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 | Biology
CLASS | Chondrichthyes
SUBCLASS | Elasmobranchii
SUPER-ORDER | Selachiformes
ORDER | Lamniformes
FAMILY | Cetorhinidae
GENUS | Cetorhinus
SPECIES | Maxiumus
CETORHINUS MAXIMUS (Gunnerus) is known by as wide a variety of common names as the waters in which it is found. In many cases, these names describe aspects of its typical behaviour or biology (e.g. “sailfish or sunfish”). It is now most commonly referred to as the basking shark (An Liamhán Gréine).
The basking shark was first documented in the northeastern Atlantic in the mid- to late 1700s by naturalists including J.E. Gunnerus (bishop of Trondheim) and Thomas Pennant (author of British Zoology, 1769). The latter is credited with giving the shark its current common English language name (Fairfax 1998). It was not until later years that the traditional oral names of Gaelic origin were recorded in Ireland and Scotland, many of which offer local insights on fisherman perception of the shark's prevailing characteristics: Liop an dá lapa ("unwieldy beast with two fins"), Liabhan Mór ("great leviathan"), Liabán gréine ("great fish of the sun")(McNally, 1976). In northern Spain, the basking shark is appropriately referred to as “Peregrino”, meaning "pilgrim". This name has strong historic significance for the timing of basking shark sightings in this region of the globe, as pilgrims for Santiago de Compostella usually arrive into northern Spain during the early summer months and are gone again by Autumn (Compagno, 1984).
As such common nomenclature indicates, a majority of current knowledge on this species is derived from early physiological studies and ongoing observations of their seasonal surface-feeding behavior. Like other elasmobranchs (sharks, skates, and rays), basking shark skeletons are characterized by cartilage, rather than bone. Characteristic of larger species, it also possesses a small suspended brain, exceptional olfactory senses, and a large number of tubules known as the Ampullae of Lorenzini that allow this species to detect the electromagnetic field projected by other living organisms (Fairfax, 1998). Many aspects of its physical character are otherwise not fully understood, lack of knowledge associated with the belief that it spends much of its life at extreme depths (UK Government CITES proposal, 2002). However, in spite of its size, the basking shark is known to be one of three planktivorous shark species, in addition to the whale shark (Rhincodon typus) and the megamouth shark (Megachasma pelagios). Among these, it is the only obligate ram filter-feeder, as the other two are primarily suction-feeders (Diamond, 1985; Compagno, 1990). Thus, while it does possess several rows of small teeth, basking sharks feed primarily through use of gill rakers that are carried by the gill arches, through which it filters plankton as it opens its mouth to feed in plankton-rich waters. Its primary prey consists of large zooplankton, specifically copepods, and current research shows the basking shark presence is often positively correlated with the density of a specific copepod species: calanus finmarchicus (Sims & Quayle, 1998).
Otherwise, key life history characteristics such as growth rate, natural mortality and fecundity have been studied in limited detail but are at present assumed rather than measured (Canadian Science Advisory Secretariat, CSAS, 2008). The biological knowledge currently available is mainly due to the efforts of early naturalists and, in more recent years, the zoologists L.H. Matthews and H.W. Parker. In 1947, these researchers conducted a number of post mortem analyses at Soay (Scotland) on the invitation of Gavin Maxwell, owner of the historically Soay-based basking shark fishery (Fairfax 1998).
In appearance, basking sharks resemble many large shark species, including the great white (C. carcharias). Cetorhinus maximus has a relatively narrow girth and pointed snout. It average size is 5-7 meters, although specimens have been recorded at 11 meters. The liver, for which it was prized commercially, is estimated to make up approximately 25% of its body weight. This organ acts as a “hydrostatic float” to keep the shark at near neutral buoyancy. The basking shark also uses a large heterocercal tail (caudal fin) for propulsion, while its paired pectoral and smaller pelvic fins control its ascent and descent throughout the water column. It also possesses two dorsal fins, the first of which gives the shark its characteristic profile. This fin has been recorded at over a meter in height and is sometimes known to droop over when above the water surface. Males and females are distinguished by the presence of two claspers on the male. These claspers, although used in mating activity, are not actually used to clasp the female; rather, these are used to deliver spermatophores into the female in a similar way to a mammalian penis (Fairfax 1998, Shark Trust, 2008).
COURTSHIP AND REPRODUCTION
Courtship behaviour and reproduction cycles have not yet been empirically studied, and current knowledge is based on limited eyewitness accounts. In fact, there is only one peer-reviewed eyewitness account of copulation from the east coast of North America. However, perceived courtship has been reported many times with differing theories on which behaviours are related to mating. Nose-to-tail following, parallel swimming and breaching are all theorised as being linked to courtship. These behaviours have been recorded in most of the basking shark hotspots around Britain and Ireland and are closely associated with thermal fronts (Sims et al, 2000). Otherwise, there is also only one recorded eyewitness account of birth by two Norwegian fishermen, Hans Goksoyr and Jonas Sordal. They were towing a harpooned basking shark when ‘ ….it threw a live young shark roughly as large as a common habrand. The young shark was alive and immediately started swimming at the surface. Then another young shark and so on to five young sharks and a sixth was dead’ (Kunzlik, 1988).
Studies led by researchers in the UK on basking shark genetic patterns and gene regions offer limited results due to the small number of samples taken. However, current genetic analysis does allow for differentiation between two mtDNA gene regions, which indicate subpopulations in northern temperate waters and southern temperate waters (Noble et al, 2006). Similar studies have also estimated the current reproductive population basking sharks (those contributing genetic material to the global population) at less than 10,000 individuals (Hoelzel et al., 2006). Perhaps most importantly, genetic analyses have contributed to conservation efforts by facilitating the enforcement of CITES regulations by allowing the detection of basking shark materials at low concentrations in processed products. As a result, the presence of basking shark fins has been confirmed in the Hong Kong and Japanese markets, and a positive relationship has been discovered between numerous fin trader categories and valuable parts of basking sharks (Magnusson et al, 2007).