Find out more about threats to Maui's dolphins not caused by people.
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Small population effects
Different types of disease can impact on the Maui’s dolphin population. There are natural diseases, diseases that are transferred from other species or land-based run-off, and stress induced disease. While the latter two may be anthropogenic in origin there is minimal evidence to confirm the origin of diseases such as Brucella and Toxoplasmosis.
An assessment of health of Hector’s dolphins during a 2004 trial tagging study at Banks Peninsula found that most results were within expected ranges or not significantly different when compared with similar species. The major result was the diagnosis of Brucella abortus (or a similar organism) in one tagged animal. Brucella is a pathogen of terrestrial mammals that can cause late pregnancy abortion, and has been seen in a range of cetacean species elsewhere. In 2006 Brucella was identified in a dead Maui’s dolphin and this could potentially have serious ramifications for this critically small population.
Toxoplasmosis is a parasitic disease that spreads through ingestion of infected meat, ingestion of material contaminated with faeces from cats, or by transmission from mother to foetus. Toxoplasmosis can cause death, behavioural changes, still births and reduced reproductive rate. The main source of infection for dolphins is likely to be through freshwater run-off from surfaces contaminated with cat faeces. It is unknown what role fish play in the potential infection pathway, but it is known that Toxoplasma oocysts can survive for months in the water.
From the DOC Hector’s and Maui’s dolphin incident database and necropsy work from Massey University, 5 of 25 Hector’s dolphins and 2 of 3 Maui’s dolphins had fatal toxoplasmosis (it was the primary cause of death). Further testing showed that of dolphins that died of other causes, 61% were also infected with Toxoplasma.
Whale lice are found on freshly dead dolphins and at close range they can be seen on living dolphins as small dark brown spots, but do not appear to cause any harm. Several species of nematodes and lungworms have also been found in Hector’s and Maui's dolphins. There is no evidence that these parasitic infections could have caused death.
Pneumonia has been noted in several Hector’s dolphins, and may have played a role in the deaths of the animals. This may be indicative of accumulation of other stressors (not normally lethal on their own) that lead to pneumonia in these dolphins.
Sharks are thought to be the main predators of Hector’s and Maui’s dolphin. Shark species known to consume these dolphins are great white, blue, and broad-nosed seven-gilled sharks. Orca, mako sharks and bronze whaler sharks may also predate Hector’s and Maui’s dolphins, but there are no known instances of this occurring.
There are two reported instances of white sharks eating Maui’s dolphins off the North Island’s west coast, including an instance in the waters off New Plymouth in 2005 where a white pointer was caught incidentally in a net and a Hector’s or Maui’s dolphin was found in its stomach. In both instances it is unknown if the dolphins were killed by the shark or scavenged after the dolphins died.
Hector’s dolphins have been found in the gut contents of seven-gilled sharks and blue sharks. A seven-gilled shark caught in the Manukau Harbour was found to have Maui’s dolphin remains within its stomach. Due to the small size of the shark it is presumed to have scavenged the remains.
Pathological reports of dead Hector’s dolphins suggest extreme weather events have been a possible reason for the separation of calves from their mothers, resulting in the death of the calf. Increases in the frequency of extreme weather events are predicted due to climate change, which has the potential to adversely affect Maui’s dolphins.
Small population effects
Given the small size of the Maui’s dolphin population it is vulnerable to small population effects such as stochastic and Allee effects. Stochastic effects refer to the inherent variability in the survival and reproductive success of individuals, which can result in fluctuating population trends for small populations. Therefore, small populations with very low growth rates are much more sensitive to random variations in survival and reproduction, and random environmental change.
Stochastic effects are different from Allee effects (or “depensation” effects) that small populations may also experience if the survival or reproduction of individuals is compromised when they are at low abundance and therefore low densities.
Small populations also suffer from reductions in genetic variability, also referred to as inbreeding depression. Loss of genetic diversity increases sensitivity to environmental change, and can also lead to increased extinction risk.
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