Reward reductions found more aversive by rats fed environmentally contaminated salmon.

Pacific salmon stocked in Lake Ontario concentrate persistent toxic chemicals such as PCBs, DDT, DDE, mercury and dioxin. The present experiments support earlier findings that consumption of these salmon by laboratory rats increases their behavioral reactions to negative events. For 20 days rats were fed a diet consisting of 30% Lake Ontario salmon or a control diet of Pacific Ocean salmon or no salmon. They were then trained to run down an alley to receive a large 15-pellet or small 1-pellet food reward (6 trials/day). Following 72 trials the 15-pellet groups were shifted to 1 pellet for 90 trials, and showed a contrast (depression) effect: they ran more slowly than the groups always given 1 pellet. Rats previously fed Lake Ontario salmon showed a much larger contrast effect than the two control groups. These results were replicated in a second experiment, and a group fed a 10% diet of Lake Ontario salmon for 60 days showed the same size contrast effect as the group fed a 30% diet for 20 days.