Seasonality in the deep sea and tropics revisited: what can otoliths tell us?

Abstract

The accepted idea that fish otoliths from supposedly aseasonal environments, such as the deep sea and tropics, do not present seasonal growth increments is questioned and re-evaluated. The main seasonal fluctuation in deep seas is a result of the transfer of organic material from the productive upper layers to the abyssal depths. There is some evidence for seasonal patterns of otolith growth, although the validation of seasonal structures is limited owing to the difficulties inherent in deep-water studies. Tropical regions have an extremely high diversity of aquatic environments but in fact very few are aseasonal, and almost all areas have strong hydrological seasons, mainly annual, interacting with temperature variations. These climatic fluctuations have undoubtedly an effect on otolith growth, therefore leading to a succession of different incremental zones. The lack of previous knowledge on this impact of seasonality is probably a result of the previous low level of exploitation or economic interest of target species, and consequently the absence of age-based assessments. This trend has now changed possibly as a result of the increased exploitation of the resources, and to the greater involvement of international and national fishery management bodies. The number of studies showing apparent or validated seasonal marks has recently increased for these environments.