Review and Assessment of Walleye Genetics and Stocking in Alberta
Fiona D. Johnston, Andrew J. Paul
Walleye (Sander vitreus) are native to Alberta occurring in large rivers and lakes throughout the province. However, overexploitation has resulted in the decline of many populations. Furthermore, walleye abundance in the province is limited by availability of large lakes that provide suitable walleye habitat and because Alberta falls near the northern limits of the species’ range. To address declining walleye populations and further extend their distribution, walleye have been stocked into 118 different waterbodies in the province. This report examines effects stocking may have on genetic diversity of walleye in Alberta.
Four hazards are associated with loss in genetic diversity from any stocking program: extinction, loss of within-population genetic variability, loss of among-population genetic variability and domestication selection. Each step in the hatchery processes (i.e., broodstock selection, broodstock collection, mating, rearing or release) is associated with a genetic hazard. The vulnerability of a population to a particular hazard depends on the product of its risk (probability an event that produces the hazard occurs) and the consequence of the hazard. Assessing vulnerability can identify key weaknesses in the hatchery process and areas for improvement which provide the greatest benefit to maintenance of genetic diversity.
In a survey of agencies responsible for walleye stocking in Canada and the United States, maintenance of genetic diversity is an important management concern. However, most agencies have only unofficial policies to prevent movement of walleye across major watershed boundaries. Few jurisdictions have comprehensive guidelines to minimise negative influences of stocking on genetic diversity from various components of the hatchery process. Several states are currently undergoing studies to examine existing and historical genetic diversity. With this information, management agencies intend to further identify: populations that represent distinct genetic groups; the vulnerability of these groups to alteration from a stocking program; and, how diversity can best be maintained.
Walleye stocking in the province of Alberta has occurred since 1926. The spatial pattern of stocking has tended to move walleye genes from northern lakes and watersheds to southern ones. The extent to which these genes have introgressed into southern populations is largely unknown. The only genetic work on walleye completed in Alberta suggests three important patterns. First, river and lake population of walleye are genetically distinct. Second, genetic diversity of walleye within the province may be substantial and unique to different drainage basins. Third, stocked walleye may have introgressed into both recipient lake populations and connected riverine populations. These patterns need to be confirmed using additional genetic techniques and populations.
We believe the most appropriate way of proceeding with genetic analyses intended for management purposes in the province requires two initial steps. First, define evolutionary significant units (ESUs) based on a defined protocol (examples are provided in the text). The ESUs should be based on a combination of genetic, ecological and geographic information that designates populations with respect to: a) reproductive isolation and b) evolutionary lineage. Second, goals of a stocking program must be evaluated with respect to the relative importance of stock performance (e.g., survival, growth and reproductive success of stocked progeny) versus maintenance of genetic diversity, although these are not necessarily mutually exclusive goals. For example, jurisdictions that have no native walleye will likely place little emphasis on maintenance of genetic diversity and most efforts into performance. In contrast, jurisdictions with native populations should place more emphasis in walleye stocking on maintenance of diversity rather than short-term performance.