Every so often departmental staff are asked the question: 'Are seals to blame for reduced fish catch?' A recent study, undertaken by the Department of Conservation (DOC) has looked at the diet of New Zealand fur seals/kekeno around Banks Peninsula, so that DOC can better understand the interaction between these seals and east coast South Island fisheries. This was the first study of this kind in the Banks Peninsula area and it has provided a useful insight into what Banks Peninsula seals are eating.
Of the different types of seal that visit New Zealand's shores, the NZ fur seal (Arctocephalus forsteri) is the most commonly seen. Brought to the brink of extinction during the sealing years, these seals have slowly begun to increase in numbers. Nowhere is this more evident than where humans and seals share common shoreline space where, during the day, seals are often seen resting on rocky platforms and sandy beaches.
Fur seals mainly feed on oily fish, such as the lanternfish (Lampanyctodes hectoris), which is a small (5-7.5 cm) fish, that rises from deep water off the continental shelf at night-time. Oily fish are an important part of a seal's diet - as the fats found in them help to increase seal's body condition in readiness for mating and breeding. As the continental shelf is approximately 40 km offshore from Banks Peninsula, fur seals of this region will spend a few days or more fishing out at sea before returning to shore to rest.
Young fur seals tend to feed on fish species that are closer to the coast, as they are not as confident at long distance swimming and diving as their more accomplished parents are. Often pups and yearlings will chase small shoals of fish up creeks and rivers. This means their typical diet includes fish such as juvenile pilchards and yellow-eyed mullet. So, what impact are seals having on fish stocks?
The study examined seal faecal samples (otherwise known as scats) and seal vomit (called regurgitate), collected from Otanerito and Te Oka bays during the four seasonal periods of summer, autumn, winter and spring of 2008. Scats and regurgitates were inspected for the remains of fish ear bones or otoliths, and squid/octopus beaks. Otoliths and beaks are species specific, so this technique was used to determine which ear bones and beaks came from what kind of fish or squid.
The results have shown that the significant proportion (70-80%) of the hard parts indentified in the samples were of lanternfish and arrow squid (Nototodarus sloanii). Ahuru/pink cod was also a common prey item found in scats in winter and spring seasons (approx. 30%).
Hoki, gurnard, barracoutta, red cod (Akaroa cod) and octopus were also present but made up only a small proportion (5-10%) of their diet in all seasons.
The results of this study mirror similar results found in other regions around NZ and show that, although seals do take some fish that are of interest to recreational fishers, they predominantly feed on fish that have little or no commercial value (the exception being arrow squid).
This is good news for fishers, it's good news for seals and it's good news for tourism (seals are a significant attraction in the area). It's also good news for DOC - we always like to find situations where wildlife and humans can enhance the lives of each other.
Studies like this form the basis for future work in the area. Further study is needed on how seal diet is affected by climatic conditions; such as El Niño Southern Oscillation (ENSO) events, and how the age and sex of a seal plays a role in its dietary requirements.