Trap-neuter-release or Trap-kill-$5?

By - Wayne Linklater 30/01/2013

To my horror, but thanks to Dr. Gareth Morgan’s recent announcement that he will donate $5 to Bob Kerridge’s SPCA for every cat they euthanize rather than release, I discovered that some factions of the SPCA are releasing cats into our cities, towns and countryside – lots of cats. Bob Kerridge – CEO SPCA – is considered a hero by a few for it. They call it trap-neuter-release – TNR

But TNR is not a solution

TNR programs do not stop the cat problem because cat numbers can only decline when a cat dies. But more cats are abandoned and migrate into the colony from other places to replace those that die [1]. Indeed, the presence of TNR in a neighbourhood is likely to encourage some domestic cat owners to release and abandon their cats – knowing that they will be cared for. The problem, therefore, is not solved.

TNR is not a viable solution when stray and feral populations are large [2] like in our major cities and wilderness. Success depends on de-sexing the majority of the population and getting them before they breed. Achieving those capture rates is extraordinarily difficult. Cats try not to be caught. A proportion of cats will not be caught and continue to breed.

TNR of so many colonies and so many cats takes constant monitoring, coordination and extraordinary resources. When that investment is not achieved or periodically fails to be maintained, which is inevitable for most colonies some of the time, cat colonies recover and grow again.

The ‘cat lovers’ who care for stray and feral cat colonies are also unlikely to be motivated to push TNR colonies into decline and eventual disappearance, even if it were possible, because they would be out of a cause or out of a job. It is more likely that they will just manage colonies of stray and feral cats in perpetuity. Indeed, in programs around the world those TNR programs that do reduce colonies end up with many small managed colonies requiring on-going investment, not the removal of the stray and feral cat problem [3].

The few TNR programs that were reportedly successful were only small populations where adoption campaigns could also remove a large number. The capacity of the human population to absorb cats is limited. We already have one of the highest cat ownership rates in the world.

TNR protects disease and predators of our native wildlife

Disease and parasites are a major problem of stray cat colonies in cities. They vector diseases to other domestic pets and people, including toxoplasmosis – a disease carried by cats which infects 40% of New Zealanders at some time in their lives and cannot be cured.

De-sexed stray and feral cats continue to impact native wildlife and on a much greater scale than anyone imagined. Stray and feral cats cause the greatest harm because they hunt to survive.

TNR does not address the human health and native wildlife problems – stray and feral cats even de-sexed continue to carry and transmit disease and kill native wildlife. By managing colonies of stray and feral cats we are effectively maintaining a reservoir of infection, disease and native animal predators for the future – madness.

Bob! – take Morgan’s $5 – you will be doing New Zealand and New Zealanders a favour. If the SPCA doesn’t accept the offer they’re mad.

How mad? – see my next post.



1 Castillo, D. and Clarke, A.L. (2003) Trap/neuter/release methods ineffective in controlling domestic cat “colonies” on public lands. Natural Areas Journal 23, 247-253

2 Guttilla, D.A. and Stapp, P. (2010) Effects of sterilization on movements of feral cats at a wildland urban interface. Journal of Mammalogy 91, 482-489

3 Jongman, E. and Karlen, G. (1996) Trap, neuter and release programs for cats: A literature review on an alternative control method of feral cats in defined urban areas. In Urban Animal Management Conference Proceedings, pp. 81-84, Australian Institute of Animal Management Inc.




0 Responses to “Trap-neuter-release or Trap-kill-$5?”

  • Hi Wayne,

    Your comment that TNR isn’t a solution isn’t the full story, which is a common theme we’ve seen throughout Gareth Morgan’s campaign where the full story is more complex than Cats To Go would lead us to believe.

    TNR can be effective, though certainly it is not always the case. A couple of examples of where it was though include:

    As I said, not quite as clear cut as you make it sound.

    • Dear Gwyn,
      Thank you for your comment. The Levy et al. study you cite is instructive. Thank you for contributing it to the discussion.

      Note in the Levy et al. study that the TNR took the population from 68 to 22 cats in over 6 years. In that time almost half (47%) of the cats were adopted. Recall that I said in my article, “The few TNR programs that have been successful were only small populations where adoption campaigns could also remove a large number“. In this study it was not TNR that reduced the population but re-homing. Re-homing is realistic for a small single population but not possible across an entire city of populations with thousands of cats.

      Note also that 11% of the cats were also euthanised and 15% disappeared. The latter may have died but also emigrated to join other colonies. So in total more than half of the population was either adopted or euthanised (58%) and up to 15% emigrated to other colonies. It is instructive that despite this TNR effort and the euthanising or adoption of more than half the cats that the colony of 68 cats was only reduced not eliminated after 6 years of effort. You will recall in my post that I said “The ‘cat lovers’ who care for stray and feral cat colonies are also unlikely to be motivated to push TNR colonies into decline and eventual disappearance even if it were possible because they would be out of a cause or out of a job. It is more likely that they will just manage colonies of stray and feral cats in perpetuity. Indeed, in programs around the world those TNR programs that do reduce colonies end up with many small managed colonies requiring on-going investment, not the removal of the stray and feral cat problem“. The cat colony in the Letty et al study was not eliminated but continued to be a reservoir for disease and predators of native wildlife requiring ongoing resources to manage.

      Thus, although the Letty study did reduce the size of a university campus colony of cats it also illustrates all of the problems of TNR that my article describes. These problems prevent it from being a solution to the cat problem at the national or even city scale.

      The second study you cite is not an evaluation of the success of a TNR program. It involves just a 2 year study – not long enough. Note also that it involved 101 cats and, again, a large number were adopted (32%).

      Neither of these articles address the other problems of TNR as a solution: cat welfare, encouraging cats to be abandoned, ongoing native wildlife predation, ongoing disease risk, and ongoing investment.

      The conclusion is clear cut. TNR is not a solution to the massive problem of feral and stray cats.


  • Thanks for the response Wayne.

    As a first point, as a science writer you should know that few things in science are clear cut and this is certainly not clear cut.

    An example from a paper highly critical of TNR points out from the conclusion of a 10 year study in Rome that it was a “‘waste of time, energy, and money’ if abandonment of owned cats could not be stopped”. The study itself was showing declines of 16 to 32% across the 103 tracked cat colonies.

    Other studies have run into similar issues, especially where the cat colonies are highly visible and attract more people to dump cats.

    Certainly, TNR is not a perfect solution and if it were to work it would need a significant amount of improvement. But its failures point to wider issues around responsible issue of cats that would need to be managed regardless of whether a TNR or Catch and Kill policy was used.

    Which of course begs the question – how effective are catch and kill policies? Just like TNR I would suspect that on its own it would be ineffective, but it’s an area that needs more study. Certainly the vacuum effect that TNR advocates suggest has been shown to be non-existent and you rightly highlight the issue around there not seeming to be a goal to actually move to eventually eliminate these cat colonies.

    My point is, as it has been on this whole debate, is that the headlines people are throwing about are not entirely representative of the science they’re claiming backs them.

    When it comes to TNR the science shows that TNR may be able to reduce the population of cat colonies, but to successfully do so it requires a significant amount of resources and, obviously, the issue of responsible cat ownership to be dealt with. Which is likely the same issue that ultimately faces a catch and kill regime – until you address the route cause of the problem with a legal and regulatory framework, there is no clear cut solution to this.

    Even then I’m uncomfortable using the wording “clear cut” because as long as we own cats as pets there is going to be the risk that owners are going to be irresponsible with them either through purposeful or unintentional neglect which will still see cats dumped.

    As a final point, please don’t confuse what I’m saying as an endorsement for TNR. You quite rightly point out that there are significant animal welfare, permanent elimination, wildlife impact issues and so on with the practice of TNR. Rather I’m trying to point out that talking in absolutes isn’t always the best way to explain the actual studies behind the headlines.

    • Thank you Gwynn,
      I enjoyed reading your blog – certainly one to follow and I will follow it with interest. A long response I am afraid – perhaps this should be a blog itself. The issues you raise are important.

      Science communication is a different role from science research.

      In my scientific career at a university, uncertainties are my focus. Growth in knowledge and improvements in understanding mean that scientists’ work and writing (scientific peer-reviewed publication) focuses on what is not known in order to know what questions need to be asked and answered. We set about answering those questions only to generate more questions. For this reason scientists are often poor communicators of science. We are trained to think about what is not known, not what is known. Indeed, I have said before that a scientist’s immediate response to a question ‘do you know… ?’ is often ‘No’ – even when they know more than the questioner. Thus, we frustrate people from other walks of life by responding to a question with caveats, uncertainties and more questions, rather than ‘clear cut’ answers. In many ways resolving this problem depends on the wider public understanding how science and scientists work (or some scientists also becoming science communicators).

      In my role as a science communicator, however, I function differently. It is my place to understand how the wider readership ‘works’ and their expectations of me. The readership wants me to ‘cut through the crap’ – to exercise my professional judgement and get to the point and the important bits relevant to the issue of the day. As a science writer I understand that the uncertainties scientists talk about are not always relevant to the answer. They result from scientists considering the intense detail with mostly negligible influence on the outcome people care about and the focus of debate. I make a professional judgement when writing about which uncertainties are important to report and which are not. I avoid treating all uncertainties as equal (in the way that the news media does with different points of view – i.e., claiming ‘balance’ but giving minority views with dubious evidential support equal time). Not all evidence is equal and not all uncertainties are equally important.

      In my writing about the cat problem I have synthesised scientific knowledge for a largely non-science audience. I have made professional judgements about the relevance of different information. My professional judgement is that TNR is not a solution to the cat problem at its current national and city-wide scale. I am not dealing in ‘absolutes’ but the current state of evidence leads me to a clear cut answer. You seem not to like my being ‘clear cut’ but I do like it because I think it is evidential.

      My concern with your comment is that if I followed your advice I would be unable to serve my readership with my professional judgement and communicate it clearly. Nevertheless, your approach is exactly what I will be doing in my office in Monday morning when, behind closed doors and alone with my data, I will be trying to untangle why rhinoceros in South African reserves are not breeding as fast as expected – focussing on the uncertainties, not the answers already known.

      The other details in your most recent comment about TNR reinforce the evidence that it is not a solution. We are in agreement on the TNR issue – its not a solution.

      Thanks again for your comments. I really like this issue of science and science communication. It fascinates me. I hope you will blog about it more.