6: Life History Diversity
Bror Jonsson, Nina Jonsson, and Robert E. Gresswell
Trout, char, huchen and taimen, and Lenok Brachymystax lenok belong to the subfamily Salmoninae (systematics in Nelson 2006) and exhibit diverse life histories reflecting adaptations to occupied habitats. In particular, they exhibit adaptations to life in cold, nutrient-poor waters with seasonally variable food resources and spawning on stony bottoms in fast-flowing streams and rivers or exposed shores of oligotrophic lakes. However, by making excursions to richer feeding areas, growth opportunities are improved for individuals, groups of individuals, or entire populations (Figure 1). Salmoninae exhibit natal philopatry (homing; Northcote 1997), which is a behaviorally important isolating mechanism (sensu Mayr 1963). Concomitantly, isolation facilitates adaptations to local environmental conditions of the spawning and nursery areas (Bond et al. 2014), where reproductive success and density-dependent survival of the offspring are typical selective forces (Elliott 1994). Although these cold, freshwater stream networks typically support few fish species, competition among salmonine species is often an important density regulator (Finstad et al. 2011). Within populations, there are significant relationships between population density, growth, and adult body size (Lobón-Cerviá 2007). This is significant because individual growth and size are major determinants of reproductive success and the size of subsequent cohorts (Jonsson and Jonsson 2011).
The diversity of lotic and lentic fresh and estuarine habitats act as templates for Salmoninae life histories (Southwood 1977; Jonsson and Jonsson 2011). Here, we summarize how different life stages of these species reflect these adaptations. The review focuses on ontogenetic development, spawning, migrations, growth and adult age and size, feeding, reproduction, and life span. Because the life histories of these fish tightly intertwine with their habitats, habitat degradation and alteration, including introduction of nonnative species, are major threats to population persistence. Often, populations comprise both migratory and nonmigratory individuals (partial migration; sensu Jonsson and Jonsson 1993). This life history decision appears associated with the growth rate of the young fish, here exemplified with data from the Voss River (Figure 2). Migratory trout and char often perform long migrations in freshwater, estuaries, and coastal sea for feeding. Environmental stimuli, such as photoperiod, water temperature, and discharge, function as cues influencing the timing and success of the migrations. Migrations are dangerous and energetically costly, and movements between distant environments influence the probability of survival. Thus, individuals in some populations do not migrate: there are trade-offs between early maturation, resulting in a nonmigratory life style, and migration involving later maturation (Jonsson and Jonsson 2011).