Spatial differences in the trace element concentrations of Irish Sea plaice Pleuronectes platessa and whiting Merlangius merlangus otoliths

Otoliths of plaice Pleuronectes platessa L. from five sites in the eastern Irish Sea and of whiting Merlangius merlangus (L.) from the eastern and western Irish Sea were analysed by inductively-coupled plasma mass-spectrometry (ICP-MS) to determine the concentrations of 13 trace elements B, Mg, Al, Sc, Ti, Cr, Mn, Ni, Cu, Zn, Sr, Ba, and Pb. The most abundant of the elements measured in all plaice otoliths were Sr (1112 ng g−l), Mg (16 ng g−l), B (15 ng g−l) and Al (12 ng g−l). The most abundant otolith elements measured in all the whiting otoliths at the two sites were Ni (20 ng g−l), Al (14 ng g−l ), Zn (10 ng g−l), Cr (6 ng g−l), and Mn (4 ng g−l). In both species significant differences in otolith composition were found between sampling sites. Discriminant analysis revealed separation between groups of plaice in the eastern Irish Sea indicative of metapopulation structure associated with known spawning grounds. The otolith composition differences also appeared related to the contamination history of the locations. For example, the otoliths of plaice from sewage sludge dumping grounds and the mouth of the River Mersey had significantly higher concentrations of B, Al, Sc, Ti, Mn, Ni, Sr and Ba than the otoliths of plaice from sites further north and west. The concentrations of Cu in plaice otoliths and Ba in whiting otoliths were positively correlated with the concentration of mercury in fish muscle.