Fish Species
Species Identification & Comparison
Learn how to distinguish easily confused fish such as sea trout vs. salmon and flounder vs. plaice.
Sea Trout vs. Atlantic Salmon
The classic confusion along coastal shores and river mouths. Master the 4 key biological identifiers.
Sea Trout (Salmo trutta)
- Caudal & Dorsal Fins: Broad, thick wrist with a straight or convex tail fin margin without a deep notch.
- Body Shape & Lateral Line: Black spots both above and below the lateral line, often on the gill covers.
- Mouth & Dentition: Upper jaw bone extends backward past the rear edge of the eye.
- Danish Record: Up to 15+ kg
Atlantic Salmon (Salmo salar)
- Caudal & Dorsal Fins: Slender caudal wrist ("handle") with a distinctly forked, concave tail fin.
- Body Shape & Lateral Line: Few or no spots below the lateral line, rarely on gill covers.
- Mouth & Dentition: Upper jaw reaches only level with the back edge of the eye pupil.
- Danish Record: Up to 20+ kg
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Tip: Tail wrist grip test: If you can firmly lift the fish by the tail without your hand slipping, it is almost certainly a salmon!
European Plaice vs. Flounder vs. Dab
Denmark's top 3 flatfish. Distinguish them easily by skin texture and bone tubercles.
European Plaice
- Caudal & Dorsal Fins: Completely smooth skin on the top side. 4–7 prominent bony knobs behind the eyes.
- Body Shape & Lateral Line: Bright orange-red spots on brown background.
- Mouth & Dentition: Small mouth adapted for crushing shellfish.
- Danish Record: Up to 5 kg
European Flounder
- Caudal & Dorsal Fins: Rough sandpaper-like skin with sharp prickles along the lateral line and fin bases.
- Body Shape & Lateral Line: Dull brown, olive or faintly reddish spots.
- Mouth & Dentition: Asymmetric mouth.
- Danish Record: Up to 3 kg
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Tip: Sandpaper test: Rub your thumb from tail to head. If it feels coarse like sandpaper, it is a flounder!
Atlantic Herring vs. European Sprat
Two crucial pelagic forage species along Danish coasts.
Atlantic Herring (Clupea harengus)
- Caudal & Dorsal Fins: Belly scales are smooth. Dorsal fin starts directly above the front of the pelvic fins.
- Body Shape & Lateral Line: Silvery flank with no dark row of spots.
- Mouth & Dentition: Lower jaw projects forward.
- Danish Record: Up to 40 cm
European Sprat (Sprattus sprattus)
- Caudal & Dorsal Fins: Sharp serrated keel scales along the belly. Dorsal fin originates behind pelvic fins.
- Body Shape & Lateral Line: Blue-green back with brilliant mirror-silver belly.
- Mouth & Dentition: Small mouth.
- Danish Record: Up to 16 cm
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Tip: Belly keel test: Run your finger forward along the belly keel. If it feels rough like a saw blade, it is a sprat!