Release methods
Whenever we translocate an animal, we add artificial elements to its life history. Hand-rearing an orphan or catching and relocating an adult are interventions that will have repercussions for its future life. Time in captivity, capture, transport and release are stressful occurrences that can potentially harm the animal.
Translocation means sudden, forced dispersal. Only if we understand the risks and minimise them as far as possible our actions can be efficient and of conservation significance.
Short-term stress seems unavoidable in any relocation event; however exposure to a variety of stressors for an extended period of time – an unfamiliar, unpredictable new environment and the need to cope with management interventions – would qualify as chronic stress. (Parker et al, 2015)
Chronic stress associated with the translocation process can increase vulnerability to diseases, particularly those at a translocation site to which the animal has not been exposed to before, might trigger an outbreak of a latent health problem, can compromise the animals’ immune-competency and create a general decline in health. (Hing et al, 2016, Kemp et al, 2015, Parker et al, 2015)
We can safely assume that stress is always at least a contributing factor in mortality and ‘frantic’ dispersal frequently observed following release.
Judy Clarke’s study (Clarke, 2011) investigated translocation success or failure and infectious diseases in conjunction; however the role of stress was not a focal issue and only one release strategy (immediate release) was employed.
Release strategies need to be specifically designed for the species in question as different species have varying needs and challenges and will react differently to management strategies. Scientific testing would also imply that a range of methods are compared which would be difficult as success or failure is related to intrinsic and extrinsic factors which might differ in similar but still different release locations.
Survival heavily depends on site characteristics and in particular predator exclusion (or otherwise) and the ecology/behavioural traits of the animals. (Moseby et al, 2014)
Translocations and reintroductions are time-consuming through preparation, animal transfer activities and long-term management such as monitoring, which also make them expensive.
Still, reviews claim that a large percentage fails.
Western ringtail possums for instance are a critically endangered species and numbers are too low to risk more failures. If we want to keep investing in translocation/reintroduction efforts, we need to look at the wide range of possible reasons for failure.
A main difficulty in doing this seems to be the necessity to measure our results. The relocated individuals need to be identifiable and they have to be followed closely in order to have definite proof of survival or death. A radio-collar still seems to be the only viable option - even when taking the high incident of premature battery failure into account.
However, newer research on western ringtail possums established a link between the collar and an increased risk of death (Yokochi, 2015).
Also the necessary frequent retrieval of the animal to change batteries could add a stress factor that might increase the likelihood of death – a scenario so far not investigated by research.
We might actually need to sacrifice some certainty for a ‘cleaner’, more successful result, which could also make a different set of objectives and criteria for success or otherwise necessary.
The available literature is quite ambiguous about the definition of release methods.
The terms ‘delayed release’, ‘soft release’, ‘immediate release’ and ‘hard release’ are the most often used. ‘Delayed and soft release’ and ‘immediate and hard release’ are often respectively used for the same practice, however the details can be extremely variable.
I personally prefer ‘delayed release’ and ‘immediate release’ as these terms give insight into the workings of the practice while the terms ‘soft’ versus ‘hard’ seem to implicitly judge the process. No release practice will ever be ‘soft’ on a disoriented, frightened animal faced with a situation it might not be able to cope with. Release can be the most stressful element of the reintroduction process and is therefore always potentially harmful.
Research into this topic is very limited but bandicoots seem to be a preferred species for the investigation into mammal release practices. I find this quite surprising as bandicoots are prone to suffering from myopathy, a stress induced condition that causes degeneration of skeletal and/or cardiac muscles and can be fatal (pers. comm. wildlife veterinarian Dr Howard Ralph).
Immediate release is the most common practice in research projects and the only practice used so far for western ringtail possums. However, this statement might already be disputed as some ringtails in the past were released in possum boxes or artificial dreys.
Some argue that any kind of support given on release constitutes ‘soft release’ and the provision of boxes falls into this criterion. However, the release would still be ‘immediate’ as the animal is free to use the box or leave.
Research projects involving releases of animals are also rarely done in an urban context where boxes might improve success rates. Nesting boxes in a conservation estate/bush setting have however not been useful aids for western ringtail possums. (Moore, 2007)
Immediate release is usually seen as the least costly and labour intensive method. However, even this statement is biased as failed relocations and loss of threatened species could also be regarded as a costly waste of resources.
To investigate whether a delayed release practice could improve survival, a clear definition of the term would be necessary – unfortunately it can only be given in a subjective manner as the literature is inconsistent if not outright contradictory.
Delayed release usually includes measures that provide for a more gradual transition to the wild, which necessarily include food provision at least while animals cannot forage for themselves. Some studies use the term ‘acclimatisation’ for this first phase.
In theory the temporary containment in a protective enclosure at the release site and provision of water and food might increase familiarity with the new environment and local conditions, allow recovery from the stress of handling and transport before release and decrease stress and ultimately increase survival rates of the relocated animals.
Disoriented animals in an unfamiliar environment tend to show extreme dispersal movements directly after release which is considered to play a role in mortality soon after release and can prevent establishment of a colony. (Moseby et al, 2014)
Reintroduction stress can also lead to significant weight loss (Dickens et al, 2009) and for instance one study on Eastern barred bandicoots showed that survival of those that lost less than 10% of their release weight was greater than for those who lost more. (Kemp et al, 2015)
So far studies have not shown a consistent link between delayed release and successful reintroduction programs (De Milliano et al, 2016) and according to IUCN guidelines (IUCN, 2013) delayed release is more likely to be useful for reintroductions of captive-bred or captive reared animals to reduce the potentially hazardous transition to a life in the wild.
However, when disappointing results are scrutinised further, some very obvious shortcomings of the release set-up can sometimes become evident.
Eastern barred bandicoots were released either after 7 days of acclimatisation near their future release site (not at it!) with food and water or immediately released after 3 days in a pen. The holding period for the immediate-release animals was necessary as transmitters were glued and sutured to the backs of the bandicoots only 3 days before transport to the site. This acclimatisation phase seems extremely short particularly after a stressful surgical procedure.
The animals were frequently trapped for weighing and checking of pouches and intensively radio-tracked. The transmitters caused skin ulcerations, loosened or fell off early in the process. (De Milliano et al, 2016)
The small sample size of only 12 (6/6) seems to have very limited statistical power and 22 days of research duration seems far too short for conclusive results.
The research still found some evidence that immediately released bandicoots tended to disperse about double as far as the other group in the first night and that delayed released animals remained closer to their release sites; however this was attributed to individual variations. (De Milliano et al, 2016)
Delayed released animals lost weight gradually – beginning at acclimatisation, whereas the immediately released animal lost weight at a faster rate over the first week. There was no evidence however that any pattern of weight loss is preferable and that the animals were fed an artificial diet until shortly before transfer to the site seems not to have been considered as important even though other research points out that wild diets should be mimicked even in the captive phase. (Swaisgood, 2010)
Other research has confirmed that changes in weight differ between delayed and immediately released animals; however the pattern was different for different species. (Moseby et al, 2014)
As the animals were all released into a predator-free environment 4 delayed and 5 immediately released animals were alive after 22 day. One delayed-released bandicoot was injured by a trap and died.
The conclusion of the study was that acclimatisation provides little substantive benefit and could even be seen as wasting valuable resources.
I agree that this experiment has not delivered proof of any benefits of delayed release – only the negative consequences of extreme stress on both bandicoot groups. Limitations and shortcomings of the setup seemed to have been too severe to be able to provide any real evidence for or against either of the tested release methods.
However, it has shown that if introduced predators are excluded and the habitat values are adequate, most animals will survive in the short term.
As reintroduced animals will adapt with time and establish themselves in their new environment, threatening processes may have differing effects over time.
A 22-day study would not even cover an average establishment phase let alone give any indication about long-term persistence and survival of the animals.
I would also dispute that most of the studies looking into release techniques are comparable – even though that is exactly what is being done.
It seems obvious that birds and mammals react differently, even different species of birds and different species of mammals will show different outcomes and what might benefit one species might be detrimental to another.
Holding periods in acclimatisation pens or aviaries can be as diverse as 2 to 4 days in one study and 95 to 345 days in another. I would argue that the time factor renders these 2 studies incomparable and it seems no more than a coincidence that animals in both were delayed released.
In general, species ecology and behavioural traits and the habitat values of a release site have to determine the release strategy. (Moseby et al, 2014)
