This is probably due to the fact that the gill area of large fish is proportionally smaller to their body size than small fish, so that their gills have to work harder to get enough oxygen under very low oxygen conditions.
(This; is probably; due to the fact that the gill area of large fish is proportionally smaller to their body size than small fish) (the gill area of large fish; is; proportionally smaller to their body size than small fish) (their gills; to work harder; to get enough oxygen under very low oxygen conditions) Context(their gills to work harder,List([136, 167))):(their gills; to work harder to get; enough oxygen; under very low oxygen conditions)
their gills to work harder,List([136, 167))
This the gill area of large fish their gills their gills
is probably is to work harder to work harder to get
due to the fact that the gill area of large fish is proportionally smaller to their body size than small fish
proportionally smaller to their body size than small fish
to get enough oxygen under very low oxygen conditions
enough oxygen under very low oxygen conditions