- Recently, there has been concern over the decline of the Pacific lamprey, Lampetra tridentata, in the northwestern United States. However, effective management has been impeded by data gaps in basic biology, especially in the early life stages. Consequently, in 2004 and 2005 I examined reproductive ecology, larval recruitment, and lamprey monitoring methods in the South Fork Coquille River, a coastal Oregon stream.
In Chapter 2 I monitored spawning populations at large (9.2 km) and small (focal area) scales. Relationships between adult counts at the two spatial scales and adult and redd counts at the large scale were analyzed. Weekly adult, redd, and carcass counts and tagging were also used to describe spawning and residence times, movement, size, and sex of mature adults. Large-scale adult and redd counts were highly correlated (2004, r2 = 0.867; P = 0.0069; 2005, r2 = 0.877; P = 0.0002); as were large-scale and focal area adult counts over both years combined (r2 = 0.690, P = 0.0001) and in 2004 (r2 = 0.753, P = 0.0250), but not in 2005 when densities were much lower (r2 = 0.065, P = 0.5069). Average residence time in spawning areas was less than a week for males and shorter for females, since >90% of recaptured fish were male. Two-thirds of dead fish (2:1) were male, versus only one-half of live fish (1:1), indicating additional sex-specific differences in postspawning behavior. No seasonal or spatial patterns in sex ratio or adult length were detected. Both adult and redd counts have inherent errors related to observer variability, movement during surveys, night spawning, and variable visibility due to weather and flow. To make adult and redd counts more useful for population monitoring their errors need to be better quantified and their relevance to life-cycle dynamics better understood.
In Chapter 3 I monitored intra-annual cohorts of spawning adults and emergent larvae at a single spawning area to examine annual and seasonal patterns of spawning, larval recruitment, and early life survival. In 2004 spawning occurred from April 6–June
3 (59 d) and larval emergence occurred from May 6–June 28 (54 d). In 2005 both spawning and emergence were later and more protracted, from April 25–July 3 (70 d) and May 15–July 25 (71 d), respectively. Over both years, larval recruitment was highly variable and only marginally correlated with spawning stock (r2 = 0.149, P = 0.0512). Survival until larval emergence was significantly related to spawning stock size, discharge during spawning, and their interaction. Survival generally declined with increasing spawning stock and decreasing discharge, both apparently related to negative density-dependent effects, which resulted in highly variable early life survival. For example, in April 2004, 65% of larvae were produced by 28% of spawners, while in May, 35% of larvae were produced by 70% of spawners. Egg predation by speckled dace, Rhinichthys osculus, increased with temperature, but contrary to expectations, had no detectable effect on survival until emergence.
This study provided justification for a multi-life stage approach to monitoring Pacific lamprey populations and understanding their dynamics. Application of this approach can provide insight into density-dependent survival and the roles of biotic and abiotic factors in larval production. Applied to the South Fork Coquille, Pacific lamprey larval production appeared to have an upper limit related, in part, to spawner density.