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Whales and Dolphins

Natural history

Most cetaceans lead a K-selective life history strategy, represented by long life expectancy, reaching sexual maturity late and reproducing few offspring with lengthy gaps between breeding years. All cetaceans give birth to only one calf; this is related to energetic limitations of reproducing in marine environments. The amount of nutrient needed for the calf from the mother and the amount of available nutrients available to the mother dictate low fecundity. Physiological adaptations to living in marine environments have led to cetaceans evolving compact streamlined bodies; therefore there is no room within the mother for more than one calf. Calves need to be large and fully formed to have the best chance for survival. Cetaceans nurse their offspring with a high fat content milk and calves are dependent on their mothers for about a year. Cetaceans are highly intelligent and social, with strong bonds between family members reinforced by constant interactions. Behaviour and hunting techniques are passed on through generations from adults to juveniles. There are some general differences between Mysticeti and Odontoceti life histories.

Mysticeti are all large bodied with long life expectancies; the longest lived mammal is thought to be the bowhead whale, where the oldest one recorded was at least 211 years old. Mysticeti make long migrations from cooler productive waters in which they feed, to warmer tropical or subtropical water where they calf and breed; the only exceptions being Bryde’s, bowhead, and the Indian Ocean Population X of humpback whales. Mysticeti engulf large quantities of water, containing high concentrations of plankton, small fish or crustaceans, in their mouths. Their expanding buccal cavity allows them to engulf huge quantities of water; they then close their mouths and expel the water through the baleen, trapping their prey which is then swallowed. The water and food within are filtered through racks of baleen, which are bristly plates of keratin that fray. Foraging behaviour differs between species, some species populations feed cooperatively, more notably this is associated with humpback whales which employ techniques such a bubble netting or the use of loud feeding calls to corral prey.

Odontoceti have more varied life history strategies than Mysticeti. Their body size and life expectancy ranges from less than 25 years for the small (<6.23ft/1.9m) harbour porpoise to over 70 years for the large (<60ft/18.3m) sperm whale. Their use of habitat is diverse and primarily species dependent, ranging from shallow coastal and deep pelagic water to estuarine and fresh water systems. The foraging behaviour varies with habitat. For example sperm whales and beaked whales use echolocation to hunt at extreme depths (often up to or over 6000ft/1.8km)as there is no light in these deep waters. Similarly river dolphins use echolocation to forage in often murky water, although agility and fast movements are imperative to capture their prey in these environments, which is a stark contrast to the slower sluggish movements of deep divers. There are also well documented cases of co-operative foraging within Odontoceti species, notable examples include pods of bottlenose dolphins and killer whales hunting.

Taxonomy and Range

Kingdom: Animalia, Phylum: Chordata, Class: Mammalia, Order: Cetacea, Suborders: Mysticeti; Odontoceti.

There are currently about 90 recognised cetacean species globally of which the majority are Odontoceti. Whales, dolphins and porpoises are terms used historically to describe different families of cetacean, but the taxonomic definition of whales, dolphins and porpoises is not clear. The taxon Mysticeti contains only whales and only those that possess baleen instead of teeth. Mysticetes or baleen whales all tend to be relatively large, feeding on plankton and small fish. The taxon Odontoceti contains whales, dolphins and porpoises, all of which are toothed. The main difference between a toothed whale and a dolphin is size. Large toothed whales such as the sperm whale are considerably bigger than dolphins such as the common bottlenose dolphin or killer whale. The term dolphin means beaked and dolphin have a beak-like snout, whereas toothed whales do not. Porpoises tend to be smaller than dolphins, with a less prominent and more triangular dorsal fin compared to a dolphin’s more curved or wave-shaped dorsal fin. Dolphin teeth are cone-shaped while porpoise teeth are flat and spade shaped.

Table of the main differences between Odontoceti and Mysticeti

Characteristic	Odontoceti	Mysticeti
Feeding	Echolocation, fast	Filter feeder, slower
Size	Smaller (except sperm & beaked whales)	Larger (except pygmy right whale)
Blowhole	One	Two
Dentition	Teeth	Baleen plates
Melon	Ovoid, in anterior facial region	Vestigial or none
Skull and facial tissue	Dorsally asymmetric	Symmetric
Sexual dimorphism	Some species have larger males	Females always larger
Mandible	Symphyseal (fused anteriorly)	Non-symphyseal
Pan bone of lower jaw	Yes	No
Maxillae projection	Outward over expanded supraorbital processes	Under eye orbit, with bony protuberance anterior to eye orbit
Olfactory nerve and bulb	Absent	Vestigial
Periotic bone	External to skull, fused with tympanic bulla	Fused with skull

The habitat and range of cetaceans is varied and often unique to individual species. Cetaceans can be found in most marine environments from polar regions to temperate and tropical zones. There are also four species of non-marine river dolphins. Some species such as the river dolphins are restricted in their range to specific habitats, however other species traverse vast migrations through many different marine habitats. For example, some humpback whale populations migrate from cooler productive water in temperate zones where they feed, to warmer less productive tropical zones where they calf and breed. Still other species follow oceanic currents tracking prey, such as the Atlantic spotted dolphins. Locally occurring species within the Cayman Islands are largely transient, rarely coming close to shore; these species have a large range not restricted to the Caribbean. Exceptions may include species of beaked whales whose local range may be restricted to deep foraging water such as the Cayman trench.

Status

Distribution: In the Cayman Islands, cetaceans are found offshore, usually in small pods. Distributions of regionally occurring species are likely limited to pelagic environments, possibly with tendency toward brief stops at oceanic atolls and deep water trenches. Small mother and calf groups have been observed for several Odontoceti species which appear to have regional distributions limited only to pelagic environments.

Conservation: Recent local studies in the Cayman Islands have discovered that cetacean occurrences are brief and rare. The lack of frequent occurrences inshore of cetaceans is anomalous and warrants further investigation; however a likely reason for infrequent sightings may be noise pollution from heavy shipping traffic and a paucity of prey species. Relative abundances of occurring species are being estimated but are likely to be significantly lower than other regions within the wider Caribbean. The conservational status for cetaceans occurring in the Cayman Islands can be seen in the table below, descriptions are summarised from the IUCN Red List.

Legal status: there is currently no specific legal protection for cetaceans on a national scale.

Current Factors Affecting cetacean species

Global target fisheries: In the wider Caribbean region there have been target fisheries for cetaceans historically, the extent to which they are targeted currently in this region is unknown and likely unreported. The main species targeted include humpback whales (Saint Vincent), killer whales, pygmy sperm whales, sperm whales and Bryde’s whales. Although a global whaling fishery does not exist today, some countries still hunt cetaceans; the three largest of these are Japan, Norway and Greenland.

Incidental catch: Incidental catch or by-catch is one of the largest sources of cetacean declines today. Cetaceans are often associated with fish such as tuna which are targeted by fishermen. Of these practices gill-nets accounted for around 84% of cetacean by-catch between 1990 and 1999 (Read et. al. 2006); global by-catch estimates of over 300,000 cetaceans during this period begun to decline due to fisheries management and closed areas. However continued use of bottom gill-nets will continue to result in high numbers of cetacean by-catch.

Habitat degradation: Coastally occurring species are particularly vulnerable to habitat degradation, which is caused by a number of factors. For example trawling vessels destroy benthic environments and reduce prey availability, and deforestation increases sediment transportation into marine environments smothering corals and reducing biodiversity. The resulting loss of habitat for prey species adversely effects populations of cetaceans. The effect climate change has on marine environments may also be a cause of habitat degradation, such as the warming of the coral reefs.

Prey depletion: Odontoceti occupy the highest trophic levels and exert a top-down effect normally. Where the availability of prey species for cetacean populations is severely reduced, there is a bottom-up community structuring, whereby the lack of prey species abundance limits the population size of predators (cetaceans in this instance). Populations with restricted or localised ranges will be amongst the most susceptible to prey depletion; these include many coastal species as these areas tend to be more heavily fished. Declining cetacean populations have been directly linked to declining fish stock particularly in the Mediterranean (Bearzi et. al. 2006).

Marine pollution: The term pollution can be applied to any foreign body or variable that is introduced into a habitat or ecosystem, with a detrimental effect on the populations and communities. There are four known pollutants that affect cetaceans throughout the globe:

1. Trace and Heavy metals: High levels of heavy metals are found in most cetaceans which have had tissue samples analysed. Metals such as mercury, cadmium, zinc, lead, silver, aluminium, titanium and copper accumulate in tissues and organs of cetaceans, mostly derived from consumed prey. As cetaceans cannot readily excrete heavy metals and are therefore bio-accumulators. Heavy metals are known to enter the marine system from terrestrial runoff (where they have been present in herbicides and pesticides), and marine anti-foulants, paints and oils. Cadmium is a known carcinogen and all trace metals in high concentrations are known to adversely affect the health of mammals. Cetaceans actively pass on trace metals to calves through suckling, so the effects of these pollutants may be accumulated over many generations affecting whole populations globally.

2. PCB: Polychrorinated biphenyls (PCBs) are a large group of synthesised synthetic hydrocarbons. PCBs are used mainly in industrial processes; however seepage of these substances into marine environments over the past six decades has caused them to become a major source of environmental contaminant. PCBs are extremely stable and do not breakdown biologically for excretion. They are present in nearly all algal species and are passed through the food chain to most animal species. As PCBs are lipid soluble they bio-accumulate in most animals, where they disrupt the hormone balance leading to reproductive problems and weakened immune system.

3. Marine debris: Marine debris enters the marine environment in many forms, the largest of which are plastics which are ubiquitous in all oceans. Debris can have multiple effects on cetaceans including entanglement and habitat destruction through smothering and mechanical abrasion. One of the most common afflictions is starvation through digestive tract blocked with ingested plastics. Discarded fishing gear, particularly nets are known to inflict much cetacean casualty each year. The extent marine debris affects cetaceans is immeasurable, although given the longevity of plastics and yearly accumulation is expected to be high.

4. Noise pollution: Cetaceans use sound as a primary sense, they are extremely sensitive to a broad spectrum of frequencies. Noise pollution attributed to seismic surveys and military sonar have been shown to distress cetacean populations related to mass emigration and in some cases mass strandings. Low frequency sonar has been shown to disturb cetacean feeding and breeding activities in the west of Scotland. High levels of sound produced by intense marine traffic are also expected to affect the likelihood of cetacean occurrence, particularly in shallow coastal systems.

Boating accidents: The International Whaling Commission recognised some 538 records of cetacean deaths attributed to boat strikes in recent decades up until 2010. The true numbers of mortality or injuries attributed to ship strikes are largely unreported. In many cases involving large vessels, crew are unaware that a strike has occurred often until they reach harbour to find the dead animal stuck to the bow, as such it is not hard to imagine that many animals that are killed but float free will be unnoticed. Accidents where shipping traffic passes through areas of high cetacean abundance are more likely. In some cases mitigating management has reduced the likelihood of cetacean strikes to the minimum by shifting traffic lanes. In many cases strikes may be avoidable by imposing restrictions on maximum vessel speeds. Some recent incidences of dead cetaceans washing up on the Cayman Islands may be attributed to ship strikes.

Opportunities and Current Local Action for cetaceans

Marine Conservation International (MCI) in collaboration with the Cayman Islands Department of Environment (DOE) has been conducting research since 2009 on locally occurring Cetaceans around the three Cayman Islands.

The main objectives were to determine baseline population data on resident cetacean species, whilst cataloguing all species occurring in Cayman waters. The relative abundance is very small, with ten cetacean species confirmed from scientific survey methods, public sightings and strandings. The occurrence of several strandings over the past five years led to an investigation into threats facing locally occurring cetaceans; these have been identified above. From data collected by the different methods described above, the spatial and temporal occurrences of cetaceans are mapped and analysed to determine seasonal patterns and congregator areas.

Further on-going activities include educational programs involving local schools, businesses, sport groups and natural history societies. Talks and presentations on cetaceans and their importance to the Cayman Islands are given. Incorporated in the awareness campaign is the production and distribution of educational materials including leaflets and identification charts. Several short televised interviews on cetacean conservation have been released on national television, promoting the project, educational awareness and encouraging the public to participate in reporting sightings. Although cetaceans face little negative stigma, there is a general lack of understanding as to cetacean vulnerabilities to environmental and anthropogenic threats.

A public sightings scheme set up by the DOE and MCI has been of great importance in generating occurrence data for many species. It encourages member of the public to become involved in on-going research work, helping to raise awareness.

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