The basking shark (Cetorhinus maximus) is the largest fish in UK waters and the second largest fish globally, reaching 12 m in length and weighing up to 4 t (Sims, 2008). Basking sharks have been shown to feed where water masses meet, such as at interfaces between bodies of water, including tidal and oceanic fronts, typically with strong thermal gradients (Sims, 2008; Priede & Miller, 2009). At these fronts, plankton are typically found in higher concentrations (Sims & Quayle, 1998). Basking sharks follow their prey horizontally and vertically, and are known to forage over large distances often observed singly, or in pairs, but can be also found in large groups (Wilson, 2004; Crowe et al., 2018). Sightings in the Northeast Atlantic are usually recorded between April and October, with the peak number of sightings between June and August (Witt et al., 2012).
The European continental shelf is a particularly important habitat for basking sharks and although they can demonstrate acute site fidelity to coastal regions, movements can be widespread (Doherty et al., 2017a; Doherty et al., 2017b; Sims et al., 2003; Southall et al., 2006). In the northeast Atlantic area they are considered Endangered by the IUCN (Fowler, 2009). Basking sharks have been shown to travel long distances to locate aggregated zooplankton, that can occur at frontal systems that can form in continental shelf and shelf-edge habitats (Sims & Quayle, 1998; Sims et al., 2006). In shallow inner-shelf regions of the Northeast Atlantic, near thermal fronts, sharks have been known to exhibit reverse diel vertical migration (DVM), where they are typically near the surface during daylight hours. This is most likely due to the reverse DVM of basking sharks prey, Calanus sp. in the area (Sims et al., 2005).
In the UK, high relative densities of surface sightings occur on the west coast of Scotland, around the Isle of Man and on the southwest coast of England (Witt et al., 2012; Speedie & Johnson, 2008; Speedie et al., 2009). There are two established hotspot sites in Scotland, which are the islands of Hyskeir and Canna situated in the Sea of the Hebrides, and the island of Coll (Gunna Sound, between Coll and Tiree) in the Inner Hebrides (Doherty et al., 2017a; Speedie et al., 2009). The Hyskeir and Canna hotspot support high numbers of sharks involved in putative courtship behaviour and is a site where breaching activity has been recorded (Speedie et al., 2009). The functional role of breaching has however yet to be resolved, but might involve intra-species communication to find mates or deter competing conspecifics for breeding opportunities (Sims et al., 2000). The occurrence and distribution of basking sharks to the west of the Shetland Islands (Fig. 1a; 60.5° N, 2.1° W), remains poorly understood with only few sightings documented in this region over the past 20 years (Witt et al., 2012). A paucity of data potentially reflects the remote nature of this island archipelago, but also that data from offshore regions is limited by reduced sightings effort and subsequent reporting to public recording programmes, such as those operated by the Marine Conservation Society (UK) and the Shark Trust (UK). This study reports upon the sighting of basking sharks breaching to the west of Shetland, observed during marine wildlife observations made during 2D seismic survey activity.