Science Advisory Report 2021/011

Recovery Potential Assessment for Lower Fraser River White Sturgeon 2020

Summary

• Lower Fraser River White Sturgeon is the Designatable Unit (DU) of White Sturgeon (Acipenser transmontanus) that reside in the mainstem of the Fraser River, and accessible tributaries and lakes, from the marine estuary to Hells Gate. Most of the information about this DU has been obtained from the “core assessment area” defined in Figure 1.
• Modelling results indicate that the abundance of age 7 to 55 (fork length 60 to 279 cm) has declined by 25% since 2006, to a 2019 estimate of 44,809 fish. The estimated abundance for juvenile (60 to 99 cm fork length) White Sturgeon has declined by > 70% from 2004 to 2019.
• Reconstructions of historic juvenile recruitment to age 7 have shown variable (increasing and decreasing) trends within an overall range similar to the estimated change since 2000.
• Somatic growth rates have been declining; the average annual growth for 60-179 cm (fork-length) White Sturgeon from 2016-2019 (3.4 cm/year) is 69% of that from 2010-2012 (4.9 cm/year).
• The total population of Lower Fraser River White Sturgeon is forecast to continue to decline if juvenile recruitment remains at similar levels to those estimated for the last 10 years (i.e. 2010 to 2019).
• Long maturation times for White Sturgeon indicate that any changes to early juvenile recruitment (i.e. recruitment to age 1) could take up to 20 years before impacting adult life stages (i.e., 2040 and beyond).
• Several threats were identified and evaluated with the following threats classified as medium level risks: past in-river gravel extraction, White Sturgeon bycatch in FSC (Food Social, and Ceremonial) fisheries, shoreline modification, and food availability.
• Past in-river gravel extraction was identified as a medium threat and may have reduced productivity of some spawning areas, particularly with substantial gravel removals from 2000 to 2010.
• White Sturgeon bycatch in FSC net fisheries was identified as a medium threat, particularly in relation to the use and operation of set nets.
• Shoreline modification was identified as a medium threat: many of these modifications (e.g. tidal/flood gates, dykes, rip-rap) have limited White Sturgeon access to a significant amount of off-channel rearing habitat.
• Food availability was identified as a medium threat because both White Sturgeon growth rates and juvenile abundance have declined during years when the biomass of Chum Salmon and Eulachon entering the Fraser River had declined. Salmon (multiple species) and Eulachon are important sources of food and nutrients for the lower Fraser River ecosystem. The need to further explore relationships between White Sturgeon abundance/growth and other salmon species was identified.
• With the uncertainty in the causes of declining juvenile recruitment, it is difficult to determine which mitigative measures would be most effective at reversing this decline. It is likely that multiple measures will be required.
• Examples of mitigation measures and alternative activities and their anticipated effects on White Sturgeon mortality and recruitment are provided in Table 4 of this document.
• A survival threshold of 10,000 adults (age 22-55, 160-27 cm fork length) was identified as the minimum number of adults needed for medium to long-term survival of the DU.
• Three complementary candidate recovery thresholds were identified: 1) 20,000 adults (age 22-55, 160-279 cm fork length), 2) total abundance of 60,000 (age 7-55, 60-279 cm fork length), and 3) a positive trend in juvenile abundance over a 50-year time window.
• Although the population is expected to be above the survival threshold into the foreseeable future; if juvenile recruitment declines further (i.e., to half of the 2010-2019 levels), adult abundance could drop below the survival threshold within 50 years.
• Six recruitment hypotheses were designed to explore a range of possible future recruitment scenarios. Using the posterior sample of age-specific abundances, posterior predictive projections were developed under each scenario to determine the likelihood of achieving survival and candidate recovery thresholds under a given hypothesis.
• Due to a long time lag between implementing mitigation measures and observing the corresponding result in adult abundances, there were some differences in the effectiveness of mitigation measures to achieve total versus adult candidate recovery thresholds within the 50-year projection window. A longer projection window (e.g., 70 years) may be needed to confirm the level of certainty for adult candidate recovery thresholds.
• To achieve the candidate recovery threshold of 60,000 total abundance within a 50-year window, and prevent any further decline in the population, juvenile recruitment must be doubled from the average recruitment estimated for 2010-2019.
• Critical to the sustainability of this population, harm needs to be reduced relative to current levels. This may be addressed through changes to habitat that would improve juvenile growth and survival, changes to fisheries, and increases in food availability.

This Science Advisory Report is from the September 22-24, 2020 regional peer review on Recovery Potential Assessment – White Sturgeon, Lower 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.

Science Advisory Report 2021/011 (PDF, 1,061 KB)

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