Forests make fish fatter

New research from the Tanentzap group freely available in the journal Nature Communications has shown that freshwater fish, an important source of nutrition for humans, are in part produced by forests.  The study focuses on small boreal lakes, which contain upwards of 60% of the world's freshwater.  It suggests that any reductions in forest cover in the boreal ecoregion, such as from industrial activities, will threaten the production of healthy fish populations.

Small streams that drain forest floors bring microscopic particles of vegetation and soil into water.  These get broken down by bacteria, which are then eaten by small invertebrate animals that are main food source for small fish.  The research uses a gradient of forest cover in Canada to show that more of this forest organic matter is brought into lakes as the surrounding landscape is vegetated.  This produces more bacteria in the near-shore water, which can support more zooplankton, and thus provide more food to small fish.  Young fish survive winters and escape predators better if they are larger, so these effects are predicted to carry forward into larger and older animals.

The research also uses natural variation in the molecular mass of primary production from land versus water to estimate the proportional of terrestrial resources used by fish.  At least 34% of fish biomass was supported by terrestrial vegetation, increasing to 66% with greater forest cover.  This suggests that fish increasingly use forest food subsidies as they become available in the small nutrient poor lakes that are characteristic of the boreal ecoregion.