AN ANALYSIS OF POPULATION STRUCTURE IN PACIFIC MOLE CRABS (HIPPA PACIFICA DANA)

¹ Marine Science Institute, University of California, Santa Barbara, California 93106

1. Essentially all of a population of mole crabs (Hippa pacifica Dana) was removed from an isolated beach at Enewetak Atoll, Marshall Islands. Estimates of poplation size, based on catch pattern, indicate that 87 to 93% of the animals were bait-trapped out of their restricted habitat. The sequential trapping permitted a number of comparisons between initial sample data and actual population structure.

2. The first three samples were large and yielded highly consistent data when compared to one another, but some aspects of population structure were nonetheless non-representative. For example, the percentage of crabs within each modal size class in the total population differed markedly from that estimated by initial samples. The location of those modal size classes did not change during the removal process.

3. After the first three samples (where male percentages were about 29%), sex ratio fluctuated wildly in individual samples, apparently as a consequence of size discrepancy in arrival at the bait. Eventually, males comprised about 48.4% of the 4011 animals removed.

4. Several other aspects of population structure were accurately estimated in early samples, including: mean size at onset of egg production, percentage of mature females carrying eggs, the sex ratio-size class relationship (for larger crabs), and the mean maximum size of males.