Growth and physiological response of juvenile tidewater goby to interspecific competitive interactions

Recovery of endangered fish species can be complicated by threats from introduced species. Intentional\ud reintroduction to historic habitat has been used as a recovery tool; however, these fish face competition\ud from fishes that have established in their native habitat since extirpation. This study sought to investigate\ud the physiological response of tidewater goby (Eucyclogobius newberryi), an endangered fish species, to the\ud presence of threespine stickleback (Gasterosteus aculeatus), a native species, and rainwater killifish\ud (Lucania parva), a non-native species. I began my investigation with a fully factorial design to examine\ud social interactions between juvenile fishes. Over a 28-day period, growth parameters (e.g. weight, length,\ud and relative body condition) and stress hormone levels (cortisol) were assessed under replete-food\ud conditions at a salinity of 15 ppt. All species had positive growth, and while each species had different\ud growth patterns, fish species assemblage did not impact growth or cortisol levels. Behavioral observations\ud suggested that competitive interactions could occur over prey if food levels were lower. To investigate this\ud potential, competition of L. parva and G. aculeatus with E. newberryi was assessed at two food-limited\ud levels. Juvenile fish were held in aquaria for 28 days under 50% and 75% feeding levels at a salinity of 25\ud ppt. Growth parameters, survival, and indicators of stress (e.g. cortisol, glucose, and lactate) were assessed.\ud Reduced food availability significantly affected weight, body condition, and stress indicators in E.\ud newberryi; however, this effect was similar across species competition treatments. Survival of E. newberryi\ud was severely impaired in the presence of G. aculeatus relative to L. parva and conspecifics. Our results\ud indicate two different mechanisms affected E. newberryi growth and survival; active scramble competition\ud at the 75% feeding level versus sustained elevated stress response at the 50% feeding level. Findings for\ud this research indicate greater competitive impact on E. newberryi from native species, with the non-native\ud L. parva also performing better than E. newberryi under low food levels