Variation in otolith strontium and calcium ratios as an indicator of life-history strategies of freshwater fish species within a brackish water system

Abstract

A possible life-history strategy of freshwater fish in brackish waters would be one of migrations between higher salinity rearing/feeding areas and low salinity spawning areas. This would allow the more euryhaline adults and sub-adults a greater foraging range, while increasing the chance of survival of highly stenohaline eggs and larvae. We tested the feasibility of using the variation in otolith strontium/calcium ratios (Sr/Ca) as a method to investigate migrations of freshwater species living under oligo-mesohaline conditions. Zander ( Stizostedion lucioperca L.) and common bream ( Abramis brama L.) adults were collected in the brackish Kiel Canal and its largest free-flowing freshwater tributary, the Haaler Au. Furthermore, reference fish were obtained from a closed freshwater lake. Otoliths were thin sectioned and polished. Strontium and calcium concentrations were measured along transects between the otolith core and outside edge at approximately 10 μm intervals using a Cameca SX-50 wavelength dispersive electron microprobe. Strontium/calcium ratios found in the reference fish otoliths were fairly constant and low (common bream: x ̄ =0.0025, S.D.=0.0005 and zander: x ̄ =0.001, S.D.=0.0004). The Sr/Ca ratios in otoliths from brackish waters were more variable (common bream: x ̄ =0.0032, S.D.=0.0023 and zander: x ̄ =0.002, S.D.=0.001). Lowest Sr/Ca ratios were measured in core zones (<0.0003). Differences in patterns were observed, suggesting individual variations in migratory histories. Our results show that analysis of Sr/Ca has great potential for describing the migratory histories of freshwater fishes within brackish water systems.