Otolith elemental composition and adult tagging reveal spawning site fidelity and estuarine dependency in rainbow smelt

Abstract

Observations of homing and straying in marine organisms based on traditional Eulerian approaches may fail to resolve dispersal kernels or be unable to differentiate homing from invariant local residency. The roles of spawning site fidelity and straying in structuring populations of anadro- mous smelt Osmerus mordax were examined in coastal Newfoundland through the analysis of otolith elemental composition and a series of adult tagging experiments. Otolith elemental baselines were generated from freshwater residents, estuarine juveniles and laboratory-held individuals (32 psu), using both a single-element (i.e. Sr:Ca and Ba:Ca) and multivariate approach (i.e. discriminant function analysis). Ten spawning fish of unknown dispersal history were sampled from each of 6 spawning locations, and otolith composition was examined using laser ablation inductively coupled plasma mass spectrometry at 12 to 16 locations across each otolith. Single-element and multivariate approaches both indicated estuarine residency predominated, with limited marine (0.7%) signatures indicative of rare marine movements. Multiyear spawning site fidelity was examined through a fin- clipping (2003, n = 7076) and visual implant elastomer (VIE) experiment (2004, n = 13 524), encom- passing each of the 6 spawning locations. We observed limited straying, with 90 to 99% annual spawning site fidelity. Tag returns declined significantly with distance from location of tagging, and declines were linear over small (<50 km) distances (VIE: p < 0.001, r 2 = 0.99; fin clipping: p < 0.001, r 2 = 0.56). We conclude that population structure in anadromous smelt is maintained by small-scale habitat associations limited to a single estuary, supporting a hypothesis of demographic isolation among estuaries.