Chemistry of otoliths from juvenile menhaden Brevoortia patronus: Evaluating strontium, strontium:calcium and strontium isotope ratios as environmental indicators

Laboratory studies were conducted to establish the utility of otolith chemistry for tracing gulf menhaden Brevoortia patronus environmental histories. Menhaden larvae were hatched from eggs and reared to juveniles under constant conditions at 3 temperatures (18, 22, 26°C) and 3 salinities (20, 26, 33.4). Whole otoliths from experimental fish along with their rearing waters were analyzed by ICPMS (inductively coupled plasma mass spectrometry) and TIMS (thermal ionization mass spectrometry) to determine the effects of temperature and salinity on Sr concentration, Sr/Ca and 87Sr/86Sr. A comparison of elemental composition by otolith type revealed significantly lower Sr concentrations and lower Sr/Ca ratios in the asteriscus versus the sagitta and lapillus. X-ray diffraction analyses for each otolith type determined that menhaden sagittae and lapilli are aragonite while their asterisci are vaterite. This suggests the type of calcium carbonate matrix deposited can affect the concentrations of trace elements incorporated into otoliths. In laboratory reared menhaden no significant relationships between Sr concentration or Sr/Ca ratios and temperature, salinity or menhaden growth rate were detected (ANOVA, p = 0.05). We found no indication that 87Sr/86Sr ratios of menhaden otoliths were affected by water temperature. 87Sr/86Sr did reflect the salinity of the rearing water; however, the relationship between Sr isotope ratios and salinity limits the utility of Sr isotope ratios as a precise indicator of salinity to low salinity environments (< 20) or over wide ranges of salinity. A comparison between Sr isotope ratios of menhaden otoliths and menhaden vertebrae, along with an analysis of a vertebra from a reproductive adult bay anchovy Anchoa mitchilli, showed there was no significant difference between otolith and vertebra 87Sr/86Sr ratios, indicating that other bony structures might be suitable for strontium isotope analyses in small individual fish.