Environmental, biological, and methodological factors affecting cholinesterase activity in walleye (Stizostedion vitreum).

Organophosphorus (OP) insecticides have high acute toxicity toward many nontarget vertebrate and invertebrate organisms, but direct measurement of OPs in environmental samples is difficult because their concentrations may fall below detection limits within hours to days after entering aquatic ecosystems. Because OPs exert toxicity through cholinesterase (ChE) inhibition, which may persist for up to several weeks, ChE inhibition has been widely used in aquatic ecosystems as a biomarker for OP exposure in aquatic organisms. However, the biological, environmental, and methodological factors affecting ChE activity have not been well documented and must be considered and understood before ChE activity can be used as a dependable indicator of OP exposure to aquatic organisms. This study examined the influence of water temperature, size of larval and juvenile walleye (Stizostedion vitreum), stress, long-term storage, postmortem changes, and methods of euthanasia on ChE activity. Water temperature (17.2, 20.9, and 24.6 degrees C), stress, long-term storage (up to 180 days), postmortem changes, and method of euthanasia had no effect on ChE activity of walleye. There was a strong positive correlation (r = 0.87) between whole body ChE activity and total length (7.2-17.9 mm) for larval walleye, but a negative correlation between brain ChE activity and total length (59-164 mm) for juvenile walleye (r = 0.75). Because size, age, and development may affect ChE activity, fish of similar size should be used when evaluating the effects of ChE inhibitors. If fish of similar size are not available, it is recommended that relations between size, age, and development be understood so estimates of variation in ChE activity can be made.