by Howard Martenstyn
Journal of the Department of Wildlife Conservation of Sri Lanka
For as long as I can remember I have had a deep fascination for the ocean and its magnificent creatures. My many expeditions led to a photographic documentation of their every move, followed by a desire to research their behaviour and environment, which eventually led me to extend my research notes into a book on my
passion. The result being a 272-page coffee table sized book, the first of its type, aptly entitled Out of the Blue.
Based on current taxonomy, 28 marine mammal species have been reported in Sri Lankan waters, with a further two tentatively identified; in all 13 species of whales (rorquals, sperm whales and beaked whales), 15 species of dolphins (blackfish and oceanic dolphins), one porpoise, and a dugong. Further scientific research, combined with sightings from whale-and dolphin-watching expedition logs is likely to reveal or confirm additional species and/or subspecies. Also, future research may show that a confirmed species, may not to be present and establish the taxonomic status of several species and subspecies that have yet to be fully elucidated.
Identifying Marine Mammals
Marine mammals in the wild are rarely easy to identify. Even under ideal conditions, observers may not enjoy more than a glimpse: a splash, a spout, a brief view of a dorsal fi n, head, back or flukes – seen, more often than not, at a considerable distance. Rough weather, glare from reflected sunlight, mist, twilight and other poor visual conditions frequently compound the problem. To confuse matters further, closely related cetacean species often appear so much alike that even experts are sometimes confused. In the case of certain little-known species, a reliable taxonomy has never been established; even a good look at a live mammal and comparison with an ‘in hand’ specimen will not necessarily settle the question. No surprise, then, that even experts must often log a marine mammal sighting as ‘unidentified’, ‘probable’ or ‘possible’, particularly in the case of closely related species and sub-species. It is better to log a sighting in this way, accompanied by a
detailed description of what one has actually seen, than to hazard an identification only to produce an erroneous record.
Identifying a marine mammal often comes down to a process of elimination. Location, habitat and the family to which the species belongs (more easily identified than the species itself) are important factors in this process. Remember that one physical feature or observed trait is rarely enough for a positive identification. Gather as much information as possible before coming to any conclusions.
Movement and Migration
As is the case with many other animals, several marine mammal species are migratory, often making astonishingly long journeys across the oceans of the world. Many large cetaceans, such as blue whales and humpbacks, migrate between high and low latitudes, moving from their chilly Antarctic habitat to tropical and sub-tropical regions in order to breed. Not all members of a species need be migratory; among humpback whales, for example, it is known that juveniles do not migrate along with breeding adults, but remain close to their feeding-grounds all year round, exploiting the relative lack of food competition while their elders are absent during the calving season.
Many cetacean species inhabit vast marine regions. In some species, migration and inter-breeding between populations occur on a regular basis, whereas in others, populations remain more distinct: overlapping migrations may occur, but specific populations still remain isolated from each other.
Some cetacean species do not undertake long seasonal migrations but instead make short, frequent journeys. These are normally associated with feeding behaviour. Pelagic night feeders like spinner dolphins are known to make daily inshore commutes, with the result that they are among the species most frequently seen by whale watchers and fishermen. In fact, fishermen often follow dolphin pods to locate shoals of tuna – a practice known since at least Ancient Greek times.
Still other marine mammals are nomadic, staying on the move all year round with no fixed abode nor any pattern to their wanderings. Their movements may be influenced by oceanic conditions and the availability of food. Individuals and populations of most nomadic species keep within a well-defined territory, but some, like sperm whales, are long-distance wanderers. Sperm whale movements are not truly migratory; instead, the whales seem to make seasonal shifts within a ‘home range’ of about 1,500km (800nm) in extent. However, the home range itself may shift over the years. Despite centuries of pursuit by whalers, sperm whale movements are not at all well understood. These great whales are found in all the world’s oceans, and individuals may well traverse the entire globe during the course of their 70-80 year lives.
Certain blackfish species of dolphins such as short-finned pilot whales are nomadic in nature. In some areas, bottlenose dolphins may have limited home ranges; in others, they may be nomadic, accompanying pilot whales in seasonal geographical movements. Orca sightings indicate that they regularly transit Sri Lankan waters and in one case the same pod has been repeatedly photographed over the years. Around Sri Lanka,
most oceanic dolphins such as, Fraser’s dolphins and rough-toothed dolphins, are highly pelagic in their movements.
Movement Between Habitats
The location of marine mammal habitats is defined by food availability, oceanic characteristics and geography. For some cetacean species, critical habitats may relate to static topographic features such as slope and depth. For others, they may be defined by ever-changing oceanographic conditions such as currents, water temperature and salinity. For example, areas of food abundance may result from upwelling and secondary biomass production. (Critical habitats are those in which marine mammals feed, socialise, rest, breed and raise their young; indispensable sections of migratory routes may also qualify as critical habitats).
Great whales tend to migrate seasonally from one well-defined habitat type to another. Orcas and other cetacean predators may move through a variety of different ecologies as they follow prey animals that are themselves in the course of migration.
Estuarine and many coastal mammals, such as humpback dolphins and dugongs, have more restricted ranges than pelagic species, and often spend all their lives within a single habitat. In Kalpitiya, humpback dolphins enter the estuary to feed on fi sh such as mullet. They prefer to enter the estuary with the flood tide and return to the sea with the ebb tide. Similar humpback dolphin movements have been observed along Adam’s Bridge across the maritime boundary with India. Dugongs too are known to enter shallow feeding areas at high tide, particularly during the spring tide to access seagrass beds not normally accessible. They sometimes travel long distances between seagrass meadows in the region of their habitat.
There can be little doubt that there are a number of significant pressures that act independently and cumulatively to influence not only the migration and movement of cetaceans but also their long-term population, distribution, abundance and survival.
Migrations Through Sri Lankan Waters
Experts disagree on from where and by what routes whales arrive and depart the waters around Sri Lanka. Reliable migration data on cetaceans in the Indian Ocean is thin. Many hypotheses have been and continue to be proposed in various publications. The information given below is based on the latest studies and data; however, it must be stressed that none of the possible cetacean migrations hypothesized or discussed herein
can be confirmed without further information becoming available.
The first question concerns what types of habitat the waters off Sri Lanka offer migrating cetaceans. Should our waters be classified as breeding or feeding grounds? Observations and records to date support both hypotheses, but the weight of evidence leans more heavily towards Sri Lankan waters being used as a feeding area. The available data, however, is very species-dependent. The topic is discussed in detail in a separate volume (Martenstyn, H. 2013. Sri Lanka Marine Mammal Records 2013, Centre for Research on Indian Ocean Marine Mammals - CRIOMM), in which information is given for each species and the various possibilities are discussed based upon collation and analysis of over three thousand seven hundred sighting records.
Cetaceans may enter or depart Sri Lankan waters from known habitats such as the northern Arabian Sea (Gulf of Oman), the Laccadive Sea around the Chagos-Laccadive Ridge, off the coast of East Africa (Somalia), the Bay of Bengal around Ninety East Ridge, off western Australia and even the Antarctic,which is a recognised summer feeding zone for migratory cetaceans such as blue, fin, sei, humpback and minke whales. In contrast, Bryde’s whales (taxonomy not fully elucidated), regularly sighted round Sri Lanka, are known to remain at tropical latitudes throughout the year.
It appears that the majority of blue whales and sperm whales in Sri Lankan waters leave the eastern coasts of the island towards the end of April and the southern coasts by May. Extenuating circumstances such as changes in seasonal weather patterns or plankton depletion may cause them to move out earlier. Does this data suggest that most blue whales seen in these waters from October to April are pygmy blue whales, while those seen from June to August are Antarctic blue whales, appearing in significantly smaller numbers? Are
there any resident blue whales, Bryde’s whales or even humpback whales in Sri Lankan waters? Humpbacks have been recorded migrating over distances of more than 8,000km (4,300nm), so their migratory destinations could be as far away as the Southern Ocean – or as near as the Gulf of Oman, where a population of humpbacks is thought to be resident.
The answers to these, and many other questions on the migrations and movements of cetaceans, must
await the results of studies yet to be conducted. In the meantime, the comings and goings of these
awe-inspiring mammals remain an intriguing mystery.
You may be interested in downloading a copy of Sri Lankan Wildlife journal Volume 09 No. 1., 180 pages.