The effect of parasitism by a blood-feeding isopod on the otolith chemistry of host fish

Otolith chemistry is widely used to discriminate fish stocks or populations, although many of the factors that determine trace-element concentrations within the otolith remain poorly understood. We investigated the effect of a blood-feeding isopod ectoparasite, Ceratothoa sp., on the otolith chemistry of yellowtail scad, Trachurus novaezelandiae. We sampled 65 fish from three subpopulations of T. novaezelandiae from Jervis Bay in south-eastern Australia, and used laser ablation (LA)–inductivelycoupled plasma mass spectrometry (ICPMS) to measure otolith lithium (Li) : calcium (Ca), magnesium (Mg) : Ca, strontium (Sr) : Ca and barium (Ba) : Ca from four consecutive summer and winter growth bands. Otoliths of parasitised fish were characterised by significantly lower Li : Ca and Mg : Ca, and higher Sr : Ca, than those of unparasitised individuals from the same subpopulation. The consistency of trends in otolith chemistry across ablation points and among subpopulations suggests that there is a consistent physiological mechanism through which Ceratothoa parasites affect the otolith chemistry of infected individuals. It is likely that a range of physical, metabolic, chemical and behavioural processes act in concert to influence the otolith chemistry of parasitised fish. Given the ubiquitous distribution of parasites in the marine environment, differential rates of parasitism among fish stocks, populations or migratory contingents may be an important but unappreciated factor driving stock- or population-based differences in otolith chemistry.