Seasonal Catch Rates of the Endemic Olympic Mudminnow in Wetland Habitat

Abstract The Olympic mudminnow (Novumbra hubbsi) is a small freshwater fish, endemic to western Washington State. Although the species is listed as a Washington sensitive species, the lack of routine monitoring has resulted in poor understanding of population dynamics over time needed to support management and conservation actions. Olympic mudminnow commonly live in wetlands and associated low-gradient channels with tannic water and soft substrates, making conventional electrofishing and seine sampling approaches challenging and potentially inaccurate. Alternatively, minnow traps can easily be set in a wide range of depths and habitat types and allow for more systematic, repeated sampling. The study purpose was to conduct monthly surveys over a 19-month period to help develop standardized sampling methods. Sampling using baited Gee minnow traps set overnight was conducted in a wetland complex that flows into Eld Inlet in south Puget Sound, Washington. Catch rates of Olympic mudminnow were highest in late summer to early autumn (August through November) when water depths were low and water temperatures were decreasing. Trap mortality was relatively low for both Olympic mudminnow and amphibians but increased during warm spring and summer months. Eighty percent of the Olympic mudminnow captured were between 44 and 56 mm total length, and there was no difference in size between males and females. Gravid females were observed from October to June. Overall, minnow trapping appeared to be an effective method for monitoring Olympic mudminnow populations year round; however, sampling in the autumn appears to be advantageous because catch rates were high, and mortality was low.

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