Spatial variability in ocean‐mediated growth potential is linked to Chinook salmon survival

Early ocean survival of Chinook salmon, Oncorhynchus tshawytscha, varies greatly inter‐annually and may be the period during which later spawning abundance and fishery recruitment are set. Therefore, identifying environmental drivers related to early survival may inform better models for management and sustainability of salmon in a variable environment. With this in mind, our main objectives were to (a) identify regions of high temporal variability in growth potential over a 23‐year time series, (b) determine whether the spatial distribution of growth potential was correlated with observed oceanographic conditions, and (c) determine whether these spatial patterns in growth potential could be used to estimate juvenile salmon survival. We applied this method to the fall run of the Central Valley Chinook salmon population, focusing on the spring and summer period after emigration into central California coastal waters. For the period from 1988 to 2010, juvenile salmon growth potential on the central California continental shelf was described by three spatial patterns. These three patterns were most correlated with upwelling, detrended sea level anomalies, and the strength of onshore/offshore currents, respectively. Using the annual strength of these three patterns, as well as the overall growth potential throughout central California coastal waters, in a generalized linear model we explained 82% of the variation in juvenile salmon survival estimates. We attributed the relationship between growth potential and survival to variability in environmental conditions experienced by juvenile salmon during their first year at sea, as well as potential shifts in predation pressure following out‐migration into coastal waters.     

Demographic changes in Chinook salmon across the Northeast Pacific Ocean

The demographic structure of populations is affected by life history strategies and how these interact with natural and anthropogenic factors such as exploitation, climate change, and biotic interactions. Previous work suggests that the mean size and age of some North American populations of Chinook salmon (Oncorhynchus tshawytscha, Salmonidae) are declining. These trends are of concern because Chinook salmon are highly valued commercially for their exceptional size and because the loss of the largest and oldest individuals may lead to reduced population productivity. Using long‐term data from wild and hatchery populations, we quantified changes in the demographic structure of Chinook salmon populations over the past four decades across the Northeast Pacific Ocean, from California through western Alaska. Our results show that wild and hatchery fish are becoming smaller and younger throughout most of the Pacific coast. Proportions of older age classes have decreased over time in most regions. Simultaneously, the length‐at‐age of older fish has declined while the length‐at‐age of younger fish has typically increased. However, negative size trends of older ages were weak or non‐existent at the southern end of the range. While it remains to be explored whether these trends are caused by changes in climate, fishing practices or species interactions such as predation, our qualitative review of the potential causes of demographic change suggests that selective removal of large fish has likely contributed to the apparent widespread declines in average body sizes.     

Interannual variability in the feeding and condition of subyearling Chinook salmon off Oregon and Washington in relation to fluctuating ocean conditions

Chinook salmon (Oncorhynchus tshawytscha) is one of several economically‐important species of salmon found in the Northeast Pacific Ocean. The first months at sea are believed to be the most critical for salmon survival, with the highest rate of mortality occurring during this period. In the present study, we examined interannual diet composition and body condition trends for late‐summer subyearling Chinook salmon caught off Oregon and Washington from 1998 to 2012. Interannual variability was observed in juvenile salmon diet composition by weight of prey consumed. Juvenile subyearling Chinook salmon were mainly piscivorous, with northern anchovy (Engraulis mordax) being especially important, making up half the diet by weight in some years. Annual diets clustered into two groups, primarily defined by their proportion of invertebrate prey (14% versus 39% on average). Diet composition was found to influence adult returns, with salmon from high‐invertebrate years returning in significantly larger numbers 2–3 yrs later. However, years that had high adult returns had overall lower stomach fullness and poorer body condition as juveniles, a counterintuitive result potentially driven by the enhanced survival of less fit individuals in better ocean conditions (top‐down effect). Ocean conditions in years with a higher percentage of invertebrates in salmon diets were significantly cooler from May to August, and bottom‐up processes may have led to a fall plankton community with a larger proportion of invertebrates. Our results suggest that the plankton community assemblage during this first fall may be critical in predicting adult returns of Chinook salmon in the Pacific Northwest.     

Food habits and marine survival of juvenile Chinook and coho salmon from marine waters of Southeast Alaska

Little is known about the food habits of juvenile Chinook (Oncorhynchus tshawytscha) and coho (Oncorhynchus kisutch) salmon in marine environments of Alaska, or whether their diets may have contributed to extremely high marine survival rates for coho salmon from Southeast Alaska and much more modest survival rates for Southeast Alaskan Chinook salmon. To address these issues, we documented the spatial and temporal variability of diets of both species collected from marine waters of Southeast Alaska during summers of 1997–2000. Food habits were similar: major prey items of both species included fishes, crab larvae, hyperiid amphipods, insects, and euphausiids. Multivariate analyses of diet composition indicated that the most distinct groups were formed at the smallest spatial and temporal scales (the haul), although groups also formed at larger scales, such as by month or habitat type. Our expectations for how food habits would influence survival were only partially supported. As predicted, Southeast Alaskan coho salmon had more prey in their stomachs overall [1.8% of body weight (BW)] and proportionally far fewer empty stomachs (0.7%) than either Alaskan Chinook (1.4% BW, 5.1% empty) or coho salmon from other regions. However, contrary to our expectations, coho salmon diets contained surprisingly few fish (49% by weight). Apparently, Alaskan coho salmon achieved extremely high marine survival rates despite a diet consisting largely of small, less energetically‐efficient crustacean prey. Our results suggest that diet quantity (how much is eaten) rather than diet quality (what is eaten) is important to marine survival.     

Influences of ocean conditions and feeding ecology on the survival of juvenile Chinook Salmon (Oncorhynchus tshawytscha)

Recruitment variability in many fish populations is postulated to be influenced by climatic and oceanographic variability. However, a mechanistic understanding of the influence of specific variables on recruitment is generally lacking. Feeding ecology is one possible mechanism that more directly links ocean conditions and recruitment. We test this mechanism using juvenile Chinook Salmon (Oncorhynchus tshawytscha) collected off the west coast of Vancouver Island, British Columbia, Canada, in 2000–2009. Stable isotopes of carbon (δ¹³C), an indicator of temperature or primary productivity, and nitrogen (δ¹⁵N), an indicator of trophic position, were taken from muscle tissues of genetically stock‐identified salmon. We also collated large‐scale climate indices (e.g., Pacific Decadal Oscillation, North Pacific Gyre Oscillation), local climate variables (e.g., sea surface temperature) and copepod community composition across these years. We used a Bayesian network to determine how ocean conditions influenced feeding ecology, and subsequent survival rates. We found that smolt survival of Chinook Salmon is predicted by their δ¹³C value, but not their δ¹⁵N. In turn, large‐scale climate variability determined the δ¹³C values of salmon, thus linking climate to survival through feeding ecology, likely through qualities propagated from the base of the food chain.     

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

Variation in prey quantity and quality can influence growth and survival of marine predators, including anadromous fish that migrate from freshwater systems. The objective of this study was to examine the energy dynamics of subyearling Chinook salmon (Oncorhynchus tshawytscha) following freshwater emigration. To address this objective, a population of Chinook salmon and their marine prey were repeatedly sampled from June to September over 2 years in coastal waters off Oregon and Washington. Subyearlings from the same population were also reared under laboratory conditions. Using a bioenergetics model evaluated in the laboratory, we found that growth rate variability in the field was associated more with differences in northern anchovy (Engraulis mordax) consumption and less with variation in diet energy density or ocean temperature. Highest growth rates (2.43–3.22% body weight/day) occurred in months when anchovy biomass peaked, and the timing of peak anchovy biomass varied by year. Our results support a general pattern among subyearling Chinook salmon occurring from Alaska to California that feeding rates contribute most to growth rate variability during early marine residence, although dominant prey types can differ seasonally, annually, or by ecosystem. In the northern California Current, faster growth appears to be associated with the availability of age‐0 anchovy. Identifying factors that influence the seasonal development of the prey field and regulate prey quantity and quality will improve understanding of salmon growth and survival during early marine residence.     

Early ocean distribution of juvenile Chinook salmon in an upwelling ecosystem

Extreme variability in abundance of California salmon populations is often ascribed to ocean conditions, yet relatively little is known about their marine life history. To investigate which ocean conditions influence their distribution and abundance, we surveyed juvenile Chinook salmon (Oncorhynchus tshawytscha) within the California Current (central California [37°30′N) to Newport, Oregon (44°00′N]) for a 2‐week period over three summers (2010–2012). At each station, we measured chlorophyll‐a as an indicator of primary productivity, acoustic‐based metrics of zooplankton density as an indicator of potential prey availability and physical characteristics such as bottom depth, temperature and salinity. We also measured fork lengths and collected genetic samples from each salmon that was caught. Genetic stock identification revealed that the majority of juvenile salmon were from the Central Valley and the Klamath Basin (91–98%). We constructed generalized logistic‐linear negative binomial hurdle models and chose the best model(s) using Akaike's Information Criterion (AIC) to determine which covariates influenced the salmon presence and, at locations where salmon were present, determined the variables that influenced their abundance. The probability of salmon presence was highest in shallower waters with a high chlorophyll‐a concentration and close to an individual's natal river. Catch abundance was primarily influenced by year, mean fork length and proximity to natal rivers. At the scale of sampling stations, presence and abundance were not related to acoustic indices of zooplankton density. In the weeks to months after ocean entry, California's juvenile Chinook salmon population appears to be primarily constrained to coastal waters near natal river outlets.     

Competitive interactions among multiple non‐native salmonids and two populations of Atlantic salmon

Competitive interactions with non‐native species can have negative impacts on the conservation of native species, resulting in chronic stress and reduced survival. Here, juvenile Atlantic salmon (Salmo salar) from two allopatric populations (Sebago and LaHave) that are being used for reintroduction into Lake Ontario were placed into semi‐natural stream tanks with four non‐native salmonid competitors that are established in Ontario streams: brown trout (S. trutta), rainbow trout (Oncorhynchus mykiss), Chinook salmon (O. tshawytscha) and coho salmon (O. kisutch). Brown trout and rainbow trout reduced the survival and fitness‐related traits of Atlantic salmon, whereas Chinook salmon and coho salmon had no impact on these traits. These data support theories on ecological niche overlap and link differences in observed aggression levels with competitive outcomes. Measurements of circulating hormones indicated that the Atlantic salmon were not chronically stressed nor had a change in social status at the 10‐month time point in the semi‐natural stream tanks. Additionally, the Sebago population was better able to coexist with the non‐native salmonids than the LaHave population. Certain populations of Atlantic salmon may thus be more suitable for some environments of the juvenile stream phase for the reintroduction into Lake Ontario.     

Year-to-year variability in ocean recovery rate of Columbia River Upriver Bright fall Chinook salmon (Oncorhynchus tshawytscha)

Unusually large returns of several stocks of fall Chinook salmon (Oncorhynchus tshawytscha) from the U.S. Northwest commonly occurred during the late 1980s. These synchronous events seem to have been due to ocean rather than freshwater conditions because natal rivers of these stocks were geographically disconnected. We examined year‐to‐year variability in cohort strength of one of these stocks, Upriver Bright (URB) fall Chinook salmon from the Columbia River Hanford Reach for brood years 1976–99 (recovery years 1979–2002). We used the ocean recovery rate of coded‐wire‐tag (CWT) fish as an index of cohort strength. To analyse year‐to‐year variability in the ocean recovery rate, we applied a log‐linear model whose candidate explanatory variables were ocean condition variables, fishing effort, age of recovered fish, and fish rearing type (hatchery versus wild). Explanatory variables in the best model included fishing effort, and the quadratic term of winter sea surface temperature (SST) measured from coastal waters of British Columbia, Canada during the fish's first ocean year. The coefficient of the quadratic term of SST was significantly negative, so the model shape was convex. Our findings can be used to infer year‐to‐year variability in cohort strength of other fall Chinook salmon whose life history and ocean distributions are similar to the URB fish.     

Covariation between the average lengths of mature coho (Oncorhynchus kisutch) and Chinook salmon (O. tshawytscha) and the ocean environment

We used the average fork length of age‐3 returning coho (Oncorhynchus kisutch) and age‐3 ocean‐type and age‐4 stream‐type Chinook (Oncorhynchus tshawytscha) salmon along the northeast Pacific coast to assess the covariability between established oceanic environmental indices and growth. These indices included the Multivariate El Niño‐Southern Oscillation Index (MEI), Pacific Decadal Oscillation (PDO), Northern Oscillation Index, and Aleutian Low Pressure Index. Washington, Oregon, and California (WOC) salmon sizes were negatively correlated with the MEI values indicating that ultimate fish size was affected negatively by El Niño‐like events. Further, we show that the growth trajectory of WOC salmon was set following the first ocean winter. Returning ocean‐type British Columbia‐Puget Sound Chinook salmon average fork length was positively correlated with the MEI values during the summer and autumn of return year, which was possibly a result of a shallower mixed layer and improved food‐web productivity of subarctic Pacific waters. Size variation of coho salmon stocks south of Alaska was synchronous and negatively correlated with warm conditions (positive PDO) and weak North Pacific high pressure during ocean residence.     

Warm oceans exacerbate Chinook salmon bycatch in the Pacific hake fishery driven by thermal and diel depth-use behaviours

Fisheries bycatch impacts marine species globally and understanding the underlying ecological and behavioural mechanisms could improve bycatch mitigation and forecasts in novel conditions. Oceans are rapidly warming causing shifts in marine species distributions with unknown, but likely, bycatch consequences. We examined whether thermal and diel depth-use behaviours influenced bycatch of a keystone species (Chinook salmon; Oncorhynchus tshawytscha, Salmonidae) in the largest fishery on the US West Coast (Pacific hake; Merluccius productus, Merlucciidae) with annual consequences in a warming ocean. We used Generalized Additive Models with 20 years of data including 54,509 hauls from the at-sea hake fishery spanning Oregon and Washington coasts including genetic information for five salmon populations. Our results demonstrate that Chinook salmon bycatch rates increased in warm ocean years explained by salmon depth-use behaviours. Chinook salmon typically occupy shallower water column depths compared to hake. However, salmon moved deeper when sea surface temperatures (SSTs) were warm and at night, which increased overlap with hake and exacerbated bycatch rates. We show that night fishing reductions (a voluntary bycatch mitigation strategy) are effective in reducing salmon bycatch in cool SSTs by limiting fishing effort when diel vertical movements bring salmon deeper but becomes less effective in warm SSTs as salmon seek deeper thermal refugia during the day. Thermal and diel behaviours were more pronounced in southern compared with northern salmon populations. We provide mechanistic support that climate change may intensify Chinook salmon bycatch in the hake fishery and demonstrate how an inferential approach can inform bycatch management in a changing world.     

Changing salmon: An analysis of body mass,abundance, survival,and productivity trends across 45 years in Puget Sound

Pacific salmon and trout (Oncorhynchus spp., Salmonidae) of the Puget Sound region of Washington State, USA, have experienced recent and longer‐term (multidecadal) variability in abundance while supporting robust fisheries. As part of the post‐season salmon management process, population‐specific estimates of total adult abundance to Puget Sound (Strait of Juan de Fuca) for pink (O. gorbuscha), chum (O. keta), coho (O. kisutch), sockeye (O. nerka), and Chinook (O. tshawytscha) salmon and steelhead trout (O. mykiss) are calculated annually. We compiled annual estimates of body mass, abundance and survival of hatchery‐ and naturally produced salmon from 1970 to 2015 to compare spatial and temporal patterns across species. Average weights of adult salmon and steelhead returning to Puget Sound, with the exception of coho salmon, have decreased since the 1970s. Temporal trends in abundance, survival and productivity varied by species and origin (hatchery vs. naturally produced). Generally, abundance and survival rates of natural‐origin species decreased whereas those of hatchery‐produced species did not, which is in contrast with other studies' general conclusions of decreasing survival among Puget Sound salmonids. Species diversity has decreased in recent years, with salmonids that rely on a short freshwater rearing phase in the natural environment (hatchery‐produced fish and naturally produced pink and chum) representing >90% of total returns in most years. This new information reveals patterns of body size, abundance, survival and productivity across species, life history and rearing type over the past 45 years and, in doing so, demonstrates the strength in multidecadal, multifactor time series to critically evaluate salmonid species.     

Influence of larval transport and temperature on recruitment dynamics of North Sea cod (Gadus morhua) across spatial scales of observation

The survival of fish eggs and larvae, and therefore recruitment success, can be critically affected by transport in ocean currents. Combining a model of early‐life stage dispersal with statistical stock–recruitment models, we investigated the role of larval transport for recruitment variability across spatial scales for the population complex of North Sea cod (Gadus morhua). By using a coupled physical–biological model, we estimated the egg and larval transport over a 44‐year period. The oceanographic component of the model, capable of capturing the interannual variability of temperature and ocean current patterns, was coupled to the biological component, an individual‐based model (IBM) that simulated the cod eggs and larvae development and mortality. This study proposes a novel method to account for larval transport and success in stock–recruitment models: weighting the spawning stock biomass by retention rate and, in the case of multiple populations, their connectivity. Our method provides an estimate of the stock biomass contributing to recruitment and the effect of larval transport on recruitment variability. Our results indicate an effect, albeit small, in some populations at the local level. Including transport anomaly as an environmental covariate in traditional stock–recruitment models in turn captures recruitment variability at larger scales. Our study aims to quantify the role of larval transport for recruitment across spatial scales, and disentangle the roles of temperature and larval transport on effective connectivity between populations, thus informing about the potential impacts of climate change on the cod population structure in the North Sea.     

Relating spatial and temporal scales of climate and ocean variability to survival of Pacific Northwest Chinook salmon (Oncorhynchus tshawytscha)

Pacific Northwest Chinook, Oncorhynchus tshawytscha, have exhibited a high degree of variability in smolt‐to‐adult survival over the past three decades. This variability is summarized for 22 Pacific Northwest stocks and analyzed using generalized linear modeling techniques. Results indicate that survival can be grouped into eight distinct regional clusters: (1) Alaska, Northern BC and North Georgia Strait; (2) Georgia Strait; (3) Lower Fraser River and West Coast Vancouver Island; (4) Puget Sound and Hood Canal; (5) Lower Columbia Tules; (6) Columbia Upriver Brights, Willamette and Cowlitz; (7) Oregon and Washington Coastal; and (8) Klamath River and Columbia River Summers. Further analysis for stocks within each of the eight regions indicates that local ocean conditions following the outmigration of smolts from freshwater to marine areas had a significant effect on survival for the majority of the stock groups analyzed. Our analyses of the data indicate that Pacific Northwest Chinook survival covaries on a spatial scale of 350–450 km. Lagged time series models are presented that link large‐scale tropical Pacific conditions, intermediate‐basin scale northeastern Pacific conditions, and local sea surface temperatures to survival of Pacific Northwest stocks.     

Effects of the North Pacific Current on the productivity of 163 Pacific salmon stocks

Horizontal ocean transport can influence the dynamics of higher‐trophic‐level species in coastal ecosystems by altering either physical oceanographic conditions or the advection of food resources into coastal areas. In this study, we investigated whether variability in two North Pacific Current (NPC) indices was associated with changes in productivity of North American Pacific salmon stocks. Specifically, we used Bayesian hierarchical models to estimate the effects of the north‐south location of the NPC bifurcation (BI) and the NPC strength, indexed by the North Pacific Gyre Oscillation (NPGO), on the productivity of 163 pink, chum, and sockeye salmon stocks. We found that for salmon stocks located in Washington (WA) and British Columbia (BC), both the BI and NPGO had significant positive effects on productivity, indicating that a northward‐shifted bifurcation and a stronger NPC are associated with increased salmon productivity. For the WA and BC regions, the estimated NPGO effect was over two times larger than the BI effect for pink and chum salmon, whereas for sockeye salmon the BI effect was 2.4 times higher than the NPGO. In contrast to WA and BC stocks, we found weak effects of both horizontal ocean transport processes on the productivity of salmon stocks in Alaska. Our results indicated that horizontal transport pathways might strongly influence population dynamics of Pacific salmon in the southern part of their North American ranges, but not the northern part, suggesting that different environmental pathways may underlie changes in salmon productivity in northern and southern areas for the species under consideration.     

Fecundity trends of Chinook salmon in the Pacific Northwest

Fecundity is an important demographic parameter that contributes to the productivity of anadromous fish stock dynamics. Yet, studies on fecundity patterns in Pacific salmon (Oncorhynchus spp.) often only include a few years of data, limiting our ability to understand spatio-temporal trends. Here, we used data on 43 hatchery Chinook salmon (O. tshawytscha, Salmonidae) populations in Washington State to evaluate whether average fecundity changed over the past three decades. We then used data from a subset of stocks (18) to evaluate the relationship between fecundity and body length. Our results revealed significant changes in fecundity across the 25-year study period with most stocks showing declines in fecundity over the past decade. Results further showed that Chinook salmon have decreased in length over this same period and that annual variation in mean length explains a majority (62%) of annual variation in mean fecundity. Specifically, we estimated that a 1-mm reduction in length results in 7.8 fewer eggs (95% CI = 6.6–8.9). Given that the majority of Pacific Northwest Chinook salmon in the environment and harvested in fisheries originate from hatchery releases and that nearby hatchery and wild populations generally have similar ocean distributions, these results likely reflect patterns for many populations not included. Combined, our results highlight the need to consider changes in body size and egg production when assessing the dynamics of anadromous fish populations and designing management or conservation plans, particularly for depressed populations.     

Effects of juvenile size at release and early marine growth on adult return rates for Hokkaido chum salmon (Oncorhynchus keta) in relation to sea surface temperature

Using path analyses, we investigated relationships between size at release from hatcheries, the early marine growth of juveniles, and adult return rates for chum salmon from five river stocks of Hokkaido, Japan, in relation to sea surface temperature during ocean residence. Marine growth was estimated using scales collected from 11 760 adults of age 0.3 (1980–2004). The growth and survival of each stock appeared to have a different suite of regulatory processes. Interannual variability in return rates was mainly regulated by size at release in two stocks from the Sea of Okhotsk. A similar relationship was found in one stock from the Sea of Japan, but growth during coastal residency also affected their return rates. In two stocks from the Pacific coast of Hokkaido, variability in return rates was not related to size at release or to the coastal growth of juveniles, but with offshore growth in the Sea of Okhotsk, the nursery area for juveniles after leaving Japanese coastal waters. Whereas coastal growth tended to be negatively correlated with size at release in some stocks, offshore growth was positively associated with the August–November sea surface temperature in all stocks. This study confirmed that mortality of juvenile salmon occurred in two phases, during the coastal residency and the late period of the growing season, but the relative importance of both phases varied by stock and region, which probably regulated year‐class strength of Hokkaido chum salmon.     

Juvenile river residence and performance of Snake River fall Chinook salmon

An animal's performance during its early life stage can greatly influence its survival to adulthood. Therefore, understanding aspects of early life history can be informative, particularly when designing management plans to rebuild a population. For a threatened population of fall Chinook salmon (Oncorhynchus tshawytscha) in the Snake River of Idaho, we reconstructed the early life history for 124 returning wild and hatchery adults using information recorded in their otoliths. Of our sampled wild adults (n = 61), 43% and 49% reared within the Snake River and Clearwater/Salmon rivers. We also found that only 21% of our sampled wild adults exhibited the historically common subyearling out‐migration strategy, in which juveniles exit freshwater shortly after hatching, while the remaining wild adults exhibited the yearling out‐migration strategy (i.e., individuals delay their freshwater exit). As expected, yearlings had, on average, a significantly larger body size than subyearlings at ocean entry. However, 35% of wild yearlings overlapped in size with wild subyearlings suggesting that spending more time in freshwater might not necessarily result in a larger body size. Lastly, we observed that variability in fork length at Snake River egress and ocean entry were best explained by migration strategy and where it reared, followed by hatch year and sex. Results from this study highlight the utility of adult otoliths in providing details about early life history, an understanding of which is critical to the conservation of Snake River fall Chinook salmon.     

Influence of river conditions during seaward migration and ocean conditions on survival rates of Snake River Chinook salmon and steelhead

Petrosky CE, Schaller HA. Influence of river conditions during seaward migration and ocean conditions on survival rates of Snake River Chinook salmon and steelhead.
Ecology of Freshwater Fish 2010: 19: 520–536. © 2010 John Wiley & Sons A/S Abstract – Improved understanding of the relative influence of ocean and freshwater factors on survival of at‐risk anadromous fish populations is critical to success of conservation and recovery efforts. Abundance and smolt to adult survival rates of Snake River Chinook salmon and steelhead decreased dramatically coincident with construction of hydropower dams in the 1970s. However, separating the influence of ocean and freshwater conditions is difficult because of possible confounding factors. We used long time‐series of smolt to adult survival rates for Chinook salmon and steelhead to estimate first year ocean survival rates. We constructed multiple regression models that explained the survival rate patterns using environmental indices for ocean conditions and in‐river conditions experienced during seaward migration. Survival rates during the smolt to adult and first year ocean life stages for both species were associated with both ocean and river conditions. Best‐fit, simplest models indicate that lower survival rates for Chinook salmon are associated with warmer ocean conditions, reduced upwelling in the spring, and with slower river velocity during the smolt migration or multiple passages through powerhouses at dams. Similarly, lower survival rates for steelhead are associated with warmer ocean conditions, reduced upwelling in the spring, and with slower river velocity and warmer river temperatures. Given projections for warming ocean conditions, a precautionary management approach should focus on improving in‐river migration conditions by increasing water velocity, relying on increased spill, or other actions that reduce delay of smolts through the river corridor during their seaward migration.     

Seasonal variability of cephalopod populations: a spatio‐temporal approach in the Western Mediterranean Sea

Cephalopod populations show wide temporal fluctuations in abundance, which have usually been investigated at inter‐annual scales related to environmental variability. However, cephalopods are also strongly linked to seasonal environmental fluctuations owing to their short life cycles and single seasonal breeding. Therefore, population abundance critically depends on the success of breeding and recruitment from the previous year and the optimization of resources in a narrow period of favorable conditions. This adaptation of population dynamics is paramount in marked oligotrophic systems, such as the western Mediterranean Sea. We used monthly landings per unit effort (LPUE) to explore the spatio‐temporal variability in seasonal patterns of three cephalopod species (Illex coindetii, Eledone cirrhosa and Octopus vulgaris). Common trends across the study area were characterized for each species. In all cases, seasonal patterns were geographically aggregated in relation to differences in local environment (i.e., primary production and surface hydro‐climatology). Variability in the mean seasonal pattern over time was also investigated under contrasting environmental or population regimes. The mean seasonal trend was more pronounced in regimes of high‐population densities, suggesting a density‐dependent control that can modify the strength of the environmental forcing in the seasonal patterns. Our study also evidences a spatial synchrony in the seasonal fluctuations of LPUEs. Scales of synchrony ranged from 70 to 200 km, indicating a patchy‐aggregated spatial pattern as a part of complex population structures in the western Mediterranean. Improving our understanding of seasonal dynamics of cephalopods across temporal and spatial scales may lead to improved forecasts and management strategies.