Catshark photogrammetry for aquarium growth monitoring

Measuring a shark without arranging a capture could be valuable for public aquarium teams. However, a sharp photograph is not automatically a reliable measurement. Research on small-spotted catsharks and underwater stereo video shows why calibration, body position and the population used for validation matter. This practical review considers how to develop a growth-monitoring method while keeping body length, sexual maturity and clinical assessment as separate questions.

Decide which measurement will support a decision

An imaging project should begin with a defined variable, rather than a camera purchase. Total length, distance between anatomical landmarks and movement through an enclosure are different measurement tasks. Each requires appropriate views and its own validation. An image that is perfectly adequate for recognising an individual may still be unsuitable for estimating its length.

The diversity of elasmobranch body forms makes this particularly important. A method developed for a catshark cannot simply be rescaled for a ray. Landmarks, curvature and visible body surfaces differ between species. A useful first document therefore identifies the species, the variable, acceptable posture and the management decision that the result is intended to inform.

We also suggest separating the welfare objective from the measurement objective. Reducing the need for capture may be a worthwhile aim, but that anticipated benefit does not establish the accuracy of the resulting values. A project needs both quality control for the measurements and observation of how animals respond to the equipment and recording session.

Read the catshark evidence within its boundaries

Rogers and colleagues investigated Scyliorhinus canicula using a camera with two parallel lasers providing a scale. They used morphometric relationships to relate a first dorsal fin measurement to total length. Their work included animals from fisheries bycatch and a live-animal trial involving male catsharks. The live component should not be mistaken for a validation across both sexes and all shark species.

The approach demonstrated potential for measuring free-swimming animals, but establishing it involved reference measurements from handled fish. Maturity assessment also depended on the sex and relationships examined. The paper does not establish a universal photographic assessment of female maturity or nutritional condition, and those claims should not be attached to the technique when presenting it to animal care teams.

For aquarium use, retain the validation population alongside any conversion equation. Record species, relevant sex information, size range and acquisition conditions. If the animals being assessed fall outside that context, reassess the conversion before generating a new set of apparently precise values. Keeping the equation without its biological scope removes information needed to judge whether it is applicable.

Test the geometry, not just image sharpness

Viewing angle and position within the image influenced measurements in the catshark study. Two visible laser points do not correct every perspective problem. A scale reference has to be interpreted within the geometry of the scene; it cannot turn a markedly oblique or curved animal into a standard lateral view.

Stereo video uses paired views to reconstruct positions in three dimensions. Dunkley and colleagues described an open-source system tested on Galápagos reefs, with both temporal synchronisation and spatial calibration. Their procedure calls for calibration at each deployment because even small changes in camera position can alter measurement errors. The reef study provides a technical foundation, not a clinical validation for every aquarium species.

We recommend defining an acquisition zone and checking known-size objects at several positions and distances within it. Keep those results with the equipment configuration. This practical exercise can reveal systematic error before an apparent change in fish size prompts a husbandry response. Repeat the check when the configuration changes rather than assuming that the previous calibration remains valid indefinitely.

Write the rejection rules before measuring the fish

Image selection needs an explicit procedure. Depending on the chosen method, reasons for rejection may include a hidden landmark, excessive body curvature or another animal crossing the outline. The exact criteria need to match the technique. Their value lies in being applied consistently, including when an image appears to support the growth pattern the team expected to see.

Record lighting conditions to help compare sessions. An ethogram can define the behaviours to record during acquisition; those observations alone are not sufficient to establish a stress response.

Individual identification is equally important for longitudinal monitoring. Retain the source image, assigned identity, date, reader and reason for rejecting a frame. A session that produces no usable image should be allowed to produce no measurement. Filling a gap with a weak estimate would conceal a real limitation of the observation process.

Comparing different readers on the same image set is a useful additional quality check. Good agreement between readers does not, by itself, exclude a shared bias, but it shows whether the landmarks can be applied consistently. Calibration and reader comparison therefore answer complementary questions. A method needs to be checked for both systematic error and consistency of use.

Interpret a growth series alongside animal care records

A small change in length needs to be considered against measurement uncertainty. Before modifying a feeding plan or concluding that growth has slowed, check whether the sessions were comparable. A different camera position, equipment adjustment or reading rule can introduce a break in the record that resembles a biological change.

Body condition requires a separate assessment. Neither of the publications discussed here validates a universal clinical condition indicator from a shark silhouette. Feeding observations, behaviour and veterinary examinations provide complementary information. Reports should clearly distinguish the measured variable from the interpretation attached to it, so that a photographic observation is not inadvertently presented as a diagnosis.

A practical reporting format can keep the estimate, quality status and relevant husbandry observations together without merging them into an unexplained score. The team can then decide whether to continue routine observation, repeat a recording under suitable conditions or request a clinical assessment. Those decisions should follow the question being asked, rather than the mere availability of a new image.

Photogrammetry is most useful when it produces traceable measurements with understood limitations. Vetofish can help public aquarium teams select landmarks, define acceptance criteria and connect imaging results with health monitoring. The aim is a workable method for the animals actually being assessed, with validation reviewed before extending it to another species, body form or clinical purpose.