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.behavior ͉ temperature ͉ growth ͉ trade off A s our climate continues to warm, there must be corresponding direct effects on animal populations because the two fundamental physiological components of growth, consumption and metabolism, are strongly affected by temperature (1). Populations are often highly adapted to their local climates, and so increases in temperature in excess of their evolved optimum value for growth (2) must reduce maximum consumption rates and increase metabolic rates (1). Resultant decreased growth may lead to negative effects on important ecological processes that depend on body size. For instance, low growth rate and small body size reduce survival of juvenile fish faced with risks of starvation and predation (for review, see ref.3). On the other hand, compensatory (or catch-up) growth is common in fishes (4), resulting from greater foraging effort to increase food consumption (5, 6). Thus, fish may behaviorally compensate for poor growing conditions caused by climate. However, although greater foraging effort will increase growth, it can also result in greater mortality rates because of greater visibility to, and encounter rates, with predators (7, 8).Although we now have a broad perspective on the potential impacts of a warming climate on a variety of plant and animal populations, including fish (9, 10), there is an absence of truly large-scale manipulative experiments in vertebrate animal populations. As a result, we have a good understanding of the effects of climate warming on processes that impact individuals, bu...

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