Microincrement analysis in otoliths of Notothenia rossii fingerlings from the South Shetland Islands to estimate early life history timings and to validate annulus formation

In this study, otolith microstructure analysis on 7 blue phase fingerlings and 26 brown phase fingerlings selected from a total sample of 364 early juveniles caught mainly in summer from 2003 to 2010 at Potter Cove, South Shetland Islands, allowed to provide insights on early life history timings and to validate annulus formation. Although in the literature it has been reported that N. rossii in the South Shetland Islands hatches in spring, our daily ring back counting in otoliths from the date of capture showed two main periods of larval hatching separated by 5-6 months, one in late summer between February and March and another in winter between July and August. The maximum and minimum ages estimated for the pelagic blue phase and demersal brown phase fingerlings were respectively 227 and 240 days, indicating a timing of demersal settlement of about eight months from hatching. The age/length frequency distribution of fish sampled in spring 2010 showed the simultaneous presence of two different cohorts, belonging to biological ages 0+ and 1+ that hatched respectively in summer and winter. The growth rate of N. rossii fingerlings in Potter Cove was estimated between 0.26 and 0.31 mm/day, which is equivalent to that reported for fingerlings caught at South Georgia, and considerably higher than the growth rates known for other nototheniids within similar size range. Based on the early life events associated with the two hatching periods in N. rossii, two main types of life cycles are hypothesised for South Shetland Islands fingerlings. All the pelagic blue phase fingerlings (age group 0+) hatched in July-August (winter cohort), entering in Potter Cove in February-March. The brown phase fingerlings (age group 0+) hatched in February-March (summer cohort), entering in the cove already in spring, i.e. from September-October onwards. Finally, early juveniles (age group 1+) hatched in July-August, entering in the cove the following year to spend the second winter inshore.