The ocean is 600,000 gallons. Its not like counting fish in a small tank. the need to mark the ones they caught. They need to repeat this process a few times to get an accurate population estimate. Who knows how many fish are there. It also said that there were only 6 students working on the project. Unless you have actually had to do pop counts in large bodies of water, you won't understand the time it takes to get an accurate estimate.