Effects of Caffeine on Fish Learning

Abstract

Pollution is an increasing threat to health and biodiversity, especially chemical pollution in the air, land, and water. One such example is caffeine, which is a main active ingredient in coffee and is ingested by humans worldwide for its stimulant effects and cultural significance. This widespread caffeine ingestion coupled with incomplete removal during wastewater treatment results in high concentrations of caffeine in the environment. Aquatic organisms living in waterways receiving wastewater effluent are often exposed to caffeine continuously. Given this long-term and widespread exposure, caffeine is an emerging contaminant of concern. However, most research investigating the effects of caffeine on aquatic organisms use caffeine doses that are much higher and caffeine exposure durations that are much shorter than those found in the environment. Also, most caffeine exposure studies also rely on relatively simple behavioural endpoints and make use of neotropical species. In contrast, I exposed fathead minnow (Pimephales promelas), a common freshwater fish in North America, to environmentally relevant concentrations of caffeine (0 ng/L; 1,000 ng/L; 10,000 ng/l) for 35 days. Caffeine exposure did not affect morphology (e.g., length, mass, growth) or metabolism (maximum metabolic rate, resting metabolic rate, and aerobic scope), but decreased their hepatosomatic index (liver investment). While caffeine did not affect the number of trials taken to associative or reversal learn, or the latency of fish to avoid an aversive trawl, three weeks of exposure to low caffeine concentrations may have decreased anxiety. Taken together our results suggest that future studies perhaps with different endpoints are needed clarify our understanding of how caffeine influences metabolism, anxiety, and learning. Overall, our results provide evidence that complex behavioural endpoints such as aversive learning can be used in ecotoxicological studies.