This is a modern misconception. Japanese people ate very little fish in the era depicted in this story. The people who ate fish the most were day laborers in the port cities and wealthy people. Sushi, for example, was a blue-collar food in the Edo era and only found favor with the middle classes during the late-Taisho to early-Showa era, exactly the time period covered in his manga. In fact, most Japanese ate very little meat of any kind, with chicken and quail being the most common available. The dominant source of protein were tofu and eggs. This was the reason behind the government campaign to popularize eating beef and pork during this era.