Creation extends beyond the mere formation of life; it requires a guarantee of survival and continuity. Thus, a divine blessing is introduced for the first time, functioning as an infusion of goodness and divine power embedded directly into the nature of the creatures to ensure they can endure and multiply [רס״ג, ביאור יש״ר, שד״ל].
A question arises as to why fish and birds required a specific blessing for reproduction, unlike the vegetation created before them. The primary approach among commentators explains that while plants reproduce naturally and automatically, animals require an active drive for courtship and mating, necessitating a blessing to awaken this reproductive instinct [ביאור שטיינזלץ, רש״ר הירש]. Another perspective focuses on the survival of these species. Because humans are destined to hunt and consume fish and birds, a special blessing was essential to prevent their extinction [רש״י, מזרחי, רבנו בחיי, ברכת אשר]. Others point to their unique biological nature. They reproduce by laying eggs, a miraculous process that requires special divine power to sustain life in an egg detached from the mother's body. Furthermore, sea fish lack natural defenses and prey upon one another, compelling them to reproduce in massive quantities simply to survive [מלבי״ם, שד״ל]. Alternatively, the inherently cold nature of water contrasts with the warmth required for reproduction, requiring a specific divine decree to enable fish to multiply within their frigid environment [אור החיים].
This raises a subsequent question regarding why the beasts and cattle of the earth, created later, did not receive a similar blessing. Some commentators suggest that the blessing bestowed upon the creatures of the water and air indirectly encompassed land animals, as all share a common reproductive nature [רמב״ן, רבנו בחיי]. In contrast, others argue that land animals were deliberately excluded so that the serpent, which was destined to be cursed, would not partake in this blessing [רש״י, שפתי חכמים]. Another explanation posits that withholding a specific blessing of multiplication from land animals was intended to protect humanity. If predatory beasts were to multiply excessively, they would threaten human settlement, a danger not posed by creatures confined to the water and air [חזקוני].
When God communicates this blessing, it is not through literal speech, as animals lack the intellect to comprehend words. Rather, it is a decree of His will, permanently embedding within them the capacity and drive to reproduce [רד״ק, רש״ר הירש, ביאור יש״ר]. This divine directive consists of two distinct stages, being fruitful and multiplying. The command to be fruitful establishes the fundamental ability to produce offspring and avoid barrenness. However, if this were the only decree, a pair of animals might produce only a single offspring. Therefore, the command to multiply is added, granting the blessing of numerous offspring that will survive and populate the world [רש״י, רמב״ן, רס״ג, ביאור יש״ר].
The directive to fill the waters in the seas acts as the actual transmission of this divine power [רד״ק, רשב״ם]. It specifically emphasizes the seas, teaching that only the vast oceans are large enough to contain the immense population of fish, whereas rivers and lakes would be insufficient [רמב״ן, רד״ק, קאסוטו]. Regarding the birds, the blessing dictates that they multiply on the earth. Even though birds were formed from the water and some dwell upon it, their reproduction takes place on solid ground, where they build their nests and lay their eggs [רמב״ן, רד״ק, סעדיה גאון, קאסוטו]. Notably, the blessing given to the fish is delivered as a direct address using a double plural, while the birds are addressed in the third person with a single term for multiplication. This distinction highlights that fish were granted a level of fertility vastly greater than that of birds [קאסוטו, אלשיך, רשב״ם].