Colonization of new environments should promote rapid speciation as a by-product of adaptation to divergent selective regimes. Although this process of ecological speciation is known to have occurred over millennia or centuries, nothing is known about how quickly reproductive isolation actually evolves when new environments are first colonized. Using DNA microsatellites, population-specific natural tags, and phenotypic variation, we tested for reproductive isolation between two adjacent salmon populations of a common ancestry that colonized divergent reproductive environments (a river and a lake beach). We found evidence for the evolution of reproductive isolation after fewer than 13 generations.

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We quantified the juvenile rearing and migratory patterns of individuals from a population of fall-spawning Chinook salmon Oncorhynchus tshawytscha in Oregon's Salmon River estuary using otolith microchemistry and microstructure. The study confirmed the daily periodicity of otolith growth increments in a natural fish population under field conditions and validated fundamental assumptions about increased otolith strontium : calcium values during entry into saline waters. The otolith results indicated that more than 75% of the subyearling Chinook salmon captured near the mouth of the Salmon River had entered the estuary during the summer and that two-thirds of these fish had spent more than a month in the estuary before capture. Unlike in other Oregon coastal estuaries, in which the fingerling-migrant portion of their life histories is dominant, approximately two-thirds of Salmon River Chinook salmon in upper-estuary marshes were early fry (,50mm) migrants. A much smaller proportion at the river mouth suggests that many fry migrants did not survive to the lower estuary or passed undetected during ocean emigration. Nonetheless, the otolith results reveal a substantially greater contribution of estuarine-rearing fry to the out-migrant population at the Salmon River than has been reported in other Oregon coastal basins. A small component of fall-migrating fish with long freshwater residence times also occurred at the river mouth. Most of these individuals had migrated quickly through the estuary. Rather than revealing a series of discrete ''types'' defined by the predominant rearing patterns in the population, the individual otolith results depict a continuum of freshwater and estuarine life histories that is consistent with reports of considerable phenotypic plasticity in Chinook salmon. Otolith analysis offers the potential to quantify the relative contributions of different juvenile rearing patterns to adult returns.

Effects of different prey taxa and daily ration levels on fish growth and the relationship between fish growth rate and mean otolith increment width were investigated for juvenile chum salmon (Oncorhynchus keta) in saltwater aquaria. Growth was positively correlated with ration, and food conversion efficiency was much higher for fish fed the harpacticoid copepod, Tigriopus californicus, than either the calanoid copepod, Pseudocalanus minutas, or the gammarid amphipod, Paramoera mohri. Otolith increments were produced daily for at least the first 160 d after hatching and there was a direct relationship between mean daily otolith increment width and fish growth rate. These results illustrate the possibility that otolith microstructure recapitulates juvenile chum growth histories during estuarine residence.

The complementary use of otolith chemistry and radiotelemetry demonstrates that bull trout Salvelinus confluentus from the Hoh River, Washington, exhibit considerable life history variability. Adult bull trout lived exclusively in the river, inhabited freshwater for prolonged periods and later became anadromous, or were anadromous and made multiple migrations between freshwater and salt water. Twenty of 40 radio‐tagged juvenile bull trout emigrated to the ocean at lengths ranging from 243 to 360 mm (mean, 287 mm), which is the first published verification of anadromy at this life history stage. Otolith chemistry analyses of 105 bull trout that were incidentally killed in commercial gill‐net fisheries revealed that 85% had migrated from freshwater to the sea at least once and that 75% had migrated multiple times. Anadromous females produced 95% of all individuals examined, but both anadromous and nonanadromous females produced progeny that were anadromous. Age at first seaward migration ranged from 3 to 6 years, 88% first emigrating to sea in their third or fourth growth year. For ages 3 and 4, anadromous individuals were larger than those that remained in freshwater. A wide size range (287–760 mm, 0.2–4.9 kg) of bull trout were killed in commercial fisheries; ages 3–5 composed 88% of the total bycatch. Relocation data from radio‐tagged juvenile and adult bull trout provided important insights on anadromous movements that helped to validate inferences drawn from widely oscillating strontium levels in otolith chemistry. In view of the direct mortality in gill‐net fisheries, an understanding of the age‐specific movements and life history variability of anadromous bull trout will be crucial to future conservation efforts, which should focus on improved monitoring of recreational and commercial bycatch in Pacific salmon fisheries.