Mass of Tadpoles

Observation - Mass of Tadpoles - UK and Ireland. Description: In a shallow ditch, saw a sold mass 2 sq ft of black tadpoles. More tadpoles were streaming in to the mass. Is this usual behaviour? There wasn't anything obvious to attract them that I could see. Was was about 1ft deep. The tadpoles didn't seem to h

In a shallow ditch, saw a sold mass 2 sq ft of black tadpoles. More tadpoles were streaming in to the mass. Is this usual behaviour? There wasn't anything obvious to attract them that I could see. Was was about 1ft deep. The tadpoles didn't seem to have developed legs yet but were quite plump looking. Alas I couldn't get a decent picture as the water reflected. Did my best.