Releasing sawfish: track before drawing conclusions

Releasing an animal from an aquarium is not complete when a transport container opens. A defensible outcome requires evidence that the animal survives, reaches suitable habitat and resumes behaviour compatible with life in the wild. An Australian study of largetooth sawfish (Pristis pristis) shows both that return is possible and that a few detections cannot establish success on their own.

For public aquariums, the decision starts long before transport. It involves the animal’s origin, time in captivity, health, behavioural competence, release site, permits and the ability to measure what happens next. Release should therefore be treated as a monitored conservation intervention, not as automatic compensation for an earlier wild collection.

Ten sawfish provide two trajectories to compare

Buckley and colleagues monitored ten juvenile largetooth sawfish in Australia’s Adelaide River catchment. Five had been collected from the wild, housed in a public aquarium for six to ten months and then released. Five similarly sized wild sawfish formed a reference group. Acoustic transmitters and tracking were used to compare movement rates, habitat use and apparent survival.

Largetooth sawfish are large euryhaline rays whose juveniles use river and estuarine habitats. Their toothed rostrum, body size and life history make capture, transport and monitoring unusually demanding. The species is Critically Endangered. The IUCN Shark Specialist Group reports major historical declines across all five sawfish species, driven principally by fishing mortality and habitat loss.

Aquarium-released sawfish moved more slowly, were more sedentary and occupied smaller activity spaces than wild animals. Their diel patterns and broad habitat-use patterns were nevertheless similar. Captivity had neither erased every natural behaviour nor left the released animals behaviourally identical to their wild counterparts.

Survival is not the same as ecological recovery

Two of the five aquarium-released sawfish were followed to 391 and 117 days. None of the five wild sawfish remained alive at the end of their respective monitoring records. Estimated mean survival duration was 157 days for aquarium fish and 58 days for wild fish.

Those figures do not show that captivity improves survival. The sample was very small, aquarium fish had grown before release, and loss of a telemetry signal does not always distinguish mortality from transmitter loss or movement beyond receiver coverage. The authors considered greater body size a possible reason for lower predation risk among aquarium fish. That explanation remains specific to the animals, site and conditions studied.

Telemetry nevertheless revealed what watching the release could not: several fish survived and used the river network, but their movement differed from that of wild controls. A meaningful assessment should separate at least four questions. Is the animal alive? Is it feeding? Does it occupy expected habitats? Does it recover movement patterns compatible with its life cycle?

Other species produce different outcomes

Two later studies reinforce the absence of a universal answer. A subadult tiger shark released after two years in an aquarium survived, while its biologger recorded more turning and lacked some of the vertical oscillations seen in a nearby wild shark. With one released animal and one comparison animal, the study demonstrates the value of high-resolution monitoring, not a general rule for sharks.

Four giant trevally released after eight years on display at uShaka Sea World were detected by acoustic receivers for at least three months and up to more than six years. Two adopted seasonal movements similar to wild conspecifics, including travel towards a known spawning aggregation. This encouraging result concerns a different species, environment, husbandry history and programme.

Together, the studies guard against two opposite errors: assuming that captivity causes irreversible loss of wild competence, or assuming that an apparently vigorous animal will automatically resume normal life. Time in captivity, age, size, geographic origin, feeding history, enclosure design and release conditions can all influence the outcome.

Build the decision before release day

A project needs a written rationale. Why was the animal collected or admitted? Why is release now preferable to continued care, transfer or another option? What conservation benefit is expected, and which result would count as failure? For a threatened species, the intervention belongs within the relevant authorities and recovery programme rather than a stand-alone communication event.

The team should document identity, provenance, time in captivity, health assessments, growth, diet, treatments, behaviour and incidents. Clinical stability does not prove that an animal is ready to find food, avoid predators or select habitat. Repeated behavioural observations therefore complement veterinary assessment.

Site and season should be chosen from species ecology: salinity, temperature, flow, depth, prey, predators, connectivity and capture risk. In the sawfish paper, the authors discuss release into freshwater reaches of the natal river and before a size that would constrain later migration. That recommendation belongs to the Australian system studied. It is neither a universal threshold nor a transferable protocol without local assessment.

Make monitoring part of welfare planning

Transport, acclimation and release require species-specific veterinary and logistical planning. Fitness criteria, water-quality limits, restraint, oxygenation, duration, stop rules and the response to poor recovery should be agreed in advance. Tagging is an intervention in its own right, so transmitter mass, attachment and possible effects must also be justified.

A monitoring plan should combine several time scales: immediate recovery, detection around the release site, medium-term movement, habitat use and evidence of survival. Receiver coverage, operating dates and possible causes of signal loss must be recorded so that non-detection is not automatically labelled mortality. Where appropriate, visual observations, video, environmental sampling and reporting networks can complement acoustic data.

Results should feed back into collection decisions. If released animals remain unusually sedentary, teams should examine behavioural preparation, enrichment, time in captivity and site choice before repeating the intervention. If monitoring cannot answer the stated question, the scientifically honest result is “undetermined”, not “successful”.

Responsibility continues after the animal enters the wild

The sawfish study shows that some individuals can survive for months after leaving an aquarium while retaining movement differences. The tiger shark and giant trevally studies show other possible trajectories. Collectively, they redefine the endpoint: entering the wild alive begins the evaluation rather than completing it.

Vetofish can support public aquariums with pre-release health assessment, transport and acclimation planning, welfare indicators, stop criteria, and joint interpretation of veterinary and telemetry evidence. This work should be integrated with permits, scientific partners and the conservation programme responsible for the species.