Trait divergence in river and reservoir populations of Australian smelt (Retropinna semoni)

Dam construction has been a major driver of ecological change in freshwater ecosystems. Fish populations have been shown to diverge in response to different flow velocity habitats. However, adaptation of fish populations to different flow velocity habitats in rivers and reservoirs have not been widely explored. Understanding how fish populations have diverged in response to ecosystem changes, such as altered flow velocity, can help predict the effects of human impacts on freshwater systems. The aim of this thesis was to evaluate divergence of body and fin morphology, prolonged swimming speed performance and physiology in response to different flow velocities by comparing these traits among six river and five reservoir populations of Australian smelt (Retropinna semoni), a small-bodied fish from south-eastern Australia. Australian smelt are a suitable model species for this research because: 1) they are a short-lived species (~ 3 years) and have a short generation time (~ 6 to 12 months); 2) they are widely distributed and common throughout rivers and reservoirs in south-eastern Australia and 3) populations are highly genetically structured and therefore potentially adapted to local habitats. Reservoir habitats selected for this study have existed for at least 50 years, during which time, divergence of resident Australian smelt from river populations may have occurred over multiple generations. Using geometric and traditional morphometric methods, I showed that mean body shape was significantly different among river and reservoir populations of Australian smelt based on Procrustes distances (D_ρ=0.007 – 0.017; F = 26.822; P < 0.001). Rivers populations had deeper bodies, larger heads, narrow caudal peduncles and terminal (horizontal) mouth position compared to reservoir conspecifics which had narrow, fusiform bodies, small heads, relatively deep caudal peduncles and superior (ventral) mouth position. The relationship of swimming speed performance with body morphology and fin aspect ratio was assessed in Australian smelt from a subset of three river and two reservoir populations using critical swimming speed (U_crit) tests in a recirculating swim tunnel. River populations achieved significantly higher mean swimming speeds (U_crit = 46.61 ± 0.98 〖cm s〗^(-1)) than reservoir conspecifics (U_crit= 35.57 ± 0.83 〖cm s〗^(-1); P < 0.05). Caudal fin and pectoral fin aspect ratios were significantly higher for river (1.71 ± 0.04 and 1.85 ± 0.03 respectively) than for reservoir populations (1.29 ± 0.02 and 1.33 ± 0.02 respectively; P < 0.05). The relationship of U_crit with body morphology (Partial Least Squares regression components, from geometric morphometric analyses), fin aspect ratios and standard length was evaluated using the best subset selection approach. Caudal fin and pectoral fin aspect ratios were the strongest predictors of U_crit (β=19.421 and 5.142; t = 9.633 and 3.036 respectively; P < 0.01). Physiological differences were evaluated from a subset of three river and two reservoir populations of Australian smelt. Citrate synthase (a metabolic enzyme) activity in lateral muscle tissue (F = 18.451; P < 0.001) and gill mass (F = 16.498; P < 0.001) were significantly higher in river populations. These fish also had higher U_crit values (F = 73.778; P < 0.001) than reservoir populations. Body shape differences among river and reservoir populations of Australian smelt support previous literature that morphological divergence occurs consistently among populations of freshwater fish from river and reservoir habitats. However, the direction of body shape divergence among Australian smelt populations (i.e. deeper or shallower body in a given habitat) was inconsistent with theoretical predictions and previous studies of body shape in river and reservoir populations of Cyprinella lutrensis, Lepomis gibbosus, Bryconops caudomaculatus and Biotodoma wavrini. In contrast, body shape patterns observed in river and reservoir populations of Australian smelt, were consistent with others such studies of Oncorhynchus nerka, Gasterosteus aculeatus and Labidesthes sicculus. Fin aspect ratios, but not body shape, were strongly correlated with critical swimming speed. Higher citrate synthase activity and gill mass in river populations may reflect the higher swimming speed requirements of living in flowing water. The formation of permanent lentic waterbodies following dam construction may be driving rapid morphological, swimming performance and physiological divergence among river and reservoir populations of Australian smelt. Understanding the effect of divergence in body morphology, fin shape and physiology on swimming performance in river and reservoir populations of Australian smelt has provided new insights into how freshwater fishes have adapted to different flow velocity habitats following dam construction. These results highlighted the need to consider swimming kinematics and swimming mode as central to future research into fish morphology, physiology and swimming performance. Understanding how freshwater fishes adapt and persist is important because it provides insight into how the impacts of dam construction on freshwater fishes could be managed.