Biology 404 Purpose: (1) To apply mark-recapture methods to field populations of the Yellow shore crab Hemigrapsus oregonensis and the Purple shore crab, Hemigrapsus nudus. (2) To examine the precision and accuracy of the Petersen, Schnabel, and Jolly-Seber methods on laboratory populations of known size. Accurate estimates of population parameters for mobile animals can be obtained by mark-recapture methods (Krebs 1999, Chapter 2). These methods all make a series of assumptions about the population being sampled, and if these assumptions are not correct the resulting estimates may be biased. In this exercise we will illustrate the use of three of the common mark-recapture methods for artificial populations of known size in the laboratory and also apply one of these methods to a field population of shore crabs.