There have been two recorded outbreaks of bacterial infections within the New Zealand sea lion colonies. During these outbreaks, pups are most at risk.
The first disease outbreak occurred in 1998 and resulted in the loss of 53% of pups during their first month of life. A different disease caused a series of outbreaks in 2002 and 2003, and killed 32% and 21% of pups during their first month of life, respectively.
Disease is unlikely to be the main cause of sea lion decline, but it may hinder the ability of the population to recover and may make sea lions more susceptible to other threats. Disease is a serious risk for this species due to its highly restricted range.
Fisheries are among the most relevant threats to sea lions. The main fisheries that operate within the New Zealand sea lion’s subantarctic foraging range are the subantarctic squid, the scampi trawl fisheries, and the southern blue whiting fishery.
The subantarctic squid trawl fishery has historically captured the largest number of sea lions, but efforts have been made to reduce the frequency of incidental captures. This fishery operates in the foraging area of the Auckland and Campbell islands shelf between February and May, coinciding with the first four months of sea lion nursing.
Accidental capture in fisheries
Because females have a restricted foraging range while nursing, they are more likely than males to encounter squid trawl nets around the Auckland Islands. Research shows that female sea lions have a lower survival rate than males. Many of the females caught at this time of year are also pregnant or nursing, leading to a further decline in pup numbers.
The Ministry for Primary Industries (MPI) sets a maximum limit on sea lion deaths every season; if this limit is exceeded the fishery can be closed for the season. DOC and MPI are working with the fishing industry and conservation groups to limit this bycatch through improved fishing practices.
The squid trawl fishery has developed tools to try and reduce the number of sea lions caught in trawl nets. These tools are called SLEDs, or sea lion exclusion devices. They are designed to allow small species, like squid, to become trapped in the net, but block sea lions from entering the net and allow them to exit through an escape hatch.
However, the efficency of SLEDs has been questioned. Animals that escape a net via a SLED may become injured by the device, which could impact their survival, or they may drown. The ejection of sea lions also means that fishermen and fisheries observers are unable to count the number of sea lions that encounter the net.
- Pups falling into holes
- Males aggressions
Some of the fish species targeted by fisheries, like squid, are the same species eaten by New Zealand sea lions. Additionally, female sea lions eat about 20% of their body mass - that’s up to 80 kg of food per day - when nursing their pups. The combination of these two factors puts New Zealand sea lions particularly at risk from prey competition with fisheries.
A decline in prey species due to fishing could lead to poor nutrition in nursing pups and adult sea lions. There is also the concern that females must exert extra energy as they search for prey that becomes sparser. Extra energy expenditure during foraging may make New Zealand sea lions and their pups more susceptible to other threats like disease.
Sea lions that are onshore are often resting from physiologically taxing foraging trips. They may also be caring for their young or socialising. When people and dogs disturb sea lions, they interrupt sleep or other important behaviours.
There have also been a few cases of New Zealand sea lions that have been deliberately shot and killed by humans on Stewart Island and the Otago Peninsula.