"The disturbance in the sea caused the fish to emigrate, to disappear from here," said Alfonso Alvear, 69, a fisherman and president of a neighborhood association here.
Because steelhead are facultatively anadromous, and the anadromous fish may emigrate to the ocean at a wide range of ages, managers in these systems are particularly concerned with the potential impacts of management actions on life history variation.
For Mokelumne River fish following Scott Creek decision rules, we predict a small number of fish adopting the resident pathway, with most fish smolting and emigrating at either age 1 or 2. Thus, it appears there are more residents on the Mokelumne than we would predict for fish behaving according to rules evolved in a coastal stream.
We model length-dependent ocean survival of emigrating fish by (A1) with l measured in mm, based on a fit to Shapovalov (1967) and Bond et al. (2008) as described in Satterthwaite et al. (2009).
This may reflect actual differences in mortality as well as the increased likelihood of smaller fish to permanently emigrate to locations with reduced encounter probabilities.
That is, there is a large difference between the expected lifetime fitness of a fish that emigrates at age 1 and a fish that emigrates at age 2. As a result, the change in fitness associated with a switch between the anadromous and resident life histories may be larger or smaller than that associated with a switching of smolt ages within the anadromous life history.
Outside of these windows we assume life history trajectories are fixed; thus (as long as fish are not spawning or emigrating to the ocean at time t): (1) where s(t) is freshwater survival from time t to time t + 1 and l′ l,g,e,t) describes the expected length at time t + 1, given expected growth from starting size l and physiological states g and e.
The overall lifetime success of Foreign fish was estimated to be only 31% that of Local fish under communal rearing conditions (38% if emigrating parr were assumed to survive), which is similar to the value of 35% reported in a previous experiment by McGinnity et al.
Ranges for freshwater values of both ratios were high, indicating most fish moved across chemically heterogeneous habitats after emigrating from estuarine habitats.
