Hooker's Sea Lions Face Challenges
Die-off in 1998 Followed by Disease and Fishery By-catch Losses
Jun 28, 2007
A deadly combination of events is working against the New Zealand sea lion. New Zealand or Hooker's sea lions (Phocarctos hookeri) were affected by a die-off of unknown origin in 1998. Since then disease outbreaks in the few remaining breeding colonies combined with ongoing by-catch in the squid fishery trawl nets have resulted in population declines.
The 1998 Die-Off
In 1998, a die-off hit both Enderby and Dundas Islands where the majority of the breeding population of New Zealand sea lions is found. Approximately 50% of the pups were lost along with numerous adult females. While a lack of sampling for algae means that a Harmful Algal Bloom (HAB) can’t be proven, there is satellite imagery suggestive of an HAB and there was a confirmed red tide in the Aukland area around the same time. HABs have been linked to losses in other marine mammal populations.
Poor Nutrition and Bacterial Disease Outbreaks
Since then, outbreaks of Klebsiella pnuemoniae in 2002, 2003 and 2006 have resulted in high pup mortality. Several factors may be contributing to these outbreaks. Once numerous throughout the islands of New Zealand, adult female Hooker’s sea lions are now restricted to feeding in the area around the few remaining rookeries (breeding colonies). Numbers of high caloric fishes are limited in this area, resulting in a diet composed largely of cephalopods, which are of lower nutritional density. New Zealand sea lion pups are completely dependent on mother’s milk for the first several months and those nursing from nutritionally compromised mothers will be deficient as well. And therefore more susceptible to disease. In addition, the pups remain on land, usually huddled in closely together, so bacteria is easily spread from animal to animal.
Entanglement in Squid Fishery Trawl Nets
In the first 2 weeks of the 2007 squid fishery in New Zealand, 15 Hooker’s sea lions were killed, despite efforts to reduce incidental entanglement. On average, 77 animals are killed in trawl nets that can be as large as 60 meters high by 150meters wide. Squid fishing presents a serious threat to all marine life in the area. For the sea lions it is triply deadly. Many of those killed in the nets are adult females as the fishery occurs during pupping season near the rookeries. Each female killed represents a potential loss of three animals-the female herself, the dependent pup at the rookery and, because breeding occurs shortly after the females give birth, the fetus that would be next year’s pup. Some protection is in place as the fishery can be closed when the year’s quota (93 for 2007) of accidental by-catch of sea lions is reached.
The New Zealand sea lion is listed as Vulnerable by the IUCN and as Threatened under the New Zealand Marine Mammals Protection Act of 1978. Efforts to control losses to the squid fishery are a small but vital part of preventing the loss of this marine mammal species.
