Science Advisory Report  2016/040

Recovery Potential Assessment for the Upper Fraser River White Sturgeon Designatable Unit - 2016

Summary

• The Upper Fraser White Sturgeon Designatable Unit (DU) now encompasses the Mid-Fraser, Upper Fraser and Nechako Nationally Significant Populations (NSPs).  Differences between the three populations, including potential genetic differences, support the continuance of management at the sub DU scale (e.g. development and management of objectives at the sub DU scale).  Population abundance targets for each sub DU must be achieved in order to ensure the success of the population at the DU scale.
• White Sturgeon is the largest, longest-lived freshwater fish species in North America. It is a slow-growing, late maturing species.
• The Upper Fraser White Sturgeon DU extends from Hell’s Gate upstream through the Mid- and Upper Fraser River (upstream of Prince George) and Nechako River systems.
• Important rearing habitat has been identified for adult and sub-adult sturgeon in each of the three sub-DU areas but important habitat for spawning and larval and juvenile sturgeon remains an uncertainty.  No spawning sites have been confirmed for the Mid- and Upper Fraser sub-DUs and only one spawning site is known for the Nechako sub-DU.
• Current estimates of adult sturgeon abundance within the DU are 185 in the Upper Fraser, 749 in the Mid-Fraser, and 630 in the Nechako. Juvenile sturgeon are present in both the Mid- and Upper Fraser River, demonstrating that recruitment likely is occurring in these Fraser River mainstem sub-DUs.  However, in the Nechako, ongoing failures of natural recruitment are apparent as very few wild juvenile sturgeon exist. Juvenile sturgeon abundance is unknown for the entire DU.
• The federal recovery strategy for White Sturgeon has identified threats for the survival or recovery of Upper Fraser White Sturgeon. Both habitat quality and quantity have declined in some areas with negative effects on survival and recruitment and overall carrying capacity. This is especially true in the Nechako where spawning habitat has been lost and/or is no longer functional. Even when habitat and food resources have not declined, there are significant risks to long-term viability when populations are small, as is the case here (at both sub-DU and DU scales).
• Recommended population abundance targets have been identified for each of the sub-DUs: 185 adults for the Upper Fraser, 749 adults for the Mid-Fraser, and 1,000 adults for the Nechako.
• Current adult estimates are at the population abundance targets for both the Mid- and Upper Fraser sub-DUs but below the target for the Nechako sub-DU where recruitment failure persists. Given that White Sturgeon is a very long-lived and late-maturing species, the changes in juvenile recruitment may not be reflected in adult abundance for decades, particularly relevant for the Nechako sub-DU.
• Currently, hatchery production is the primary source of juvenile sturgeon in the Nechako system.  Although, there appears to be limited exchange among the three sub-DUs within the Upper Fraser, adaptive management of Nechako River hatchery operations should ensure hatchery-produced sturgeon do not overwhelm the other sub-DUs in this metapopulation.
• Modeling that includes both habitat productivity (Habt) and human induced mortality (HM) allows visualizations of potential trade-offs should changes to either occur. There is very little scope for improved habitat productivity for the Mid- and Upper Fraser sub-DUs. However habitat improvement is essential in the Nechako sub-DU where spawning habitat restoration efforts are required to restore naturally sustained recruitment.
• Since human induced mortality was considered low for all three sub-DUs, there is limited scope to reduce it further, except perhaps the Mid-Fraser where there is more uncertainty.
• The Nechako sub-DU is well below its recovery target of 1,000 adult fish, so there is limited scope for additional allowable harm to this sub-DU and any should be in support of conservation objectives. Potential allowable harm for Mid- and Upper Fraser sub-DUs was evaluated by modeling changes in HM while maintaining current Habt values. If HM increased 2× for the Mid-Fraser sub-DU or 3× for the Upper Fraser sub-DU over current HM levels, it would have substantial negative effects on the abundance of White Sturgeon.

This Science Advisory Report is from the March 30 - April 1, 2016 Recovery Potential Assessment – White Sturgeon, Upper Fraser Designatable Unit. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

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