The answer is very small crystals
One way would be to limit who can receive a passport or visa to travel to another country and another would be at the entrance to the country they may not be admitted for example due to inadequate funds to live. In my case, when I travelled by bus from Costa Rica to Panama, the Panamanian authorities required that I either have a ticket out of the country or a return ticket back to San Jose, Costa Rica.
The number of minnows would increase.
Hope this helps!
A. Basins are the landforms that are most likely to have coal, oil and gas deposits. Much of the countries natural gas, oil and coal are deposited in the bottom of basins.