Circulus formation rate in scales from sea trout (Salmo trutta L.)

Abstract

Circulus formation rate in the scales of sea trout during different times of the year has been studied in populations from several Danish marine areas. Analysis of the structure in the scales can provide information on fish age, age at seaward migration, number of spawns, and growth rate during different life stages. The circulus formation stops completely from the beginning of December to the middle of April. This period is represented on the scale by a winter check. A zone with small distance between circuli is seen centrally for this winter check. This zone is normally accounted for as a winter zone, but it is formed before December. During the rest of the year from April to December, the circulus formation rate is approximately one circulus per week. This is true for both the first and second year at sea. The statistical variance in circulus counts between individuals is constant throughout the growing season, and most of the differences between these circulus counts consequently can be related to differences in starting date after winter or after smolt run. The constant circulus counts during the growing period make it possible to relate a circulus to a certain period of this season, but the winter stop in circulus formation rate make this impossible for the winter period. The winter stop most probably is related to coldwater temperatures. The duration of the period with cold water may vary considerably from north to south within the distribution area of the trout. Consequently, the onset and stop of circulus formation may also vary considerably with latitude. In the present study, however, analysis of several different populations from different marine areas show only minor differences between annual numbers of circuli in immature trout. Due to an early stop in circulus formation related to the maturing processes, spawning changes the number of circuli formed during the spawning years considerably. This stop takes place earlier and earlier in the year as the sea age of the trout increases. The difference in circulus counts between small 0SW spawners and immature trout allows for an almost complete separation of these two groups without internal dissection of the trout. Circulus counting also allows for separating between spawning marks, winter checks, summer checks, and other checks in the scale because these phenomena can be dated. Growth rates calculated from scales are possible for short periods using circulus counts, but precaution has to be taken because of the possible lack of linear relationship between scale growth and fish growth during small periods of time.