Fish Diseases and Management

7.2.4 Degree days

While many qualitative similarities exist within the metabolism of drugs in fish and mammals, there exists a pharmacokinetic complexity that aquatic animals are poikilotherms (cold blooded), and as such, their metabolic rate and activities are affected by differences in water temperature. This complexity is not found in the much better understood mammalian and avian (homeotherms) physiological systems, which remain within a nearly constant temperature range. Decreasing the water temperature generally slows the metabolic actions of aquatic animals, causing slower drug absorption, metabolism, and excretion. One important consequence of this effect is the possibility of longer residue retention in aquatic animal tissues following treatment. Also, fish eat less when the temperature cools below their optimum level, leading to lower dietary intake of a drug in feed. Lower intake, combined with slower drug absorption because of a reduced metabolic rate, may lead to both reduced drug effectiveness and lower tissue residue levels. These factors require that the metabolic state of the fish and the predicted changes in the environment of the fish over the weeks following treatment to be considered carefully when planning the treatment.

Key numbers that are calculated from the drug residue depletion and safety data are the allowable tissue tolerance level [or maximum residue level (MRL)] and the withdrawal period for the drug. The withdrawal period is the amount of time required following the administration of the last treatment for measurable residues of the drug or a metabolite to decrease to below an established safe level. This safe level is the MRL, or tolerance. Fish appear to metabolise drugs at about one tenth the rate in mammals and the temperature optimum for many of these reactions is lower than that of the mammals usually the temperature of the natural environment of the fish. Because of the variability of drug excretion, especially with temperature, degree days have been advocated for estimating withdrawal time. Careful attention to the withdrawal period is a very important aspect of treatment administration, and producers need to maintain complete records that show the treatment completion dates on aquatic animal stocks. These stocks must be identifiable from the time of treatment until harvest. If a group with one withdrawal completion date is mixed with a second group that has a later date, then the whole group must now be assumed to have the later withdrawal date.

Degree days are calculated by adding the mean daily water temperature in ° C for total number of days measured. If mean temperature was 11 ° C, for 50 days immediately after stopping the treatment, the degree days would be 550. At higher optimum temperature for the fish, the required degree days could be achieved in less number of days. This however should be calculated experimentally for different chemicals in different fish species.