Evidence suggests that the variability in recruitment of adult Pacific salmon is related to smolt survival during the first ocean year. Specifically, the first few weeks and first marine winter may be two critical periods of high mortality during early marine life. Mortality during early marine residency has been attributed to predation and size-dependent factors while high mortality during the first winter may be due to energy deficits and failure to reach a certain size by the end of the growing season. My study assessed factors influencing overwinter mortality and early marine growth in juvenile Chinook salmon (Oncorhynchus tshawytscha) from Marble River, Quatsino Sound, British Columbia. Juvenile salmon were collected during November 2005 and 2006 (fall) and March 2006 and 2007(winter). Mortality rates over the first winter derived from catch per unit effort across seasons ranged between 80-90% in all years. These are the first estimations of overwinter mortality in juvenile Pacific salmon. Fish size distributions showed no evidence of size-selective overwinter mortality between fall and winter fish in either 2005-2006 or 2006-2007. Otolith microstructure analyses showed no significant difference in circulus increment widths during the first four weeks after marine entry. Similarities in increment width indicated that early marine growth did not differ between fall and winter fish during early marine residency in 2006. These observations show that the high overwinter mortality rates of juvenile Chinook salmon in Quatsino Sound are not size-dependent. Total plankton biomass was significantly lower in the winter season but size distribution, gut fullness and energy density data did not show evidence of starvation. No correlation was found between early marine growth, size, energy accumulation and high mortality in Marble River juvenile Chinook salmon during their first ocean winter in Quatsino Sound. Possible factors influencing these high mortality rates may include non size-selective predation, disease, local environmental influences or an as yet unknown source. Future work should continue to focus on understanding the relationship between early marine survival and adult recruitment. The expansion of growth comparisons geographically and chronologically while determining the effects of predatory mortality on juvenile Chinook salmon along the north Pacific continental shelf and beyond are imperative to fully understanding this complex marine life stage.

Body size, mediated through biotic and abiotic factors affecting growth, is fundamental in determining survival as larger animals are usually less vulnerable to predation, starvation, and extreme environmental conditions (Peterson & Wroblewski 1984; Sogard 1997). Size-selective mortality is a prevalent force regulating marine survival for many anadromous salmonid species, including ESA-listed Chinook salmon (Oncorhynchus tshawytscha) in Puget Sound, WA. The “critical size – critical period” hypothesis suggests that marine survival of anadromous Pacific Salmon (Oncorhynchus spp.) is controlled by two size-selective survival bottlenecks – one during the first marine summer and another during the first marine winter (Beamish and Mahnken 2001). Previous research has indicated a strong positive relationship between the size of juvenile ESA-listed Chinook salmon (O. tshawytscha) in Puget Sound and their survival to adulthood, indicating that early marine growth drives survival (Duffy 2009). Before investigating the drivers of early marine growth, however, it is imperative to understand whether size-selective mortality occurs prior to July in Puget Sound. If so, we may be able to augment marine survival by directing conservation and restoration efforts toward the habitats or regions of Puget Sound where size-selective mortality occurs. Additionally, we must account for any size-selective mortality in estimating early marine growth, as observed weight in July would reflect an artificially inflated “apparent” growth if smaller individuals were experiencing disproportionately high mortality. In this study, we repeatedly sampled nine stocks of both wild and hatchery-origin sub-yearling Chinook salmon during their outmigration into and rearing in Puget Sound. We used scale morphometrics to determine if size-selective mortality is affecting sub-yearling Chinook salmon during their first marine summer rearing in Puget Sound, and if so, where and when that size-selective mortality occurs. We found no evidence of size-selective mortality occurring between habitats or between sampling periods within habitats, suggesting that weight of juvenile Chinook as measured in July is representative of early marine growth and that size-selective mortality occurs later in the summer or outside Puget Sound during the first marine winter. We then focused on understanding differences in growth rates across time, among habitats, and among stocks of juvenile Chinook salmon, and used bioenergetic models to determine the relative influence of prey quality, prey availability, and temperature on early marine growth rates We found that sub-yearling Chinook were larger and grew faster in offshore than in nearshore habitats, and that this difference in growth rate was likely due to differences in prey availability and may have been exacerbated by higher nearshore temperatures. The results of this study can be used to direct restoration and conservation efforts aimed at supporting early marine growth of juvenile Chinook in Puget Sound, and can augment our understanding of distribution patterns and feeding behaviors of Pacific salmon during critical growth periods.

The early marine phase following freshwater emigration has been identified as a critical period in salmonid (Oncorhynchus spp.) life history, characterized by high but variable mortality. Consistent with the “growth-mortality” and “bigger-is-better” hypotheses, at least some of the mortality during the critical period appears to be size-dependent – with smaller or slower growing individuals less likely to survive than larger, faster growing conspecifics. Size and growth are flexible morphological traits that vary with prey availability, yet there is incomplete information on the temporal and spatial match/mismatch between juvenile salmonids and their marine prey in the Northern California Current Ecosystem. This work addressed a gap in the understanding of seasonal variability in prey community composition, abundance, and quality during early marine residence. Three studies were conducted using a population of subyearling (age-0) Chinook salmon (O. tshawytscha) from the upper Columbia River in order to evaluate the effects of prey on salmon growth, biochemistry, and performance. The first was a laboratory study that tested for growth rate and swimming speed differences in salmon reared on three treatment diets followed by three fasting treatments to assess the effects of variability in summer diet quality and winter diet quantity. Significant differences in growth were detected among fasting treatments but not diet treatments. Also, larger salmon with more storage lipids swam faster than smaller leaner fish following fasting, indirectly supporting the notion that growth during the critical period provides a carryover benefit important for overwinter survival. Salmon fatty acids and bulk stable isotopes of carbon and nitrogen were measured throughout the experiment to provide estimates of turnover and incorporation rates. The next study was a longitudinal field study that measured variation in salmon size and prey field community throughout the early ocean period (May – September) over two years of high marine survival (2011 and 2012) to better understand the relationship between prey community composition and salmon growth. Maximum growth rates were associated with high biomass of northern anchovy (Engraulis mordax) which peaked in abundance at different times in each year. The final bioenergetics modeling study combined data from the laboratory and field studies to evaluate the relative importance of prey availability, prey energy density, and temperature on salmon growth. Variation in feeding rate was related most with growth rate variability and least with prey energy density. Throughout their range, subyearlings can grow at high rates in the ocean (>2% body weight per day) by consuming both invertebrate and marine fish prey. However, when marine fish prey are highly abundant they likely provide an energetic advantage over invertebrate prey by reducing overall foraging costs. Quantifying the abundance, size, diet, and distribution of juvenile salmonids relative to their prey field throughout early ocean residence will contribute to a better understanding of seasonal differences in trophic interactions that are associated with differences in annual growth and survival rates. Moreover, an integrated approach that combines sampling of prey with measurements of predator growth, diet, fatty acids, and stable isotopes provides a useful framework for assessing trophic dynamics and evaluating the effects of climate variability and change on predator and prey communities.

From 2008 to 2011, migrating acoustic-tagged juvenile yearling Chinook salmon smolts (Oncorhynchus tshawytscha) were detected on receivers deployed across the Columbia River and continental shelf at Cascade Head (Oregon), Willapa Bay (Washington), and Vancouver Island (British Columbia). The telemetry data were used to estimate survival and record migration parameters. These were evaluated against oceanographic and freshwater hydrologic variables in statistical and individual-based models. Plume survival was found to be variable, but daily survival rates were more constant and survival was effectively modeled as exponential decay. Correlates of early marine survival that do not have direct effects may act on plume survival by controlling the period of exposure to plume predation. In 2011, half of smolts released were exposed to total dissolved gas levels (TDG) above 120%, the water quality limit for TDG below Columbia River dams. This exposure appears to have negatively affected daily survival rates in the lower river and plume, and has important implications for a proposal to increase the TDG limit to 125% to support spring fish passage. Finally, consistent with the critical size, critical period hypothesis of salmon production, it appears that smolts select habitat to maximize their growth as they migrate north through the plume, rather than selectively using local currents to speed their passage. These findings shed new light on perennial questions in salmon early marine ecology. They lay the groundwork for future research aimed at understanding the effects of changing oceanography and freshwater hydrology on salmon migration and survival.

A study of the population ecology of Columbia River fall chinook salmon, Oncorhynchus tshawytscha (Walbaum), was made in an attempt to determine the cause of a serious decline in this run which occurred in the early 1950's. Fluctuations in abundance of major salmon runs the North Pacific were examined to detect any coastwide pattern. Only chinook salmon in Cook Inlet, Alaska, and chum salmon from Oregon to southwestern Alaska showed a similar trend. The following life history stages broken down into pre- and post-decline years were examined: (1) marine life including distribution and migration, growth and maturity, survival rate, oceanography, and commercial and sport fisheries; (2) upstream migration including river fisheries, gear selectivity, size and age composition of the run, escapement, and influence of dams, diseases, and water quality; (3) reproduction and incubation including spawning areas and spawning and incubation conditions; and (4) downstream migration which included predation, dams and reservoirs, diseases, flow, turbidity and temperature, and estuary life. Salient points of the analysis were: (1) a change in the maturity and survival pattern based on tagged and fin-clipped fish recovered before and after 1950; (2) a significant negative correlation between sea-water temperature during a year class' first year at sea and subsequent survival; (3) a large increase in the ocean fisheries coincident with the decline in the run; (4) catch-effort statistics of the ocean fishery show a near classic example of the effect of overexploitation; (5) estimates of the contribution of Columbia River chinook to the ocean fisheries based on tag recoveries could be underestimates rather than overestimates; (6) a significant inverse correlation between estimated ocean catch of Columbia River fall chinook and numbers entering the river; (7) size and age composition of the ocean and river catches decreased coincident with the decline in the run; (8) the gill-net fishery shows little size selectivity by age, size, or sex in the dominant group; (9) fluctuations in abundance of hatchery stocks are related to differences in survival between fingerling and adult; (10) hatchery, lower river, and upriver populations fluctuate in abundance in much the same pattern; (11) optimum escapement is between 90,000 and 100,000 adults, a value that was exceeded during most years; (12) a highly significant negative correlation between numbers of spawners and return per spawner; (13) most of the early dams had no direct effect on fall chinook and the decline in productivity occurred when river conditions were relatively stable; (14) temperatures at time of migration and spawning for fall chinook have not increased enough to be a serious mortality factor; (15) little relationship between flow, turbidity, and temperature at time of downstream migration and subsequent return was evident except that high temperatures and high flows (and turbidities) tended to produce poorer runs during certain time periods; and (16) predation and delay of smolts in reservoirs are largely unknown factors, but circumstantial evidence suggests that they were not important in regulating fall chinook numbers during the period of the study. Finally, variables that appeared to bear some relationship to fluctuations in abundance of fall chinook were submitted to multiple regression analysis. For the predecline period (1938-46 brood years), sea-water temperature and ocean troll fishing effort were significant variables (R2 = 0.74). For post decline years (1947-59 broods), troll had the most influence on total return with ocean temperature and escapement having lesser effects. For the combined years, troll intensity and ocean temperature were the significant variables (R2 = 0.572). Entering interaction of river flow at downstream migration with the other variables brought R2 to 0.754 which means that 75% of the variability in the returning run could be accounted for by these three factors. Return per spawner was so heavily influenced by numbers of spawners that the other factors assumed negligible importance. Equations were derived that predicted the returning run in close agreement with the actual run size. Substituting a low and constant troll fishing effort in the equation resulted in the predicted run maintaining the average predecline level. The increase in ocean fishing was the main contributor to the decline of the Columbia River fall chinook run as shown by correlation, by analogy, and by the process of elimination. To demonstrate why other chinook runs have not shown similar declines, it was shown that due to several unique features in Columbia River fall chinook life history they are exposed to much more ocean fishing than other populations. It was emphasized that these conclusions should not be extrapolated to the future or to other species or runs of salmon.