No, that is completely impossible. The reasoning behind it is because if its a large group of fish they're swimming back and forth ect, ect, it would be like counting a bowl of rice, but not being able to take any out. To get around these counting problems, scientists take data from just a small portion of the population called a sample. They take several samples and then use the average size of those samples to calculate an estimate of the entire population size.
If you need help feel free to ask!