Before we eat the Fish we say…

יְהִי רָצוֹן מִלְּפָנֶיךָ, ה’ אֶלֹקֵינוּ וֵאלֹקֵי אַבוֹתֵינוּ, שֶׁנִּפְרֶה וְנִרְבֶּה כְּדָגִים

Which means….

May it be your will, Hashem our G-d and G-d of our forefathers, that we should be fruitful and multiply like fish.

The first Mitzvah (Commandment) in the Torah is to be fruitful and multiply. Fish have lots of babies and we pray to Hashem that we too will be Zoche (Merit) to have many Jewish children.