Yaakov blessed Efraim and Menashe, Yosef’s sons, with the bracha of HaMalach HaGoel (Breisheet 48:16):
The Angel who redeemed me from all evil, should bless the lads, and let my name be called on them, together with the name of my fathers, Avraham and Yitzchak. May they be like fish multiplying within the land (bikerev Ha’aretz).
The Vilna Gaon points out that the tribes of Efraim and Menashe were not significantly larger than the other tribes while B’nai Yisrael were in the wilderness as well as when they were in Arvot Moav before entering the Land of Israel.
Once B’nai Yisrael entered Israel, the prophecy came through- the tribes of Efraim and Menashe multiplied like fish.
After the land was conquered, even before it was distributed it says (Yehoshua 17:4): “The children of Yosef spoke to Yehoshua, saying, ‘Why have you given me an inheritance of only one lot and one portion, when I am a numerous people, for God has blessed me so much.’” In the short time that we have been in the Land of Israel, we have already become a big nation.
Yehoshua answered (17:15): “If you are such a numerous people, go up to the forest…”
We learn in Bava Batra 118a: Yehoshua said to them. “Go and hide in the forests, so that the Ayin HaRa, Evil Eye will not have control over you.” Yehoshua was afraid that such a quick and wondrous proliferation would provoke the Evil Eye to take action against them.
Why did their tribes increase at such a quick rate once they got to Israel? One reason is because of the blessing of “HaMalach HaGoel” from Yaakov, that they will be like fish multiplying within the Land.
In Israel we always have to worry about demographics. Although there are a lot of Olim, immigrants, there are also a lot of Yordim, people that leave. One way to make sure that the Jewish population continues to rise is by having children. High birth rates are often found in the Haredi and Dati Leumi communities. In Israel in general, there is a focus on the centrality of the family.
May we continue to raise lots of children in the Modern State of Israel and may those who are trying to conceive be blessed with healthy babies.