DESCRIPTION
The goal of this project is evaluate the role and drivers of juvenile, size-selective mortality as it relates to the overall marine survival of ESA-listed Puget Sound Chinook that inhabit the San Juan Islands. This will be done by: a) identifying the critical periods of growth and associated habitats; and b) determining whether temperature, food supply, energetic quality of food, or competition are the primary factors limiting growth. Our work will be performed during the juvenile marine residence period in the nearshore and offshore areas of the San Juan Islands where ESA-listed, Whidbey Basin and Nooksack Chinook populations are present. We will collaborate with in-river, estuarine, nearshore, and some offshore sampling efforts in the Whidbey Basin and Bellingham Bay to capture as much of the range of juvenile marine residence for the targeted populations as possible. Emerging genetic techniques will be used to better discriminate the Chinook stocks than could be done previously. This work will help refine the focus on the priority habitats/ecological conditions to protect and restore (expanding to the offshore) by providing more detail about the relationship between specific habitats/areas, their ecological conditions, and the growth and survival of salmon that utilize the San Juans. This project will compliment similar work proposed for Central/South Puget Sound as part of a collective effort to investigate declines in marine survival of Puget Sound Chinook.
This project successfully assessed the role and drivers of juvenile, size-selective mortality of ESA-listed Puget Sound Chinook that inhabit the San Juan Islands. Juvenile Chinook were collected in 2014 and 2015 from in-river, nearshore and offshore habitats, starting from the Nooksack and Skagit Rivers out into Whidbey Basin/North Puget Sound and through the San Juan Islands. The Snohomish and Nisqually Rivers and Central and South Puget Sound were analyzed as part of the Puget Sound wide growth and size-selective mortality effort but aren't addressed here due to these populations limited use of the San Juan Islands, as shown by the genetic identification work done as part of this project. Overall, little evidence of size selective mortality was found among juvenile life stages from marine entry through early August throughout Puget Sound, including the San Juan Islands. Skagit and Nooksack River Chinook dominated the composition of Chinook captured at the study sites, followed by Glenwood Hatchery (Orcas Island) and Strait of Georgia Chinook and a small percentage from other areas of Puget Sound. The diets of Chinook in the San Juan Islands nearshore areas had greater proportions of fish in them, and the Chinook relied less on terrestrial insects and marine invertebrates when compared to Chinook eating in like nearshore areas adjacent their natal Skagit and Nooksack watersheds. The mean proportion of fish in the diets of Chinook captured in the San Juan Islands was ~0.50 while it was 0.11 and 0.10 for the Skagit and Nooksack, respectively. Furthermore, the proportion of stomachs with fish in them was higher the San Juan Islands (0.61) compared with the other regions (Skagit: 0.15 and Nooksack: 0.14). Analyses of IGF-1 concentrations suggest increased growth rates for Chinook inhabiting the San Juan Islands nearshore habitats when compared to nearshore and offshore habitats adjacent to the natal watersheds. Both temperature and prey type/quality influenced the observed increases in individual growth rates. Increased IGF-1 concentrations were related to individual Chinook size and the presence of fish in diets, primarily Pacific herring and Pacific sandlance. Where fish were available as prey and where Chinook salmon were large enough to incorporate fish into their diets, individuals had higher observed IGF-1 concentrations compared to Chinook that were smaller, fed on invertebrate prey, and/or inhabited other regions. Bioenergetics simulations also found temperature and potentially prey quality influenced the higher observed absolute growth rates for fish capture in the San Juan Islands. Water temperatures in the San Juan Islands were often lower than other regions/habitats and were more stable seasonally than those of nearshore habitats in the other regions. Surface temperatures in the San Juan Islands nearshore habitat remained close to the peak or optimum for growth potential. Further testing is being done to clarify the bioenergetics results and refine the assessment of types of forage fish consumed by Chinook. These data suggest that San Juan Islands Chinook recovery efforts should build better connections with the Skagit and Nooskack recovery plans, and priorities for actions should be evaluated in the context of all actions for Skagit and Nooksack populations. These data also suggest that the San Juans nearshore should be considered as providing a unique advantage relative to other marine areas, with herring and sandlance, and cooler waters, available that promote feeding and growth. These data suggest that San Juan Islands Chinook recovery efforts should build better connections with the Skagit and Nooskack recovery plans, and priorities for actions should be evaluated in the context of all actions for Skagit and Nooksack populations. These data also suggest that the San Juans nearshore should be considered as providing a unique advantage relative to other marine areas, with herring and sandlance, and cooler waters, available that promote feeding and growth. Due the influence of circulation, water volume and bathymetry on the San Juan Islands nearshore, actions to sustain current nearshore water temperatures (e.g. riparian cover), even in the face of climate change, may not be warranted. Instead, salmon recovery actions in the San Juans should primarily focus on supporting the productivity of the herring and sandlance they provide. . Instead, salmon recovery actions in the San Juans should primarily focus on supporting the productivity of the herring and sandlance they provide.
