Is Larval Walleye River Residence Time Sufficient to Establish a Natal-Site Signal in Otolith Chemistry?

We seek robust methods to identify the spawning site origin of individual Lake Erie walleye. By matching composition of the core of the otolith of an adult fish to water chemistry at spawning locations, we can trace adults to their natal sites. Previous work has suggested, however, that 20 days of residence may be necessary for otolith chemistry to come to equilibrium with the ambient water, while we expect walleye larvae to reside in their natal river for only a few days. To resolve this issue, we spawned Lake Erie walleye in the laboratory, reared the eggs and larvae at three levels of strontium, and analyzed otolith strontium concentrations of larvae of different ages. In addition, we analyzed otoliths of walleye larvae collected from two Lake Erie spawning rivers over three years to provide a field check of experimental results. Otolith strontium concentrations differed among strontium treatments in fish as young as 4 days post-hatch. Origins of field-caught larvae also could be distinguished using otolith strontium levels. We suggest that the previous estimate of 20 days did not consider elemental uptake during egg incubation, and otolith microchemistry will be a useful tool in discriminating among Lake Erie walleye spawning stocks.