REFLECTIONS ON A RARE BASKING SHARK ENCOUNTER
Dr. Simon Berrow narrates his experience with basking sharks in the fall off West Clare. The species is often seen feeding in this area in the spring - but the seasonal timing of this experience was not the only unusual component...
© Chelsea Gray
SPECIES | Distribution
A map depicting the global distribution of basking sharks (areas showing high likelihood of occurrence are marked in red).
Basking sharks can be found worldwide in temperate waters, normally between 6-16 degrees C (42-60 degrees F). In the northern Atlantic, they are most frequently sighted during the months of April – September, often in shallow coastal habitats. Aggregations (groups) of individuals will be observed during this time in highly frequented locations known as basking shark "hotspots", which include Inishowen waters (Martin & Clark, 2008, CSAS 2008). In the Pacific, basking sharks are mainly reported during the winter months, between October – May. Historically, frequent sightings occurred on the coast of California, especially near the Santa Barbara Channel Islands. There is one published record of a basking shark in Mexican waters, but California is usually regarded as the southern end of the shark’s Pacific distribution (Sandoval-Castillo, J. et al, 2005). Similarly, until the late 1960’s basking sharks were also commonly sighted and encountered in the northern end of their eastern Pacific range, especially in British Columbia. However, observations of this species on the California and Canadian coast have dramatically reduced in number since that time (CSAS, 2008).
Research in the UK estimates that basking sharks spend up 36% of their time on the surface during their seasonal hotpot residence (Sims et al, 2003), though areas of high shark activity may not always be characterized by visible feeding behavior (Southall et al, 2005). However, significantly less is known about basking shark overwintering behavior because these organisms tend to move away from coastal areas following the summer season in the northeastern Atlantic (Sims et al., 2003; Gore et al, 2008). Until recently, this species was considered to hibernate offshore (Parker and Boseman, 1954). However, recent studies suggest sharks undergo extensive migrations from September to May (Gore et al., 2008; Skomal et al., 2009; Doherty et al., 2017). The current accepted migratory theory is of a west to east (deep-water to coastal) migration in April and a gradual northward trend following increases in sea surface temperature and copepod density until late August. This is when the sharks are believed to return to deeper waters to the west. Opposite directions apply in the western Atlantic zone.
Though they are capable of traversing entire ocean basins (the longest recorded distance is approximately 10,000 horizontal kilometers; Gore et al., 2008), basking shark migratory strategies vary widely. For example, sharks tagged off the coast of Scotland have been observed to travel anywhere from the Bay of Biscay in Europe to the northern coast of Africa (Doherty et al., 2017). Others in this region have also been known to travel across the Atlantic Ocean (Gore et al., 2008). Similarly, sharks originally tagged off the coast of Cape Cod in the United States have migrated to the Caribbean and Brazil during winter months (Skomal et al., 2009). Potential drivers are thought to include thermoregulation, feeding at deeper ocean layers, or travel to mating grounds.
HOTSPOTS, MIGRATION AND CONSERVATION
Generally, basking sharks have been recorded to be somewhat habitual in their choice of feeding areas and timing of their return to coastal hotspot locations. Unfortunately, such localised population groups have historically been and may continue to be highly susceptible to target fishing (Sims & Reid, 2002). Previously targeted by fisheries in Norway, the United Kingdom, the United States, Canada, Japan and New Zealand, the basking shark is currently listed on the IUCN Red List as globally vulnerable and endangered in the northeast Atlantic region (IUCN, 2011). Furthermore, studies that have observed trans-Atlantic migrations (Gore et al., 2008) have led to considerable debate on the possibility of a single genetic population in the North Atlantic. This would have significant implications for conservation bodies and the scale on which protective measures are implemented.