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.
The first time ES followed this protocol we marked 420 crabs with permanent felt markers within 4 5 meter wide transects in a cobble beach but found none on the recapture of 350. We found it highly improbable that none of the orginal crabs were recaptured. We questioned if the marks had stayed on. We captured 10 crabs, marked them, and kept them overnight in a container with cobbles. In the morning when we examined the crabs, we found that none of the marks had remained. We then explored a variety of marking systems that did not harm the crabs. We found a short piece of cotton thread tied on a leg and cut short remained in place for three or four days before the thread disintegrated.
ES students have used this system of marking because it is both non-invasive and quick. It appears to leave no lasting effects on the crab.
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.
The first time ES followed this protocol we marked 420 crabs with permanent felt markers within 4 5 meter wide transects in a cobble beach but found none on the recapture of 350. We found it highly improbable that none of the orginal crabs were recaptured. We questioned if the marks had stayed on. We captured 10 crabs, marked them, and kept them overnight in a container with cobbles. In the morning when we examined the crabs, we found that none of the marks had remained. We then explored a variety of marking systems that did not harm the crabs. We found a short piece of cotton thread tied on a leg and cut short remained in place for three or four days before the thread disintegrated.
ES students have used this system of marking because it is both non-invasive and quick. It appears to leave no lasting effects on the crab.