By Otago Marine Science 31/03/2020

Prof Elisabeth Slooten, Prof Steve Dawson

This has been a terrible summer for Hector’s dolphins.

The first indication was very low numbers of dolphin sightings during late spring and early summer. The Otago University Marine Mammal Research Team has carried out routine dolphin surveys at Banks Peninsula for more than 30 years. In all that time, we have never seen such low dolphin numbers during summer.

Hector’s dolphins change their distribution with the seasons. During winter, they are found almost evenly distributed throughout waters less than 100 metres deep. In summer, they become much more concentrated close inshore and are routinely found inside harbours like Akaroa Harbour, Manukau Harbour, Otanerito Lagoon and other large bays and harbours.

Seasonal changes in dolphin distribution

These seasonal movements are driven by the distribution of their food; the fish and squid that the dolphins eat. Many species that Hector’s dolphins eat, come inshore to spawn during late spring and summer. At Banks Peninsula, lower than usual water temperatures until about mid-January may have affected the dolphins’ prey, and therefore dolphin movements. However, this summer was very different – with very low numbers of dolphins close to shore even compared to normal winter numbers.

The next thing we noticed was that the number of calves was dramatically lower than normal. Over the last 30 years, about 4.4% of the dolphins that we see on our standardised surveys are calves. This summer, it was only 0.8%, half of the previous lowest number of calves we’ve seen.

So what does this mean? The reproductive rate of Hector’s dolphin, like any other animal in the wild, varies from year to year. This can be due to poor feeding conditions influencing the condition of females, and hence their likelihood of getting pregnant, or simply random variation. But it’s not just that fewer calves are born in some years – the survival of those calves also varies from year to year. Because the dolphins were distributed further offshore than normal in early summer, just when females start giving birth, pregnant and suckling females would have been diving deeper and working harder to reach the bottom – where much of their prey is found. This would have been tough on them and their calves, and may have resulted in lower calf survival.

Normally, good years to some extent make up for bad years. This is population biology 101. You can calculate an average reproductive rate for a large population of dolphins, but this ignores the fact that in a bad year the population is much more vulnerable than in a good year. Likewise, a small population of a few hundred dolphins or fewer, is much more vulnerable than a large population. Simply, it is more vulnerable to environmental variability and to human impacts, including dolphin deaths in fishing gear.

Video showing dolphins following a trawler and diving to the net

Another serious problem is that the risks to dolphin survival increase when they spend more time further out to sea. They are much more likely to get caught by gillnets (which are banned from the shoreline to 4 nautical miles offshore) and trawlers (which are restricted within 2 nautical miles). This could be a very bad year for bycatch of Hector’s dolphins.

The mother and calf in the photo below were following a trawler in waters about 60 metres deep. The mother was travelling very fast in between deep dives down to the trawl net and the calf was struggling to keep up with her.

Mum and calf Hector’s dolphin following a trawler

The very low numbers of Hector’s dolphin calves this summer have heighted concerns for this Endangered species, which is only found in New Zealand waters, and comes at a time when further protection has been delayed. A decision on better dolphin protection was originally expected in 2013, delayed until late 2019 and then delayed again. The latest government statement was that an announcement concerning the Threat Management Plan review for Māui and Hector’s dolphin is expected “in the coming months”.

The Akaroa dolphin watching company Black Cat Cruises has weighed in: “There is now even more reason for the Government to ensure increased protection for Hector’s dolphins is written into the Threat Management Plan” their Chief Executive, Paul Milligan, says.

“We would normally expect to see dolphins right into Akaroa Harbour, but this summer we’ve often had to leave the harbour and go past the Akaroa Heads to find dolphins”, says Milligan. “I remember skippers 15 years ago who saw dolphins swimming right up to the wharf, but this is the first year I’ve ever seen them this spread out”.

Hector’s dolphin calf with its mother

Our research team at Banks Peninsula includes PhD student Lindsay Wickman and MSc student Will Carome. Two research boats cover all of the peninsula’s inshore waters. The  alarming decrease in calf observations this season took us all by surprise. We kept saying to each other, where are all the calves this year? Most days we found no calves at all, which is unheard of in January.

Click on this picture to see some drone video of Hector’s dolphins

We’re working on it, but we don’t yet know exactly why the number of calves was so low. What is clear, however, is that this observation underscores the vulnerability of the population, and increases the urgency for improved protection.

Tour operators and scientists say the priority must be on achieving increased protection for Hector’s dolphins. The IUCN, the international organisation that lists endangered species, has recommended banning gillnets and trawling in all waters less than 100 metres deep. These fishing gears are known to kill dolphins.

Additional information:  

  • Hector’s dolphins and their sub-species Māui Dolphin are the only cetacean (whales and dolphins) species found only in New Zealand. They are endangered, with a declining population. Deaths in fishing nets (gillnets and trawling) have reduced Maui dolphin to less than 10% of its original population and Hector’s dolphin to less than 30%.
  • Compounding the problem is that Hector’s dolphins only breed once every 2-3 years, so any death in the population is hard to replace.
  • Black Cat Cruises is asking the Government to:
  • Extend the Banks Peninsula Marine Mammal Sanctuary (introduced in 1988)
  • Ban gillnets in the harbours
  • Extend the current gillnet and trawl exclusion area out to a depth of 100 metres.
  • Black Cat Cruises undertook the first research ever into the specific economic value of dolphin tourism to both New Zealand and Canterbury. The research revealed that Hector’s dolphin eco-tourism is worth almost $25 million to the New Zealand economy each year, with another $3-$6 million in associated tourist activity. This also sustains the equivalent of 476 jobs in the national economy.