The mysterious Hector’s dolphins

This April, let’s celebrate Hector’s dolphins (Cephalorhynchus hectori). They are the only cetaceans endemic to New Zealand, which means they are only found there. Hector’s dolphins actually include two subspecies: the endangered South Island’s Hector’s dolphins and the critically endangered Maui dolphin. Let’s find out more about Hector’s dolphins in this post.

Credit: Anjette Baker

One of the smallest cetaceans

Hector’s dolphins are among the smallest dolphins in the world. Their adult size can vary between 1.2 and 1.6 m (which corresponds to 3 ft 11 in – 5 ft 3 in). They only weigh up to 60kg (132 lb)! When babies are born, they only weigh up to 10 kg (22 lb). Their small size and the fact that they live in coastal environments is one of the reasons why they are endangered. They face both bycatch and collision risks from smaller boats. Hector’s dolphins usually spend their entire life within the first 15km (9.3 miles) from shore. They also live in shallow areas, above 100m (300 feet). Some groups reside in small areas of about 30km (18.6 miles) of coastline.

Some big balls

Male Hector’s dolphins have incredibly large testes compared to their body size. This large testes-to-body ratio suggests Hector’s dolphin males compete against other males to mate with multiple females. In this study, scientists found that their largest Hector’s dolphin weighed 41.5kg (91.5 lb), with testes totaling 1.2kg (2.6 lb). If we use the same ratio for an adult male human of around 90kg (200 lb), the average weight in the USA, his testes would weigh 2.6 kg (5.7 lb)!! That’s some big balls…

Endangered to Critically endangered

Fishing-related threats

The South Island’s subspecies are endangered, with less than 15000 individuals left, and only 55 adults remain in the Maui population! Indeed, these small dolphins face many threats. The biggest threat to their survival is by far fishing with bycatch and entanglements. As a result of both commercial and recreational fishing activities, Hector’s dolphins usually end up getting caught in gillnets and trawls. Researchers even affirmed that entanglements caused a loss of genetic diversity in this species of dolphin. In an effort to save Hector’s dolphins, the ministry of fisheries of New Zealand took action and banned the use of gillnets within 4 miles of the coast. They also created several sanctuaries around the South Island to help the populations recover.

Hector's dolphins gillnets
Entanglements in gillnets are among the biggest threats to Hector’s dolphins’ survival – Credit: Steve Dawson

Tourism and boats

Since Hector’s dolphins inhabit shallow coastal environments like harbors and bays, collision risks are high. Newborn dolphins are extremely vulnerable and can get injured by boat propellers. Some young Hector dolphins have died as a result of incautious tourists.

Genetic diversity

The loss of genetic diversity due to the declining number of Hector’s dolphins is another threat to their survival. The overall population is fragmented into smaller groups that do not exchange genes: it means that individuals do not reproduce outside of their group. Scientists believe that the reason behind this population fragmentation was over-fishing and habitat destruction. They have also noticed that these dolphins do not migrate along the coast. As a result, Hector’s dolphins’ subpopulations became isolated and rates of inbreeding went up.

Thank you for reading!

If you want to help Hector’s dolphins, you can visit this local page dedicated to save Hectors dolphins. They encourage taking action by emailing and/or meeting local politicians and decision makers to protect all dolphins within the 100m depth coastal areas. They also encourage local fishermen and tourists to leave the dolphins alone: “If you go fishing, do not use nets. Only use fish traps, hook and line, a fishing rod or other dolphin-safe fishing methods. When you see dolphins out on the water, slow down […]. Avoid sudden changes in speed or direction.”

Sources

  • Hamner, Rebecca M., et al. “Estimating the abundance and effective population size of Maui’s dolphins using microsatellite genotypes in 2010–11, with retrospective matching to 2001–07.” Department of Conservation, Auckland 44 (2012).
  • Pichler, Franz B., and C. Scott Baker. “Loss of genetic diversity in the endemic Hector’s dolphin due to fisheries-related mortality.” Proceedings of the Royal Society of London. Series B: Biological Sciences 267.1438 (2000): 97-102.
  • Slooten, Elisabeth. “Age, growth, and reproduction in Hector’s dolphins.” Canadian Journal of Zoology 69.6 (1991): 1689-1700.
  • New Zealand Government’s website

Thank you for reading! If you liked this post, consider reading more about endangered cetaceans here, and read about another endangered dolphin, the Irrawaddy dolphin:

The Irrawaddy dolphin – September 2020
 | Website

Anaïs is the founder of Whale Scientists. She is a PhD student at McGill University working on killer whale ecology and pollution. You can read more about her here.

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