Some factors affecting the catching power of unbaited fish traps †

Perch Perca fluviatilis L. newly introduced into waters were markedly vulnerable to traps for up to four or five days. Thereafter catches tended to be positively correlated with water temperature up to about 10.5°C and negatively correlated at higher temperatures. The catching power of traps is low; at a density of one per 354 m2 water surface, traps caught from 0.9 to 6.8 % and 0.3 to 1.4% of a perch and roach Rutilus rutilus (L.) population per day. Perch did not become ‘trap‐shy’ and fin clipping did not affect their vulnerability. Applications of 0.1 to 0.5 ppm copper sulphate resulted in up to an 11‐fold increase in catches.