Elasmobranch Activity & Home Range
Many shark species are considered nocturnal, although crepuscular (active at dawn and dusk) would probably be a better description. During the afternoon, some species can be found resting on the seabed or in caves (e.g. the whitetip reef shark, Triaenodon obesus), while others swim casually around their preferred hunting or socialising grounds. It should be pointed out that although most species of shark increase their activity rhythms during the twilight hours, they are opportunistic feeders and will not pass-up the opportunity of a meal, merely because the clock says that it’s not dinnertime yet. Indeed, groups of sharks can be seen feeding in the middle of the day off Australian or South African beaches if conditions lead to concentration of prey species.
Coastal species, such as grey reef sharks (Carcharhinus amblyrhynchos) will spend the day offshore and move onto the reef at night to hunt. Similarly, studies on electroreception in hammerheads suggest some species experience an increase in sensitivity to bioelectric fields at night, indicating a nocturnal lifestyle. Conversely, work on white shark vision by Samuel Gruber at Bimini Field Station in the Bahamas has shown that this species appears to have reasonably good colour visual acuity, suggesting they’re geared towards more a more diurnal (daylight) activity pattern.
The home ranges of elasmobranch fishes have only been studied in detail for a few species, with the lemon shark (Negaprion brevirostris) perhaps the best-studied example. A study by Sam Gruber, John Morrissey and the late Donald Nelson tracked nine sharks for varying lengths of time during the early 1980s. From the results, it appears that, in the Bahamas, this shark has a home range that varied considerably according to the individual. A mature female tracked for almost five days occupied an area of 93 square kilometres (36 sq-miles), while a juvenile female spent the majority of the 19 hours for which she was tracked within an area of 18 square kilometres (7 sq-miles). These home ranges have also been found to overlap with other species and members of the same species. Indeed, territoriality has yet to be conclusively demonstrated in any species of shark.
Studies on other sharks confirm that wide variation exists between as well as within species. Historically, it was considered that great whites off California were largely coastal sharks, which bred off southern California and then migrated a short distance south to feed on pinnipeds (seals and sealions). A paper by six marine biologists spanning three institutions found that this is not the case, though. The study, published in January 2002, found that these sharks were, in fact, highly migratory; the furthest ranging of the six sharks tagged travelled from where it was tagged off California west to Hawaii—a distance of some 4,500km (2,800 mi). In February 2004, a 3.8m (12.5 ft.) female white shark named “Nicole” by the scientists tracking her set the migration record in this species when her satellite tag recorded her off the south coast of Australia, some 11,000km (6,800 miles) from where she was tagged in South Africa 99 days earlier. To-date, the longest distance travelled by a shark was recorded by Héctor Guzmán and colleagues, who satellite tracked a female whale shark they named “Anne” just over 19,300km (12,000 miles) across the Pacific Ocean from Panama to the Philippines.
An interesting paper presented to the American Elasmobranch Society’s 1999 Annual Meeting in Pennsylvania by Michael Robinson at the University of Miami suggested that home range in sharks may be related to body size and metabolic rate. He suggested that sharks with high metabolic rates, such as the lemon shark, require a relatively larger area over which to forage for food. This is a simple ecological concept, but must also vary according to prey density and accessibility within a given area.
Studies tracking the movements and looking at home ranges in batoids are rare and we know little about their movements. We do know that skates and rays will make long distance movements, but also show a tendency for philopatry (i.e., returning the familiar areas). During their study using satellite transmitters to track the movements of manta rays (Manta alfredi) off the coast of eastern Australia, for example, Fabrice Jaine and colleagues recorded movements of up to 2,441km (1,500 miles) that took them several hundred kilometres (straight line distance) from their tagging site. In many cases, however, the rays subsequently returned to the tagging site. A genetic analysis of red stingrays (Dasyatis akajei) in Chinese and Japanese waters found structuring between the samples collected up to 720km (450 miles) apart, despite there being no physical barriers preventing the populations mixing. This suggests that the rays were largely resident—they didn’t travel far or, at the very least, returned to their natal range to breed.