Different types of disease can impact on the Māui 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 Māui 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 Māui dolphin incident database and necropsy work from Massey University, 5 of 25 Hector’s dolphins and 2 of 3 Māui dolphins examined 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 Māui 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 Māui dolphins. 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 Māui dolphin, but there are no known instances of this occurring.
There are two reported instances of white sharks eating Māui dolphin 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 Māui 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 Māui 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 Māui dolphins.
Given the small size of the Māui dolphin population, it is vulnerable to small population effects such as stochastic effects, allee effects, and low genetic variability.
Stochastic effects refer to the inherent variability in the environment and an individual’s chance of survival and reproductive success. For example, one year a dolphin may give birth to a healthy calf, whereas three years later it may give birth to a stillborn.
Additionally, a random severe weather event one year may cause the death of several dolphins, whereas for the next five years there are no severe events, allowing the population time to recover again. In large populations, events like these have little impact on the long term viability of the population. However, in a small population like that of the Māui dolphin, variability can have a large impact on the survival of the species.
Allee effects (or “depensation” effects) are experienced by small populations if the survival or reproduction of individuals is compromised when they are at low abundance. For example, it may become harder for a Māui dolphin to find a mate as the population becomes smaller and potentially more dispersed.
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.