Effects of water current on growth performance of juvenile Gambusia affinis and Tanichthys albonubes at different ration levels

This study was performed to provide further evidence of inter-species competition between Gambusia affinis and Tanichthys albonubes. We assessed the growth performances of juvenile animals of the two species at different water velocities (0 and 1.5 body length per second [bl/s]) and ration levels (repletion- and medium-rations). For each treatment, 30 experimental fish were chosen randomly and cultured in a specially designed feeding equipment at 25 ± 1 °C. Initial body lengths and masses were measured in 50 fish randomly selected from the experimental animals. During the experiment, fish were fed twice (09:00 and 21:00) daily. After 30 days, food intake, mortality rates, body lengths, dry body masses, specific growth rates, and growth efficiencies were evaluated. The results showed that feeding rates of both G. affinis and T. albonubes had no significant differences between flowing (1.5 bl/s) and static (0 bl/s) waters, both in repletion and medium rations. Daily satiation of G. affinis and T. albonubes increased in a linear trend with feeding time. Feeding rates were significantly higher in G. affinis compared with T. albonubes (P $_amp_$lt; 0.05). Mortality rates of G. affinis showed no significant difference between flowing and static waters at repletion ration while, under medium ration conditions, G. affinis died quite considerably in static water (P $_amp_$lt; 0.05). Body lengths, dry masses, specific growth rates, growth efficiencies, and condition factors in G. affinis and T. albonubes showed no significant differences between flowing and static water conditions at repletion ration. However, at medium ration in flowing water, these parameters increased significantly in G. affinis (P $_amp_$lt; 0.05), but reduced considerably in T. albonubes (P $_amp_$lt; 0.05). These findings indicate that although T. albonubes had higher growth rates with adequate food supply, G. affinis had higher feeding rates and was more adaptable to water currents and lower ration levels. This shows that G. affinis may have more advantages in feed resource competition in the common distribution of natural stream habitats.