•Salmon populations express a range of migration tendencies, including partial migration (Figure 1)
•Dams are threats to migratory salmon across their range
•Where dams bisect populations, persistence upstream requires adoption of the completely resident form
•Whether fish in these populations undergo migrations is dependent on genetic and environmental conditions
1
2
1
Using our empirical evidence of heritability and selection on the threshold fork length, we can build a model to simulate the response of an upstream population to the construction or removal of a barrier to migration.
(a) % of populations extinct
(b)
Salmon species that express both migratory and resident forms are said to exhibit “ partial migration.
”
Steelhead (a) and rainbow trout (b) are the migratory and resident forms of the same species ( Oncorhynchus mykiss ). Furthermore, offspring of resident parents can be migratory and vice versa (c). However, dams may act as migration barriers, disconnecting the resident population from the migratory population (d). In order to persist, above barrier populations must adopt a fully resident life history.
300 eggs per ♀
3,000 eggs per ♀
The marine environment is more productive than the freshwater environment. If the growth requirements and difficulty of migration can be overcome, migrants acquire greater resources that can be put toward reproduction. For residents, freshwater survival is higher, increasing the probability of making it to reproductive age.
Average fork length at decision window
Only
53% of above barrier offspring were smolts.
Jesse Adams
Resident (“Non-Smolt”)
Below barrier , smolts were
76% of the offspring
Migrant steelhead (“Smolt”)
We captured O. mykiss from above (a) and below (b) a recent barrier to migration, crossed them, and reared their offspring in common garden. At the end of 1 year, we scored offspring as resident “non-smolts” (c) or migratory “smolts” (d)
Model Avg. GLM fit of data with 95% CI
Below barrier
Above barrier
Direction of selection
Otolith 87 Sr/ 86 Sr results were binned to habitat according to their respective water 87 Sr/ 86 Sr values. Days at a site were assigned by number of 60 µ m laser analyses within the habitats water Sr-isotopic range using the estimate of ~5 µ m/daily increment. To increase temporal resolution a second transect of 60 µ m spots was analyzed parallel to the primary spot transect but offset 30 µ m yielding a maximum temporal resolution of 6 days.
Above barrier smolts were also larger, suggesting selection on the threshold length that triggers migration.
Barrier removed
Barrier constructed
Salmon generations
(e.g. 4 years/generation)
Migration model simulated for 100 generations, iterated 100 times. Barrier is added after 10 generations and removed following 50 generations.
Following barrier construction , individuals that grow larger than their inherited threshold fork length will trigger the migration threshold, moving over the barrier and out of the gene pool. Thus, populations which do not rapidly evolve larger threshold fork lengths eventually go extinct (grey lines). Populations that persist (colored lines) produce fewer migratory offspring due to threshold fork lengths exceeding average growth. Populations decrease in size due to lower fecundity of residents (Fig 2) and loss of migratory individuals over the barrier. As a result, most populations go extinct shortly after barrier construction (red line).
Following barrier removal , selection for large threshold fork lengths relaxes and the reproductive advantage of migration is restored. The migratory form can rapidly re-invade the population if the threshold has not evolved beyond a fork length an individual can achieve . Some populations may recover the migratory form as selection favors individuals with threshold fork lengths small enough to be met in growth by the decision window (blue lines). Other populations may become fixed at low rates of migration due to large threshold fork lengths (red lines). In these cases, populations will not recover to pre-barrier numbers due to the lower fecundity of the resident form.