We get this question a lot on the Fishmobile. Since our Executive Director, Dr. Kristen Lycett, worked with blue crabs as part of her Ph.D. research, we thought this would be a good opportunity to talk about what’s going on with the blue crab population in the Chesapeake Bay.
This might be old news to some, but the blue crab population in the Chesapeake Bay is the lowest it’s been since we started surveying the population back in 1990. This is the third year in a row that blue crab abundance has been below the 30 year average.
We know this because of the winter dredge survey that is done each winter through a partnership between the Virginia Institute of Marine Science, the University of Maryland Center for Environmental Science Chesapeake Biological Laboratory, and the Maryland Department of Natural Resources. This survey is performed every winter, from December through March, and the resulting data is used to estimate the abundance of blue crabs in the Bay. You can find information on the most recent survey here.
From the Maryland DNR website, we can see the graph of the yearly abundance of all crabs estimated to be in the Chesapeake Bay. This shows that the total number of crabs in the Bay has been declining since 2019. We can also see from this graph that there is a lot of variation in the blue crab population from year to year. So, let’s talk about why there is so much variation and then we’ll get into some of the reasons as to why we’re seeing such low numbers now.
The first thing is that the blue crab life cycle is complex and the success of larval and juvenile crabs is dependent on many factors. This means that if one or more of those factors isn’t favorable for blue crabs, we are likely to see reduced success in terms of larval crabs making it to adulthood.
Juvenile and adult blue crabs can regulate their internal salinity, so they are able to move about the Chesapeake Bay freely, from fresher water to saltier water. Larval crabs, however, cannot regulate their internal salinity, so they need to be in full salt water. For this reason, mature female crabs migrate to the mouth of the Chesapeake Bay, where they will sponge (produce an egg sac). The egg sac, which can contain as many as 8 million eggs, remains attached to the female until the larvae are fully developed and the eggs hatch.
Once the larval crabs hatch out of the eggs, they are free floating in the water. Most larval crabs get washed out to sea due to surface currents. They remain in surface waters along the continental shelf for about a month, during which time they develop through several stages. Crabs are invertebrates, which means they have an exoskeleton that they must shed (molt) in order to grow, so each stage is the result of the larval crab shedding its old exoskeleton. After 30 to 50 days, the crab molts into its final larval stage, the megalopa.
At this stage, the crab is barely visible to the naked eye (it’s about 1 mm in size, so still pretty tiny) but begins to look more crab-like. These larval crabs are able to move up and down in the water column, so as to position themselves to take advantage of different currents. This is important because megalopae must ride these currents back into an estuary, in order to survive and continue developing into juvenile crabs.
In the Chesapeake Bay, most mature female crabs begin producing sponges around May and June, with a second wave of egg production happening in August. Eggs take roughly 14 days to hatch, which means we end up with megalopae returning to the Chesapeake throughout the summer and fall. The wind and current patterns in the summer help the larval crabs to stay in the waters near the mouth of the Chesapeake Bay and a shift to southward winds in the fall allows the megalopae to return to the estuary.
All of this is to say that without the right wind and current conditions, the return rate of larval blue crabs back into the Chesapeake Bay will decrease. We already know that in the mid-Atlantic region, we are seeing shifts in climate. This is not a political post, we’re not talking about the why and how of those changes, just the fact that they exist. We are seeing higher average temperatures and higher average precipitation, as compared to 20 years ago. It’s less clear what the impacts of these changes are on wind speed and direction and the seasonal shifts in wind patterns, but it’s possible that long term changes in climate affect the conditions that influence the rate at which blue crabs return to an estuary. If a megalopa does not return to an estuary, it is almost guaranteed to die. It’s important to note though, that it doesn’t have to return to its home estuary; some studies suggest as much as 10% of the megalopae that return to the Chesapeake Bay were hatched in the Delaware Bay.
Ok, so that was a lot of information just to get the point across that changes in long term weather patterns, ie climate, are possibly affecting blue crab populations.
The next thing to know is that megalopae and juvenile crabs need cover in order to survive, once they enter an estuary. Megalopae actively seek out habitats that can provide cover, such as seagrass beds, to settle in. Once they find a spot, they are ready to molt and become a juvenile crab. This means they look like an adult blue crab in terms of shape, but they are only a few millimeters in size. Because they are so small, they are very likely to get eaten, possibly even by slightly larger juvenile blue crabs. This is why they seek habitat that provides lots of cover and megalopae that don’t find cover are almost all eaten.
The abundance of seagrass beds in the Chesapeake Bay is another factor that is highly variable from year to year and greatly affects the survival rate of larval and juvenile blue crabs. While the general trend for seagrass coverage in the Bay has been positive since 1984, we did see a big drop in seagrass from 2018 to 2019. While we had a slight increase from 2019 and 2020 to 2021, we’re still looking at significantly less seagrass than was present from 2015 to 2018.
Another potential factor that might affect the blue crab population is the amount of juvenile crabs that are being eaten by other animals. In particular, concern has been raised about the impact of the invasive blue catfish. Currently, data is limited, but a 2021 study done in a portion of the James River in Virginia suggested that the 1.6 million blue catfish present there were able to eat 2.3 million blue crabs in a single year.
Another recent study suggested that the Bay-wide blue catfish population was as large as 100 million in 2019. If we estimate the same proportion of blue crabs being eaten by those catfish, a population this size has the potential to remove 143.75 million blue crabs each year. Now, are blue catfish eating that many blue crabs? It’s hard to say for sure, but the possibility is there. The 2021 study found that blue crabs were primarily eaten by intermediate to large sized catfish, with the smallest catfish with blue crabs in its stomach being 9.4 inches (240 mm). As catfish got larger, they were more likely to eat crabs and to eat more crabs. So, that means a good way to potentially protect blue crab populations is to help remove blue catfish from the bay by catching and eating them, and the larger the catfish the better!
Finally, we have to talk about the blue crab fishery. Here are Phillips Wharf, we are all about supporting healthy ecosystems AND fisheries. Seafood provides an important source of jobs and food for people around the world but it has to be harvested in a way that supports the continued success of the fishery.
We acknowledge that the blue crab fishery removes a significant proportion of the adult blue crab population. However, based on the data provided by scientists and researchers, the blue crab population is not overfished. The blue crab fishery in the Chesapeake Bay follows an adaptive management strategy, meaning that the harvest regulations for the fishery change based on data from research like the winter dredge survey.
In fact, based on the low abundance reported by the survey earlier this year, catch limits were reduced. In Maryland, recreational harvest was limited to one bushel per day per boat while the commercial fishing season was closed early on November 30th, rather than the typical date of December 15th. Reduced bushel limits were also placed on male and female crabs for certain months of the year in both states. In Virginia, the winter dredge fishery, which primarily targets mature females, has been closed since 2008 and will remain closed this year.
It’s difficult to say what the blue crab population will look like over the winter and into next year. We hope that the winter dredge survey will show an uptick in abundance since harvest was reduced this year. However, as was hopefully highlighted here, there are many other factors that will affect the blue crab population so it may take a few years for these harvest reductions to be reflected in the abundance numbers reported by the survey. In the meantime, we hope to see research funding being devoted to exploring these factors, so that we can better understand their true impacts on blue crab populations and make any necessary adjustments in the blue crab management plan.
What a great article! Thank you for keeping us all informed