Aerobic metabolic capacity was similar between juvenile Chinook salmon reared at 15 and 19°C and showed little change with acute warming to 23°C.

1. The effects of water temperature on individual and group movement behaviour in prey fish can affect ecological interactions such as competition and predation, but how variability in temperature influence fish behaviour is less understood.Of particular concern is how increased warming in tidally fluctuating estuaries may impact the native and endangered delta smelt (Hypomesus transpacificus, Osmeridae).2. To help address this issue, we tested the effects of increased water temperature (fluctuating [17-21°C] and warm [21°C] acclimated treatments) on juvenile delta smelt individual and group behaviour, response to chemical alarm and predator cues, as well as capacity to evade predation. In addition, predation of delta smelt was tested in the presence of a dominant invasive competitor, Mississippi silversides (Menidia beryllina, Atherinopsidae), as well as comparative predation mortality on Mississippi silversides when isolated.3. After 7 days of increased temperature treatments, delta smelt in the warm treatment increased swimming velocity, decreased turning angle, and altered group structure with larger inter-individual distances compared to fish in the control (17°C) and fluctuating temperature treatments. Following conspecific and predator chemical alarm cues, delta smelt showed anti-predator responses. Control and fluctuating treatment fish responded to conspecific cues with increased swimming speeds, decreased inter-individual distances and near-neighbour distances, and, after 15 min, fish recovered back to baseline behaviours. In contrast, fish in the warm treatment had not recovered after 15 min, and swimming speeds were maintained at roughly 25 cm/s, close to maximum capabilities. Fish in control and fluctuating treatments showed minimal responses to predator cues, whereas delta smelt exposed to warm conditions significantly increased swimming speeds and decreased turning angle. Predation of delta smelt by largemouth bass (Micropterus salmoides, Centrarchidae)was greatest under the warm treatment, correlating with altered behaviours of | 2157 DAVIS et Al.

Human-induced thermal variability can disrupt energy balance and performance in ectotherms; however, phenotypic plasticity may play a pivotal protective role. Ectotherm performance can be maintained in thermally heterogeneous habitats by reducing the thermal sensitivity of physiological processes and concomitant performance. We examined the capacity of juvenile green sturgeon (Acipenser medirostris) to respond to daily thermal variation. Juveniles (47 days post-hatch) were exposed to either stable (15 ± 0.5 °C) or variable (narrowly variable: 13-17 °C day or widely variable 11-21 °C day) thermoperiod treatments, with equivalent mean temperatures (15 ± 0.5 °C), for 21 days. Growth (relative growth rate, % body mass gain), upper thermal tolerance (critical thermal maxima, CTMax) and the thermal sensitivity of swimming performance (critical swimming speed, U) were assessed in fish from all treatments. Accelerated growth was observed in fish maintained under widely variable temperatures compared to narrowly variable and stable temperatures. No significant variation in CTMax was observed among thermoperiod treatments, suggesting all treatment groups acclimated to the mean temperature rather than daily maximums. The widely variable treatment induced a plastic response in swimming performance, where U was insensitive to temperature and performance was maintained across a widened thermal breadth. Maximum U attained was similar among thermoperiod treatments, but performance was maximised at different test temperatures (stable: 4.62 ± 0.44 BL s at 15 °C; narrowly variable: 4.52 ± 0.23 BL s at 21 °C; widely variable: 3.90 ± 0.24 BL s at 11 °C, mean ± s.e.m.). In combination, these findings suggest juvenile A. medirostris are resilient to daily fluctuations in temperature, within the temperature range tested here.

Throughout the world, louver‐bypass systems are a common method for fish protection at water diversion sites. Their performance has been evaluated for pelagic, strong‐swimming teleost fishes, but limited information exists regarding the effectiveness of louvers for guiding sturgeon, despite being a globally threatened taxon. This study used controlled laboratory experiments to quantify louver efficiency for juvenile green sturgeon under a range of conditions. Fish guidance efficiency was influenced most strongly by fish size and approach velocity, with poor efficiency for small fish at high velocities. Low velocities and, to a lesser extent, daylight conditions allowed a greater proportion of fish to remain upstream of the louver at all sizes. High velocities resulted in the greatest contact rate between the sturgeon and louver face. Louvers can be effective for benthic, weak‐swimming fish, but site‐ and species‐specific needs should always be considered in the development or operation of a louver guidance system.