Upper thermal limits on the oceanic distribution of Pacific salmon (Oncorhynchus spp.) in the spring

Abstract

Pacific salmon are normally thought to be distributed throughout the Subarctic Pacific, an area where they form the dominant fish fauna. We use a series of generalized additive models to show that salmon exhibit a sharp step-function response to temperature in the oceanic eastern north Pacific in spring. The critical temperature defining the southern boundary varied by species: 10.4"C for pink and chum salmon, 9.4"C for coho salmon, and 8.9"C for sockeye salmon. These thermal limits occur well to the north of the southern boundary of the Transition Zone, at widely separated geographic positions within the Subarctic Domain, and at temperatures much lower than the lethal upper limit for each species. The sharp decline in abundance with temperature, and the remarkably low temperatures at which the response occurs, suggests that thermal barriers form an effective limit to the offshore distribution of salmon in spring, and can limit the distribution of Pacific salmon to a relatively small area of the Subarctic Pacific. The strength of this response is presumably the direct result of strong evolutionary selection. Future temperature changes in the North Pacific could therefore have a direct impact on the production dynamics of Pacific salmon.

Aggregation on the edge: effects of hypoxia avoidance on the spatial distribution of brown shrimp and demersal fishes in the Northern Gulf of Mexico

“…logistic; Bell & Eggleston 2005), the functional form of which may influence the estimate of the low DO threshold. I modified a semi-parametric modeling approach originally developed by Welch et al (1995) to estimate a mean DO avoidance threshold and associated variance separately for the 10 most abundant species in the trawl sampling (see also Craig et al 2010). This approach assumes only that a low DO avoidance threshold with a mean and variance from a particular distribution exists.…”

“…A large variance indicates a gradual change in CPUE with decreasing DO and a weak avoidance response. A small variance indicates a rapid change in CPUE with decreasing DO and a strong avoidance response (after Welch et al 1995) I assumed a normal distribution for the avoidance threshold but also considered a lognormal distribution because catches showed some positive skew near the edge of the hypoxic zone (see Fig. 4).…”

“…Additional details regarding the threshold model can be found in Welch et al (1995) and Craig et al (2010). The threshold model was also compared to a null (intercept-only) model and a linear model using Akaike's information criterion (AIC; Akaike 1973).…”

“…The variance about the mean threshold can be interpreted as a measure of the strength of low DO avoidance behavior, with a low variance indicating a strong avoidance response while a high variance indicates a weak avoidance response. The threshold modeling approach was originally developed to quantify the oceanic distribution of salmon in relation to temperature (Welch et al 1995) but is particularly applicable to DO, which is generally thought to operate in a threshold manner, imposing a limit on the spatial distribution of organisms (Fry 1971). Advantages of this approach are that it is based on commonly collected field data (point estimates of CPUE and DO from a survey) and does not require assumptions about the functional form of the relationship between abundance and DO, as do parametric modeling approaches.…”

Aerobic scope increases throughout an ecologically relevant temperature range in coho salmon

“…During their spawning migration, adult coho salmon are thought to conform to the OCLTT hypothesis by achieving peak AS at the water temperatures most commonly encountered during their upstream migration to freshwater spawning areas Farrell et al, 2008). We measured AS in adult coho salmon from the Chilliwack River, British Columbia, Canada at 10°C, a temperature commonly encountered both during ocean life (Welch et al, 1995) and during the upstream migration to spawning areas, and after at least 2 days of exposure to 15°C, the extreme upper limit of historically experienced temperatures during their 125 km (∼5-7 day) upstream migration to the spawning area. Use of these two key temperatures, coupled with long-term thermal datasets from the relevant watersheds, enabled us to test the prediction that the AS of adult coho salmon peaks at the temperature(s) most commonly encountered during the upstream migration to spawning areas (here, ∼10°C) and declines at a higher temperature (15°C).…”