The footage presented here can be analysed in depth to quantify swimming alignment, nearest‐neighbour distances, velocity and tail beat frequency to provide a more comprehensive analysis of these parameters for both predator and prey.…”