Why did the son-in-law send Spanish mackerel to his mother-in-law? Any allusions?

The custom that a son-in-law sends Spanish mackerel to his parents-in-law has its origin.

Before the opening of the island city, there was a simple story among the people. It was about a child named Xiao Wu whose parents died and was adopted by a loyal and charitable old man. When he grew up, he grew up to be an honest young man because of the influence of environment and family. The old man who adopted him betrothed his daughter to Xiao Wu as his wife. Xiao Wu works hard every day, farming in the morning and evening, and fishing during the day to repay the kindness of the old man. One spring, during the fish season, the old man suddenly fell ill and wanted to eat fresh fish when he was dying, but the weather was bad and it was windy every day, so he couldn't go fishing. Seeing that the old man's condition was getting worse and worse, Xiao Wu had to risk his life to go to sea. The elderly at home are in danger, and the daughter has been saying, "Mom, mom, don't die yet, Xiaowu will be back soon." The old man nodded: "Good boy, it's hard for Xiao Wu, that's it, that's it …" The old man swallowed before he finished. Just then, Xiao brought back a big fresh fish, but the old man had passed away. The couple were heartbroken and wept bitterly, so they had to cook the fresh fish and offer it to the old man's coffin. Since then, Mr. and Mrs. Xiao Wu have dedicated this big fish just caught in early spring to the grave of the old man every year, and named it "just a fish" according to the lament that the old man said "just a fish" before he died.

After the news spread, people got inspiration from Xiao Wu's experience, that is, the old people must be full of what they can eat, and the old people must eat what they want, especially the Spanish mackerel in spring. Therefore, it has become a long-standing custom to send Spanish mackerel to honor in-laws in spring, which has been passed down to this day.