Sähn, Leif Niko (2008) Temperature effects on Baltic larval cod (Gadus morhua L.) derived from laboratory experiments. (Diploma thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 76 pp.

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Abstract

The present diploma thesis was part of the RESTOCK project initiated by the Danish Institut for Fisheries Research (DIFRES). The project aims to enhance the natural eastern Baltic cod (Gadus morhua L.) stock through yearly releases of first feeding yolk-sac larvae into the Bornholm Basin to support local fisheries. Baltic cod yolk-sac larvae were hatched at 7 ± 0.5°C and two independent experimental series were performed with newly hatched and progressively starved larvae, respectively. In the first experiment (L2A), Baltic cod yolk-sac larvae were transferred to temperatures of 7, 9, 11, 13 and l 5°C directly after hatching and subsequently deprived of food. Volk absorption and time to starvation were accelerated with increasing temperature. The relation between yolk absorption rate and temperature was directly proportional and described by a positive linear regression model. Time to starvation was inversely related to temperature and increased exponentially with decreasing temperature. Growth in standard length was found to be low upon yolk reserves and final standard length did not seem to be affected by temperature. Calculated yolk utilization efficiency showed differences between temperatures but no distinct trend was obvious. In the second experiment (L3A) Baltic cod larvae were held at 7 ± 0.5°C in deprivation of food. Every day, progressively starved larvae were abruptly transferred to experimental temperatures of 5, 7, 9, 11 (and l3° C) in presence of food. Effects of progressive starvation and subsequently applied abrupt temperature changes on ability and incidence of first feeding, mortality, yolk absorption and length growth were determined after 24 h of incubation. First feeding was affected by temperature and maximal feeding incidence was reduced with increasing temperature difference (Li T) between incubation and release temperature. Maximal feeding incidence was obtained at 9 ± 0.6 days post hatching. No effect of abrupt temperature changes in the studied range was determined on mortality after 24 h. Yolk absorption responded quickly to changes in temperature indicating increased metabolic activity at higher temperatures. Standard length showed no difference between temperatures after 24 h incubation on nearly all days. lt was concluded that larvae can be released to temperatures in the studied range, but food concentrations should be higher at increased temperatures. Based on feeding incidence, the optimal time of release during the RESTOCK project is between day 4 and 8 post hatching, a result supported by morphological and behavioural studies.

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