The time to exhaustion is the amount of time that a fish can swim at a prescribed velocity and is typically estimated from swim speed tests. Swim tests have generally shown that, when swimming above sustained speeds, as the swimming velocity increases the amount of time the velocity can be maintained decreases. Time to exhaustion may be reported as a constant value derived in the swim test, a default value selected by FishXing Biologists, or as function of the swim speed equation.

When the relationship between exhaustion times and swim speeds has not been defined as part of a swim speed study, reasonable approximations are presumed to be 20 seconds to 200 minutes for prolonged speeds, and less then 20 seconds for burst speeds (Beamish 1978).

Determining Time to Exhaustion in Swim Speed Tests

In fixed velocity (endurance) tests, the time to exhaustion is directly measured for each velocity tested. In increasing velocity (critical) tests velocity is increased in a series of steps at specific time increments. The assumption required to use this data to assess fish passage is that the length of the time increment in the critical velocity test corresponds to the endurance time in a fixed velocity test. This assumption implies that fish are not fatigued at lower speeds, but fatigue rapidly as speeds increase (Hunter and Mayor 1986). Brett (1964) showed that for small fish fatigue curves from fixed and increasing velocity tests with 60 minute time increments have similar slopes thus supporting the validity of this assumption. However, fixed velocity tests had slightly higher endurance times than those predicted from increasing velocity tests.

Selecting Time to Exhaustion in FishXing

When you select a set of swim performance data from the Literature Swim Speeds Table a time to exhaustion will be automatically entered into the time to exhaustion field. If the swim speed study reported time to exhaustion, this data will be entered  If time to exhaustion was not reported in the study, then the default is arbitrarily set at 10 seconds for burst speeds and 30 minutes for prolonged speeds. These default values are near the center and the lower end of the commonly observed time ranges associated with burst and prolonged speeds, respectively. Since FishXing generally defaults to the middle to lower end time ranges there is less chance of overestimating the time to exhaustion.

Constant Swim Speed

When using fixed swimming speeds our recommendation is to use the reported time. Entering shorter times will result in underestimating passage capabilities and entering longer times may not be valid because they are outside the range of those tested. If you enter a time to exhaustion outside the range used in the original study a warning will be displayed.

Endurance Swim Speed

On the other hand, when using equations with time as a variable, such as the Hunter and Mayor (1986) equations, it may be desirable to vary the time to exhaustion.  Since swimming speed and time to exhaustion are related, lowering the time to exhaustion will increase the swimming speed. It may be desirable to run the analysis using multiple times to exhaustion within the acceptable range of values and compare passage performance.