Growth and Physiological Response of Juvenile Tidewater Goby to Interspecific Competition

Intentional reintroduction of an endangered fish species has been used as a recovery tool; however, these fish face competition from fishes that have established in their native habitat since extirpation.  This study sought to investigate the physiological response of tidewater goby (Eucyclogobius newberryi), an endangered species, when in the presence of threespine stickleback (Gasterosteus aculeatus), a native species, and rainwater killifish (Lucania parva), a non-native species.  Following a preliminary experiment, behavioral observations suggested competitive interactions could occur over prey.  To investigate this potential, competition of L. parva and G. aculeatus with E. newberryi was assessed at two food-limited levels. Juvenile fish were held in aquaria for 28 days under 50% and 75% feeding levels. Growth parameters, survival, and stress indicators (i.e. cortisol, glucose, lactate) were assessed. Feeding levels significantly affected weight, body condition, and stress indicators in E. newberryi; however, this effect was similar across competition treatments. Survival of E. newberryi was severely impaired in the presence of G. aculeatus relative to L. parva or other conspecifics. Our results indicate two different mechanisms affected E. newberryi growth and survival; where active scramble competition occurred at the 75% feeding level while sustained elevated stress response occurred at the 50% feeding level.