Persistent regional variation in populations of a tidepool fish

Understanding the factors that drive population dynamics is particularly challenging in systems that are connected via propagule dispersal because the rate of arrival of new individuals is both notoriously unpredictable and potentially critical in determining future dynamics. Furthermore, patterns of propagule recruitment can be fundamentally modified by post-recruitment interactions. We investigated how variation in both the recruitment of juveniles and local community structure affected the demography of an intertidal fish in two oceanographically distinct regions in Oregon. From 2001–2003, we observed persistently higher densities of adult tidepool fish Clinocottus globiceps (Girard) at Cape Foulweather, a stronger-upwelling region compared to Cape Perpetua, a weaker-upwelling region. In 2002, recruitment was roughly two times higher in the Cape Perpetua region. By the following spring, densities of adults were more than three times higher in the Cape Foulweather region, implicating intense post-recruitment mortality in the Cape Perpetua region as the cause of regional differences in adult densities. We manipulated, using a factorial experimental design, the presence and absence of dominant sessile organisms in tidepools at one site in each region in the spring of 2002 and explored whether differences in tidepool communities could be related to regional variation in fish abundance. Fish recruited disproportionately to pools with low cover of mussels in both regions. By the following spring, adults were more abundant in pools with mussels in one region and equally abundant in pools with or without mussels in the other region. Possible explanations for regional differences in C. globiceps populations include regional variation in the cover of intertidal invertebrates, in local biological interactions such as predation, or in coastal bathymetry that may be linked to variation in physical processes controlling recruitment and over-winter survival.