Tooth isotopes and statistical models helping scientists understand NZ sea lion decline
In December 2013, Dr Brittany Graham received one of the best Christmas presents she could ever have wished for.
Three dusty boxes that had been sitting untouched for years in a cupboard at Massey University arrived at her Wellington office – the result of a plea to scientist Laureline Meynier for material on which to conduct some state-of-the-art scientific research.
The boxes contained several hundred New Zealand sea lion teeth, each individually bagged and labelled with the animal’s age, sex and where it was caught.
There it was: the raw material that would enable Dr Graham, a NIWA marine ecologist, to go back through time and help solve why a once healthy, prolific species is now struggling.
Teeth as historical libraries
“These teeth are such gems, to have access to them is an incredible opportunity.”
Teeth can tell scientists a lot about a species. They have bands that grow annually, a bit like rings in tree trunks, that Massey staff had previously analysed to determine the animals’ age. Because the sea lions were captured between 1987 and 2005 and were of different ages, they represent an impressive record.
Dr Graham is drilling into the teeth, taking tiny samples from the bands to analyse using highly sophisticated new pieces of technology at NIWA. Using teeth as historical libraries would reveal a whole new layer of information about what these sea lions ate, where they ate it, and possibly if there were changes in ocean conditions over their lifetime.
“This is a way of looking into the past we haven’t used before. These teeth will provide information not only on whether ocean conditions have changed, but also whether there have been changes to the structure of the marine food web.
“We can now look back through time and understand what was happening in the marine food web around the Auckland Islands and deduce if any changes have affected the New Zealand sea lions. In other words, this is a new tool that provides a glimpse at the past 20 years from a sea lion’s perspective.”
What happened to New Zealand's sea lions?
New Zealand sea lion teeth were once valued by Māori as fish hooks. Remains have been found in pre-European Māori and Moriori middens, indicating they were once used as food.
Fur seal hunters first came to New Zealand for several years from 1788 when Sydney-based merchants realised they had a potential commodity for trading. By 1797, interest had waned because the Bass Strait seal colonies were closer and more accessible.
But by 1803, the seals of Bass Strait were almost all gone, and the hunters again looked south. Fur and leather was in demand in England as was oil. The boom times for the fur seal trade lasted until about 1810 with reports of 140,000 killed in the Antipodes Islands in just three years. It is likely that sea lions, known also as hair seal and later as Hooker sea lions, were targeted once the fur seal stocks were depleted.
Records show three vessels returned from hunting on the Auckland and Campbell Islands in 1825 with 3670 sea lion skins. By the next year the trade was all but over and the population almost wiped out.
It took until the end of the 19th century for it to show signs of recovery, but while fur seals are now abundant, the picture remains alarming for sea lions.
10,000 left and dropping
There are about 10,000 sea lions left and that figure is dropping. Once prevalent around the New Zealand coastline, nearly all breeding now occurs on the Auckland Islands, and to a lesser extent Campbell Island, in the sub-Antarctic. Sea lions display strong fidelity to breeding and foraging sites and recolonisation on the mainland is likely to be slow. Despite this, small colonies have recently re-established on the Otago Peninsula, the Catlins and Stewart Island, giving cause for optimism.
The Auckland Islands colony is the largest and most intensively studied; it has also been declining rapidly for a number of years. This year, only 1575 sea lion pups were born here, around half the number born in the late 1990s. Pup counts at Campbell Island have grown rapidly over the same period, but scientists are uncertain as to why the two colonies are faring so differently.
Female sea lions produce just one pup a year and must feed it for almost a year after birth. This is only possible by regular breaks in nursing to find more food. The need to return regularly to feed their pup tethers the mother to feeding grounds near the breeding colonies. This may leave them vulnerable to factors which alter the quality and quantity of prey. At times, the female will skip breeding to ensure a continuous supply of milk to the pup they do have.
In 1997, sea lions were officially declared a threatened species. More than 15 years later, the reasons for their dire state remain unclear. A number of possible causes have been identified but there is much still to learn.
Is the fishing industry to blame? Are too many fish being caught, leaving sea lions without enough food? What about squid fishing? Have the numbers caught in the nets of the Auckland Islands squid fishery driven the decline? Is disease affecting pup mortality? How has climate variability or change affected their environment? Are great white sharks, possibly their only natural predator, driving down numbers? Is something else going on?
Depending on who you ask, the answer is all of the above but in different combinations and emphasis. Opinion varies as much among scientists as it does among environmental advocates and fishing industry representatives. Politicians have also weighed in.
Last month, Conservation Minister Dr Nick Smith announced he was developing a threat management plan for sea lions. Its purpose, he said, was to “review all the risks and explore all possible measures to ensure their survival”. Everyone, he said, had to work together.
So, what to do?
NIWA’s Dr Jim Roberts is among a small group of scientists trying to answer that.
“I’ve been thinking about sea lions pretty much constantly for the last two years,” he says, insisting that he still thinks they are “the nicest species to work with”.
Dr Roberts has been undertaking postdoctorate work at NIWA focusing on developing statistical models to identify the causes of sea lion decline at the Auckland Islands. This includes a project funded by the Department of Conservation to develop models to identify key demographic rates, for example survival and pupping rates, that may be driving population change.
The demographic models make use of observations of sea lions that have been marked in some way. Since the early 1980s, pups born at the Auckland Islands have had plastic tags attached to their flippers a few weeks after birth. Every year a field team returns to the breeding rookeries to record which ones have returned and monitor their breeding behaviour.
“We can develop demographic models that use these data to assess changes in survival through time based on the proportion that return year after year,” Dr Roberts says.
Using this 'mark-recapture data' scientists can also estimate changes in reproductive rates through time and shifts in age at first pupping, which also explain some of the variability in pups born each year. Dr Roberts says it is frequently overlooked that the pup counts were increasing through the mid to late 1990s, so they need to find out what changes occurred in the demographic rate which could help to explain the more recent decline.
“Assuming you have a closed population, then changes in population size must be driven by local births and deaths. So, if you have a decline in the number of pups being born, one explanation might be that a reduction in survival has lowered the number of breeding age animals.
“Another might be that the proportion of breeding age females producing pups each year has declined and survival of the species has remained the same. Or it could be a combination of the two.”
The model estimates of pupping rates for the Auckland Islands colony are very low for a seal species, and low fecundity is likely to be one of the main causes of population decline. Fewer pups being born is thought by scientists to be caused by changes to the amount of food available to lay down the blubber required to rear a pup. Changes in diet may also account for the variation in the condition of lactating females and the quality of milk they have produced since the late 1990s.
Commercial squid fishing began in the sea lion foraging areas in the late 1970s, with estimated captures of the mammals topping 100 in some years. But in about 2003, Sea Lion Exclusion Devices or SLEDs were introduced. Essentially they operate as a trap door in the net, allowing the sea lion to escape, and have been fitted to all squid trawl nets fishing at the Auckland Islands since 2004.
Dr Roberts says while the SLEDs have been successful in reducing the number of sea lions caught, and research has indicated that encounters with SLEDs are not likely to cause major injuries, no one really knows what proportion of those that make it through survive.
Even so, mortality rates of sea lions caused by fishing are likely to have reduced at the same time the population has been in steep decline.
In addition to the reduction in pupping rate “we are also seeing a decline in survival in the Auckland Islands which may also explain the decline in population size. It also seems there has been a reduction in survival of those up to two years old since the early 1990s, with particularly low survival of cohorts born after 2005.
“If there is variation in pup survival that’s really interesting because pups are dependent on their mothers for food. It may be telling us that females are unable to find the resources they need.”
Then there’s disease. First a bacterial epidemic was detected which, in 1998, killed 600 pups at Dundas, the largest colony in the Auckland Islands. In 2002 and 2003, many dead pups were found to be infected with Klebsiella pneumonia.
The next phase of the project will relate changes in survival and pupping rate variation in pup mass, maternal condition, milk quality, fishery captures, the incidence of disease and others to identify the ultimate causes of the decline.
“We now have multiple strands of evidence pointing towards nutritional stress – potentially driven by changes in the quantity and quality of food negatively affecting the productivity of the Auckland Islands sea lion population,” Dr Roberts says.
Sea lions eat an array of species including octopus, squid, hoki, red cod, rattails, jack mackerel, crustaceans, salps, fur seals and penguins.
“They have an extremely varied diet and it’s very different depending on where you look.” They eat what’s available, and what’s available not only varies across the foraging distribution of the species but also through time, over seasons and from year to year.
Dr Roberts says as our understanding of what’s driving sea lion population change improves, scientists are increasingly recognising the importance of both describing and understanding changes in the ocean climate of the Campbell Plateau and how these may relate to our apex predators.
“Everything is interlinked and we need to better understand how our marine ecosystems are plumbed together.”
One way of achieving that is to develop multi-species models that can be used as a fisheries management tool to assess the potential drivers of change to the marine food web.
Analysis of ancient DNA at Otago University has indicated sea lions were once more genetically diverse and bred all around New Zealand. It is likely that the reduction in the number of breeding populations will have affected their resilience to catastrophic events and habitat changes. Dr Roberts believes sea lions are possibly more vulnerable than we think with their eggs “in only a few baskets”.
But his research, and that of other scientists, appears to be shedding new light on what’s causing the population change in sea lions and other apex predators inhabiting New Zealand waters.
On the sub-Antarctic islands the animals rule. But they need the help of scientists working together – and the willingness of all players in the Southern Ocean to keep it that way.
Drilling a sea lion’s tooth is exacting work. First, the tooth is cut in half lengthwise, then the pieces are highly polished to remove any external contaminants before a 2mm-thin slice is removed and mounted on a slide.
“Then we take some very detailed, high-resolution images of the thin section so we can best locate the different growth bands to micro-drill”, says Dr Brittany Graham.
The next procedure is to prepare the drilled material for stable isotope analysis. It requires many steps using different chemicals, temperatures and extractions to ensure the sample is in the best form for analysis.
The final material is analysed using a mass spectrometer. By linking a gas chromatograph to this high tech equipment, scientists are able to obtain isotope signatures of individual compounds. This specialised equipment is part of NIWA’s newly improved and expanded ecological isotope laboratory, spearheaded by Dr Sarah Bury.
The data produced from this type of analysis takes understanding one step further.
When a sea lion leaves home, where does it go?
Scientists now have a few ideas, thanks to an electronic tagging project and some good glue.
Before you can stick a tag on a sea lion, you have to wait for it to moult. Then you glue it to the top of the back and keep your eye on a computer screen.
The first scientific effort to track sea lions at Campbell Island began in 2012 with a NIWA-led project that saw five animals electronically tagged. Last year, another seven were tagged and in July this year, the aim is to tag at least 10. All have so far been juvenile sea lions, but the aim is to tag larger animals this year.
Project leader Dr Leigh Torres, who had previous experience attaching tags to albatrosses, says that the data have, so far, shown that once a sea lion establishes a pattern of feeding, it will confine itself to that area. And once they find a pattern of feeding that works for them, they confine themselves to it and continue to fish there for consecutive years.
“The site fidelity they showed was a surprise. We were also surprised by how much feeding happened on the shelf closer to shore. We expected them to go further away,” Dr Torres said.
“They all have different feeding strategies and will target different prey. But they all show site fidelity.”
This year, the intention is also to look at how the sea lions interact with the southern blue whiting fishing season centred around Campbell Island that takes place from August to October.
The aim is to determine how foraging patterns might change with the arrival of the fleet, and how much each overlaps.
“Over time we will start to understand how foraging habits might shift.”