Talwinian winter is called the Ruadrath, approximately "Elf-folk," by Merseian explorers.
Ancestors of the Ruadrath:
inhabited the continental shelf;
developed both lungs and gills to cope with annual floods and droughts;
sheltered from summer heat in the sea;
however, being better walkers than swimmers (spending more time on land), came to estivate at sea.
A Ruad is unconscious all summer and a mere swimming animal until the seasonal return to land triggers memories and intelligence. Then Rrinn remembers that he leads Wirda's pack and is married to Cuwarra, that supplies and equipment have been left in a building a safe distance from shore etc.
Different packs protect themselves from predators in different ways during estivation, e.g.:
boulders rolled across crevices;
When Rrinn awakes, he is too deep for sunlight but sees other swimmers "...as blurs of blackness." (Poul Anderson, Young Flandry (New York, 2010), p. 317) I wondered whether Rrinn had rediscovered John Milton's "...no light, but rather darkness visible..." However, it is explained that illumination comes from organisms planted at the sides of the cage.
Most of Chapter Sixteen has the awakening, swimming, remembering, then land-traversing, Rrinn as its viewpoint character but, before that, the omniscient narrator explains:
"When first they woke, the People had no names. He who was Rrinn ashore was an animal at the bottom of the sea." (p. 317)
Anderson's vocabulary again: "...hyaline clarity..." We might remember the Splendor Hyaline from Narnia. (My computer does not recognize this adjective.)