Wednesday, September 4, 2013

Juvenile Blue Crabs Numbers are Low in Maryland this Summer

From Smithsonian Environmental Research Center summer intern Katie Sinclair:

Since 2007, the Fish and Invertebrate Ecology Lab at SERC has been collecting data on the abundance of blue crabs in Maryland tributaries of Chesapeake Bay. Every summer, the lab uses crab tows—a meter-wide net on skids that glides along the bottom--to collect crabs and fish. The blue crabs are counted and measured, and the presence of fish species is noted. As part of my summer internship with the Smithsonian Environmental Research Center, I helped with the blue crab project. This year, we sampled 2 sites in each of 7 areas across the bay, doing 12 crab tows per site. Sampling took place from late June to early August. We also sampled at a new site in Herring Bay.
The average number of crabs per site was much lower than during the period from 2010-2012. For example, there were six times as many crabs in the Rhode River in summer 2012 compared to this summer (see Figure 1). This finding was consistent with the very low number of juvenile crabs reported earlier this year in the 2013 Chesapeake Bay Winter Dredge Survey, which is conducted throughout the bay by Maryland Department of Natural Resources and the Virginia Institute of Marine Sciences. The lowest number of crabs was found in Middle River, which was the northernmost site (see figure below). Because young crabs migrate into the bay from the open ocean, sites like upper Tangier Sound that are closer to the mouth of the bay and have extensive marsh and seagrass habitats often have higher numbers of crabs.

Although we do not yet know for certain why the abundance of crabs is so low this summer, evidence suggests it is due to low numbers of reproductive females during the summer 2012 spawning season.  In 2008, new protections on harvesting adult female blue crabs were enacted, which has led to higher numbers of female blue crabs in four of the five years since (according the Winter Dredge Survey). However, the winter of 2012 was the one year in which the number of spawning-age females was very low. With few females in winter 2012, it is likely that reproduction was also very low in the summer of 2012. Our survey is designed to target crabs that are roughly one year old. Because of this, it appears likely that the low number of crabs caught in our survey (and in the 2013 Winter Dredge Survey) was the result of low reproduction in summer 2012. Other factors such as weather, harvest, predation, habitat loss or water quality could also have played a role.

While it is yet unknown exactly why there was a low number of female blue crabs found in the 2012 Winter Dredge Survey,  the relatively high number of females in four of five years since 2008 suggests that the protections placed on female blue crabs are working. Long-term monitoring data from SERC’s summer crab survey, the Winter Dredge Survey and other sampling programs show that protecting the population of adult females is one way to ensure that the blue crab, an important ecological and commercial species, remains abundant throughout the Chesapeake Bay.

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