No evidence that the widespread environmental contaminant caffeine alters energy balance or stress responses in fish

Anthropogenic sources of environmental pollution are ever-increasing as
urban areas expand and more chemical compounds are used in daily life. The
stimulant caffeine is one of the most consumed chemical compounds
worldwide, and as a result, has been detected as an environmental
contaminant in all types of major water sources on all continents.
Exposure of wildlife to environmental pollutants can disrupt the energy
balance of these organisms, as restoration of homeostasis is prioritised.
In turn, energy allocated to other key biological processes such as growth
or reproduction may be affected, consequently reducing the overall fitness
of an individual. Therefore, we aimed to investigate if long-term exposure
to environmentally relevant concentrations of caffeine had any energetic
consequences on wildlife. Specifically, we exposed wild eastern
mosquitofish (Gambusia holbrooki) to one of three nominal concentrations
of caffeine (0, 100, and 10,000 ng/L) and assayed individuals for
metabolic rate, general activity, antipredator and foraging behaviour, and
body size as measures of energy expenditure or energy intake. We found no
differences in any measured traits between any of the given exposure
treatments, indicating that exposure to caffeine at current environmental
levels may not adversely affect the energy balance and fitness of
vulnerable freshwater fish.