Soon after their arrival, human settlers started to hunt the New Zealand fur seals (Arctocephalus forsteri) from the islands’ shores and in its sounds. The pinnipeds were harvested for their meat by Maori, as well as by Europeans, who in addition found them quite useful for their fur (hence their name). In fact, the fur seal’s pelt, being especially dense and warm, was considered the most valuable pelt of all seals.
Unsurprisingly, with export numbers of about 930,000 skins in the late 1700s and early 1800s the roughly 2 million New Zealand fur seals roaming around the islands were brought close to extinction. However, with less than 10% of the original population size remaining the sealing industry in New Zealand, as in the rest of the world, eventually faded in importance in the 19th century.
Today, the population of fur seals has been brought back from the brink of extinction and we can see and hear hem enjoying the sun (and sometimes the rain) on rocky shores all along the New Zealand coastline. And lucrative tourism, for example around the town of Kaikoura, is evolving around the colonies.
While seals are protected under the Marine Mammals Protection Act 1978, there are also some people expressing concerns. Fishermen claim that the seals compete with them for fish stocks and cause financial damage to the industry, and suggest that some form of management should be implemented. Across the Tasman Sea in South Australia, people were even debating a culling scheme for the ‘rats of the sea’. Increasing seal numbers may not only influence the fishing catches, but may have adverse effects on other marine wildlife, such as penguins and pelicans.
To assess the reasonability of these claims and to find out more about the fur seal diet in general, Adrian Paterson and his colleagues from Lincoln University (LU) went out to seal colonies all across the country and looked for evidence.
But how to find out what these ocean dwellers eat when they’re known to dive up to more than 200 meters deep, forage up to 200 kilometers off the coast, and consume their prey out in the open ocean?
While seals may hunt and feed in the ocean, their food waste will find its way out of their bodies eventually; something that happens both in the ocean and on shore. Examining the contents of fur seal faeces is the method of choice.
Previously, the most common technique of faeces sampling was the so called ‘hard part sampling’, in which the seals’ poo was examined for bony content, like fish skeletons and heads or squid beaks.
However, with more precise technology emerging and the increased accessibility and accuracy of bio-molecular methods, the DNA of fish, squids, and mussels that the scientists found in the faecal samples could be examined for the first time in New Zealand.
Applying this method of molecular scatology has some advantages over classic hard part sampling. First, bigger fish, whose bony heads usually are left uneaten by the seals, and second, cartilaginous species, like sharks, rays, and skates, which have no bony parts at all, can be identified. Last, but not least, less common prey species are able to be recorded as well.
With the poo DNA analysis, Dr Paterson and his colleagues, particularly PhD student Arsalan Khoyi Emami, were able to compose a list of prey items containing 90 different species, revealing high variability of common prey items between different the colonies and throughout the year. These results indicate a serendipitous and quite generalist hunting behaviour for the seals. In general, this means that the seals simply eat whatever is around and that they feed on a quite broad spectrum of prey. About 16 % of the prey species were reported in previous studies, while the other 84 % are first-time records. These results underline the superiority of the DNA sequencing method and its advantages over hard parts sampling mentioned above.
Without relying solely on bony contents, the importance of cartilaginous species in the diet of NZ fur seals could be demonstrated for the first time. In other words, there are not only sharks feeding on seals in our oceans, the seals also have some shark from time to time. Interestingly, some of the shark species identified, like the pelagic tiger shark (Galeocerdo cuvier) and two species of cow sharks (Hexanchidae), reach about the same size or become much larger than adult seals when they are fully mature. On top of that, they are known to feed on seals.
Some shark nurseries are known to be in shallow waters, so young sharks of these species are quite accessible for the seals and are small enough to be consumed by them. Furthermore, Cape fur seals in South African waters have been observed feeding on sharks that were far larger than their usual prey items. In these cases, the seals were only aiming for the energy-rich livers and the fish-filled stomachs of the sharks, while the rest of the body was left behind.
In terms of mutual fish demand of seals and fishers, the scientists found an overlap of about 10 %. The fish species that were of commercial interest by humans and nutritious interest by the seals include for example (Filet-o-Fish-) Hoki, Barracouda, Jack Mackerel and species of Warehou. Thus, species wise, the fur seals compete only for a small proportion of the total prey. Yet, it remains unclear to which extend these species contribute to the seals diet, since the applied method doesn’t allow any quantitative assessment on the biomass contribution of the different fish species.
In conclusion, it seems that the claims of the fishermen have some basis, but without any quantitative assessment, it would be too soon to state the reason for economic damages in fisheries in increasing seal populations. Additionally, most of the commercially relevant species found in the scats are ranked as ‘worst choice’ fish in Forest&Bird’s “Best Fish Guide 2017“. The main reason why they are listed in this category is the overfishing of stocks and the lack of accurate monitoring of fish population sizes.
Fishermen demanding seal management should make sure that the economic damage they complain about is not self-made. With more investments in fish population surveys and catching quotes on the survey results sustainable fish harvesting could be accomplished. This at the same time would bring more clearance in the discussion of seal management since. It would ensure that the fishermen are fishing along the recovery potential of stocks and ongoing decreases in catch sizes and economic damages would be caused by other influencers.
Studies from Australia state that little penguins (Eudyptula minor) contribute significantly to seal diet and that colonies of these birds perished when seals started to breed at the same spots . Sharks are globally declining due to overfishing in our oceans and little penguins in New Zealand already experience pressure by introduced predators from land, meaning increased predation in the sea could potentially become a serious threat. This implies that with further growing population sizes of seals, we have to completely understand the role these top predators play and, if necessary, how seal management schemes for the future will look like.
The author Dominik Schwab is a postgraduate student in the Master of International Nature Conservation taught jointly at Lincoln University and University of Göttingen. He wrote this article as part of his assessment for ECOL 608 Research Methods in Ecology.