Our main objective for monitoring the sandy beach is to establish baseline information about the density and distribution of the Pacific mole crab. By monitoring, we can track changes (or lack of changes) in abundance over time. The sampling methods are described below.
Setting Up the Sample Area:
A 50-m line is placed above the swash zone, parallel to shore. This line should be placed in the same location each time the beach is surveyed. A GPS or compass can be used to find this location. Choose five random numbers between 0 and 50. Locate these positions along the 50-m line and stretch a 10-m transect, perpendicular to the 50-m line, into the swash zone. The 10-m transect should start at the top of the swash zone. A flag should be placed securely in the sand every meter along the transect.
Collecting a Sample:
One sample should be collected at each of the 10 flags along the transect. To collect a sample, wait until the waves have receded. Quickly press a core into the sand at or next to one flag. Core to a depth of 10 cm (at northen CA sites only) or 20 cm (all other sites). Tilt the core diagonally, lift, and use one hand to hold the sand in place. Place the sample into sieves and pour water overtop to wash the sand away.
Recording the Data:
If mole crabs are present in a sample, determine the size and gender of each crab. Determine the size by measuring the length of the carapace with a caliper (to the nearest millimeter). Determine gender by carefully lifting the telson. Females are identified by the presence of modified legs, or pleopods, under the telson. Ovigerous females are identified by an orange mass of eggs. Males lack both pleopods and eggs. Gender cannot be determined if crabs are less than 9 mm. These tiny crabs are considered “recruits.” Record data on data sheets and return animals to the water away from the transect.
In this optional extension to sandy beach monitoring, the presence of a parasitic worm (Phlyum Acanthocephala) is investigated. A fishing permit or scientific collecting permit is needed to conduct this activity. About 30 crabs should be collected and placed in a zip lock bag. Place the ziplock bag in a freezer for more than 4 hours. Thaw and dissect crabs. Count the number of parasites present in the midgut of each crab.