Distribution of Juvenile Coho Salmon in Relation to Water Temperatures in Tributaries of the Mattole River, California
In an attempt to define the upper thermal tolerance of juvenile coho salmon (Oncorhynchus kisutch), we examined the relationship between the presence of this species and the summer water temperatures in 21 tributaries of the Mattole River of northwestern California. We characterized the temperatures of each tributary using dataloggers, waterproof electronic devices that record hourly water temperatures throughout the summer. Juvenile coho salmon presence was determined by snorkeling pools, following peak summer water temperatures.
We used two different parameters to summarize the water temperature regime in the streams because both are currently used in the literature, and it is useful and informative to compare them in a management context. First, we averaged daily maximum temperatures for each seven day period and then selected the highest average for the entire summer (MWMT - maximum weekly maximum temperature). Second, we averaged daily mean temperatures for each seven day period and selected the highest average for the summer (MWAT - maximum weekly average temperature). It turns out that the greater the daily fluctuation (amplitude), the more different the two measurements are.
We found juvenile coho salmon in 9 of the 21 streams sampled. There were no coho detected in any of the streams with an MWMT above 18.0°C (or an MWAT above 16.7°C). The four coldest streams (MWMT <16°C or MWAT <14.5°C) all contained coho salmon. MWMT and MWAT provided good-fitting logistic regression models for the presence or absence of coho salmon. Both models correctly classified 18 out of 21 streams. Two of the three streams misclassified by the logistic regressions lacked coho salmon (likely for reasons other then temperature).
Thermal regimes in streams supporting coho salmon in the Mattole do not approach the critical thermal maxima for this species determined by short-term laboratory and field tests (e.g., Konecki et al. 1995), which have exceeded 29°C. Long-term survival and growth under natural conditions where food may be limiting and competitors are present may be possible only in a narrower range of temperatures than what can be tolerated under artificial conditions for brief periods. Eaton et al. (1995) estimated that the maximum temperature tolerance for coho salmon is 23.4°C, using a database with observations from throughout the United States. However, these data were mostly from larger streams and rivers and included adult coho salmon. Our research focused only on the nursery areas of young fish in small tributaries.
Upstream reaches of six of the nine tributaries of the Mattole containing coho salmon also support some of the few remaining populations of two cold water adapted stream amphibians in the Mattole, the tailed frog (Ascaphus truei) and the southern torrent salamander (Rhyacotriton variegatus) (H. H. Welsh and G. A. Hodgson, unpublished data). The presence of these amphibians in streams with juvenile coho salmon supports the idea that the latter are a possible surrogate species (Landres et al. 1988; Simberloff 1998; Caro and O`Doherty 1999) for other members of the coldwater adapted biota of the Mattole. However, these amphibians may require cooler thermal regimes than coho salmon and commonly occupy smaller channels and springs in cooler upstream habitats inaccessible to fish (Welsh et al. 2000). In the four Mattole reaches containing tailed frogs, MWMT averaged 14.6°C, and MWAT averaged 13.8°C. In the two reaches conning southern torent salamanders, MWMT averaged 14.5°C, and MWAT averaged 13.8°C. Summer stream temperature data from across the southern range of both amphibian species indicated that the maximum temperature at which they occurred was 15°C (Welsh and Lind 1996; H. H. Welsh and A. J. Lind, unpublished data).
This study suggests that MWMT greater than 18.1°C or MWAT greater than 16.8°C may preclude the presence of coho salmon in the Mattole. It seems very unlikely that streams with temperatures above these thresholds could provide appreciable rearing habitat for coho salmon in the absence of extremely high food availability, which might mitigate elevated metabolic rates of fish in these streams (Bisson et al. 1988).
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