Science Advisory Report  2013/019

Recovery Potential Assessment of Silver Chub (Macrhybopsis storeriana) in Ontario

Summary

• In Ontario, Silver Chub is limited to Lake Erie, Lake St. Clair, and the extreme southern portion of Lake Huron. It may also occur in connecting rivers (i.e., St. Clair and Detroit rivers). It has not been collected in Lake Huron and Lake St. Clair since 1983 and 1994, respectively. The majority of the Lake Erie occurrences have originated from the western basin.
• Silver Chub occurs in water temperatures up to 25.9°C and spawns when water temperature is between 19°C and 23°C. Silver Chub has been recorded from a wide range of water depths (2.3-24 m) and turbidity levels (0-4.5 m Secchi depth). Silver Chub occupy areas with clean gravel, sand, and silt bottoms.
• Adult Silver Chub fed primarily on Hexagenia mayfly nymphs prior to invasion of Round Goby (Neogobius melanostomus), then switched to a diet composed mainly of dreissenid mussels.
• To achieve ~99% probability of persistence, given a 15% per generation chance of catastrophic (50%) decline, requires ~444,000 adult Silver Chub and at least 84 km² of suitable habitat.
• Current available habitat in the Western Basin of Lake Erie is estimated at 3000+ km². The quality of this habitat (suitability for Silver Chub) is unknown.
• The greatest threats to the survival and persistence of Silver Chub in Ontario are anthropogenic in origin such as nutrient loading, turbidity and sediment loading, contaminants and toxic substances, and habitat removal and alteration. Furthermore, the presence of numerous invasive species may pose a threat to the survival and persistence of the Silver Chub in Ontario. The threats may not always act independently on Silver Chub populations; rather, one threat may directly affect another, or the interaction between two threats may introduce an interaction effect.
• The growth rate of a growing Silver Chub population is most sensitive to changes in the survival of immature individuals. Stable or Declining populations are most sensitive to changes in cumulative adult survival, and are more sensitive than growing populations to changes in fecundity of older individuals.
• Current population abundance in Canada is estimated at ~662,000 (266,000 – 1,620,000 95% confidence). This mean abundance (but not the lower confidence bound) exceeds the recommended recovery target (MVP).
• Silver Chub in the Western Basin of Lake Erie has been in decline, on average, since 2000, at a mean rate of 20% annual decline. More recently population trajectory is highly uncertain; the average growth rate from 2007 to 2012 was 4% annual growth with a large 95% confidence interval of 31% annual decline to 55% annual growth.
• At the current abundance (mean estimate of 622,000), the expected time to extirpation of a population declining at a rate of 20% annually is 58 years (36 – 95 years, 95% confidence).
• To avoid a decrease in population growth rate larger than 1%, transient harm (one-time removal of individuals) should not exceed 23.5% reduction in YOY abundance, or 15% reduction in adult abundance, or 8.5% reduction in total abundance within a seven-year period.
• Assuming a population growth rate of 4% annually, chronic harm (long-term reductions to vital rates) should not exceed 3% of YOY survival or 2% of adult survival or 3% of fecundity.
• A number of key sources of uncertainty exist for this species related to population life history, distribution and structure, habitat preferences, and factors limiting their existence. Resolving these sources of uncertainty would greatly enhance our understanding of Silver Chub in Ontario. Specifically, stage-dependent survival rates are not known (and were based on allometries for this report). Fecundity estimates are out of date (1950s) and  may no longer be accurate.

This Science Advisory Report is from the March 5, 2013 Recovery Potential Assessment of Silver Chub (Macrhybopsis storeriana). 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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