Shore Crab Mark Recapture
ES classes follow a procedure that was developed by Charlie Krebs. Capture-Mark-Recapture is a process commonly used in ecology for estimating large populations that can not readily be counted. Purple Shore Crabs (Hemigrapsus nudus) and Green Shore Crabs (Hemigrapsus orego- nensis) are commonly found under cobbles and boulders on rocky shores. They move quickly, seeking cover when rocks are lifted. Marks will not remain on the carapace of the Purple Shore Crab. They can be marked using colored cotton thread tied onto their leg. The cotton will break down and release in three to four days.
Method: Five meter wide sections of the intertidal zone (between 1 to 1.5 meter elevation) is marked off by piles of rocks. A ten minute period is given to the collection group to gather as many crabs as possible. Emphasis is placed on doing no harm to the crabs during the collection and marking process. Thread is tied onto the from legs of the captured crabs. The crabs are then released near the area they were captured. The same activity is repeated 25 hours later. Comparisons are made between the marked and not marked on the second day with the total marks to the estimated total population. When more than one group conducts this survey, we use a different color thread for the groups. Mixes of the colors give and indication of the mobility of the population.
ES classes have conducted these studies in numerous locations. We found that marking the carapace of the crabs was problematic since many of the marks washed off. We tried drying the carapace before marking with somewhat better success. We kept a number of marked crabs overnight in a marine aquarium to see if all the marks had remained. Most marks had gone so we started to tie short pieces of thread into the crab legs as a marking method.
ES classes follow a procedure that was developed by Charlie Krebs. Capture-Mark-Recapture is a process commonly used in ecology for estimating large populations that can not readily be counted. Purple Shore Crabs (Hemigrapsus nudus) and Green Shore Crabs (Hemigrapsus orego- nensis) are commonly found under cobbles and boulders on rocky shores. They move quickly, seeking cover when rocks are lifted. Marks will not remain on the carapace of the Purple Shore Crab. They can be marked using colored cotton thread tied onto their leg. The cotton will break down and release in three to four days.
Method: Five meter wide sections of the intertidal zone (between 1 to 1.5 meter elevation) is marked off by piles of rocks. A ten minute period is given to the collection group to gather as many crabs as possible. Emphasis is placed on doing no harm to the crabs during the collection and marking process. Thread is tied onto the from legs of the captured crabs. The crabs are then released near the area they were captured. The same activity is repeated 25 hours later. Comparisons are made between the marked and not marked on the second day with the total marks to the estimated total population. When more than one group conducts this survey, we use a different color thread for the groups. Mixes of the colors give and indication of the mobility of the population.
ES classes have conducted these studies in numerous locations. We found that marking the carapace of the crabs was problematic since many of the marks washed off. We tried drying the carapace before marking with somewhat better success. We kept a number of marked crabs overnight in a marine aquarium to see if all the marks had remained. Most marks had gone so we started to tie short pieces of thread into the crab legs as a marking method.