A new study by Dr Stefan Meyer from the University of Otago, published in the NZ Herald recently, claimed to have "game-changing" new evidence linking the decline of New Zealand sea lions to the Auckland Islands squid fishery.
As a group of scientists working closely with New Zealand sea lions at the Auckland Islands and their data, we are concerned by articles such as this one that misinform the public and policy makers about the causes of their decline, with potentially damaging consequences for their conservation.
We highlight two key areas in which the authors of this study have misrepresented the evidence and misinformed the public.
In linking the decline in pup counts to fisheries the authors used the wrong data. Using the correct data produces a totally different conclusion.
The latest study by Meyer et al. compared the numbers of pups born annually with factors that could potentially impact on sea lions. To explore a scenario where sea lion exclusion devices (also known as SLEDs) were ineffective, they selected interaction rate-the number of sea lions interacting with each trawl - and found that when this was high in one year the pup count was reduced in the next year. This approach would have been valid if the fishing effort was the same each year, except that it is highly variable, ranging from 4500 trawls in 1996 to 400 in 1999. A more meaningful proxy for sea lion deaths under this scenario would have been annual estimates of total sea lion-squid fishery interactions, which accounts for changes in fishing effort through time. Just as with fish population, you need to know how many you have caught, not how hard they were to catch.
When you relate total interactions to changes in pup counts, there is no relationship.
Using this more meaningful proxy for sea lion deaths completely nullified the main conclusion of their study. This data set was freely available to the authors, in fact they used it to generate interaction rate, but they chose to not to include it in their analysis.
We are mystified as to how the peer review process failed to pick up on this obvious "game-changing" flaw in their study. We are disturbed by how it is now being used to recklessly misinform the public about what must be done and what should be ignored to improve the conservation of an endangered endemic species.
They hastily dismiss other more probable causes of the decline.
The 2017 New Zealand sea lion Threat Management Plan (TMP) was informed by a custom-built model that assessed the population effects of mortality from key threats. Even for the most pessimistic fisheries scenario where all sea lion interactions with commercial trawls resulted in death, fisheries-related mortality was not nearly sufficient to explain the 50 per cent decline in the number of breeders. Clearly something much bigger is been impacting on this population.
The TMP assessment model also found that pup survival declined from around 80 per cent in the early 1990s to around 40 per cent in the late 1990s and has remained low ever since. Full investigations into the causes of pup deaths have been conducted since 2006. So far, a total of 438 dead pups have been assessed, of which 55 per cent were diagnosed to have died as a direct result of infection by the bacterium Klebsiella pneumoniae. The next most common cause of death was trauma or drowning in mud holes, killing 21 per cent of pups. As far as we know, the scale and duration of this bacterial endemic is unprecedented in a sea lion species. The TMP risk assessment model found that of all the threats assessed, alleviating Klebsiella-related mortality would have the greatest positive effect on the population.
The Auckland Islands population also displays many of the classic indicators of nutritional stress, including poor condition of mothers and pups, delayed age at first breeding and years of very low pupping rate. We urgently need to identify the causes of this so that we can understand the potential effects of changing ocean climate and resource competition with fisheries.
Why did Meyer et al dismiss these other threats? Will it be to the advantage of New Zealand sea lion conservation to dismiss threats other than fishing as unimportant, despite the weight of evidence to the contrary? We don't think so.
Good progress is being made to understand the root causes of the catastrophic decline at the Auckland Islands including the roles of disease, nutrition and fisheries. But we still haven't resolved the best management options for reversing the downward trend.
It is essential that simplification and misinformation are not allowed to confuse our collective efforts to understand and conserve New Zealand's own sea lion.
Dr Jim Roberts (Niwa), Dr Simon Childerhouse (Blue Planet Marine) and Prof Wendi Roe (Massey University)