Assessing Intertidal Populations of the Invasive European Green Crab

Green crabs are an incredibly harmful invasive species. Understanding spatial temporal patterns in their population dynamics is essential for predicting and managing the ecological and economic impacts of this species. This is a standardized method for assessing crab populations that doesn't require specialized or expensive equipment is accessible to a wide range of users and is easily adapted to suit the needs of the investigator.

This research can inform shellfish and coastal habitat conservation and restoration as well as green crab fishery development research. Handling and identifying live crabs can be challenging. So familiarizing yourself with the characteristics of interest and practicing handling live crabs prior to the surveys can be useful.

Understanding the specific area of shoreline and crab characteristics targeted in this protocol is critical to ensuring standardization, which is something that is easier to absorb visually. Helping to demonstrate the procedure, will be Jessie Batchelder a field technician from my laboratory. At the predicted low tide time, run a 50 meter transect tape vertically from the low intertidal zone to the high intertidal zone.

Divide the resulting distance into a high, middle and low section. The low intertidal section, parallel to the shoreline is the target sampling area. Within the low intertidal zone, measure a distance of 100 meters parallel to the shoreline and establish permanent markers delineating this zone, using a rebar or natural permanent landmarks, such as immovable boulders, ledge or dock pilings.

Upon arriving at the survey site, locate the 100 meter section of low intertidal shoreline where the survey will be conducted. Unpack gear and organize data sheets and field guides. Optionally, measure the water temperature, using a waterproof digital thermometer in the shallow water adjacent to the sampling area.

Measure salinity by placing several drops of water, collected adjacent to the sampling area onto the refraction prism of a salinity refractometer. Record water temperature in degrees celsius and salinity in parts per thousand on the intertidal survey data sheet or directly in the intertidal green crab project on the Anecdata app. Begin the survey by haphazardly tossing the one square meter quadrat within the predefined low intertidal zone area.

Record a visual estimate of the percent of both movable rock and algae canopy cover within the quadrat to the nearest quarter percent. Within each quadrat, lift movable rocks or cobble and carefully move aside algae to look for crabs, making sure to replace all rocks and algae as they are found. Collect all of the crabs found and store them in a bucket until the entire quadrat has been searched.

Identify the species of each crab using the intertidal crab field guide, and record it using the species codes listed on the intertidal survey data sheet or on the intertidal green crab project on the Anecdata app. Use vernier calipers to measure the caliper's width of each crab across the widest part of the calipers, spinning from tip to tip of the terminal spines to the nearest one millimeter. For crabs with the calipers width of greater than 10 millimeters, examine the abdomen of the ventral side of the crab to determine sex.

Male crabs tend to have a narrow-pointed abdomen and female crabs tend to have a wider beehive-shaped abdomen. For all crabs, record the number of claws, number of legs, shell condition as determined by whether the calipers resists or gives when finger pressure is applied and the presence or absence of extruded eggs for females. Optionally, record the color for green crabs, identify pre-molt shell condition for green crabs, using external pre-molt indicators.

Pre-molt green crabs are within three weeks of molting and are of particular interest to the emerging soft shell green crab fishery. Return all crabs to the habitat within the quadrat, once all measurements and characteristics have been recorded. Continue haphazardly tossing the quadrat within the predefined low intertidal area until a total of 10 meters squared has been sampled.

Move forward along the low intertidal area, ensuring the quadrats are separated by a minimum of one meter so that resampling does not occur and a maximum of 10 meters so that the survey area does not exceed 100 meters. In 2019, this protocol was used to conduct monthly intertidal green crab surveys at three locations from May to November and at one location from May to August. The data collected indicated wide variations in spatial and temporal green crab population density and sex ratio.

Significant differences were also noted in cumulative science frequency among the sites. Shell condition changed seasonally within each site with peak pre-molt and soft shell phases for males in the spring and for females in the early summer and fall. The protocol was also used in 2018 and 2019 by Georgetown Central School, third and fourth grade students who surveyed the same site in Georgetown, Maine each year in October.

They observed a shift in the population from being dominated by five to 15 millimeter calipers width sized crabs in 2018 to 15 to 30 millimeter calipers, width sized crabs in 2019. They also observed an increase in the percent occurrence of the invasive Asian shore crab from 3.5%in 2018 to 9%in 2019. When attempting this protocol, keep in mind that targeting a standardized area along the shoreline is critical to collecting data that can be compared across spatial and temporal scales.

This protocol can be conducted in conjunction with or inform many other types of shoreline monitoring, such as saltmarsh, shellfish or shallow subtidal species monitoring. This technique has allowed many user groups ranging from researchers to educators, to citizen scientists to understand spatial temporal patterns of shoreline populations of native and invasive crabs.