Effects of trout aquaculture on water chemistry of tropical montane streams in Ecuador

Recent growth in inland aquaculture may have unintended consequences for freshwater ecosystems. To examine how trout aquaculture affects montane stream water chemistry, we sampled multiple locations above and below 14 trout farms on the eastern and western slopes of the Andes Mountains near Quito, Ecuador. We quantified changes in water temperature, dissolved oxygen, conductivity, pH, total dissolved nitrogen, and total dissolved phosphorus (TDP). Trout farms varied in size (1 to 60 raceways), elevation (similar to 1,500 to 2,500 m above sea level), and surrounding forest cover (7 to 56% mean canopy cover). Samples collected immediately above and below trout farms showed small decreases in mean dissolved oxygen (6% decrease) and pH (0.1 unit decrease), a small increase in mean water temperature (0.2 degrees C), but no changes in conductivity. We observed a 30% increase in total dissolved nitrogen from immediately above to below trout farms. Changes in both dissolved oxygen and nitrogen dissipated with distance downstream from the trout farm. TDP increased by 39% immediately below trout farms, was positively associated with the number of raceways per farm, and did not decrease with distance downstream. Future work should address how the observed nutrient increases affect Andean stream communities and ecosystem processes, particularly alongside the direct effects of introduced trout and concurrent land use change, including livestock grazing.

Automated summary

The study found that trout aquaculture affects dissolved oxygen and nitrogen concentrations in surrounding mountain stream water, with these effects dissipating further downstream from the fish farms. Additionally, total dissolved phosphorus concentrations in stream water increase downstream from trout farms.