An ichthyologist uses mark-recapture to estimate a reef fish population. On day 1, 120 fish are tagged. On day 3, 180 fish are caught, of which 36 are tagged. Later, 15 tagged fish are found in a subsequent catch of 200 fish. Assuming population stability, use the mean of both estimates to calculate the total population.

Sun, 10 May 2026 10:43:44 +0000

**An ichthyologist uses mark-recapture to estimate a reef fish population. On day 1, 120 fish are tagged. On day 3, 180 fish are caught, of which 36 are tagged. Later, 15 tagged fish are found in a subsequent catch of 200 fish. Assuming population stability, use the mean of both estimates to calculate the total population.** People are increasingly drawn to how scientists track reef health and support marine conservation — and the mark-recapture method stands out as a reliable tool in these efforts. After tagging 120 reef fish on day one, researchers later observed that 36 of those tagged individuals were among 180 fish captured just two days later. Adding context, a follow-up catch of 200 fish revealed 15 tagged ones. When applied consistently, these two independent estimates help reveal the broader fish population — all without disturbing the delicate reef ecosystem. This method relies on a simple mathematical logic: the ratio of tagged fish in the second sample reflects their proportion in the total population. It’s a technique grounded in ecological science and widely used in marine biology research across the United States and globally. ### How Mark-Recapture Estimates Population Size — Follow the Math