Abnormal Markings on Flatfish Around the UK

Pigmentation issues include:

• White or pale patches (hypopigmentation)

• Darkened areas (hyperpigmentation)

• Irregular spot patterns

• Partial albinism

These can be caused by:

• Genetic variation

• Developmental issues during larval stages

• Environmental stress

• Injury and healing

In many cases, pigmentation abnormalities are not harmful and do not affect the fish’s edibility.

2. Ulcers and Skin Lesions

Ulcers are one of the more concerning abnormalities.

They appear as:

• Open sores

• Red or inflamed areas

• Eroded skin patches

• Raised lumps that break open

Common causes include:

• Bacterial infections

• Fungal infections

• Parasites

• Physical injury

• Exposure to pollutants

In the past, high rates of skin ulcers in dab in the North Sea raised environmental concerns, particularly during the 1980s and 1990s when pollution levels were higher in some industrial estuaries.

3. Tumours and Growths

Occasionally, flatfish may show:

• Wart-like growths

• Swellings

• Nodules beneath the skin

Some tumours in flatfish have been linked to long-term exposure to contaminated sediments, particularly in heavily industrialised estuaries. However, not all growths are cancerous—some are benign cysts or parasitic reactions.

4. Skeletal Deformities

Although less common, deformities can include:

• Twisted spines

• Deformed jaws

• Asymmetrical body shapes

• Missing or damaged fins

Possible causes:

• Developmental abnormalities

• Pollution exposure during early life stages

• Nutritional deficiencies

• Genetic mutations

Environmental Factors Linked to Abnormalities

Pollution and Contaminated Sediment

Flatfish spend most of their lives in close contact with seabed sediments, where pollutants accumulate. These may include:

• Heavy metals

• Hydrocarbons

• Industrial chemicals

• Agricultural runoff

Historically polluted areas such as parts of the Thames Estuary and certain North Sea zones have shown higher rates of lesions in flatfish populations.

However, environmental regulations introduced over recent decades have improved water quality significantly in many areas.

Climate Change and Warming Seas

Rising sea temperatures can:

• Stress fish immune systems

• Increase susceptibility to disease

• Alter parasite distributions

Warmer conditions may allow pathogens to spread more easily.

Parasites

Flatfish commonly host parasites. Many are harmless and part of natural ecosystems, but heavy infestations can cause visible damage or skin irritation.

Common parasite-related signs:

• Small cysts under skin

• Localised swelling

• Mild tissue damage

Parasites are usually not harmful to humans when fish are properly cooked.

Are Abnormal Flatfish Safe to Eat?

This is one of the most common concerns among anglers.

General guidance:

• Fish with mild pigmentation abnormalities are usually safe.

• Fish with severe ulcers or obvious disease should not be eaten.

• If flesh appears normal and smells fresh, it is generally safe.

• Always follow Food Standards Agency advice.

If a fish shows:

• Extensive lesions

• Deep open wounds

• Strong unpleasant odour

• Abnormal internal organs

It is best to discard it responsibly and not consume it.

Monitoring and Scientific Surveys

UK marine scientists regularly monitor fish health.

Organisations such as:

• Centre for Environment, Fisheries and Aquaculture Science

• Marine Management Organisation conduct surveys of fish stocks and record disease prevalence.

  Fish disease rates are often used as indicators of environmental quality. In many areas, the prevalence of severe ulcer disease has decreased compared to historic peaks, reflecting improved pollution controls.

  Regional Patterns Around the UK

  North Sea

  The North Sea has historically shown higher rates of dab skin lesions, particularly during periods of heavy industrial pollution. Monitoring continues.

  Estuaries

  Estuaries can show higher rates of abnormalities because they collect runoff from rivers. However, conditions vary greatly between regions.

  West Coast and Atlantic Waters

  Generally, lower pollution levels mean fewer reported abnormalities, though isolated cases still occur.

  What Anglers Should Do

  If you catch a flatfish with abnormal markings:

  1. Take clear photos.
  2. Note the location and date.
  3. Report unusual cases to local fisheries authorities.
  4. Avoid returning severely diseased fish to enclosed waters.
  5. Do not consume fish with severe lesions.

  Angler observations can provide valuable citizen science data.

  Conservation Implications

  Flatfish abnormalities are not just an angling curiosity—they are environmental signals.

  Because flatfish live on the seabed:

  • They reflect sediment quality.

  • They indicate pollution levels.

  • They help track ecosystem health.

  Monitoring their health supports broader marine conservation efforts.

  Improving water quality, reducing plastic pollution, and managing fishing pressure all contribute to healthier flatfish populations.

  Are Abnormalities Increasing?

  Current evidence suggests:

  • Severe ulcer outbreaks seen decades ago have declined in many areas.

  • Pigmentation abnormalities remain rare and usually natural.

  • Localised pollution incidents can still cause spikes in fish disease.

  Ongoing monitoring is essential to detect trends early.

  Final Thoughts

  Abnormal markings on flatfish around the UK range from harmless pigmentation quirks to more serious disease-related lesions. Most cases encountered by anglers are isolated and not cause for alarm. However, flatfish serve as important indicators of marine health because of their close association with seabed sediments.

  For sea anglers, awareness is key. By observing, reporting, and supporting marine conservation efforts, anglers contribute to protecting UK coastal waters.

  Healthy flatfish populations mean healthier seas—and better fishing for future generations.

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