Summary 1. Droughts have become prevalent in the south‐eastern U.S.A. and are predicted to become more common in the future. Drought conditions have particularly strong negative effects on sessile aquatic organisms with limited dispersal ability. This study explored the linkages between physiological tolerance, behavioural response and survival of three species of freshwater mussels subjected to drought. 2. To assess physiological tolerance, we measured survival under desiccation at 25, 35 and 45 °C in the laboratory. To assess behavioural responses, we tracked horizontal and vertical movement of mussels in a drying stream reach. 3. Uniomerus tetralasmus showed the greatest desiccation tolerance, Lampsilis straminea was intermediate, and Pyganodon grandis had the lowest tolerance at all temperatures. 4. In the drying stream reach, U. tetralasmus showed little horizontal movement and quickly became stranded. The other two species tracked the receding water. It was not until the pool had been reduced to c. 10% of its original size that ≥50% of P. grandis and L. straminea became stranded. 5. Uniomerus tetralasmus and L. straminea burrowed in response to becoming stranded; however, both species burrowed only shallowly (3–4 cm), presumably because burrowing to greater depths did not convey increasing thermal refuge. Pyganodon grandis rarely burrowed. No P. grandis survived the 15‐week drought, while 45% of L. straminea and 77% of U. tetralasmus survived. 6. Three strategies emerged for freshwater mussels to cope with drought conditions: tracking (intolerant), track then burrow (semi‐tolerant) and burrowing (tolerant). Results suggest that drought poses the greatest threat to intolerant trackers, while tolerant burrowers are the most resistant to drought conditions.

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We parameterized and evaluated a bioenergetics model for saugeye (walleye Sander vitreus 3 sauger S. canadensis) by using laboratory experiments in an effort to improve predictions of prey consumption. First, we measured daily prey consumption rate and growth of age-0 and age-1 saugeyes fed two daily rations (ad libitum and 50% of maximum) at five temperatures ranging from 108C to 288C. Additional experiments quantified routine respiration rates and waste losses for three ages of saugeye (ages 0, 1, and 2) at five temperatures ranging from 108C to 288C. Mean daily rates of prey consumption (g g À1 d À1 ) by saugeyes increased from 108C to 258C, declining at 288C. Respiration rates (g O 2 g À1 d À1 ) increased over the entire range of water temperatures. Waste losses were minor for saugeyes as egestion averaged 3.5% of consumed energy and energy lost via excretion was 4.5% of assimilated energy. We evaluated the accuracy of bioenergetics model predictions of saugeye prey consumption using daily prey consumption and corresponding growth data from our first set of experiments. Model estimates of prey consumption rates (g g À1 d À1 ) closely followed observed trends, providing reasonable estimates of cumulative prey consumption across temperature and fish size. The saugeye model provided improved estimates of consumption compared with a model published for walleyes (Kitchell et al. 1977), especially when water temperatures were in excess of 258C. The differences we observed in predictive performance between the two models resulted from higher thermal optima for saugeyes compared with walleyes, and waste constants for saugeyes were two to three times lower than those calculated from the walleye model. These differences may largely be responsible for the walleye model's overestimation of consumption. Saugeye thermal optima are warmer than those of either parent species, and saugeye is better suited for warm, productive midwestern U.S. reservoirs. The saugeye model developed herein will improve the ability of managers to more accurately predict the consumptive demand of in situ saugeye populations and better tailor stocking rates to match available prey biomass.

Summary Suspended solids are one of the most widespread pollutants in streams, but little is known about their effects on freshwater mussels. In a previous study, we showed that increasing concentrations of total suspended solids (TSS) resulted in a decreased proportion of gravid female mussels; however, the generality of this effect and the specific stage at which reproduction was inhibited remain unknown. In this study, we examined stage‐specific disruption of reproduction in a short‐term brooding mussel species (Reginaia ebenus) and a long‐term brooding species (Ligumia subrostrata). High TSS concentrations disrupted reproduction in both the short‐ and long‐term brooding species, but the mechanism appeared to differ between species. A high proportion of female Reginaia ebenus (range = 0.33–0.93) were fertilised across the entire TSS gradient (11–92 mg L−1), but few glochidia developed at TSS >20 mg L−1. In contrast, only a low proportion (0–0.28) of female Ligumia subrostrata were fertilised at high TSS concentrations, but all gravid females eventually produced fully developed glochidia. Excessive inputs of sediments and/or nutrients probably disrupt early reproductive stages of freshwater mussels via elevation of inorganic or organic TSS concentrations and may be important drivers of freshwater mussel declines.

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