Triclosan (TCS) is an emerging contaminant of concern in environmental studies due to its potential adverse effects on fish behavior. Since avoidance has been shown to be a relevant behavioral endpoint, our aims were: (i) to determine if TCS is able to trigger an avoidance response in Poecilia reticulata; (ii) to predict the population immediate decline (PID) caused by TCS exposure, by integrating lethality and avoidance responses; and (iii) to verify the overestimation of risk when mortality is assessed under forced exposure. Fish were exposed to TCS in a forced exposure system, to assess mortality, and to a TCS gradient in a non-forced exposure (NFE) system. Two NFE scenarios were simulated: (#1) a spatially permanent gradient, including low and high concentrations; and (#2) a scenario with high concentrations, simulating a local discharge. The fish avoided TCS concentrations as low as 0.2 μg L (avoidance of 22%). The AC50 obtained from scenario #1 (8.04 μg L) was about 15 times more sensitive than that from scenario #2 (118.4 μg L). In general, up to the highest concentration tested (2000 μg L), the PID was determined by the avoidance. Mortality from the forced exposure was overestimated (48 h-LC50 of 1650 mg L), relative to the NFE. The reduced mortality in a non-forced environment does not imply a lower effect, because part of the population is expected to disappear by moving towards favorable environments. TCS is a potential environmental disturber, since at environmentally relevant concentrations (<2 μg L) it could cause a decline in the fish population.

Information on how atrazine can affect the spatial distribution of organisms is non-existent. As this effect has been observed for some other contaminants, we hypothesized that atrazine-containing leachates/discharges could trigger spatial avoidance by the fish Poecilia reticulata and form a chemical barrier isolating upstream and downstream populations. Firstly, guppies were exposed to an atrazine gradient in a non-forced exposure system, in which organisms moved freely among the concentrations, to assess their ability to avoid atrazine. Secondly, a chemical barrier formed by atrazine, separating two clean habitats (extremities of the non-forced system), was simulated to assess whether the presence of the contaminant could prevent guppies from migrating to the other side of the system. Fish were able to avoid atrazine contamination at environmentally relevant concentrations (0.02 μg L), below those described to cause sub-lethal effects. The AC (atrazine concentration causing avoidance to 50% of the population) was 0.065 μg L. The chemical barrier formed by atrazine at 150 μg L (concentration that should produce an avoidance around 82%) caused a reduction in the migratory potential of the fish by 47%; while the chemical barrier at 1058 μg L (concentration that produces torpidity) caused a reduction in the migratory potential of the fish by 91%. Contamination by atrazine, besides driving the spatial distribution of fish populations, has potential to act as a chemical barrier by isolating fish populations. This study includes a novel approach to be integrated in environmental risk assessment schemes to assess high-tier contamination effects such as habitat fragmentation and population displacement and isolation.

The ability of aquatic organisms to sense the surrounding environment chemically and interpret such signals correctly is crucial for their ecological niche and survival. Although it is an oversimplification of the ecological interactions, we could consider that a significant part of the decisions taken by organisms are, to some extent, chemically driven. Accordingly, chemical contamination might interfere in the way organisms behave and interact with the environment. Just as any environmental factor, contamination can make a habitat less attractive or even unsuitable to accommodate life, conditioning to some degree the decision of organisms to stay in, or move from, an ecosystem. If we consider that contamination is not always spatially homogeneous and that many organisms can avoid it, the ability of contaminants to repel organisms should also be of concern. Thus, in this critical review, we have discussed the dual role of contamination: toxicity (disruption of the physiological and behavioral homeostasis) vs. repellency (contamination-driven changes in spatial distribution/habitat selection). The discussion is centered on methodologies (forced exposure against non-forced multi-compartmented exposure systems) and conceptual improvements (individual stress due to the toxic effects caused by a continuous exposure against contamination-driven spatial distribution). Finally, we propose an approach in which Stress and Landscape Ecology could be integrated with each other to improve our understanding of the threat contaminants represent to aquatic ecosystems.