Ageing two myctophid fishes using otolith from king and macaroni penguins in the Marion Island and its implication to feeding preference of penguins on those myctophids

Myctophids apparently represent the second largest (after krill) and most widely distributed biological resources in the Antarctic waters, which are the important food sources of top predators, particularly penguins. However, the information on age of those fishes are very limited and inconsistent. Moreover, the data on trophic relationship between penguins and myctophid fishes are scarce, so more quantitative and explicit studies on those relationships are required in order to assess the role of myctophids in the Antarctic ecosystem. Using the otolith samples collected from King and Macaroni penguins in the Marion Island, the present study examined the age determination and precision of age estimation on two myctophid fishes, Electrona carlsbergi and Protomyctophum bolini. The results demonstrated that otolith section can be used to discriminate the fish species. The coefficient of variation (CV) and average percent error (APE) of ring readings on otolith section are stable for E. carlsbergi but decrease for P. bolini between two readers through repeat reading runs 1 to 3. The age structures of E. carlsbergi are 1.0 to 5.5 years and 2.0 to 5.0 years from King penguins and Macaroni penguin, respectively. The age structures of P. bolini are 0.5 to 4.0 years and 1.5 to 4.0 years from King penguins and Macaroni penguin, respectively. Compared to Macaroni penguin, King penguins fed more on small E. carlsbergi and P. bolini.