Mechanisms for climate-induced mortality of fish populations in whole-lake experiments

Abstract

The effects of climate change on plant and animal populations are widespread and documented for many species in many areas of the world. However, projections of climate impacts will require a better mechanistic understanding of ecological and behavioral responses to climate change and climate variation. For vertebrate animals, there is an absence of whole-system manipulative experiments that express natural variation in predator and prey behaviors. Here we investigate the effect of elevated water temperature on the physiology, behavior, growth, and survival of fish populations in a multiple whole-lake experiment, by using 17 lake-years of data collected over 2 years with differing average temperatures. We found that elevated temperatures in excess of the optimum reduced the scope for growth through reduced maximum consumption and increased metabolism in young rainbow trout, Oncorhynchus mykiss. Increased metabolism at high temperatures resulted in increased feeding activity (consumption) by individuals to compensate and maintain growth rates similar to that observed at cooler (optimum) temperatures. However, greater feeding activity rates resulted in greater vulnerability to predators that reduced survival to only half that of the cooler year. Our work therefore identifies temperature-dependent physiology and compensatory feeding behavior as proximate mechanisms for substantial climate-induced mortality in fish populations at the scale of entire populations and waterbodies.

Fig. 1.

Mean July air temperatures from 1969 to 2005. Solid line represents the significant (P < 0.05) linear increase in temperature over time. Labeled are the 2 years for which we present comparative survival, growth, and behavior of young trout in small lakes (1998, n = 8 lakes; 1999, n = 9 lakes).

Fig. 2.

Average (across eight or nine lakes) hourly temperatures in the littoral zone from stocking in early July to mid-August in the experimental lakes in 1998 and 1999. Every 100 h represents just over 4 days for a total of ≈37 days (or 900 h) total duration. Hourly temperatures are shown to illustrate the variation in water temperature over the entire day and to show lack of temperature overlap between years.

Fig. 3.

Temperature-dependent rates of metabolism (dashed line) and maximum consumption (solid line) for rainbow trout (see Methods). Note that the maximum growth rate and the scope for growth (difference between maximum consumption and metabolism) decline with increases in temperatures above 17.5°C and become zero at 25°C.

Fig. 4.

Survival of age-0 trout. (Top) Survival in relation to growth rate. (Middle) Survival in relation to average water temperature. (Bottom) Proportion of time spent moving by individual age-0 trout in the littoral zone (where most reside) in relation to average water temperature. Each datum represents the estimate obtained for an entire trout population in a given experimental lake; n = 17 lake populations in total.