Length back-calculation and growth patterns of juvenile Baltic sprat (Sprattis sprattus L.) ; Längenrückberechnung und Wachstumsmuster juveniler Ostseesprotten (Sprattus sprattus L.)

The present thesis investigated the recruitment-relevant post-larval and juvenile life-stages in Baltic sprat by examining individual age and growth patterns recorded in otolith microstructures. Length back-calculation on the basis of daily otolith micro-increments is an important tool to investigate mechanisms influencing mortality in a yearly cohort. A new length back-calculation method, the Metamorphosis Inflection Point (MIP) method, was developed taking into account a non-linear ratio between fish length and otolith length on an individual basis. A change in the relationship between fish length and otolith length co-occurred with the otolith length when maximal increment widths were deposited on the otolith and with the mean fish length when the increase in body height was maximal during the development. This coincidence of three changing morphometric features formed the back-bone of the new back-calculation algorithm. Length growth in young sprat features two peaks, one smaller in the larval and one larger in the early juvenile stage. The MIP algorithm assigned the minimum in length growth to the maximum in otolith growth at metamorphosis. Compared to the results of the linear biological intercept method, higher length growth rates were found during the larval and the juvenile stage. In an experiment, length, dry and wet mass and otolith growth during the early juvenile stage was examined under ad libitum conditions at different constant temperatures in the laboratory. The main goals were to investigate length and otolith growth under ad libitum feeding conditions between 16°C and 22°C and to quantify the amount of food consumed under ad libitum conditions. Juveniles were found to require ten times more food than post-metamorphic sprat. Optimal otolith growth in relation to temperature was described with a quadratic function. Results of the experiment were used to reconstruct feeding conditions from increment patterns of wild caught juvenile sprat from the Western Baltic Sea in 2003 and 2007. Assuming ...