Does Daily Growth Affect the Rate of Manganese Uptake in Juvenile River Herring Otoliths?

Manganese is a commonly found constituent in fish otoliths, yet a direct correlation with ambient water concentrations as observed for other elements has proven elusive. There is compelling evidence that Mn uptake is related to the availability of reduced Mn (Mn2+) resulting from hypoxia (Limburg et al. 2015) but that fish growth rate also plays a role. We tested the latter possibility by examining otoliths from anadromous river herring (Alewife Alosa pseudoharengus and Blueback Herring A. aestivalis) in New York waters. A significant correlation was found between the ages at which shifts in otolith daily growth increments (assumed proportional to somatic growth) and shifts in otolith Mn: Ca ratios were detected. Furthermore, significant positive Spearman's rank correlations were observed between mean otolith daily growth and otolith Mn: Ca. Species-and site-specific differences were evident, but significant positive correlations were found for these relationships within each species and at most sampling sites. Ambient water concentrations were weakly correlated with otolith Mn: Ca, suggesting that both exogenous and endogenous factors influence Mn: Ca in otoliths. Thus, while endogenous mechanisms (i.e., growth rate) likely influence Mn uptake, exogenous availability is the ultimate control.