New Zealand sea lions and the threat of misinformation
New Zealand is lucky to have its own sea lion. They were nearly driven to extinction more than 150 years ago by the first human settlers and then by commercial sealing—a story shared with nearly all seals.
But the New Zealand sea lion has been slow to recover and they still number little more than 10,000 individuals.
Recently, we have seen the alarming decline of the main breeding population at the Auckland Islands, which has halved in size over just 15 years. Advocates have focused on a simple message: that it was caused by the squid trawl fishery. NIWA scientist Dr Jim Roberts says the answer is likely to be more complex.
Dr Roberts is a member of the scientific community seeking to understand threats to New Zealand sea lions.
"Real progress been made to understand the decline at the Auckland Islands in recent years, including the twin roles of bacterial disease and food availability, in addition to better understanding the effect of captures in commercial trawls.”
These discoveries have informed the development of a Threat Management Plan for New Zealand sea lions by the Ministry for Primary Industries and the Department of Conservation.
The Threat Management Plan has attracted lots of interest, but Dr Roberts says “recent comments about New Zealand sea lions are out of touch and supported by claims for which there is weak scientific evidence.”
"A number of myths are commonly spread and they are confusing our efforts to conserve the species”.
Myth # 1 – New Zealand sea lions mainly eat squid
Actually, southern arrow squid make up less than one fifth of the diet of New Zealand sea lions. Furthermore, survival and breeding rates of sea lions at the Auckland Islands were poor during a period of high squid abundance. This is not consistent with squid fishing causing the decline in sea lion numbers through competition for food. However, there is strong evidence of nutritional stress in sea lions at the Auckland Islands and it is possible that commercial fish catches may be a factor since other key prey, including hoki and red cod, are caught. The abundance of these species will also be responding to changes in climate, making it very difficult to determine the ultimate causes of nutritional stress. This task is made even harder when dietary studies are misrepresented.
We have really good information about what sea lions eat and we need to make better use of it.
Myth # 2 - The Threat Management Plan ignores cryptic Sea Lion Exclusion Device mortality
Since 2006, Sea Lion Exclusion Devices have been standard equipment in squid trawls around the Auckland Islands. They allow sea lions to escape from nets and have led to a major reduction in observed captures, but there is concern that sea lions drown or die from injuries after passing through a Sea Lion Exclusion Device. It has been claimed that these deaths have been ignored by the Threat Management Plan process, but this is untrue. The risk assessment looked at the effects of commercial trawl mortality, including a scenario where 100% of sea lions interacting with gear died, their pups ashore died and all future pups were lost. Even this most pessimistic scenario does not explain the whole of the sea lion population decline.
The Auckland Islands population has been dealing with something much bigger than trawl mortality and we urgently need to know what this is.
Myth # 3 – Bacterial disease has not killed many pups since the 2002 and 2003 epidemics
Bacterial disease was first noticed killing pups at the Auckland Islands in 1998, with epidemics in 2002 and 2003 attributed to Klebsiella pneumoniae. This disease has since become endemic (a constant presence in the population) and is the main killer of pups during the summer field season. The duration of this endemic is unusual for a seal species and coincides with a protracted period of low pup survival.
Bacterial disease is a major threat and research is needed to understand what we can do about it.
Myth # 4 – Smaller populations don’t merit as much attention
Encouragingly, over the past 20 years we have seen the recolonisation of Stewart Island and the New Zealand mainland. We have also seen the rise of the Campbell Island population, where a third of all pups are now born. Having many breeding sites helps protect a species against catastrophic events. However, half of pups born at Campbell Island have been dying in the first few weeks and the causes—provisionally starvation and drowning in wallows—clearly deserve our attention. Because of their close proximity to humans, the Stewart Island and mainland populations face a very different set of threats.
We must afford these fledgling populations the protection they need to persist and realise their growth potential.
Dr Roberts says these myths are harmful because they distort the science and may divert resources into ineffective conservation measures. Although not specifically addressed by the Threat Management Plan, misinformation is a genuine threat to the conservation of New Zealand sea lions.
"Thankfully, we have a wealth of information and we need to make best use of it," he says.
More information can be found here: www.niwa.co.nz/nzsealions
Dr Jim Roberts
NIWA fisheries scientist
Ph 04 386 0395