Genèse 1:22

And He blessed them: Because [man] eliminates and hunts from them and [he] eats them, the blessing was needed. And [so,] also the animals needed a blessing, except that because of the snake that would receive a curse in the future; for that reason He did not bless them, so that it would not be included.

"Be fruitful:" It is an expression of fruit; which means to say, "make fruit."

"And multiply:" If He had only said, "be fruitful," one would have given birth to one and no more; and [so] comes "and multiply," so that one [can] give birth [to] many.

And He blessed them, saying, Increase and multiply, and fill the waters of the seas, and let the fowl multiply upon the earth.

"and He blessed" - the meaning of "be fruitful and multiply" [i.e., command form] is "may you be fruitful and may you multiply". Likewise, "Die on the mountain" Deuteronomy 32:50, for this is not within his [Moshe's] power or at his behest.

AND G-D BLESSED THEM SAYING. He decreed the blessing on them and said of them that they should be fruitful and multiply, meaning that they should bring forth abundantly, that one creature should bring forth many like itself. The purport of the blessing is procreation, even as it says, And I will bless her, and she shall be a mother of nations. In connection with plants also, the term “blessing” applies: Then I will command My blessing upon you in the sixth year. However, it does not say so on the third day [when the plants and trees were created] because all created living beings were only a single pair, male and female, according to their kind, and therefore they were in need of a blessing to bring forth abundantly; but in the case of plants, they sprang up over the face of the entire earth in great abundance, just as they exist today. Nor did He mention a blessing on the sixth day for cattle and beasts because in the decree of abundancy which He decreed for the moving souls in the waters there were included the moving souls on the earth, as all living souls that do not speak are in the same class of creation. And our Rabbis have said that they [the fish and fowl] were in need of a blessing because people hunt them and eat them.

AND FILL THE WATERS IN THE SEA. He blessed them that in their abundancy they would fill the seas, the streams and the pools. Or it may be that their “filling” is to be in the seas only for in the streams they are few.

AND LET FOWL MULTIPLY IN THE EARTH. Although the fowl were created out of the waters, their blessing — that they be fruitful and multiply — was to be on the earth for there is no fowl that lays its eggs in the waters and has them grow there. Even those fowl which abide always in the waters and derive their food from them lay their eggs on the earth, and there they are born.

ויברך אותם אלוקים, you will note that all the categories of living creatures whom G’d created were blessed by Him.

ומלאו את המים, this is in the imperative mode, but in a relatively weak conjugation such as אמרו לאלוקים in Psalms 663, or in Hoseah 2,5 רדפו מהר. If the Torah had written mal-u instead mil-u this would have been an imperative in the strong conjugation, comparable to G’d commanding Moses and Aaron with the words dabb-ru (Exodus 12,3.)

ירב. The word means “may it increase in numbers,” an alternate way of saying yirbeh. We have a similar construction in yifen and yifneh respectively, meaning: “he will turn.” Any verb that can accept the letter ה at the end, may appear both with or without that letter.

ויברך אותם אלוקים, seeing there were so many of them they would not achieve their purpose in this universe without such a Divine blessing.

ויברך אותם...פרו ורבו, the land-based mammals were created on the sixth day, on the same day man was created, seeing that both categories of creatures make their habitat on the dry land.

את המים בימים, the waters which are in the oceans; for in the rivers and streams the fish do not reproduce at such rapid rates. The directive by G’d to multiply was not addressed to them directly, seeing they do not possess the intelligence to receive and understand such messages. What the Torah means is that G’d’s goodwill towards them was equivalent to His having addressed such a blessing to them directly.

והעוף ירב בארץ, the emphasis here is on the word הארץ, i.e. even though its origin is in the waters the birds will grow and generally behave more like land-based creatures.

ויברך אותם אלהים, G–d blessed them.” We find the same expression in respect to the Sabbath, in Genesis 2,3. Our sages learn from this that any new phenomenon deserves to be blessed, even when one purchases or receives new garments, etc., and they composed a benediction for the respective phenomena.