MEPS 119:25-35 (1995)  -  doi:10.3354/meps119025

Sagittal otoliths of young-of-year American shad Alosa sapidissima from the Hudson River estuary, New York, USA, were transected with an X-ray-dispersive microprobe to examine temporal patterns of strontium, a micro-constituent found in otolith aragonite. Otoliths were assayed from fish reared on known diets (freshwater zooplankton, followed by artificial diet containing marine fishmeal) in fresh water. The switch from freshwater plankton to artificial diet resulted in a significant rise in Sr:Ca ratio in the otolith (mean increase 3.2-fold, p < 0.001) both for fish reared at 12.5*C and those reared at 22*C, although there was no significant difference in Sr:Ca increases between the 2 temperature treatments. In a field study, Sr:Ca values of otoliths from wild fish caught in the freshwater reaches of the Hudson were low (mean 0.79 x 10^-3 Sr:Ca +/- 0.32 SD, range 0.00 to 1.46 x 10^-3). Six fish captured in a single trawl in the lower estuary on 25 September 1990 had low Sr:Ca values on the inner parts of their otoliths (corresponding to younger age: mean Sr:Ca = 0.98 x 10^-3 +/- 0.38 SD), but the strontium content increased 3- to 5-fold (mean Sr:Ca = 3.62 x 10^-3 +/- 0.71 SD) on the outer parts, corresponding to dates when the fish were older. The change in strontium content is consistent with the movement of fish from freshwater to seawater, as seawater has a higher Sr concentration and ratio of Sr:Ca. The same change was seen in the otolith of a female adult shad and has been documented in other diadromous fishes. Three of these juveniles had such anomalous early growth patterns as to suggest their origin in a natal river other than the Hudson. The estimated seawater entry dates of these fish were as early as late June. This is the first demonstration of such early outmigration, and possibly vagrancy, in young-of-year of this species, and contributes to our understanding of American shad life history.

American shad . Otolith microchemistry . Strontium:calcium ratios . Anadromy . Migration