Bless the lads.
In them may my name be recalled,
And the names of my fathers Abraham and Isaac,
And may they be teeming multitudes upon the earth.”
וידגו לרוב וגו'. הכוונה על דרך מה שפירשתי בפ' בראשית בפסוק (א' כ"ב) ויברך אותם וגו' האמור בדגים כי הוצרך ה' לברכם לצד שמקום דירתם הוא נגדיי בקצה האחרון לבחינת הלידה לזה הגדיל בהם כח המוליד להוליד אפילו במים, וכאן אמר יעקב שידגו כדגים בריבוי הלידה ויהיה להם גדר זה בארץ, וכפי זה תהיה ברכתם גדולה מברכת הדגים להיותם במים והבן.
וידגו לרוב בקרב הארץ, "and they will multiply on earth just like fish." Jacob's considerations may have been similar to the blessings G'd bestowed on the fish in Genesis 1,22.1 explained there that a special blessing for the fish was called for due to their habitat being less suited for successful and enduring procreation than the conditions prevailing on dry land. G'd therefore increased the fish's natural reproductive powers to counter the negative conditions prevailing in their habitat. Jacob too accorded Joseph's sons greater powers of reproduction when he blessed them by comparing their fruitfulness to that of fish. Inasmuch as Ephrayim and Menashe did not reside in regions which are hostile to human reproduction, Jacob's blessing was even stronger than G'd's blessing for the fish.
From this we learn that before this blessing the Jewish forefathers didn't have lots of children, when we look at Avraham, Yitzchak and Yaakov who had only one or two children with their official/wanted wives. So this shows us that the forefathers understood that the big nation they got promised by 'ה wasn't immediate but was meant for the future.
So we can take this to our world and remember ourselves that success needs time.