In this study, the identification of major ontogenetic events and the duration of the fresh water and estuarine phases before the seaward exit of young allis shad Alosa alosa were investigated retrospectively, using the otoliths of spawners. Daily growth increment periodicity in the otoliths was validated with an accuracy of 4 days (9Á4%) for fish >30 days. The specific limits of the Sr:Ca ratios in the otoliths, indicating fish movements between salinity 'compartments', were up to 1Á1 Â10 À3 for the freshwater phase, 1Á1 to 1Á9 Â 10 À3 for the estuarine phase and 1Á9 Â 10 À3 for the marine phase. In early life history, a major event occured at 22 days, which was marked on the otoliths by a change in the direction of the otolith growth axis. This event was related to a change in larval swimming behaviour. Juvenile freshwater residence time was between 58 and 123 days, with a median duration of 88 days. Contrary to what has been estimated in previous studies, the present study showed that the estuarine phase lasted from 4 to 36 days, with a median value of 11 days. Seaward exit occurred in fish aged from 63 to 150 days, with a median age of 99 days. These results have important implications for the sustainable management of this species.

–  The timing and duration of the estuarine phase during seaward migration were addressed for allis shad and twaite shad, closely related sympatric species in the Gironde system. Otolith Sr:Ca and daily growth increments were used to retrospectively estimate the age at entrance in the estuary, the time spent in estuary and the age at seaward exit from returning adults. The two species exhibited different seaward migration strategies: allis shad entered the estuary at an older age than twaite shad (88 days vs. 59 days), stayed shorter in the estuary (11 days vs. 17 days) and exited to the sea at an older age (99 days vs. 77 days). In the Gironde system, the estuarine phase is very short and occurs within a particular time ‘window’ for the juveniles that will become future spawners. In this context, it might be suggested that the estuarine phase is a bottleneck for these sympatric shad populations.

To test if otoliths can be used to track fish migration in polluted areas, fish sampled from Onondaga Lake, heavily polluted with mercury, were used in an assay to determine whether mercury was detectable in the fishes' otoliths using synchrotronbased scanning X-ray fluorescence microscopy (SXFM). Mercury was undetectable, but selenium, rarely reported in otoliths and known as a physiological antagonist to mercury, was. Strontium was also present but appeared to be taken up independently of selenium, and thus these serve as independent biogeochemical markers. Both selenium and mercury were detected in fish tissues, but selenium was below levels considered toxic. Selenium was low in otoliths of fishes collected in nearby Oneida Lake. Synoptic surveys of water chemistry revealed that Se is regionally highest in Onondaga Lake and in particular its main inlet, Onondaga Creek. SXFM appears to be a sensitive method for detecting selenium in otoliths.

To study fish migration using otolith microchemistry, it is important to understand the relationship between elements in the otoliths and in the surrounding water, including potential interactions with other elements. Selenium (Se) is a trace element with strong affinity for mercury (Hg). To test if Se is a reliable tracer for fish migration, the effects of dissolved Se and Hg concentrations on Se incorporation in fish otoliths were investigated experimentally. Brown bullheads ( Ameiurus nebulosus ) were reared in waters spiked with various concentrations of inorganic Se and Hg. Otolith Se:Ca increased nonlinearly with dissolved Se concentrations as there was no significant difference between fish reared in low and medium [Se] waters (Se:Ca for low [Se] waters, 7.64 × 10−6; medium, 6.59 × 10−6; high, 1.24 × 10−5). Our study also provided the first evidence of a negative effect of Hg on Se incorporation into otoliths (p = 0.01), a phenomenon most evident in high [Se] waters. Because of the influence of Hg, caution should be taken when inferring fish migration based on Se.