In two places in parshat Bereishit Gd commands "be fruitful and multiply". The second place is the familiar commandment directed to humans (Gen 1:28). In the first place, the same words are used but concerning fish and birds (Gen. 1:22). Are animals commanded? If so, in what sense? If not, how do we interpret Gen. 1:22?
כב וַיְבָרֶךְ אֹתָם אֱלֹהִים לֵאמֹר פְּרוּ וּרְבוּ וּמִלְאוּ אֶת הַמַּיִם בַּיַּמִּים וְהָעוֹף יִרֶב בָּאָרֶץ:
22 And God blessed them, saying, "Be fruitful and multiply, and fill the waters of the seas, and let the fowl multiply upon the earth."
As you can see, this is not a command but a blessing. Rav Hirsch points out the difference between אמר and דבר as used here. Part of Rav Hirsch's commentary emphasizes that the blessing gives the (non-free willed) animals the power to be fruitful and multiply unlike the free willed following of a commandment which a human being is capable of. That is why the commandment to a human being uses דבר
... the fulfillment immediately followed the command. Hashem blessed the organic living creatures, i.e. He gave them the power of reproduction and multiplying, and with this power He gave them at the same time the direction and urge for it. For in the unfree living organic beings, power and fulfillment are not separated. The power itself automatically drives them to the fulfillment of that for which Hasem has given them that power.
Rav Hirsch differentiates this from the command in 1:28 and explains the difference between Adam (getting a command) and animals (getting a brachah) because it says
כח וַיְבָרֶךְ אֹתָם אֱלֹהִים וַיֹּאמֶר לָהֶם אֱלֹהִים פְּרוּ וּרְבוּ וּמִלְאוּ אֶת הָאָרֶץ וְכִבְשֻׁהָ וּרְדוּ בִּדְגַת הַיָּם וּבְעוֹף הַשָּׁמַיִם וּבְכָל חַיָּה הָרֹמֶשֶׂת עַל הָאָרֶץ:
He gave them the command to fulfill the mission of mankind on Earth. Whereas above, in verse 22 it says וַיְבָרֶךְ אֹתָם אֱלֹהִים לֵאמֹר here it says וַיְבָרֶךְ אֹתָם אֱלֹהִים וַיֹּאמֶר There in the unfree living creatures, as already remarked thereon, with the giving of the blessing, ... the fulfillment of this blessing is already automatically given.
Here, in human beings, the blessing, ... is separated from the fulfillment i.e. from using this power and ability for the purposes for which Hashem intended them. ... That which in animals is a purely physical act becomes in Man a free-willed moral act.