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 localized 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:
- Trace and Heavy metals: High levels of heavy metals are found in most cetaceans which have had tissue samples analyzed. 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.
- PCB: Polychrorinated biphenyls (PCBs) are a large group of synthesized 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.
- 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.
- 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.