Scientists estimated the population of one species of fish in a lake.
They caught 120 fish, put a small tag on the fin of each fish and released them.
One week later, they caught 150 fish. 18 of these fish had a tag.
The scientists use this equation:
population size=number in first sample×number in second samplenumber of tagged fish in second sample\text{population size} = \dfrac{\text{number in first sample} \times \text{number in second sample}}{\text{number of tagged fish in second sample}}population size=number of tagged fish in second samplenumber in first sample×number in second sample
Calculate the estimated population size.
Some tags fell off the fish between the first and second samples.
Explain how this would affect the estimate of the population size.
Give two other assumptions that the scientists make when they use this method.
Suggest why the scientists left one week between the two samples.
109 exam-style questions on OCR GCSE Biology 6.1 Monitoring and maintaining the environment, covering 6.1.1 Field investigation of distribution and abundance, 6.1.2 Human interactions and biodiversity, 6.1.3 Maintaining biodiversity, and 6.1.4 Environmental change and distribution of organisms. Each one has a worked solution and a mark scheme showing where the marks go.