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Purpose of an Eel Dissection
Dissecting a New Zealand eel (tuna) allows us to understand its internal anatomy, life stage, diet, and overall health. Because eels sit high in the freshwater food web and live for many years, their bodies can also reflect long-term river conditions such as water quality, food availability, and pollution.
Ethical Considerations and Humane Sacrifice
If there is a sufficient and sustainable population of eels in the river, individuals may be ethically sacrificed for educational or monitoring purposes.
Any eel used for dissection must be humanely euthanised before the procedure. Commonly accepted, animal-welfare–approved methods include clove oil anaesthesia (overdosing to induce unconsciousness followed by death) or freezing, which is considered humane when the eel is first rendered unconscious.
These methods minimise stress and suffering and ensure the dissection is carried out respectfully and responsibly.
Ethical handling acknowledges the cultural importance of tuna and reinforces the role of dissection as a scientific and environmental learning tool rather than unnecessary harm.
Tools Commonly Used
A basic eel dissection kit usually includes:
Scalpel
For making precise incisions along the body cavity
Dissecting Scissors
To open muscle and connective tissue safely
Forceps (Tweezers)
For lifting and separating organs
Dissecting Tray/Board
To secure the eel during dissection
Gloves and Eye Protection
For hygiene and safety
Probe or Ruler
For pointing out structures and measuring organs
How the Dissection Is Carried Out
One Method:
The eel is placed belly-up on the dissecting tray. A shallow incision is made from the vent (anus) towards the head, taking care not to damage the internal organs.
The body wall is gently opened and pinned back so the organs are clearly visible. Each organ is then identified, examined, and discussed in relation to its function and what it can reveal about the eel's condition.
External Organs and Features
The external features of a New Zealand eel (tuna) provide immediate information about its health, behaviour, and the condition of the river it lives in. These observations are often made before dissection and help build a picture of overall environmental quality.
Skin and Slime Layer
Eels have thick, smooth skin covered in a mucus (slime) layer that protects them from disease, parasites, and injury. A healthy eel will feel slippery and look evenly coloured. Dry skin, wounds, or excess mucus can indicate stress, poor water quality, or exposure to pollutants and sediment.
Colouration
Body colour varies naturally, but strong, even colour usually indicates good health. Darkened backs with lighter bellies are typical. Very pale, blotchy, or dull colouring can suggest illness, low oxygen levels, or poor habitat conditions. Silvering of the belly indicates a mature eel preparing for migration.
Eyes
Clear, bright eyes suggest good health. Cloudy, damaged, or sunken eyes may indicate disease, physical injury, or prolonged exposure to poor water quality.
Fins
Eels have a continuous dorsal, ventral, and anal fin. These fins should be intact and flexible. Torn or eroded fins can result from pollution, abrasive substrates, barriers, or high sediment levels within the river.
Gills (External View)
When visible, healthy gills appear red and moist. Pale, brown, or mucus-covered gills can suggest low dissolved oxygen, sediment clogging, or chemical irritation.
Mouth and Jaws
The mouth and jaw alignment show feeding ability. Deformities, sores, or damage may indicate disease or difficulty feeding, often linked to environmental stress or reduced prey availability.
Vent (Anus)
The vent provides clues about maturity and health. Swelling, redness, or discharge may indicate infection or internal stress, while a normal vent suggests good overall condition.
Body Condition and Shape
A well-rounded body indicates good nutrition and habitat quality.
Thin or emaciated eels may reflect poor food supply, overcrowding, or degraded river conditions.
Organs Identified and What They Tell Us
Liver
The liver is usually large and dark. A healthy liver should be smooth and firm. Pale colouring, swelling, or spots can indicate disease, stress, or exposure to pollutants such as heavy metals or agricultural chemicals.
Stomach and Intestines
These organs show what the eel has been feeding on (e.g. insects, fish, kōura). Full, well-developed digestive organs suggest good food availability, while empty or damaged guts may indicate poor habitat quality or limited prey.
Gonads (Reproductive Organs)
The size and development of the gonads help determine the eel's sex and life stage. Enlarged gonads indicate a migrating "silver eel" preparing to leave freshwater to spawn, while small gonads suggest a resident feeding stage. Female NZ eel gonads (ovaries) are large, thick, and granular, often cream to yellow in colour and taking up much of the body cavity. Male gonads (testes) are thin, flat, smooth, ribbon-like, pale or translucent, and much smaller and harder to see.
Picture of Male Gonads.
Swim Bladder
This gas-filled organ helps the eel control buoyancy. Damage, parasites, or cloudiness can reduce swimming efficiency and may reflect poor water quality or parasite presence in the river.
Kidneys
Kidneys regulate salts and remove waste. Discolouration or abnormal size can suggest long-term stress from poor water chemistry, such as high conductivity or contaminants.
Heart and Gills (If Examined)
The heart and gills indicate oxygen transport and respiration. Healthy gills are bright red and clean; pale or damaged gills may point to low dissolved oxygen, sedimentation, or chemical irritation in the stream.
What Eel Dissection Tells Us About River Health
Eels (tuna) are strong indicators of river health because they are long-lived and remain in the same catchment for many years.
Their internal condition reflects long-term water quality and habitat conditions, rather than short-term changes.
Liver and Fat Reserves
The liver and fat reserves can show exposure to pollutants and overall environmental stress. A healthy, firm liver and good fat stores suggest clean water and reliable food sources, while pale or damaged tissue may indicate contamination from runoff or degraded water quality.
Stomach and Intestines
The stomach and intestines provide insight into food availability and the health of the river food web. A varied diet points to good macroinvertebrate and fish populations, whereas empty stomachs can suggest habitat loss, sedimentation, or reduced biodiversity.
Parasites and Organ Damage
Parasites and organ damage, particularly in the swim bladder or kidneys, can signal ongoing stress linked to poor oxygen levels, high temperatures, or pollution. While some parasites are normal, high numbers often reflect weakened ecosystem conditions.
Gonad Development
Gonad development helps show whether eels are healthy enough to reach maturity and migrate. Poor reproductive development can indicate barriers, low habitat quality, or long-term environmental pressure.

Overall, a healthy eel with well-developed organs suggests a river with good water quality, stable habitat, and functioning ecosystems. Poor eel health can indicate broader environmental problems within the catchment, making eel dissection a useful tool for assessing river health.