By Liam Craig & Liam Chatfield
Are there more green crabs on the South Shore or on the North Shore of Prince Edward Island?
While swimming on the beaches of Prince Edward Island you may encounter an aggressive crab. Unlike other crabs on PEI, this one will not run away and hide, but will try to attack your feet! This ocean menace is called the European Green Crab (Carcinus maenas) and is interesting and important not just because of the danger to human toes but due to the fact that they are not native to Canada. They are threatening other crab and ocean species. Not only are green crabs are aggressive predators but they are also destroying marine life habitats by eating eelgrass and other vegetation.
In Atlantic Canada, green crabs were first noticed in 1951 in New Brunswick. By 1994 they were found in the Gulf of St. Lawrence. We first noticed green crabs on the South Eastern shores of Prince Edward Island (e.g. near Panmure Island and Basin Head). The amount of spread of Green Crabs to other beaches in PEI, particularly the North Shore, is unclear. The objective of this study was to determine and compare the number of Green Crabs versus Rock crabs on PEI beaches.
As we had only observed Green Crabs on South Shore beaches, we hypothesized that there would be more green crabs on the South Shore than on the North Shore.
We went to 8 beaches, including four from the South Shore (Basin Head, Panmure Island, Argyle Shore, Linkletter) and four from the North Shore (West Point, Miminegash, Cabot Beach and Stanhope). See Figure 1. Our goal was to find and catalogue 25 crabs at each beach for a total of 200 crabs.
Green crabs were distinguished from other crabs, primarily the rock crabs, by looking for certain characteristics. For example, Green crabs were: i) more green, brown or purple in color, ii) had 5 distinct spikes located on each side of the front of the shell, and iii) had a more circular shape to their shell. Figure 2 shows pictures we took of both a green crab and a rock crab.
We used the fisher’s test to determine whether the percentage of rock and green crabs was different between the North and South shore beaches.
At the South shore beaches we found 93 crabs. The only South shore beach where we did not find 25 crabs was Basin Head, where the current and waves were strong that day. At the North Shore beaches we found 53 of our target 100 crabs. We only found three crabs at Miminegash and none at the West Point beach due to high winds and waves making it difficult to see the bottom.
At the South shore beaches, 54 of the 93 crabs were identified as rock crabs, representing 58%. For the North shore beaches all 53 crabs were identified as rock crabs, representing 100%. The fisher’s test was used to show that the difference between the North and South Shores was statistically significant (p<0.0001).
We also considered how things would change if the missing crabs were green crabs. On the South shore beaches this would result in 46 and 47 green crabs, on the North and South shore beaches respectively. In this case, the fisher’s test did not find the difference to be significant (p >0.05).
Green crabs were first found in Atlantic Canada in New Brunswick in 1951. They spread slowly into the remainder of the Atlantic provinces generally moving from southern to more northern locations along the coastline. They were not noted in the Gulf of St. Lawrence until the 1990’s. We observed green crabs on Southeastern beaches of PEI. Our project confirmed our hypothesis that there are more green crabs on south shore beaches of PEI than on north shore beaches. We believe more green crabs were found on these beaches as they likely invaded there first as they moved from New Brunswick to Eastern Nova Scotia and around Cape Breton into the Northumberland Strait. Unfortunately, over time they may continue to invade all beaches on PEI.
Also green crabs prefer shallow water with muddy, sandy or pebble bottoms as well as areas with vegetation such as eelgrass. They also tolerate more variability in salinity, eg. the amount of salt content in the water, and temperature than rock crabs. This is more consistent with the Northumberland Strait (eg. south shore) than the beaches on the north shore. They are more aggressive predators than rock crabs so will beat their competition by leaving native crabs less food. With the destruction of eelgrass, estuaries are damaged by green crabs. This not only destroys habitats of other crabs but also of many other marine animals including small fish and shellfish such as clams, mussels, oysters, etc. Green crabs are very hardy. They can survive out of water for many days and live 4-7 years. Green crabs are truly superior adversaries.
Limitations to our study may include the number of beaches we visited and the number of crabs we counted. Had we snorkelled for a larger number of crabs or visited more beaches, our results may have been different. The weather also made it challenging to identify crabs on a few of the beaches we visited. We believe it would be valuable to repeat the study in the future.
Green crabs are listed on the top 100 global invasive species database. Invasive species destroy habitats, disrupt the food chain and displace native species. The summer following our science fair project, we travelled to Kejimkujik Seaside National Park in Nova Scotia to spend the day with National Park staff researching green crab invasion of the estuaries within the park boundaries. We took ATV’s to the estuaries then counted and measured green crabs in rowboats and canoes. There we observed the complete destruction of some of the estuaries. Eelgrass was almost completely wiped out in places and virtually no marine life was visible in these areas. As a result of our interest and feedback after this experience, they now have a program called “Gone Crabbin’: Citizen science seaside adventure at Kejimkujik Seaside”. When speaking with National Park staff at Kejimkujik Seaside, local fisherman and Paula Tummon Flynn (a student studying green crabs at UPEI), we learned that new invasive species have been found in Atlantic Canada including the Blue Crab. We personally found blue crabs when snorkelling in Florida.
Fortunately green crabs have not completely taken over. In our project in 2014-2015, rock crabs were still present on most beaches. It is also interesting that through informal observation the summer of 2018, we did not find many green crabs on South shore beaches compared to other summers. We observed none on North Shore beaches.
Our hypothesis was correct because we found 39 more green crabs on the south shore than on the north shore of Prince Edward Island.
Tummon Flynn, Paula, Ph.D. Student, Biology at UPEI. Personal Interview, February 04, 2015.
Department of Fisheries and Oceans Website.
In person conversation with fishermen at West Point and Victoria.
CBC News website
Nature conservancy of Canada. http://www.natureconservancy.ca/en/what-we-do/resource-centre/invasive-species/european_green_crab.html
Global invasive species database. http://www.iucngisd.org/gisd/?st=100ss
Parks Canada Kejimkujik National Park website. https://www.pc.gc.ca/en/pn-np/ns/kejimkujik/activ/expedi-crabes-gone-crabbin
About the Authors
Liam and Liam both attended West Royalty Elementary school at the time of completing the project. They both now attend Queen Charlotte Junior High School and started grade 8 in the Fall of 2018. They love sports, music and still enjoy hunting for crabs.