Morphological evolution reduces downstream displacement in juvenile landlocked salmon

Area of Science:

• Evolutionary biology
• Ecology
• Ichthyology

Background:

• Severe flooding causes downstream displacement of aquatic animals, impacting population persistence.
• Salmonid populations often exist upstream of barriers, isolating them after flood events.
• Spatial sorting due to differential displacement can influence population evolution.

Purpose of the Study:

• To investigate the evolution of increased body depth in juvenile amago salmon (Oncorhynchus masou ishikawae) inhabiting above-barrier habitats.
• To determine if enhanced body depth aids in resisting downstream displacement during floods.

Main Methods:

• Collected juvenile amago salmon from stations with and without downstream barriers in two rivers.
• Conducted common-garden experiments to assess morphological differences under controlled conditions.
• Performed field experiments using natural flooding events to observe displacement resistance.

Main Results:

• Juveniles from above-barrier habitats exhibited significantly deeper bodies compared to those from other stations.
• Common-garden experiments confirmed that deeper-bodied fish maintained their position better.
• Field experiments demonstrated that deep bodies enhance resistance to downstream displacement during floods.

Conclusions:

• Juvenile amago salmon in above-barrier habitats have evolved deeper bodies as an adaptation to resist flood-induced downstream displacement.
• This study provides the first evidence of evolutionary consequences of passive spatial sorting during severe climate events in fish populations.