Radak: Now it discusses what was created on day five. Birds (even though also they were created on day five) were mentioned earlier, Agav (along with the benefit of tall trees).

Rashi: It is a spacious place.

Radak: (a) They multiply more than Chayos of the field. (b) They are born at all times of the year, therefore there are more fish than Chayos.

Malbim: You did not prepare special abodes for sea creatures; bigger fish swallow smaller fish. To compensate, You made them multiply greatly. There are a multitude of eggs in the innards of one fish. Therefore, the small dwell with the big; even though every day, big fish swallow many myriads of small fish, they are not eradicated. The latter multiply so much that place was made for them in the arctic sea and distant islands.