Size, Growth, and Size‐Selective Mortality of Subyearling Chinook Salmon during Early Marine Residence in Puget Sound

Abstract

In marine ecosystems, survival can be heavily influenced by size‐selective mortality during juvenile life stages. Understanding how and when size‐selective mortality operates on a population can reveal underlying growth dynamics and size‐selective ecological processes affecting the population and thus can be used to guide conservation efforts. For subyearling Chinook Salmon Oncorhynchus tshawytscha in Puget Sound, previous research reported a strong positive relationship between marine survival and body mass during midsummer in epipelagic habitats within Puget Sound, suggesting that early marine growth drives survival. However, a fine‐scale analysis of size‐selective mortality is needed to identify specific critical growth periods and habitats. The objectives of this study were to (1) describe occupancy patterns across estuarine delta, nearshore marine, and offshore epipelagic habitats in Puget Sound; (2) describe changes in FL and weight observed across habitats and time; (3) evaluate evidence for size‐selective mortality; and (4) illustrate how marine survival of the stocks studied may be affected by variation in July weight. In 2014 and 2015, we sampled FLs, weights, and scales from seven hatchery‐origin and two natural‐origin stocks of subyearling Chinook Salmon captured every 2 weeks during out‐migration and rearing in estuary, nearshore, and offshore habitats within Puget Sound. Natural‐origin stocks had more protracted habitat occupancy patterns than hatchery‐origin stocks and were smaller than hatchery‐origin stocks in both years. Regardless of origin, subyearlings were longer and heavier and grew faster in offshore habitats compared to estuary and nearshore habitats. For all stocks, we found little evidence of size‐selective mortality among habitats in Puget Sound. These patterns were consistent in both years. Finally, the weights of subyearlings sampled during July in the offshore habitat predicted Puget Sound‐wide marine survival rates of 0.4% for 2014 and 2.0% for 2015, with stock‐specific predictions ranging from 0.18% to 11.70%.

Freshwater Tidal Forests and Estuarine Wetlands May Confer Early Life Growth Advantages for Delta‐Reared Chinook Salmon

“…Scale samples were processed according to methods outlined in Gamble et al. (). We used the two most recent complete scale circuli to back‐calculate the FL and weight of each fish using the Fraser–Lee method (Pierce et al.…”

“…We calculated daily growth rate (g·g −1 ·d −1 ) for each fish using the equation: where W t is the back‐calculated weight from the most recent circulus, W 0 is the back‐calculated weight from the penultimate circulus, and t is the number of days ( t = 6) needed to grow from one circulus to the next (Gamble et al. ).…”

“…Additionally, as part of the Salish Sea Marine Survival Project, we supplemented biweekly surveys with samples collected from the offshore subtidal zone using a purse seine. Survey methods are described in detail in Davis et al (2018b) and Gamble et al (2018).…”

Estuary habitat associations for juvenile Pacific salmon and pelagic fish: Implications for coastal planning processes

“…Because much of the habitat closer to shore (within <5 m) is subtidal and can be sampled with the 9 m or 5 m deep purse seines, an association with locations closer to shore suggests that fish were found in higher abundances in subtidal nearshore habitat on channel margins compared with open water between landforms. In other estuaries, larger sub‐yearlings and yearling Chinook and coho salmon are associated with deeper channel or channel margin habitats (10–15 m deep; (Gamble et al, ; Pinnix, Nelson, Stutzer, & Wright, ; Roegner et al, ), whereas smaller fry or sub‐yearling Chinook are associated with shallow nearshore habitat (<3 m deep; Bottom, Jones, et al, ; Hering et al, ; Roegner et al, ; Simenstad et al, ). As both, coho (73–242 mm) and Chinook (85–185 mm) salmon found in the Skeena River estuary are primarily larger age classes (subyearling or ≥1 age class—using designations from Roegner et al, and Weitkamp et al, ), results from this study are comparable to findings from other estuaries.…”

Energy dynamics of subyearling Chinook salmon reveal the importance of piscivory to short‐term growth during early marine residence

“…Size of subyearling Chinook salmon during early marine residence can be highly variable (Claiborne et al, 2014;Gamble et al, 2018;Miller et al, 2013). In this study, we used mean size and time of capture to estimate growth following freshwater emigration.…”