Estimating the escapement of small populations of steelhead Oncorhynchus mykiss (ocean‐migrating rainbow trout) is challenging and requires innovative methods. Difficulties arise from rare and episodic occurrence of individuals, high turbidity levels during migration, and the need to minimize jeopardy to the fish, which have led to a lack of population estimates, especially in California. Here we show that dual‐frequency identification sonar (DIDSON) can be used to produce escapement estimates for a small steelhead population in Scott Creek, Santa Cruz County, California, with virtually no impact on the fish. The DIDSON uses sound to form near‐video quality images and passively monitors fish without the need to handle them or constrict passage. We deployed a DIDSON and recorded steelhead passage over three spawning seasons (2008–2010). We used a decision support tool to analyze DIDSON images and compared the resulting estimates (153, 57, and 84) with raw weir counts (50, 23, and 40), mark–recapture estimates (293 ± 9, 126 ± 12, and 109 ± 34) generated over the entire migration period, and adjusted mark–recapture estimates (201, 74, and 85), which coincided with the period of DIDSON deployment. The DIDSON and weir estimates were restricted to a smaller sampling window due to the installation of downstream migrant traps causing increased incidences of milling fish that interfered with the DIDSON results late in the migration season. The DIDSON estimates were two to three times higher than weir estimates and 23% to 55% less than the full‐season mark–recapture estimates. The adjusted mark–recapture estimates followed the same trends as the full‐season mark–recapture estimates and were correlated with the DIDSON estimates. We conclude that DIDSON is an effective tool to generate steelhead escapement estimates, but it is important to collect data over the entire migration season and to consider fish behavior and potential species identification issues during analysis. Received October 6, 2011; accepted May 21, 2012
Evaluation of population genetic structure and variation is an important part of planning for the recovery and management of protected species. Data from 18 polymorphic microsatellite DNA markers were used to analyze the phylogeographic structure of protected Coho Salmon Oncorhynchus kisutch from populations throughout California. Fish from 30 locations in two evolutionarily significant units (ESUs) representing most of the extant populations in the state were studied. Multiple analyses indicated a hierarchical pattern of population structure: the greatest divergence was found at the broadest geographic scale (ESU), followed by the divergences between basins and populations within basins. The populations of the large Klamath River basin were consistently identified as a distinct phylogenetic group, nearly as divergent from all other populations as the two ESUs were from each other. All populations in different basins were differentiated from each other and a pattern of isolation by distance was found at a California‐wide scale, but not at smaller spatial scales. Similarly, most individuals were accurately assigned to their population of origin, and almost all misassignments were to an adjacent or geographically proximal basin, indicating that there is substantial gene flow within each region but much less between regions. The number of parents contributing to each population was highly variable and reflected larger patterns of genetic variation, which was found to be generally higher in the southerly, low‐elevation coastal populations than in the northern, interior, higher‐elevation populations. The results strongly support the current boundary between the two ESU regions, and the detailed understanding of phylogeographic structure provided here will help to guide the management and recovery of Coho Salmon at the southern end of their geographic range. Received August 25, 2015; accepted May 4, 2016 Published online August 12, 2016
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