Spatial covariation of fish population vital rates in a stream network

Animal populations are spatially structured in heterogeneous landscapes,
in which local patches with differing vital rates are connected by
dispersal of individuals to varying degrees. Although there is evidence
that vital rates differ among local populations, much less is understood
about how vital rates covary among local patches in spatially
heterogeneous landscapes. In this study, we conducted a 9-year annual
mark-recapture survey to characterize spatial covariation of survival and
growth for two Japanese native salmonids, white-spotted charr (Salvelinus
leucomaenis japonicus) and red-spotted masu salmon (Oncorhynchus masou
ishikawae), in a headwater stream network composed of distinctly different
tributary and mainstem habitats. Spatial structure of survival and growth
differed by species and age class, but results provided support for
negative covariation between vital rates, where survival was higher in the
tributary habitat but growth was higher in the mainstem habitat. Thus,
neither habitat was apparently more important than the other, and local
habitats with complementary vital rates may make this spatially structured
population less vulnerable to environmental change (i.e., portfolio
effect). Despite the spatial structure of vital rates and possibilities
that fish can exploit spatially distributed resources, movement of fish
was limited due partly to a series of low-head dams that prevented
upstream movement of fish in the study area. This study shows that spatial
structure of vital rates can be complex and depend on species and age
class, and this knowledge is likely paramount to elucidating dynamics of
spatially structured populations.