Theincorporation of both trace and minor-trace elements within the otolith aragonite matrix of hatchery reared sea bass Dicentrarchus labraxand the possible effects of post mortemhandling, transportation and the period of time whole fish werestored frozen were examined. Furthermore, the possible effects of temporal variability of the water chemistry within two nursery grounds and the effect on the elemental concentrations within otoliths were measured.Finally, the use ofnaturally occurring trace and minor-trace elements incorporated withinthe otolithstructures ofteleost fishand their use as natural biogeochemical tags to infer movement patternsover spatial scales were assessed. Statistically significant differences were observed in the concentrations of Mg and K measured in the otolithsof hatchery reared Dicentrarchus labraxwhen whole fish were stored frozen for a period of 6 months. Similarly, the elemental concentrations ofMn differed significantly between thestorage periods of 1 day and 12 months. Three elements Na, Sr and Ba indicated no significant change in their elemental concentrations in response to the methods of dispatch, transportation protocol and freezer storage period. Based on the concentrations of Na, Sr and Ba, indications show these threeelements are not subjectto alteration when using the most commonly used methods of euthanasia / transportation and storage duration for D.labrax. Significantinter-annual and intra-annual differences were observed in the elemental concentrations of otoliths from juvenile Pleuronectes platessasampled over a period of 7 years(2004-2010)from two nursery grounds Llanfairfechan and Llandonna in North Wales.Inter-annual (between years) variationat the site Llanfairfechan was observed for Mg ineach of the 3 years 2007-2010,similarly between the 2 years 2009-2010 and the concentrations of Na. Differences were also observed in the concentrations of Sr and between each of the 5 years 2005-2010.Inter-annual variationwas observed at the site Llanfairfechan for Na between each of the 3years 2007-2010, between the 2years 2007-2010 for K, and between the two years 2007-2009 and 2009-2010 for Sr and Ba respectively. Some degree of temporal stability could be observed for Na, Mg and Ba at Llanfairfechan and forNa, K, Sr and Ba within the site at Llandonnaover short time scales (i.e.2-3 year periods), increasing to 4 years(2004-2007) for Na and Ba at Llanfairfechan and K and Ba at Llanddona. There appeared to be sometemporal stability on an inter- iiannual scale over ashort term: i.e.2-3 years, withsome elements such as Ba being more stablefor a period up to 4 years.Significant differences in the elemental concentrations of Mg, Mn, Sr and Ba were observed in the otoliths of Salmo truttaparr sampledfrom each of36 main sea trout producing rivers in SW Scotland, NW England, Wales, Isle of Man and the east coast of Ireland which flow into the Irish and Celtic Seas.Usingquadratic discriminant function analysis (QDFA), 74% ofjuvenile trout parr were classified back to their natal rivers, with 66% of trout parr correctly classified to region. Using the elementsMg, Mn, Sr and Ba from trout parr otolithsafreshwater biogeochemical baselinewas produced to assign “blind” run parr samples to their source,with 27/39(69%) of “blind” run trout parr correctly classifiedback to their natal river. Using the biogeochemical freshwater baseline created from the QDFA adult sea trout of unknown origin were assigned to their putative natal region usingtheir period of freshwater residency. Classification was low with > 20% of adults correctly assigned to their putative region of origin. Although classification accuracy of trout parr to river/region was high and indicated the freshwater baseline was robust, the results forthe present study suggest that the poor classification of adults to their putative natal regionmay indicatemigratory patterns for adult sea trout within the Irish Sea aremore extensive than previously understood.