Evaluation of the resilience of a fishery restoration success: source-sink population dynamics of walleye, Sander vitreus, in Saginaw Bay, Lake Huron

Background/Question/Methods

Saginaw Bay watershed. The Saginaw Bay population has been supported by stocking since the1940’s crash, however, stocking ended in 2004 when extensive natural reproduction became apparent. Numerous potential spawning rivers exist, but spawning runs are only known to occur in a few and their contribution of young-of-the-year fish to the greater Saginaw Bay stock is to date completely unknown. Regional managers are concerned that the majority of reproduction is coming from a single river, located below a chemical plant dam, and the stability of the Saginaw Bay walleye restoration is clearly precarious. One source of reproduction leaves the system vulnerable to another crash, and multiple sources (i.e. spawning areas) provide greater stability and resilience to the population. Our objectives were to evaluate the resilience of the population based on the contributions from each spawning river. We captured spawning adults (n = 85, from three known spawning rivers), assumed high natal fidelity, and used their otolith microchemistry to develop a discriminant model to assess the river of origin for over 200 young-of-the-year and yearling walleye from a spatially stratified random sample of Bay fish.

Results/Conclusions

Elemental water chemistry data from the USGS (1998 to present) strongly suggest that the otolith signatures originating from the individual rivers should be distinguishable. Magnesium (Mg), potassium (K), phosphorus (P, p < 0.0001 in all cases), and sulfate (p= 0.0022) all contributed to a significant discriminant model separating the rivers having the largest spawning runs. We observed differences in mean concentrations (mg/L) of Mg, K, P, and SO₄ over time and Mg and K had coefficients of variation less than 25%.  The temporal variance of SO4 and P were higher (22-34% and 42-68%, respectively), but larger differences in mean values allowed them to be useful descriptors of individual rivers.  Differences in mean elemental water chemistry and low temporal variance suggest that early life otolith signatures will be a valuable tool for assessing population source-sink dynamics.  Understanding the sources of walleye recruitment will shed light onto the stability of this restoration success story and will also inform conservation efforts over the long-term.