Hatchery skeletal anomalies: investigate before grading

Start with an anatomical description

A deformed juvenile found during grading does not reveal when the change began or what caused it. Boglione and colleagues reviewed skeletal anomalies in European farmed fish and highlighted substantial differences between detection methods. External inspection, radiography and examination of stained skeletal preparations provide different information. A prevalence recorded with one method cannot simply be compared with a prevalence recorded with another.

The first record should identify the species, batch, developmental stage and affected region: jaws, operculum, vertebral column or fins. A consistent lateral photograph makes the observation easier to discuss. Distinguish an abnormal structure from an acquired injury and from a photographic artefact. An oblique view can make an otherwise straight body appear curved.

A label such as “deformed fish” is too broad for a hatchery investigation. Lordosis, scoliosis and an opercular abnormality are different findings. Recording the structure involved gives the veterinarian, production manager and diagnostic laboratory a useful starting point before they consider possible causes.

Reconstruct development rather than guessing backwards

Ossification is a progressive process. A subtle disturbance during early development may become conspicuous as a fish grows, while other abnormalities develop later. It would therefore be misleading to attribute every deformity seen at grading to the original egg batch.

Reconstruct the production history: spawning origin, successive rearing units, feed changes, transfers, current settings and previous grading. Earlier photographs can help identify when a change first became detectable. Age is useful, but body size and developmental stage may provide additional information when growth differs between batches.

A practical monitoring plan can use production stages at which fish are already assessed. This is an organisational proposal to adapt locally, rather than a timetable established for every marine species. Additional examinations should answer a defined question and take account of animal welfare, handling requirements and the information already available.

If early records are missing, say so. A clear statement that onset is unknown is more useful than a precise but unsupported reconstruction. The next production cycle can then include observations targeted at the uncertainty.

Match the examination to the structure

Radiographs can reveal vertebral changes that are not obvious from external body shape. They do not answer every question. Small body size and incomplete mineralisation can limit interpretation at early stages, so the technique must suit the skeletal element, developmental stage and available equipment.

Kumar and colleagues reviewed imaging and automated image analysis in model and aquaculture fish. Their account also describes challenges involving positioning, image acquisition, limited datasets and recognition of anatomical structures. An automated tool needs evaluation with representative local images; convincing demonstration pictures do not establish reliable performance in a working hatchery.

Before collecting a series, agree on the classification rules. Define normal, abnormal and unresolved observations, with reference photographs where useful. Keeping an unresolved category allows a second reading instead of forcing a doubtful fish into the wrong group.

The report should distinguish an image finding from an explanation of that finding. Detecting an altered vertebra is a result; assigning it to nutrition or water flow requires further evidence.

Keep several causal explanations open

The literature discusses nutritional influences, physical rearing conditions, genetics and interactions between them. Temperature, lighting and hydrodynamic conditions need to be considered during the relevant developmental period. A satisfactory measurement on the day of inspection does not exclude a different exposure earlier in life.

Swim bladder development may be relevant to some investigations, depending on species and anomaly type. It does not follow that every curved fish failed to inflate its swim bladder. Likewise, a feed change close to the suspected onset is a useful lead, rather than proof of a nutritional deficiency.

Changing the feed, current and temperature together makes an improvement difficult to interpret. Where the situation allows, select a supported hypothesis and plan a comparison that can distinguish its effects. Any change needed immediately for animal welfare takes priority and should be recorded with its date and reason.

An investigation can remain useful even when it does not identify a single cause. Narrowing the time window, excluding a recording error or improving anatomical classification can make the next comparison substantially more informative.

Include sampling and previous removals

Every percentage needs the number examined and an explanation of how fish were selected. Sampling only the easiest fish to inspect can distort the picture. Previous deaths, removals and grading decisions also influence the population left for assessment.

A lower visible frequency after grading may reflect removal of affected fish rather than prevention of new abnormalities. Keep that distinction explicit. Record the examination method, anatomical criteria and size distribution alongside each result, and consider comparative readings when a method changes.

Few externally visible abnormalities do not prove that internal structures are unaffected or that swimming and feeding are normal. Conversely, a subtle abnormality does not determine an individual fish’s outcome on its own. Interpretation combines anatomical severity, function and change over time.

Use the record to guide the next decision

Late grading describes the fish remaining at a particular point. Observations across development can help locate when a problem arose and decide what to examine next. The aim is a proportionate response: clarify a finding, review a production stage or test a documented hypothesis.

Our advice and support service can help aquaculture teams connect anatomical observations with hatchery records and prepare a focused discussion with the laboratory. A useful investigation may identify several contributing factors. Its practical value lies in comparable observations and changes whose effects can subsequently be assessed.