Have a break or keep going - behavioral and metabolic overwintering strategies of two invasive species of the river Rhine, Germany

Winter conditions, extended periods of low temperature and limited food availability, impose pressures to develop strategies to cope with such unfavorable environmental conditions. Overwintering strategies may embrace both behavioral and physiological adaptations and result in differential responses. To address the effects of low temperature and feeding treatments (feeding or starving) in two common invasive goby species of the River Rhine, the round goby (Neogobius melanostomus) and the monkey goby (Neogobius fluviatilis) we monitored locomotor activity and measured oxygen consumption at 5 degrees C in monthly intervals for 90days. Under still warm water conditions at 15 degrees C as a baseline, locomotor activity did not differ between the two species, but N. melanostomus had a higher metabolic rate. During the course of the 5 degrees C cold overwintering period, the behavioral responses were strikingly different between the two species: N. melanostomus maintained higher activity levels regardless of the feeding treatment, while N. fluviatilis showed a lower activity with a strong response to the feeding treatment. After 90days of simulated winter condition, the starved groups of both species increased their locomotor activity. Oxygen consumption on the other hand showed no differences between fed and starved N. fluviatilis. In contrast, N. melanostomus had a stronger response to temperature per se and for starving individuals consistently lower metabolic rates. We conclude that the two locally most abundant fish species adopted two different overwintering strategies. N. fluviatilis applies a low energy expenditure strategy resulting in a high survival rate even under starvation. N. melanostomus applies a more risky high energy expenditure strategy with increased activity levels under extended periods of starvation.