Sammendrag
A comparison of age determination from opercular bones and burned otoliths has been conducted on 460 perch (Perca fluviatilis) from five lakes (Svarttjern, Hauken, Bergevatn, Roken and Sandungen) at the Fjorda lake system in Southern Norway. The lakes are situated 389-422 m a.s.l. in well forested area, with a lake area varying from 0.06-1.98 km2, and they were acid to slightly acid before liming. Fish species inhabiting the lakes are brown trout Salmo trutta and perch Perca fluviatilis in all localities, Arctic charr Salvelinus alpinus and whitefish Coregonus lavaretus in Svarttjern and Hauken and minnow Phoxinus phoxinus in the lakes Svarttjern, Hauken and Sandungen. The ageing was conducted separately on the two compared structures, operculum and burned otoliths. Left operculum bone and the sagitta otoliths were removed and rinsed. Suitable otoliths were burned and cracked into two pieces using a pair of tweezers. Burning of the otoliths must be done before breaking because body elements (rests of the membrane which enclose the otolith/or dried up fluid from inside the membrane), may soot the breaked surface of the otolith. Ageing of the structures took place separately with some time between the readings. For the whole sample the comparison of the age determinations revealed that 13 opercular bones (2.8%) were overestimated with mainly one year. There was coincidence in the age determinations between the opercular bones and burned otoliths in 357 perch (77.0%). For the last 93 specimens (20.2%) opercular bones were underestimated compared to burned otoliths. The underestimation increased with increasing age. Greatest discrepancies were found in Lake Bergevatn, where a perch in age group 14+ was underestimated with nine years in the corresponding opercular bone. Another perch from Lake Roken in age group 16+ was also underestimated with nine years. The invisible years in the opercular bones were hidden in (semi)hyaline (translucent) band(s) on the structures. The corresponding years on the otoliths were relatively easily found by the burning technique used. For ageing perch from stunted perch populations, it is very important to be aware of the possibility to underestimate the age from the opercular bones.
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