IDENTIFICATION OF PERCH (PERCA-FLUVIATILIS), PIKEPERCH (STIZOSTEDION-LUCIOPERCA) AND RUFFE (GYMNOCEPHALUS-CERNUUS) LARVAE

A quick way to distinguish between percids at the early yolk sac stage is to count the myomeres from yolk to anus, 2 to 3 for ruffe, 4 to 6 for perch and 7 to 9 for pikeperch. Reliable identification of later l arvae involves more laborious myomere counting. Ruffe has only 13 to 1 6 preanal myomeres whereas perch and pikeperch have at least 17. The n umber of postanal myomeres is higher in pikeperch (27-31) than in perc h (23-26) and ruffe (22-24). Tn perch less than 15 to 20 mm in size, t he most useful feature is the characteristic line-shaped melanophore p attern between the myomeres. The melanophores may, however, be weak or lacking in perch from turbid waters, and in clear waters thin melanop hores may also exist in pikeperch. The position of the mouth and the l ength of jaws can be used for identifying percids after they have atta ined a total length of 12 mm The number of fin rays in the anal and se cond dorsal fin can be counted by the time the fish are 16-18 mm in si ze.