Approaches for Monitoring Pacific Lamprey Spawning Populations in a Coastal Oregon Stream

We evaluated two methods for assessing Pacific lamprey Lampetra tridentata spawning populations (visual counts of spawning adults and redds) and one method for assessing larval production (emergent ammocoete counts from drift nets) in the South Fork Coquille River, Oregon in 2004 and 2005. All three methods generally provided similar portrayals of timing, duration, and magnitude of spawning, including greater abundance in 2004 and seasonally bimodal spawning in 2005. We found a linear relationship between adult and redd counts but a high redd to adult ratio that varied seasonally in both years. The high redd to adult ratio can be attributed to short residence time in spawning areas and temperature or habitat-dependent differences in detection of adults, both of which can undermine adult count data. Redds had relatively longer persistence and larger numbers compared to adults and therefore may be a more practical survey method, but variable redd shape, size, and age, as well as superimposition, presented significant counting errors. Both adult and redd counts had no clear-cut way to quantify errors. Sampling emergent ammocoetes in the drift allowed detection of low density early and late season spawning and would be the preferred survey method when surveys of spawning adults and redds are impractical due to river size, visibility, or access. Even when spawning surveys are practical, emergent ammocoete counts may be better for detecting and monitoring small populations. Disadvantages of ammocoete sampling include nighttime work hours, extra laboratory time, and difficulties with species identification. The general absence of a stock-recruit relationship in lampreys means adult and redd counts are poor predictors of ammocoete production and emergent ammocoete abundance is a poor predictor of spawning abundance. The relationship breaks down because of variability in early survival, which is best detected using data from both spawning surveys and larval drift samples.