Estimating Intertidal populations
The mark and recapture method involves marking a number of individuals in a natural population, returning them to that population, and subsequently recapturing some of them as a basis for estimating the size of the population at the time of marking and release. This population estimate is based on the principle that if a proportion of the population was marked in some way, returned to the original population and then, after complete mixing, a second sample was taken, the proportion of marked individuals in the second sample would be the same as was marked initially in the total population. That is: R (marked recaptures) / C (total in second sample) = M (marked initially) / N (total pop. size)
The accuracy of this method rests on a number of assumptions, including the following:
A population of shore crabs along Canada's Pacific coast can be sampled twice at low tide to estimate abundance. Shore crabs are very abundant under the rocks along the intertidal zone. There are two species, as shown to the right, Hemigrapsus oregonensis (green) and Hemigrapsus nudus (red). Males can be separated from females by examining the plates on the underside of the abdomen (males have plates that resemble the Empire State Building!).
(adapted from: www.zoology.ubc.ca/~srivast/biol404/markrecaplab_05.doc
Procedure:
The mark and recapture method involves marking a number of individuals in a natural population, returning them to that population, and subsequently recapturing some of them as a basis for estimating the size of the population at the time of marking and release. This population estimate is based on the principle that if a proportion of the population was marked in some way, returned to the original population and then, after complete mixing, a second sample was taken, the proportion of marked individuals in the second sample would be the same as was marked initially in the total population. That is: R (marked recaptures) / C (total in second sample) = M (marked initially) / N (total pop. size)
The accuracy of this method rests on a number of assumptions, including the following:
- During the interval between the preliminary marking period and the subsequent recapture period, nothing has happened to upset the proportions of marked to unmarked animals (that is, no new individuals were born or immigrated into the population, and none died or emigrated).
- The chances for each individual in the population to be caught are equal and constant for both the initial marking period and the recapture period. That is, marked individuals must not become either easier or more difficult to catch.
- Sufficient time must be allowed between the initial marking period and the recapture period for all marked individuals to be randomly dispersed throughout the population (so that assumption 2 above holds). However, the time period must not be so long that assumption 1 breaks down.
- Animals are not affected by their marks (i.e., their survival, catchability, ability to migrate, reproductive ability in the time interval are all unaffected by the marks).
- Animals do not lose their marks.
A population of shore crabs along Canada's Pacific coast can be sampled twice at low tide to estimate abundance. Shore crabs are very abundant under the rocks along the intertidal zone. There are two species, as shown to the right, Hemigrapsus oregonensis (green) and Hemigrapsus nudus (red). Males can be separated from females by examining the plates on the underside of the abdomen (males have plates that resemble the Empire State Building!).
(adapted from: www.zoology.ubc.ca/~srivast/biol404/markrecaplab_05.doc
Procedure:
- Check out the tide charts and the weather before doing this activity. It is preferable to select two days with low tides during the day and days that are forecast to be calm days.
- Mark two areas within the rocky intertidal zone. Each area should be five meters wide and run from the mid-tide line to low tide line. These are marked with stone cairns. These areas should parallel each other and be placed about five meters apart. Emphasize the importance of trying to keep the stress on the crabs to a minimum.
- Two groups of students, divided equally, are given 10 minutes to collect shore crabs from each of the areas. The crabs should be kept in a tray with water and some rocks.
- Each crab should be identified with respect to species and gender and each should be marked on the carapace. Marks for each of the two areas should be different colours. Records for each section should be kept on the data sheets provided. The marked crabs are kept in a second water filled bucket.
- The marked crabs are then placed back among the rocks throughout the study area.
- This process is repeated the next day at the low tide time.
- Students are instructed to spend an equivalent amount and time in the collection process as they did on the previous day.
- Collected crabs are counted as before and an additional mark is placed on the crabs if a third collection is to be done. (A third collection increases the confidence in the population estimates.
- The marked crabs are then placed back among the rocks throughout the study area.