Electrofishing Injury to Stream Salmonids; Injury Assessment at the Sample, Reach, and Stream Scales

Abstract

Electrofishing injury rates in rainbow trout and juvenile steelhead Oncorhynchus mykiss and juvenile spring chinook salmon O. tshawytscha were quantified in samples collected in four tributaries to, and one reach of, the Yakima River, Washington. Estimated electrofishing injury rates at the reach and stream scales were generated by using sample injury rates, derived from this study, multiplied by capture probabilities and the fraction of habitat sampled. Sample injury rates in small O. mykiss and juvenile spring chinook salmon were low. Mean electrofishing injury rate in O. mykiss samples captured in tributaries was 5.1%. Only 2.0% of the juvenile spring chinook salmon captured by electrofishing in the Yakima River were injured. Larger O. mykiss (≥250 mm fork length, FL) were injured at a significantly higher rate (27.7%) than their smaller counterparts (1.2%; P = 0.023) in the Yakima River sample. Electrofishing injury rates decreased with increasing scale from the sample to the reach and stream scales. Injury rates for index reaches that we use for long-term monitoring were 4.9% for O. mykiss in tributaries, 0.7% for O. mykiss less than 250 mm FL in the Yakima River, and 11.2% for O. mykiss larger than 250 mm FL in the Yakima River. Although the injury rate at the reach scale for larger O. mykiss was relatively high, we do not believe it affects our long-term monitoring data because annual mortality of these fish is high (>60%) and because a small proportion of the total population is captured by electrofishing (18–21%). Stream scale injury rates were very low in tributaries (0.1%) and in the Yakima River for smaller O. mykiss (a mixture of juvenile steelhead and resident rainbow trout <250 mm FL; 0.1%). The estimated stream scale injury rate for larger O. mykiss (≥250 mm FL) was 2.1%. Stream scale injury rates for all groups examined were below levels that we would expect to affect our long-term monitoring data. The distribution, conservation status, size structure, lifespan and annual mortality of the population, the fraction of the habitat sampled, sampling frequency, and availability, effectiveness, and cost of alternative sampling methods, must all be balanced against the need for data when establishing research or monitoring efforts that use electrofishing.