Effects of morphology on swimming performance in wild and laboratory crosses of brook trout ecotypes

Summary 1. Trophic polymorphism is common in fish and is manifested by individual differences in habitat use, food preference, and morphology. The significance of morphological variations among ecotypes is thought to reflect individual specializations in habitat use, however, their functional relationship has rarely been examined. 2. The objectives of this study were to determine if littoral and pelagic ecotypes of brook trout differ in their swimming performances, and if so to investigate the functional significance of morphological variations on their swimming performances. 3. One-year-old laboratory-raised littoral and pelagic individuals (and hybrids), as well as wild individuals were forced to swim in respirometers at 10, 15, and 20 °C, and 16 morphological characters were measured on each fish. 4. The critical swimming test (Ucrit) revealed that pelagic trout reared in the wild are better swimmers than littoral ones. In addition, both laboratory-raised littoral and pelagic individuals had higher Ucrit performances than their hybrids. High Ucrit. was associated with short pectoral, anal, caudal and dorsal fins (wild fish) as well as with high body width (both wild and laboratory-raised) and body height (laboratory-raised). Wild littoral individuals had a steeper power curve (i.e. higher energy expenditure due to swimming) than pelagic ones; however, this was compensated by a lower standard metabolic rate for the former, resulting in comparable minimum transport cost for ecotypes. The swimming performances of laboratory-raised ecotypes were more homogenous than those of wild fish, suggesting an important environmental effect on the performances themselves. Water temperature seemly had little influence on the difference between swimming performances of ecotypes. 5. The results of the present study only partially support our hypothesis that critical swimming speed of pelagic individuals will be higher than that of littoral individuals due to their shorter pectoral and dorsal fins and streamlined body shape; high Ucrit was associated with short fins but not with streamlined body shape. Because brook trout is not considered a fast swimmer, it is possible that a higher muscular mass provides higher benefits related to thrust for swimming in the pelagic zone than costs due to drag, usually associated with a high aspect ratio. 6. Our results also support the cost reduction strategy hypothesis (that higher energy expenditure due to swimming is compensated by a lower standard metabolic rate). This implies that the lower performance, due to the higher energy expenditure, of a given ecotype can be compensated by a low standard metabolic rate, a phenomenon that could apply to all species exhibiting morphological plasticity. 7. While previous studies developed conceptual models and predictions relating morphological differences to swimming performance in fishes, this study is one of the first to test such a model. Collectively, our results show that morphological differentiation, associated with trophic polymorphism, have important functional consequences on the swimming energetics of ecotypes.