The Influence of Photoperiod and Feeding Frequency on the Distinctness of Daily Otolith Increments in Larval Whitefish (Coregonus lavaretus L.)

It has been observed earlier that in larval and juvenile whitefish (Coregonus lavaretus) from Lake Constance, Germany, otolith increments are deposited daily, whereas daily deposition could not be confirmed in larval whitefish from Lake Pyh~iselka, Finland. In these studies, Lake Constance whitefish experienced 12 to 14 hours light per day, while light intensity in Finnish lakes was under 100 lux for about two hours only and complete darkness was never observed. Therefore, I tested the alternative hypotheses that (i) lack of a clear zeitgeber or (ii) continuous feeding throughout 24 hours leads to an irregular formation of otolith increments so that daily increments cannot be counted unequivocally. Lake Constance whitefish larvae were reared under continuous illumination, and they were fed either during 24 hours or during 10 hours only. Control fish were reared under local photoperiod conditions and fed during 10 hours. After four weeks, sagittae were removed and their microstructure was analysed. Control fish had clearly defined daily increments. Fish from the continuous light and 10 hours feeding groups had some subdally increments but their age could still be determined reliably. Fish from the continuous light and 24 hours feeding groups had many narrow increments and age could not be estimated. These results lend support to the hypothesis that continuous feeding throughout 24 hours leads to the formation of multiple increments per day.