Medium-sized anadromous fish (with body lengths between 200 and 350 mm) are most likely to be caught, because they are abundant and, during the upstream migration of immature anadromous brook charr in early fall (Lenormand et al. 2004), concentrated in the river's estuary, where their exploitation is little regulated.
Migration of steelhead, the anadromous phase of rainbow trout, has mostly been eliminated because of the construction of dams downstream on the Snake and Columbia Rivers.
Warmer global temperatures will likely shift the distribution of salmon habitat northward geographically, increasing the productivity of most existing nursery lakes, but increasing the difficulty (fitness cost) of anadromous migration in southern areas as rivers warm and summer flows decrease.
The distribution of kokanee appears to be determined by lake productivity and the difficulty of anadromous migration, which in combination, likely determine fitness relative to the lake ecotype (Wood 1995).
The alternations included earlier anadromous migration of adult fish and faster maturation for successful spawning [ 40].
On the Pacific coast of North America, one of the more intensively studied cross-ecosystem interactions involves the migration of salmon (Oncorhynchus spp .. Anadromous Pacific salmon spend the majority of their lives in marine ecosystems, where they gain >99% of their mass, before re-entering complex freshwater ecosystems to spawn and die [ 3].
Intensive coastal fisheries for sockeye salmon act to increase the cost of migration for both anadromous ecotypes (see Theriault et al., 2008), effectively shifting habitat towards kokanee (to the right in Fig. 5C).
During a 100 years of fishing of anadromous individuals, we predicted evolution in this population's migration reaction norm, with the average probability of migration decreasing with increasing harvest rate.
Here, we have shown that the intensive harvesting of anadromous fish evolutionarily reduces the probability of migration.
