See here Jonathan "said his father not unkindly." Winter isn't far away. Boats will be few and the surface fish will be swimming deep.

“你瞧，乔纳森，”他父亲温和地说，“冬天快到了，船只就要少了，海面上的鱼也要钻到海底去了。”

youdao

For a while they sat, and Suchen remembered the splash of a fish breaking the surface of the water, and two egrets taking off with unhurried elegance, one after the other.

NEWYORKER: Alone

Coastal fisherman know that when a flock of seagulls is circling and diving into the sea a school of bluefish is often driving bait fish towards the surface and the anglers hasten to cast their lines into the frothy water beneath the screeching birds.

FORBES: Bevy of Black Swans Or Just A Flock of Seagulls?

Changing temperatures in the lake during the winter could have brought stagnant water to the surface, thus asphyxiating entire schools of fish.

ECONOMIST: What killed Fossil Lake?