Critical swimming speeds of wild bull trout

Abstract

We estimated the critical swimming speeds (U c r i t ) of wild bull trout at 6°, 11°, and 15°C in laboratory experiments. At 11°C, 5 fish ranging from 11 to 19 cm in length had a mean U c r i t of 48.24 cm/s or 3.22 body lengths per second (BL/s). Also at 11°C, 6 fish from 32 to 42 cm had a mean U c r i t of 73.99 cm/s or 2.05 BL/s. At 15°C, 5 fish from 14 to 23 cm had a mean U c r i t of 54.66 cm/s or 2.88 BL/s. No fish successfully swam at 6°C. Swim speed was significantly influenced by fish length. Many bull trout performed poorly in our enclosed respirometers: of 71 U c r i t tests we attempted, only the 16 described above were successful. Bull trout that refused to swim held station within tunnels by using their pectoral fins as depressors, or they rested and later became impinged against a downstream screen. Several common techniques did not stimulate consistent swimming activity in these fish. Our estimates of U c r i t for bull trout provide an understanding of their performance capacity and will be useful in modeling efforts aimed at improving fish passage structures. We recommend that fishway or culvert designers concerned with bull trout passage maintain velocities within their structures at or below our estimates of U c r i t , thus taking a conservative approach to ensuring that these fish can ascend migratory obstacles safely.