Frontiers in Marine Science (Nov 2015)
Evaluation of chemical markers for age validation of western Baltic cod (Gadus morhua) otoliths
Abstract
Age determination of Baltic cod (Gadus morhua) is subject to inaccuracies caused by inconsistencies in the periodical pattern of the growth increments in the otoliths. The need of age validation has become a major concern to ensure adequate stock assessment of this ecological and economical important species. The effect of three levels of tetracycline hydrochloride (TET) on (1) mortality and (2) the formation of clear and distinct marks in cod otoliths was determined (easily visible fluorescence marker). In addition, TET and strontium chloride (STR) at low concentration were injected together to test possible interaction effects reported from other vertebrates (STR as a long-term marker). Baltic cod were intraperitoneally injected with the different chemicals and kept for 47 days in netpens. The lowest mortality and best ring formation was observed at 100mg/kg cod wet weight compared to 50 and 25mg/kg wet weight (TET only). Preliminary analysis suggests that the simultaneous injection of TET and STR decreases the visibility of TET-rings. This is probably due to a binding interaction between both markers in the body of the fish, so that less TET is bound in the otolith. The use of tetracycline hydrochloride in the concentrations of 50 mg/kg and 100 mg/kg fish were considered the most appropriate to mark Baltic cod for age validation in large-scale mark-recapture experiments. TET and STR should not be injected together.
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