What is the distinction between "Peru" (be fruitful) and "Revu" (multiply)?
Why did Hashem find it necessary to bless the fish and the birds independently?
Rashi: Because due to the fact that they are constantly hunted and trapped for food, their numbers are depleted - He did not incorporate the Chayos because of the snake, which was destined to be cursed. 1
Kesuvos 5a: In any event, now that the fish (which proliferate rapidly) were blessed on the fifth day, virgins (in the time of the Gemara) would marry on Wednesday. And by the same token, widows would marry on Thursday and be intimate on Thursday night (Friday), because Hashem blessed Adam on the sixth day.
See Sisei Chachamim.
Why did Hashem not bless the animals and beasts on the sixth day (as He did the birds and fish on the fifth day)?
Rashi: Because of the Chayos (wild animals); which in turn, Hashem declined to bless, because of the snake, which He knew would ultimately earn a curse.
Ramban and Rashbam: The Berachah given to the birds and the fish extended to the animals as well. 1
Da'as Zekenim: Mules were created only later (Bereishis 36:24), therefore the Berachah did not apply to them, and they do not reproduce.
What is the significance of the blessing, "ve'ha'Of Yirev ba'Aretz"?
Oznayim la'Torah: Bearing in mind the Pirkei de'R Eliezer - that whatever was created from earth resides on land, and whatever was created from water resides in the water, the Torah needs to tell us that, although the birds were made from earth and water (mud), they reside on land exclusively.
QUESTIONS ON RASHI
Rashi writes: "Because [the fish and birds] are depleted by [man] who trap and eat them, they needed [Hashem's] blessing [to 'be fruitful and multiply']." If so, why was man himself blessed? Also, why were the trees not blessed?
Gur Aryeh: Mankind was blessed to "be fruitful and multiply" (1:28), because man is an intelligent being with free choice, and he might have chosen to abstain from procreation. Fruit trees perpetuate their species involuntarily, whereas animals need to actively mate; therefore the animals required a blessing.
Rashi writes: "The animals also needed a blessing, but because of the snake among them, He did not bless them." How, then, did the animals endure?
Gur Aryeh: The animals received Hashem's blessing later, after the Flood (9:7). 1 The snake had already been cursed, and so was ineligible for the later blessing.
