Evaluation of the critical thermal maximum for determining thermal tolerance of freshwater fish

Synopsis

Upper loss of equilibrium and death temperatures were determined by the critical thermal maximum (CTM) method for juvenile coho salmon (Oncorhynchus kisutch) acclimated to 5 and 15° C, and for pumpkinseed sunfish (Lepomis gibbosus) acclimated to 10 and 20° C. Temperature increase rates were 1, 6, 18, 30 and 60° C h⁻¹. Analysis of variance showed significant (P < 0.5) differences due to acclimation level, temperature increase rate, and an interaction between these variables. CTM values for coho, a stenothermal fish, were lower than those for pumpkinseed, an eurythermal fish. Generally, higher acclimations and more rapid rates of temperature increase produced higher CTM values. Upward acclimation during testing was detected only for 10° C acclimated pumpkinseed at a 1° C h⁻¹ increase rate

Assessments were made of CTM procedural relationships from test data and the literature. A 1° C min⁻¹ (60° C h⁻¹) temperature increase rate is so rapid that internal body temperatures of fish will usually lag behind media temperatures. A standard increase rate of 18° C h⁻¹ (0.3° C min⁻¹ is recommended for routine CTM determinations with fish. Other recommendations are: 1) Test fish should be small, and relatively uniform in size and condition; 2) final loss of ability to return upright should be the loss of equilibrium criterion; 3) death also should be included as a criterion in CTM determinations, with the term ‘lethal thermal maximum’ applied for death temperature; and 4) linearity should be maintained in time-temperature distributions of the heating rate. All procedures are also applicable to determinations of the critical thermal minimum. An amended description of the CTM is given that expands its scope.