recaptured. The ratio of marked to recaptured individuals in the second sample can be used to estimate the general population's size. Here is a simple formula for estimating population size (N) from mark-recapture data:N = (Total individuals marked in first sample xSize of second sample) / Number recaptured individualsSuppose a team of fisheries biologists set out to estimate the density of Largemouth bass in the Des Plaines River. At a randomly selected starting point, they electrofished a 1000 m section of the stream (its mean width was 5 m). The captured fish were measured, recorded, fin clipped, then released (these are the marked fish). Two days later, they resampled the same reach and recorded the number of marked and unmarked fish in each of 4 size classes. Here are their data:
Table1. Mark-recapture Largemouth bass data from the Des Plaines River.Size (cm)Number ofindividualsday 1Number of individuals day 2PopulationdensityestimateUnmarkedMarked< 1073642257010-2035281320-30202218> 30642Total13411855Using these data, estimate the density (number/hectare) of each size class plus the overall density. [Note: 1 hectare = 10,000 m Bluntnose minnowComplete the following table Table 2. Mark-recapture Bluntnose minnow data from the Des Plaines River. Size (cm) Number of individuals day 1 Number of individuals day 2 Population density estimate Unmarked Marked < 3 252 174 63 3-6 196 145 54 6-9 177 132 58
> 9 34 26 11 Total 659 477 186 1.Which species was more numerous is the Des Plaines River? How much more abundant?2.Based on the data, which species grows to a larger size?3.Which species do you think is the prey and which is the predator? Why?
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