As the amount of rainfall increases in rainy June, rivers flow into the sea with rich minerals from the mountains, which let the fish living in the areas where freshwater meets seawater store much nutrition. Especially Edomae “anago” (sea eel) and “shako” (a kind of mantis shrimp) are said to be the best around this season.
Also June is the season for “ayu” (sweetfish), a kind of freshwater fish relatively rare as sushi ingredient. In Edomae style, ayu is soaked in vinegar for hours to soften the bones before using for nigiri sushi.
You might also not want to forget, “shinko” (baby kohada, or baby konoshiro gizzard shad) appears around the end of June every year. So popular as to be called “the summer symbol of Edomae sushi” and many shinko lovers rush to sushi restaurants for the two months until the end of August when shinko decorates “tane-bako” (wooden box where sushi ingredients are kept on the counter).