Assessing Intertidal Populations of the Invasive European Green Crab

Summary

Understanding spatiotemporal patterns in green crab population dynamics is essential for predicting and managing the ecological and economic impacts of this harmful invasive species. This protocol was developed in an effort to create a standardized method for assessing green crab populations in the rocky intertidal zone of the Northwest Atlantic.

Abstract

Invasive species have caused major disruptions to ecosystems worldwide. The European green crab invaded North America in the 1800s and is considered one of the world’s 100 worst invaders by the IUCN. Observations of spatiotemporal green crab population dynamics are essential for predicting and managing the ecological and economic impacts of this harmful invasive species. This protocol was developed in an effort to create a standardized method for assessing green crab population dynamics in the rocky intertidal zone of New England and Atlantic Canada. The protocol was designed to be accessible to multiple users including researchers, educators, students, and citizen scientists. Although it was designed for surveying crab populations, this protocol is easy to adapt and could be used for any number of intertidal species. The resulting data collected using this protocol has a wide range of uses, including to inform ecological research, conservation efforts, mitigation strategies, and fishery development, as well as for educational outreach purposes.

Introduction

Biological invasions can potentially disrupt species interactions and ecological processes, and may have far reaching ecological^1,2,3 and economic consequences⁴. The ability to successfully predict, mitigate, and adapt to invasions strongly depends on characterizing spatiotemporal population dynamics⁵. While a range of tools exist (e.g., population genetics, stable isotopes) and are emerging (e.g., eDNA) for tracking invasive species, traditional in situ monitoring techniques continue to be widely utilized for assessing invasive species distribution and abundance.

The European green crab (Carcinus maenas) is an invasive species that was first detected in North America in 1817 and has successfully invaded ecosystems worldwide^6,7. Green crabs have a multitude of negative impacts on local ecosystems, including reducing native bivalve populations through predation^8,9, competing with native crustaceans for food and shelter^10,11,12 and destruction of eel grass habitat and subsequent changes to fish community structure^12,13,14. Compounding these issues is the link between increasing temperature and increasing green crab abundance and/or range expansion^15,16, which has had severe ecological and socio-economic consequences in areas such as the Gulf of Maine, where warming is occurring faster than 99% of the world’s other oceans¹⁷.

On the eastern seaboard of North America, green crabs range from Virginia to Newfoundland. They are most commonly found on wave-protected shorelines, estuaries, and embayments in depths ranging from the high tide level to 5-6 m¹⁸. Their presence in the intertidal zone makes them an ideal marine species for shoreline surveys. The most distinguishing characteristic used to identify green crabs is the pattern of five spines or ‘teeth’ on each side of the eyes and three spines between the eyes (see Appendix 1). Their carapace (dorsal side) is typically a mottled dark green and brown, but ventral color patterns can vary greatly (see Appendix 2).

There are many organizations, researchers, citizen scientist groups, and educators currently conducting green crab population monitoring. However, the lack of a standardized protocol makes it difficult to compare datasets and to ultimately understand green crab populations on both a local and regional scale. This protocol is designed to quantify spatiotemporal population dynamics of green crabs in the rocky intertidal zone in New England and Atlantic Canada. Ideally, the development of a standardized, inexpensive, and easily adaptable survey will promote long-term monitoring efforts by a wide range of users, including researchers, citizen scientists, educators, and students.

Although green crabs are the target species of interest in this protocol, data is also collected for native Jonah and rock crabs (Cancer borealis and Cancer irroratus), as well as the invasive Asian shore crab (Hemigrapsus sanguineus). These are crab species commonly found in the rocky intertidal zone in northern New England, and trends in their population distribution and abundance have ecological and economic significance. An Intertidal Crab Field Guide was developed alongside this protocol to aid in crab identification (Appendix 1) specific to northern New England. A data entry and storage platform called “Intertidal Green Crab Project” was also developed for this protocol using Anecdata¹⁹. Anecdata is a free online citizen science platform that provides web-based and mobile solutions for gathering and accessing observations, and provides a user-friendly platform to easily collect, manage, and share data.

Protocol

1. Timing of survey work

1. Conduct surveys between May-November, during the height of productivity in the intertidal zone.
2. Schedule surveys around negative or zero tides (generally new and full moon cycles) to allow for adequate time in the low intertidal zone (i.e., at least 2 hours).

2. Pre-survey preparation

1. Locate and print all field guides and data collection sheets (see Appendices) prior to conducting the survey if that is the preferred method. If using the Anecdata app for field guides and data collection, visit the Anecdata website and join the Intertidal Green Crab Project¹⁹. The data collection sheet and Anecdata collection categories are identical.

3. Site selection and description

1. Locate a wave-sheltered rocky intertidal site with cobble (i.e., not stable, rolled by wave action) and algal canopy habitat. Ensure that there is at least 100 m of shoreline to accommodate the planned sampling.
2. Record the location of the study site using a global positioning system (GPS) unit, or a device such as a smart phone that has GPS capability (e.g., many compass apps are free to download or are already pre-programmed on smart phones). Record site coordinates on the Intertidal Survey Data Sheet (Appendix 3) or directly into the Intertidal Green Crab Project on Anecdata.
3. At the predicted low tide time (determined from the NOAA Tide Predications website or an app such as Tides) run a 50 m transect tape vertically from the low intertidal zone (i.e., the splash zone) to the high intertidal zone (i.e., the black microalgal zone that is typically dry at high tide). Divide the resulting distance into three equal sections: high, middle, and low (Figure 1). The low intertidal section, parallel to the shoreline, is the target sampling area.
4. Within the low intertidal zone, measure a distance of 100 m parallel to the shoreline (Figure 2) and establish permanent markers delineating this zone using rebar or natural permanent landmarks such as immovable boulders, ledge, dock pilings, etc.

4. Conducting survey

1. Prior to arriving at the survey site, record the following information on the Intertidal Survey Data Sheet (Appendix 3) or directly into the Intertidal Green Crab Project on Anecdata: site name, sampling date, participants, time and height of low tide at the location/date to be sampled (determined using the NOAA Tide Predications website, or an app such as Tides), and lunar phase (determined using a lunar calendar such as www.moongiant.com).
2. Upon arriving at the survey site, locate the 100 m section of low intertidal shoreline where the survey will be conducted, unpack gear, and organize data sheets and field guides.
3. Optionally, measure water temperature using a waterproof digital thermometer in the shallow water adjacent to the sampling area.
   1. Measure salinity by placing several drops of water collected adjacent to the sampling area onto the refraction prism of a salinity refractometer.
   2. Record water temperature in °C and salinity in parts per thousand (ppt) on the Intertidal Survey Data Sheet or directly into the Intertidal Green Crab Project on the Anecdata app.
4. Begin the survey by haphazardly tossing the 1 m² quadrat within the predefined low intertidal zone area that runs parallel to the shoreline (a transect tape is not needed to conduct the survey because the sample area has already been defined). Record a visual estimate of the percent of both moveable rock (i.e., cobble/gravel that you can look underneath) and algae canopy cover (e.g. Ascophyllum or Fucus spp.) within the quadrat to the nearest quarter percent (i.e., 0, 25, 50, 75, or 100%). A rocky intertidal habitat is often patchy and can contain areas of sand, mud, ledge, or other habitats where green crabs are not found.
   1. To avoid skewing density estimates by sampling unsuitable habitat, only sample quadrats with greater than 50% movable rock, or greater than 50% algal canopy. Also avoid sampling areas where boulders or ledges are noticeably elevated above the profile of the shoreline within the low intertidal zone, as this habitat may be more representative of the mid intertidal zone.
5. Within each quadrat, lift moveable rocks or cobble and carefully move aside algae to look for crabs. Be 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.
6. Identify the species of each crab using the Intertidal Crab Field Guide (Appendix 1, or source on Anecdata project platform) and record using the species codes listed on the Intertidal Survey Data Sheet (Appendix 3) or on the Intertidal Green Crab Project on the Anecdata app.
7. Measure the carapace width (CW) of each crab across the widest part of the carapace, spanning from tip to tip of the terminal spines, to the nearest 1 mm using Vernier calipers.
8. Use the abdomen (or ‘apron’) on 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 (Appendix 1). Only record sex for crabs ≥ 10 mm CW.
9. For all crabs, record number of claws, number of legs, shell condition (i.e., hard- or soft-shell as determined by whether the carapace resists (hard) or gives (soft) when finger pressure is applied), and the presence (i.e., ovigerous) or absence of extruded eggs for females.
10. Optionally, record the color for green crabs, but not other crab species, using the color protocol developed by Young and Elliot²⁰ (Appendix 2). This protocol should only be used if the actual paint chips can be sourced and brought into the field, as printed versions can vary substantially. Identify pre-molt shell condition for green crabs using external pre-molt indicators (Appendix 4, or source on Anecdata project platform). Pre-molt green crabs are within 3 weeks of molting and are of particular interest to the emerging soft-shell green crab fishery^21,22.
11. Return all crabs to the habitat within the quadrat once all measurements and characteristics have been recorded.
12. Continue haphazardly tossing the quadrat within the predefined low intertidal area until a total of 10 m² is sampled. Continually move forward along the low intertidal area of shoreline and ensure that quadrats are separated by a minimum of 1 m so that resampling does not occur and a maximum of 10 m so that the survey area does not exceed 100 m.

5. Data management and analysis

1. If using data sheets, check all raw data sheets for errors and legibility post-survey, photocopy, scan, and archive. Use the photocopy for data entry into an excel spreadsheet (see Appendix 5 for example) or into the Intertidal Green Crab Project on Anecdata¹⁹. Store scanned data sheets electronically.
2. Conduct data analyses appropriate to the design of the study. Useful population metrics include crab density (total number of crabs divided by total number of quadrats sampled), sex ratio, cumulative size frequency, injury rate, shell condition ratio, and overall species encounter rates (e.g., % of native vs. invasive crabs).

Results

In 2019, this protocol was used to conduct monthly intertidal green crab surveys at three locations from May-November (Sandy Point, Yarmouth, ME (43°46'17.92"N, 70° 8'45.52"W), Robinhood Cove, Georgetown, ME (43°48'13.80"N, 69°44'50.97"W), and New Meadows River, West Bath, ME (43°51'17.84"N, 69°51'55.20"W)), and at one location from May-August (Damariscotta River, Walpole, ME (43°56'9.42"N, 69°34'52.75")). The data col...

Discussion

This protocol describes a survey method for assessing spatial and temporal trends of crab populations in the rocky intertidal zone that is accessible to multiple users including researchers, educators, students, and citizen scientists. The benefits of this protocol include the following: it does not require specialized or expensive equipment, the methodology is approachable for a wide range of skill levels (e.g., 3^rd and 4^th grade students have successfully used it), and it can easily be adapted to ...

Materials

Name	Company	Catalog Number	Comments
1 m2 PVC quadrat (1/2" PVC)	Any hardware/home improvement store		PVC can be sourced at any hardware/home improvement store and cut into 1m lengths to form quadrat (4 1/2" PVC elbows will also be needed to connect 1 m lengths into square)
1/2" rebar	Home depot	5152	*optional (for marking low intertidal area)
40 m Fiberglass Transect Tape	Grainger	3LJX1
5 gal bucket	Home depot	05GLHD2
Ade Advanced Optics Salinity Refractometer	Amazon		*optional
Clip board	Any office supply store or Amazon
Uei Waterproof Digital Thermometer	Amazon		*optional
Vernier calipers	Bel-Art		Many companies make calipers, however our preferred brand is Bel-Art which can be sourced on Amazon