This week I performed my mark and recaptured a sampling project to estimate the population of the goldfish pool. This was started on Tuesday with an initial sample of 35 fish. A larger sample would have been preferred however trapping was ineffective and fishing nets used were inadequate size. Captured fish were then clipped at the lower caudal fin (tail fin) and returned to the goldfish pool population.
The following Thursday a second sample was taken with a different fish net, allowing for a much larger sample to be taken. Target sample population was about 200 fish in order to yield an accurate estimation and 236 fish was the end count for the second sampling. Each fish from the second sample was then examined to determine if it was a recapture with a clipped fin from the first sample. There were 11 recaptured fish with clipped fins out of the 236 fish of the second sample. These numbers were then used with the Chapman estimator equation to determine an estimate of the fish population. The result from this was a total population of 711 fish. This estimate is inaccurate because the true population of the goldfish pool is already known to be at least 2000 fish.
The inaccurate estimate of 711 fish may have occurred for a few reasons. Some fish may have been mistaken for a clipped fish due to fish biting or other damage to the fin. Clipped fish may also have had an increased chance of being sampled a second time due to clipped fins reducing fish speed/maneuverability. A final possible reason could simply be due to the nature of a random sample in which case repeat tests should produce a more accurate estimation.