Boughton et al. 2005

Boughton, D. A., and coauthors. 2005. Contraction of the southern range limit for anadromous Oncorhynchus Mykiss. NOAA National Marine Fisheries Service, Southwest Fisheries Science Center, NOAA-TM-NMFS-SWFSC-380, Santa Cruz, California.

For many species, contraction of the geographic range limit nearest the equator is predicted by current global climate trends. Anecdotal data suggest that the southern range limit for anadromous populations of the fish Oncorhynchus mykiss has recently contracted northward; the contraction is consistent with the climate-change hypothesis, but is also consistent with the coincidental urbanization of the species’ southern range limit during the twentieth century. Distinguishing between the two mechanisms of range contraction is important in part to plan conservation measures for the species; and in part to accurately account for the impacts of climate change vs other impacts on species’ geographic distributions. Here we assess the current occurrence of anadromous O. mykiss in each coastal basin of southern California in which it occurred historically. The extent of the study is the region from Monterey Bay (the Pajaro River basin) southward to the US Border with Mexico; the grain of the study (the smallest unit for which occurrence is estimated) is the coastal basin. Two especially large basins - those of the Pajaro and Salinas rivers - were divided into sub-units. Ninety-two historic steelhead basins were identified, and occurrence was estimated in 86 of them through a combination of field reconnaissance and spot checks (snorkel surveys). The spot checks were conducted in the best occurring habitat of stream reaches with migration-connectivity to the ocean. The results indicated that between 58% and 65% of historic steelhead basins currently harbor O. mykiss populations. The pattern of basin-level extirpation showed a latitudinal gradient, with the highest extirpation rates in the south, confirming that a range contraction has taken place. Most of the extirpations (68%) were associated with anthropogenic barriers to fish migration (dams, flood-control structures, culverts, etc.). Historically, the probability of occurrence in a basin was correlated with basin area, but this correlation no longer holds. Barrier-associated extirpations were positively correlated with urban/agricultural development, whereas the “other” extirpations (i.e,. not associated with barriers) were positively correlated with mean annual air temperature in the basin. The results suggest that the range contraction is primarily an effect of lost migration opportunities arising from the urban/rural infrastructure; however climate and other stressors may have played a subsidiary role.