freshwater trout, smelt
鮎 centers on a specific freshwater fish: the ayu or sweetfish, also called freshwater trout or smelt in English. The meanings are different names for the same fish rather than distinct concepts.
鮎 combines 魚 (fish) with 占, which likely contributes the sound. The character was created to name the ayu fish.
The left side 魚 means fish, and the right side 占 looks like a fortune-telling shell. Imagine a fish that fortune-tellers use to predict good catches: the ayu.
For デン, picture a den of ayu fish swimming together in a freshwater stream: den -> デン, and the fish fill the den.
ayu (Plecoglossus altivelis); sweetfish
fat greenling (Hexagrammos otakii)
Patagonian toothfish (Dissostichus eleginoides); Chilean sea bass
razorfish (Aeoliscus strigatus)
Ryukyu sweetfish (Plecoglossus altivelis ryukyuensis)
Hyonenzu (painting by Josetsu, 1415)
young sweetfish; young ayu
young sweetfish
sweetfish going downstream to spawn (in autumn)
sweetfish sushi; ayu sushi; belly of sweetfish salted, pickled and served on rice
fourspine sculpin (Cottus kazika)
kissing loach (Leptobotia curta)
fishing for ayu
ayu fishing