Abstract
Otoliths, the small, calcified structures in the hearing/balance system of fishes, are
increasingly used to understand the life histories of individual fishes. In particular, the
chemical properties of otoliths provide insight into both environmental and physiological
factors affecting fishes, although questions remain. Some of the chemistries have been found
to be reliable tracers of certain environmental processes, such as diadromous migration and
hypoxia. Fish and their otoliths may be regarded as “mobile monitors” of their local
environments, and when scaled up to the population level, may also inform about larger scale
phenomena. We used recent findings on otolith microchemistry to study the responses of
fishes in two different, large ecosystems - the Baltic and Lake Erie - two stressful
environments. In the Baltic, we tested whether two recently diverged species of flounder
(Platichthys flesus and P. solemdali), with different salinity-adapted spawning strategies,
used sufficiently different habitats to be detected from otolith chemistry. In addition, we used
otolith Mn:Mg ratios to study flounders' exposure to hypoxia, and used Mg:Ca (proxies for
metabolic activity) and back-calculated lengths to understand possible impacts. We
undertook a parallel study on hypoxia exposure and impact on yellow perch (Perca
flavescens) in Lake Erie. Flounder species could not be distinguished based on otolith
chemistry signatures in early life; however, there was a strong geographical signal reflecting
the Baltic salinity gradient. A complex geographical gradient was observed in hypoxia
exposure in flounders. However, despite difference in exposure, the impacts in terms of
growth and metabolic activity were not related to hypoxia exposure. For Lake Erie yellow
perch, fish in the Central Basin were most heavily exposed to hypoxia and showed effects in
terms of metabolic activity. Only exposure to hypoxia exceeding 75% of a year was
associated with reduced length at age. Despite inhabiting some of the most hypoxic aquatic
systems known, neither species exhibited dramatic impacts of hypoxia exposure. This
suggests that these species have evolved adaptations that enable them to exploit these
stressful environments.