Revised length categories and standard weight equation for Northern Pikeminnow

Objective

Length and weight indices (e.g., proportional size distribution, relative weight) provide standardized benchmarks that are useful for comparing groups of fish, identifying ecological interactions, and evaluating the effect of management actions. However, the current length categories and standard weight (W_s) equation for Northern Pikeminnow Ptychocheilus oregonensis, a species of important management focus in the Pacific Northwest, were developed using limited data, unclear methods, and no validation. As such, we sought to revise the length categories and W_s equation for Northern Pikeminnow.

Methods

We used the all-tackle world record to develop length categories for Northern Pikeminnow. We compiled data from 100,663 Northern Pikeminnow from 114 populations in Idaho, Montana, Oregon, Washington, and British Columbia to develop a revised W_s equation. Most fish were measured in fork length (FL), so we converted total lengths (TLs) to FLs using the equation TL = −2.301 + 0.916(FL); r² = 0.998. We used the regression line percentile, linear empirical percentile (EmP), and quadratic EmP methods to develop 50th percentile and 75th percentile W_s equations. We then assessed length-related biases in our W_s equations and the previously published equation.

Results

We propose minimum FLs of 17 cm (7 inches; stock), 26 cm (10 inches; quality), 35 cm (14 inches; preferred), 41 cm (16 inches; memorable), and 51 cm (20 inches; trophy) for proportional size distribution calculations. The previously published W_s equation exhibited substantial length-related biases according to the Willis and weighted empirical quartile method tests. The EmP 50th-percentile W_s equation was the only equation that we evaluated that did not exhibit length-related bias. The EmP 50th-percentile W_s equation was the best performing equation (i.e., no length-related bias detected). The equation is log₁₀(W_s) = −5.258 + 3.135(FL), where W_s is in grams and FL is in millimeters. The equation is valid for Northern Pikeminnow 90–580 mm FL.

Conclusion

The length categories, length conversion, and W_s equation presented here will aid fisheries professionals in the study and management of Northern Pikeminnow populations.