Something To Know

Credit: Listal

Marine mammals are charismatic organisms both as they appear to our eyes and for the entire food web. In fact, we can recognize the most productive environments in the world even without observing oceanic motions (think of the upwelling places), simply by recognizing their presence. In fact, these organisms allocate or more often migrate to different areas depending on the stage of the life cycle. However we begin to indicate the possible lifestyles and the reasons of migration:

  • Migrate for mating and feeding;
  • Migrate to maintain their food supplies;
  • No standard migration patterns;
  • No migration, with a fixed distance from their home location all year round

In reality, however, little is known about their actual distribution, the biology and ecology of these imposing and charismatic organisms. Also because in this case sacrificing even a single specimen could be very harmful to the ecosystem. For this reason, strandings (the marine mammals are sometimes brought ashore and this event is called stranding), in this sense are useful in helping the knowledge of the researcher, to understand not only the biology of the animal, but also the distribution, reproduction and migratory patterns.

Let’s focus on New Caledonia and Vanuatu in the tropical Pacific Southwest. This is today the area with the least data collected regarding marine mammals and for this reason we talk about it.

New Caledonia (French territory) is located in the southwestern Pacific Ocean at 18-20 ° S and 163-168 ° E. It includes a main island, 400 km long and 80 km wide, and a series of smaller islands, including: Ile des Pins in the south; Belep in the north; and the Loyalty Islands (Lifou, Maré and Ouvea) to the east. A 1600 km long coral reef surrounds a huge 24,000 km2 lagoon around the main island. The average depth of the lagoon is 24 m. In contrast, at least two of the Loyalty Islands are surrounded by open ocean.

Vanuatu, whose official name is Republic of Vanuatu is an island state located in the southern Pacific Ocean. The volcanic archipelago is located about 1,750 km east of Australia, 500 km north east of New Caledonia, west of Fiji and south of the Solomon Islands.

Credit: Wikipedia

The phenomenon of stranding often arouses the interest of local newspapers which therefore prove to be a source of data also for actual subsequent scientific studies (although often these are inaccurate data). From all the data collected from these newspapers and from photographs designed to testify to the stranding, in the areas just mentioned, studies have been carried out on the distribution, on the species, on the seasonal models of the species most frequently affected by this phenomenon.

Consider the period from 1877 to 2005, when 72 strandings were registered. Most of these concerned only one specimen, but 5 mass strandings were also recorded. The latter involved 32 and 199 individuals of the Peponocephala electra species in Vanuatu in 1972, 52 organisms of the Globicephala macrorhynchus species in the Loyalty Islands in 1977, and the last two cases each concerned about 30 dolphins (Stenella longirostris) one in the Loyalty Islands in 1994 and the other in Vanuatu in 1995.

In the end, taking into consideration all the events, 22 species of marine mammals were identified in the New Caledonia-Vanuatu area:

Arctocephalus forsteri, Balaenoptera acutorostrata, B. bonaerensis (antarctic minke whale), B. borealis, B. edeni, B. musculus, Delphinus delphis, Dugong dugon, Globicephala macrorhynchus, Grampus griseus, Kogia breviceps, K. Sima, Megaptera nova densirostris, Orcinus orca, Peponocephala electra, Physeter macrocephalus, Pseudorca crassidens, Stenella attenuata, Stenella Longirostris, Tursiops aduncus, T. truncatus.

Thanks to these studies we can say that the fauna of the marine mammals of New Caledonia – Vanuatu includes mainly tropical and warm temperate species (Balaenoptera edeni, Mesoplodon densirostris, Globicephala macrorhynchus, Pseudorca crassidens, Peponocephala electra, Delphinus delphis, Stenella attenuis, Stenella Longongirir ), tropical to cold temperate species (B. borealis, Grampus griseus, Tursiops aduncus), cosmopolitan species (B. acutorostrata, B. musculus, Megaptera novaeangliae, Physeter macrocephalus, Orcinus orca), a species distributed from the temperate to circumpolar latitudes of the southern hemisphere (B. bonaerensis) and a mostly limited species in southern Australia and New Zealand (Arctocephalus forsteri). Five of the previous species (Balaenoptera acutorostrata, B. bonaerensis, B. borealis, Megaptera novaeangliae, Physeter macrocephalus) are known for seasonal migrations between the summer spawning area in the tropics, and the feeding grounds at higher latitudes. However, the migrations of Physeter macrocephalus are not as well understood as those of most whales and the migratory patterns of B. borealis and B. musculus in the southwestern Pacific have not yet been documented and need further investigation.

Although the migration pattern has not been documented, the presence of large populations of B. musculus off the Solomon Islands in 1957 has been instead observed. Instead some data of illegal whaling expeditions have mapped the distribution of this species, in an area comprising a large part of the Indian Ocean, part of the Indo-Malay archipelago and in the Tasman Sea. What we can say is that therefore, despite the lack of documentation, blue whales can be a frequent encounter in the tropical southwest of the Pacific, unless the species here has become locally extinct due to the intense illegal catches that have taken place.

Dugong dugon, according to the International Union for Conservation of Nature (IUCN), must be classified as a species vulnerable to extinction on a global scale, due to the abundant decline in its usual distribution ranges. In fact, in New Caledonia, the populations of this species have shrunk sharply over the past few decades, despite being a protected species in these waters since 1963. The current data reveal that the factors that continue to determine the decline of these specimens are: poaching and maritime traffic (also responsible for beaching not only mortality).

Arctocephalus forsteri is instead considered as a sort of wandering species, homeless as it regularly moves away from New Zealand and Three Kings Island arriving at a distance equal to 1400 km from the southern part of New Caledonia. This seems to occur in the winter period and this indicates a seasonal dispersion model probably due to climatic conditions (such as temperature) and to processes of intra-specific competition for resources.

Credit: Wikipedia

Physeter macrocephalus, on the other hand, shows a sort of seasonality in the stranding event, in fact these occur more frequently in spring and autumn. This may be indicative of the fact that beaching reflects the abundance of individuals offshore and therefore the migration process itself. This species migrates every year to the Antarctic waters in the summer (of the southern hemisphere). It is also known that males migrate to Antarctica while females and calves continue to inhabit the milder and tropical areas.

Credit: Atlasobscura

As for the causes of these strandings, they are not reported and documented in detail. Certainly attacks on large cetaceans by Orcinus Orca are frequent as well as those by the tiger shark (Galeocedro cuvier). Furthermore, many cases are also due to collisions with ships.

Surely the strandings, probably, in more recent times also occur due to the exploitation of resources by man. Think of the deep sea mining process that affects the survival of these organisms, and the ability to orientate. Furthermore, we have climate change which is having tragic effects now visible even in the places that were the fulcrum of the stability of the entire biosphere: the poles. And for a species like Physeter macrocephalus that migrates to Antarctic waters, the change in temperature probably affects or will begin to affect its entire life cycle.

I started studying marine biology, naively thinking of having to save animals from some reckless people and dangers of the sea itself. Instead, the goal now is to protect them also from myself and the whole human race.

Maria Bruno

P. Borsa, C. R. Biologies 329 (2006)

Micronesica 34(1): 27–33, 2001

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