Authors: April, Tess & Emily
In November of 2007, the container ship, Cosco Busan collided with a Bay Bridge support tower. The ship was fatally wounded, oozing 53,569 gallons of oil into the San Francisco Bay. The aftermath of this incident may have accounted for a steep drop in the sand crab population on San Francisco’s Ocean Beach in the years to follow. The oil spill may have affected the number of surviving recruits, or young sand crabs, especially since the mating season for sand crabs occurs in spring and summer, thereby affecting the number of recruits in 2008.
Our Marine Biology class participated in the LiMPETS sandy beach monitoring program on September 20th, 2014 at Ocean Beach. To survey the sand crabs living on the beach, we took 50 random samples of sand from the swash zone, or the zone of wave action. In each of these 50 samples, we observed any sand crabs that were found. We recorded their gender and assessed whether they were a recruit (young crab) or adult.
This piqued our curiosity about how the famous Cosco Busan oil spill in 2007, which occurred right near where we monitored the crabs, affected the population and number of recruits. We looked at the LiMPETS data one year after the spill to compare to the data we had gathered in September of 2014 to see how the percentage of recruits had changed.
In order to compare various data, we analyzed data from surveys that took place in October 2008 (11 months after the spill) and September 2014. We looked at the total number of crabs and total number of recruits on Ocean Beach, and we made a graph to compare the data. Based on the data and graph, the oil spill may have negatively affected the percent of recruits. In October of 2008, the year after the oil spill, 15% of crabs were recruits. In September of 2014, 27% were recruits. This data is limited by the small sample size, and can possibly be expanded by looking at the entire year instead on just one month of each year. However, other scientific studies are taking place to further study the negative effects of oil spills on crabs, especially the infamous BP oil spill.
In conclusion, the sand crab population varies a lot year to year regardless of changes in human impact or environment. However, while only 15% of the crabs were recruits the October after the oil spill, that percentage nearly doubled five years later. A possible explanation for the lower percentage of recruits during this time could be the excess of oil on the beach. Subsequently, five years later in September of 2014, a higher percentage of recruits could be explained by the lack of oil on the beach, suggesting a positive trend for the sand crab population at Ocean Beach as it recovers from the oil spill. This data, therefore, could be a sign of the sand crab population’s resilience and ability to overcome even the stickiest of situations.