MSc defence: Mads Korfitz Mortensen

Direct age determination of the Greenland snow crab (Chionoecetes opilio)

2017.11.09 | Peter Schmidt Mikkelsen

Date Wed 22 Nov
Time 14:00 16:00
Location Biologiens Hus, Bygn. 1220-1223, Aarhus Universitet, Wilhelm Meyers Allé, 8000 Aarhus C

Abstract.

For academic and fisheries scientists age information of marine stocks is crucial. In fisheries management, age is ranked among one of the most influential biological measures, as it functionally forms the basis for the back-calculations of growth rate, natural mortality and recruitment productivity. For many fish species, direct ageing has been made possible and assessed through age band counts in both scales and otoliths. Recent interest in the field of ageing and longevity of crustaceans has led to development of the ageing method. In fact, it has led to the statement that crustacean age determination is now possible. In the present study, the applicability of using band counts from the Greenland snow crab (Chionoecetes opilio) as preserved increments for age is investigated. Two fjords (Amerloq and Kangerluarsuk) located in the central-western part of Greenland were examined. Growth bands were found to be present in transverse sections of the endocuticle layer of the eyestalk, but were also found to be present in the gastric mill apparatus. Here, transverse sections of the paired zygocardiac ossicles and longitudinal sections of the meso- and urocardiac ossicles were performed. Precision was assessed by having both one and three independent age readers and estimated by calculating the coefficient of variation (CV) between age estimations. For the present study, the precision between reads was relatively similar to those reported in the literature (<15 % CV) and did not indicate a clear bias between reads. Only age readings from eyestalk 1 (fjord: Kangerluarsuk) indicated a significantly high CV of 31.2 % (n=15), potentially due to sampling difficulties. The accuracy of using different structures as preserved increments for the direct age also demonstrated relatively consistent age results. Comparison of growth band counts with independent estimates of age (size-frequency distribution analyses) were attempted. However, as no correlation between size (carapace width) and direct age was found for the species, nor for the four processed structures, the results from this study cannot alone confirm the presence of annual growth bands in these structures. In general, more research is needed to determine exactly what each of the observed bands may represent, and if the age of crustaceans accurately can be assessed.

Arctic Research Centre