The Mystery of the Missing Sea Lions

The plight of the New Zealand Sea Lion (also known as the hooker’s sea-lion, or whakahao) appears to be a never-ending fight for survival.  Their value in the 19th century to hunters for their skins and oil meant they were hunted to near extinction until sealing was banned in the middle of the 20th century.

Today, an estimated 71% of the population take refuge in the Auckland Islands. But even 400 kilometres from the nearest city, they are still threatened with extinction. Their numbers have been dwindling for the past 10 years and in the last 3 years scientists have observed a drop in the number of pups being born on the islands.

What is the reason for these declines in both numbers and fertility? Let’s take a quick look at a few sides of a controversial and diverse debate…

1 hour old New Zealand Sea Lion pup on Enderby Island (Photo: Kimberley Collins)

The Squid Fishery – Management of Sea-Lion By-catch:

The marine life of the Auckland Islands is supposedly well protected from human influence. In 1993, an area of 12 nautical miles surrounding the islands was declared a marine mammal sanctuary. In 2003, it was upgraded to a marine reserve status. Neighbouring this marine reserve, is a well frequented squid fishery, known as SQU6T. In theory, the marine reserve should protect the wildlife from being caught and killed in trawlers, but unfortunately this is not the case for the New Zealand Sea Lion.

Location of the SQU6T fishery zones surrounding the Auckland Islands (Photo: Ministry of Fisheries)

The New Zealand sea-lion love squid and will travel vast distances to get their flippers on these tasty morsels, often well outside the 12 nautical miles of the marine reserve surrounding the islands. For fishing vessels, this means that sea lions are often caught in trawling nets while foraging and for a while dead sea-lions would frequently come to the surface in a trawling net. In response, government agencies proposed a quota to control the by-catch of sea-lions, or put simply – a control on the number of sea-lion deaths caused by trawling nets.

Over the last 10 – 15 years, however, sea-lion exclusion devices (SLEDs) were developed. Their basic function is to act as an escape route for the wayward, hungry sea-lion. They have been compulsory for all vessels trawling in the area since 2007, and when comparing the number of sea-lion deaths from 2001 (6.8 per 100 trawls) and 2011 (0 deaths per 100 trawls) we can assume that they are effective.

UPDATE (12/4/12): It has been brought to my attention that the animal in the above video has been identified as a fur seal, not a NZ sea lion!

Following the apparent success of these devices, the government is proposing that there is no longer a need for by-catch management, or the quota aforementioned. A number of environmental agencies disagree with this, and want to see further research on the effect of SLEDs on sea-lion health, an increase in the size of the existing marine mammal sanctuary, and a major reduction in the by-catch quota to zero for all fisheries.

While we know that SLED devices prevent direct mortality as a result of drowning in trawling nets, their long-term effects are not well understood.

The SLEDs are basically a gate in the trawl net that prevents a sea-lion from entering, allowing them to swim out the top through a hole in the net. While sea-lions are great at accelerating, their brakes are not as well developed and when combined with darkness and the weight of water potentially making the hole open and close irregularly, it could be a recipe for disaster. For all we know, sea-lions may be seriously injured on their escape from the SLED (head injuries are the biggest worry) and so many are also calling for research on the ‘post-escape’ health of sea lions.

The Squid Fishery – Food Depletion:

Researchers working on Enderby Island have spent time bravely attaching tracking devices to female sea-lions in an attempt to understand their feeding patterns. They have discovered that females are travelling much further to access food, which can have serious effects on the health of their young.

If a female has to travel farther than usual to access food, she will use more energy and be unable to replace it with the small amount of food she is able to find. The more energy a female is using, the less milk she can produce, and so the chances of her pup’s survival is lowered.

A 1 hour old New Zealand sea lion pup on Enderby Island has its first feed. (Photo: Kimberley Collins)

Climate:

It has been suggested that The Auckland Islands are the farthest point of the New Zealand sea lion’s possible distribution and that they would do better on the mainland of New Zealand, if they were not threatened regularly by humans. This is based on comparisons made between populations on the mainland of New Zealand (specifically on the Otago Peninsula) and those on Enderby Island. Mainland populations appear to be larger and healthier, whereas the Enderby Island population appears to have stunted growth and less success in reproduction – possibly because of the difficult environment. Then again, this could be a result of the aforementioned threats from squid fisheries.

Disease:

The largest colony of New Zealand sea-lions (71% of the total population) are found on Enderby Island, a small island northeast of the main Auckland Island. Researchers have watched the breeding population in Sandy Bay for a number of years, and have witnessed a number of bacterial epidemics wipe out large proportions of the population. The most signficant of these events was in 1998 when 20% of the population and 50% of all pups born died. Such events have severe repercussions on future generations and current declines in pup numbers may be the knock-on effect of these virulent, bacterial infections.

More Information:

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5 thoughts on “The Mystery of the Missing Sea Lions

  1. Excellent post Kimi. If the squid fishing industry changed its fishing method from trawl to jig fishing, this would obviate the need for SLEDs and the potential damage they do. Also our increasingly threatened NZ Sealions would be much better protected and managed under the Marine Mammals Protection Act instead of the Fisheries Act.

    • Ahhh, I see. Might have to do a follow up post then! There’s so many aspects of this subject that it’s hard to cover them all in a blog post. I should do a sea-lion series instead of just one post.

    • Thanks! It’s hard to say for sure what’s causing these declines, so I dont want to tell people – rather let them make up their own mind and hopefully go away to do some more reading on the subject!

  2. The Sea Lion and the SLED « Sub-Antarctic Science

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