Trait comparisons with other anadromous fishes
Despite strong differences in philopatry and population genetic structure, there are similarities between the Pacific lamprey ecotypes and those of steelhead trout (anadromous Oncorhynchus mykiss ) and Chinook salmon (O. tshwaytscha ), as were described by Parkeret al. (2019). In this study, we observed even greater similarities with steelhead of these ecotypes in the Willamette River than were apparent in the Klamath River Pacific lamprey. Notably, like steelhead (Hess et al. 2016), the Pacific lamprey ocean- and river-maturing ecotypes exhibit seasonal separation where premature fish return to freshwater long before spawning in contrast to mature fish that arrive shortly before spawning. Also similar to steelhead (Micheletti et al. 2018), the Pacific lamprey ocean-maturing form is only distributed in coastal regions and the river-maturing ecotype is distributed further inland. However, it is unknown whether inland migrating Pacific lamprey exhibit both early and late arrival to spawning grounds as observed for inland migrating steelhead (Micheletti et al. 2018) and Chinook salmon (Narum et al. 2018). Finally, although we found no evidence that the homologous genes were conserved with salmonids, Pacific lamprey ecotypes were associated with a single locus of major effect as shown in steelhead (Hess et al. 2016; Micheletti et al. 2018) and Chinook salmon (Prince et al. 2017; Narum et al. 2018).
The adult body size trait that was associated with chromosome 02 genes in Pacific lamprey may share similarities with the age-at-maturity trait described in Pacific salmon (McKinney et al. 2019), steelhead (Copeland et al. 2017), and Atlantic salmon (Salmo salar , Barson et al. 2015). In salmonids, the number of consecutive years spent in the ocean before returning to freshwater as a mature adult is highly correlated to body size (e.g., Chinook salmon, Lewiset al. 2015). It has also been shown that larger, older Chinook and Sockeye salmon (O. nerka ) tend to arrive earlier at Bonneville Dam compared to the smaller 1-ocean-age adults (Anderson and Beer 2009). Similarly, larger Pacific lamprey arrive earlier at Bonneville Dam than the smaller bodied forms (Keefer et al. 2009, 2013), which may be related to life history decisions in seasonal environments. The primary growth of Pacific lamprey occurs during the ocean phase of the lamprey’s parasitic life cycle and so bigger lamprey may also be older in ocean age. There is not yet an accurate way to measure the total age or ocean age of lampreys since they lack bony structures, but this hypothesis could be tested once an aging method is developed (e.g., statolith microstructure). Collectively, the convergence of traits in salmonids and lamprey suggest strong tradeoffs between allocation of resources in capital breeding fishes, whereby long distance migration constrains maturation schedules and in at least some cases (e.g., lamprey) body size.