How to Identify Biwa Trout

To identify Biwa Trout (Oncorhynchus masou rhodurus), look at these key features.

Physical Characteristics

Body shape

torpedo

Coloration

Adult Biwa Trout typically exhibit a silvery body with a darker, bluish-green dorsal surface and lighter flanks, fading to a white or cream belly. During the spawning season, males develop more intense coloration, often with reddish or purplish hues on their flanks and a more pronounced kype (hooked jaw). Juveniles (parr) have distinct parr marks (dark vertical bars) along their sides, which fade as they mature.

Scale type

cycloid

Mouth position

terminal

Distinctive features

Relatively small, dark spots on the dorsal surface and upper flanks, fewer and less distinct than some other trout species., Forked caudal fin., Adipose fin present., Males develop a pronounced kype during spawning., Absence of distinct red spots often seen in other trout species.

Dorsal fin

Single, relatively tall dorsal fin located mid-body, typically with 10-14 rays.

Tail fin

Strongly forked caudal fin, indicative of a pelagic, active swimmer.

Pectoral fin

Paired pectoral fins positioned low on the body, moderately sized.

Distinctive fins

Presence of a small, fleshy adipose fin between the dorsal and caudal fins, characteristic of salmonids.

Identification Tips

1. 1

   Geographic Location

   Exclusively found in Lake Biwa and its tributaries in Japan. This is the primary distinguishing factor from other salmonids.

2. 2

   Coloration and Spotting

   Silvery body with a dark back, few and small dark spots primarily on the dorsal side and upper flanks. Lacks the prominent red spots often seen in other trout species.

3. 3

   Body Shape

   Streamlined, torpedo-shaped body, typical of active, pelagic predators.

4. 4

   Caudal Fin

   Distinctly forked caudal fin, which differentiates it from some other trout with less-forked or truncate tails.

5. 5

   Spawning Morphology (Males)

   During spawning, males develop a pronounced kype (hooked jaw) and more vibrant, often reddish or purplish, breeding coloration.

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