Science is a marathon: What long-term opportunistic data can tell us about New Zealand sperm whales | Conservation blog

Sperm whale strandings in New Zealand

Sperm whales (Physeter macrocephalus) are the largest species of odontocetes (toothed whales). They live in the waters around New Zealand and you can usually see them in the deep waters off Kaikoura, South Island. Sperm whale strandings have been recorded in New Zealand since 1873! In total, 596 sperm whales have stranded since then. Sperm whales can strand by themselves (single stranding events) or together, with other individuals (mass stranding events). Mass strandings are less common (24 recorded events) than single strandings (235 recorded events). Strandings can tell us a lot about a species, including what kind of social groups they hang out in and what they are eating.

The 2018 mass stranding

In May 2018, 13 male sperm whales stranded in Taranaki, North Island. One month later, another male sperm whale stranded at Clifford Bay, South Island. With the permission of iwi (local tribes of Maōri, the indigenous people of New Zealand), we took some small skin samples in order to learn more about the individuals, as well as the species as whole. We also took these events as an incentive to analyse the existing records of sperm whale strandings in New Zealand.

Genetics results

When we collect skin samples from stranded animals, we can gain information about how individuals are connected to one another, like a family tree. There is a particular type of DNA called mitochondrial DNA (mtDNA) that is passed down from mother to calf. On this mtDNA, we can find specific groups of alleles (certain DNA sequences) called “haplotypes” which we can use as indicators of any connections or differences that sperm whales have to each other. We found 7 haplotypes from the 14 male sperm whales that we assessed. One of these was a brand new haplotype that had not been recorded anywhere else previously in the world. Another one had not been previously found in New Zealand sperm whales before.

Stable isotope results

Skin samples collected from strandings also allow us to find out information about sperm whale feeding habits! While it is very informative to look at the stomach contents of dead animals, with large whales this is not always feasible, plus stomachs of stranded animals are sometimes empty. Instead, we used stable isotope analysis of skin samples, which is based on the principle that different isotopes of elements differ in their molecular weights. We use two types: stable isotopes of carbon tells us about the feeding habitat, and stable isotopes of nitrogen tell us roughly how high up the food chain a whale has been feeding.

In our case, we found that the whale that stranded in Clifford Bay had very different isotopic values compared to the other whales that stranded a few weeks earlier. This confirmed our suspicion that this single whale did not belong to the group that stranded earlier. Instead, it was likely a solitary animal which is very common for adult male sperm whales. By comparing the fluke of the male who stranded at Clifford Bay to photos of males taken at Kaikoura, the stranded male was confirmed to be a regular visitor to Kaikoura. His name was Kaha and he was well known locally by tour operators and researchers.

Demographic analyses

When we analysed all available stranding records from New Zealand, we found that sperm whales stranded all around the New Zealand coast. However, females did not often strand below 42 degree latitude, which matches the global distribution of female sperm whales where they tend to stay in warmer waters with their calves. As stranding locations relate to where sperm whales are likely hanging out, this is really important information. We know that male sperm whales spend a lot of time feeding in the Kaikōura Canyon, but previously we didn’t know much at all about female sperm whales in New Zealand. We also found that mass strandings occurred mainly on the North Island and on Chatham Island, but single strandings occurred along the entire coastline of New Zealand. Single strandings also seemed to be more common in summer, which is a general peak time for strandings of whale and dolphins in New Zealand.

What’s next for New Zealand sperm whales?

There are still more data we have not yet analysed, for example we could extract DNA from cultural artefacts such as scrimshaw or pendants made of tooth and bone, to look at how genetic connectivity has changed over time. While our results have answered some questions regarding the spatial and demographic trends of New Zealand sperm whale stranding, we are still just scratching the surface of everything there is to know about these magnificent animals!