Decline of New Zealand sea lion
The current decline in the New Zealand sea lion population is likely due to a complex interaction between human impacts, like fishing, and natural causes. For example, accidental catches of sea lions in fishing gear may be particularly harmful to population numbers if they happen at the same time as a disease outbreak.
Every year a significant number of New Zealand sea lions drown due to incidental entanglement in a number of fisheries. One of the biggest human related threats to New Zealand sea lions is accidentally becoming entangled in fishing gear.
The New Zealand sea lion has declined by over 50% in the past 15 years in the Auckland Islands. A decline in the Auckland Islands is concerning, as this is where nearly 70% of all New Zealand sea lion pups are born.
The exact cause of the decline in the Auckland Islands population is unknown, but it is likely due to a combination of both natural and human related factors.
The population on Campbell Island is thought to be stable or increasing slightly and almost 30% of New Zealand sea lion pups are born here.
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. It resulted in the loss of 53% of pups during their first month of life. A different disease caused a series of outbreaks that occurred 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.
Sharks are the New Zealand sea lions' only known natural predators. This is based on presence of wounds consistent with these predators. For example, 27% of sea lions at Sandy Bay, Enderby Island, have shark attack scars. Killer whales may also prey on sea lions, but they have never been sighted around the Subantarctic islands where most New Zealand sea lions live.
Pups are the most vulnerable to shark attacks. In addition to sharks, on occasion pups have been attacked by adult males of their own species.
A large percentage of New Zealand sea lions have shark attack scars
Severe weather and natural disasters
Changes in weather patterns may reduce availability of New Zealand sea lion’s food sources. For example, the El Nino Southern Oscillation (ENSO) is a natural weather pattern that occurs every 7- 8 years. ENSO changes normal sea surface temperatures, and as a result, impacts the availability of New Zealand sea lion prey. Lower prey availability may make sea lions more susceptible to other threats during events like ENSO.
A tsunami could also impact the population by destroying habitat and drowning individuals. The impact on the population would depend heavily on the location of the tsunami, when it occurred, and its severity.
When populations reach small numbers, lack of genetic diversity can cause populations to go into even further decline. Because so many sea lions were killed during the sealing industry in the 19th century, there is concern that New Zealand sea lions have lost much of their past genetic diversity.
However, analysis of New Zealand sea lion genes shows that there is likely still enough diversity to support the population. Additionally, there is no genetic evidence that inbreeding has occurred in the New Zealand sea lion population.
Threats caused by people
Fisheries are likely the greatest threat to sea lions caused by humans. The main fisheries that operate within the New Zealand sea lion’s subantarctic foraging range are the southern blue whiting fishery and the subantarctic squid and scampi trawl fisheries. 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 Islands shelf between February and May, coinciding with the first four months of sea lion nursing. With New Zealand sea lions foraging and trawlers fishing for the same prey, incidental captures of sea lions are inevitable.
Much less is known about interactions between New Zealand sea lions on the mainland and fisheries.
New Zealand sea lions can be affected by fisheries in two ways:
- their food sources can be reduced by fisheries catching their prey, or altering the foodweb (indirect effects)
- they can become caught in fishing nets and drown (direct effects)
Competition with fisheries for prey
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.
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 a tool 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 efficacy 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.
Contaminants/pollution and marine debris
Toxic chemicals make their way into the marine environment through run off. They may harm sea lions by either causing acute toxicity, or through chronic sub-lethal exposure. Sub-lethal amounts of toxins in marine mammals have been shown to cause immune and reproductive dysfunction. However, New Zealand sea lions have low contaminant loads compared to New Zealand dolphins, and they are well below the threshold thought to cause problems in marine mammals.
If an accident such as an oil spill were to occur near a sea lion breeding site, the resulting pollution could impact the population for many years.
Marine debris also poses a threat. Debris could be ingested by sea lions or cause entanglement, leading to injury or inability to perform functions like foraging. The effects of ocean debris on the population as a whole have not been quantified.
Impacts from pollution and litter are especially likely to occur on the mainland population of sea lions, where sea lions share habitat close to human populated areas.
A rise in sea levels due to climate change may cause sea lions to lose haul out sites they currently use for breeding.
Changes in sea temperatures may also lead to changes in sea lion prey abundance and movement patterns, impacting their food availability.
Harassment by people and dogs
Sea lions that have hauled out onshore are often resting from physiologically taxing foraging trips. They may also be caring for their young or socializing. 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 on the Otago Peninsula.
New Zealand sea lions and people often share the same beaches