In search of the dead zone: Use of otoliths for tracking fish exposure to hypoxia

Otolith chemistry is often useful for tracking provenance of fishes, as well as examining migration histories. Whereas elements such as strontium and barium correlate well with salinity and temperature, experiments that examine manganese uptake as a function of these parameters have found no such correlation. Instead, dissolved manganese is available as a redox product, and as such, is indicative of low-oxygen conditions. Here we present evidence for that mechanism in a range of habitats from marine to freshwater, across species, and also present ancillary proxies that support the mechanism as well. For example, iodine is redox-sensitive and varies inversely with Mn; and sulfur stable isotope ratios provide evidence of anoxic sulfate reduction in some circumstances. Further, S may be incorporated trophically whereas other elements appear to be taken up directly from water. This research suggests a potential means to identify individual fish exposure to hypoxia, over entire lifetimes. With further testing and understanding, in the future fish may be able to be used as “mobile monitors” of hypoxic conditions. •Otolith Mn is more abundant in otoliths from fish living in hypoxia than in normoxia.•This suggests that reduced Mn is the major form taken up.•Other redox-sensitive elements are documented in otoliths and are additional proxies.•δ34S supports Mn as a hypoxia proxy but suggests a dietary pathway for S uptake.•Otoliths may be useful to track fish lifetime exposure to hypoxia.