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Mystery of the Missing Blue Crabs: Winter vs. Summer Young

Tuesday, June 25th, 2013

Republished with permission from the Blue Crab Blog. Check out the Blue Crab Blog for the latest news regarding Maryland’s favorite crustacean.

By Katie Sinclair, Guest Blogger and Intern at the Smithsonian Environmental Research Center
The blue crab may be the most well-known denizen of the Chesapeake Bay, with the blue crab fishery one of the most productive in the region. From the late 1990s to mid-2000s, the blue crab population was in decline, with a near record low population of blue crabs recorded in 2008. The cause of this decline is not fully known, but is most likely a combination of overfishing, habitat loss, poor recruitment, and poor water quality.

Since new regulations on crab harvesting, particularly those restricting the harvest of mature females, were put in place in 2008, the population of blue crabs has increased significantly. However, a low number of juveniles were caught in the winter dredge this year, leading to a gloomy forecast for the number of harvestable blue crabs for the 2013 season.

During my summer internship at the Smithsonian Environmental Research Center (SERC), I want to investigate if this forecast is coming true. The winter dredge survey, an extensive bottom trawl survey that catches blue crabs overwintering at the bottom of the bay, is impressive for its scale and precision. The survey takes into account 3 different regions of the bay, and 1500 sites are surveyed. The data are used to calculate crab density and from that project overall crab abundance. The 2013 winter dredge survey found markedly lower numbers of juvenile crabs (crabs smaller than 2.4 in) than in previous years.  One of the key questions regarding the survey, however, is just how closely the observed winter population of juveniles correlates with the actual number of blue crabs that survive to the summer.

One of the main issues with using the juvenile index from the winter dredge survey to predict future abundance of adult blue crabs is that it does not take into account survivorship of juvenile crabs, which can vary widely from year to year. Blue crabs are competitive and cannibalistic, and a large proportion of juvenile blue crab mortality can be attributed to predation by blue crabs themselves. Using the juvenile index to predict future adult abundances does not take into consideration interactions between adult and juvenile blue crabs—a low number of juveniles could in fact be the result of increased predation pressure from the adult population. Longer term research conducted at SERC has indeed shown that mortality of juveniles is related to the density of adult crabs.

Over this summer, research will be conducted to determine how adult and juvenile abundances from the winter dredge survey correlate with the actual numbers of blue crabs observed in the summer. Crabs will be collected by net tows and their abundance and size will be recorded. Similar research conducted last summer showed that the high numbers of juvenile blue crabs found by the 2012 winter dredge survey had vanished by the summer.

Hopefully for crab-lovers, the future low abundance of crabs projected by the low juvenile index of the winter dredge survey will be found to be too low. Recruitment rates for blue crab are known to fluctuate wildly, and survivorship of larvae to juveniles depends on multiple factors: salinity, temperature, dissolved oxygen, and predation. The winter dredge report did show an increase in mature females, which suggests that management strategies designed to protect fecund females are in fact working.

Research done at SERC comparing crab abundance and mortality brings to light interesting questions regarding the overall dynamics of the blue crab populations. The comparison of observed crab abundance in the summer to the juvenile index from the winter dredge report will help us determine how accurate the juvenile crab index is at predicting future crab abundances. Studying the population dynamics of blue crabs can help us understand and preserve this valuable natural resource.

From the Field: Sea Wall Scraping Round II

Wednesday, June 19th, 2013

by Katrina Lohan

Sherry Reed (in pink) and Kristina Hill pick through clumps of oysters removed from the seawall at the Harbor Branch Oceanographic Institute. (Katrina Lohan)

Sherry Reed (in pink) and Kristina Hill pick through clumps of oysters removed from the seawall at the Harbor Branch Oceanographic Institute. (Katrina Lohan)

For our final sampling location we chose another seawall, at the marina at the Harbor Branch Oceanographic Institute. I had never been there before and the campus is beautiful! Driving into the marina from the Indian River Lagoon, the inlet is lined with mature mangrove trees, which also surround the entire marina. It was fairly windy that day, so the ride over was choppy, but once inside the inlet, the water was calm and the breeze was almost indiscernible.

It was my job to hold the boat against the seawall while Kristy and Sherry hammered oysters off the wall. I also kept track as they called out numbers of Ostrea sp.—we didn’t bother counting Crassostrea virginica because they were everywhere! Unfortunately for us, there were no Isognomon sp. along the wall, which we all found odd given that this habitat was so similar to the seawall we had previously sampled, where we found that species.

After about an hour of sampling, we were all wishing for the breeze! Without that breeze, the sun intensity felt more brutal and it was stiflingly hot. Once we had enough C. virginica and Ostrea sp. we drove around to a few other spots within that inlet to see if the Isognomon sp. were more localized, but no luck. I wanted another sediment sample from this site, but it was too deep along the wall, so Sherry found a spot in the mangroves that wasn’t too dense. I hopped out of the boat and took my sediment and water samples. I started looking for Isognomon sp. on the mangrove branches, but only found a handful… We had one last look on the rocks on our way out of the inlet, but no luck there either. Bummer! Well, two out of three species isn’t too bad!

From the Field: A Day Off?

Tuesday, June 18th, 2013

by Katrina Lohan

When Kristy and I go on these sampling excursions, we generally pack in a lot of sampling and processing into a short amount of time. We work every day, including weekends, and average 11-12 hour workdays for the two-week timeframe. So you can imagine our surprise when we finished processing all of the oysters from the second sampling location and realized that we had time in this trip for a day off!

What do two marine biologists do on their day off in Florida? Go to the beach! We inquired with the local staff as to the best place to go and were told that the Florida Inlet State Park beach was preferred as they had bathrooms and showers. We took the advice and headed to the beach for a day of R&R. I realized while sitting on the beach, mesmerized by the pounding of the waves, that it doesn’t take much for me to remember why I wanted to be a marine biologist. I am still awed by the power of water, watching the waves roll into the shore, crest, then fall, pounding into the sand and churning the shell fragments, turning them into smaller and smaller bits. I still stare at the ocean with wonder, imagining the vast and curious creatures that live in a world so completely different from the one that is familiar to me. Having these moments helps me to rejuvenate, so I can get back in the lab, continue processing my samples, and, hopefully, add to the body of knowledge about the ecology and evolution of marine creatures.

From the Field: Oyster Parasites Through a Microscope

Thursday, June 13th, 2013

by Katrina Lohan

Microscopic cysts inside a Crassostrea virginica oyster. (Katrina Lohan)

Microscopic cysts inside a Crassostrea virginica oyster. (Katrina Lohan)

In addition to taking oyster tissue to test for the presence of protistan parasites, we are also analyzing the oyster tissue for larger, but still microscopic, parasitic species such as worms, trematodes, and copepods. We have found parasites in all the species examined in all the locations we have traveled, but I got the best pictures of critters observed in oysters in Florida!

The highest prevalence and density of parasites occurred in Crassostrea virginica oysters. So far, I have seen copepods, turbellarians (probably Urostoma sp.), pea crabs, gill parasites, and cysts (probably from cestodes). I even saw what looked like a fish embryo, which was probably just brought into the gills, and an insect larva. Seeing all these critters under the microscope has made me contemplate why oysters are considered an aphrodisiac!

From the Field: Sea Wall Scraping

Wednesday, June 12th, 2013

by Katrina Lohan

Sea wall coated with oysters at South Bridge Marina. (Kristina Hill)

Sea wall coated with oysters at South Bridge Marina. (Kristina Hill)

Our second sampling location was a sea wall adjacent to a marina just below the south bridge in Ft. Pierce. The water was shallow enough along the wall that we were able to get out of the boat and stand in front of the wall, which made hammering and scraping the oysters off of it much easier!

Just as before, finding enough Crassostrea virginica oysters was easy, but finding the other oysters…took a lot more scraping. The Isognomon sp. at this location were fairly small and tucked into the crevices created by C. virginica, so they could be harder to spot. I took a step-wise approach, collecting the easiest species first, then making sure I had enough Isognomon sp., and only then did I move on and attempt to locate Ostrea sp., which easily took up the majority of our time. Most of the individuals in this species were dime-sized or smaller, tucked into the crevices or even in the dead shells of other oyster species, and they were covered in turf algae and mud. It took me (and the others) about an hour to find a sufficient number of these little guys at each of our respective sampling sites along the wall.

The funniest part of sampling along this wall was that there were lots of people around who did not hesitate to yell out! I am grateful to the multiple individuals who were concerned for our safety and mentioned the pilings in the water near the wall. (The water was clear enough to see them and we walked very slowly. Also, we all wore hard-bottomed water shoes and thick gloves.) We also had multiple boaters that were traveling in and out of the marina ask what we were doing, though I think some of them probably thought that we were just there to clean off the seawall!

Once all the oysters were collected, I took one water and one subtidal sediment sample and put them on ice. Then it was back to the lab to process more oysters.

From the Field: Dolphins!

Tuesday, June 4th, 2013

by Katrina Lohan

 A dolphin surfaces in Indian River Lagoon, Fla. (Kristina Hill)

A dolphin surfaces in Indian River Lagoon, Fla. (Kristina Hill)

We soon learned that it is mating season for dolphins and they frequently visit the Indian River Lagoon. On our boat trips to sampling locations, we have seen dolphins twice, and both times they were engaging in courtship behaviors. They are such graceful creatures.

I have to admit that I really, really want to see a manatee, another commonly spotted marine mammal in the lagoon. I haven’t spotted one yet, but I’m keeping my fingers crossed!

More on dolphin courtship >>

View manatee spotted in Florida mangroves >>

From the Field: Biofilms and Sediments

Monday, June 3rd, 2013

by Katrina Lohan

Kristy Hill uses cable ties to secure  a "biofilm collector" (a.k.a. microscope slides in a container) to a cage suspended from a local dock. (Katrina Lohan)

Kristy Hill uses cable ties to secure a “biofilm collector” (a.k.a. microscope slides in a container) to a cage suspended from a local dock. (Katrina Lohan)

As you may recall from a previous blog, part of the research for my fellowship project involves using genetic tools to look for parasites outside of their host organism in order to increase our understanding of the general ecology of these parasites.

I have been collecting water samples at all of the oyster sampling locations, but I decided that I wanted a larger diversity of habitat types. Thus, after discussing this idea with my advisors, I have decided to also sample subtidal marine sediments at the oyster sampling locations and collect biofilm samples. I will use the same genetic techniques on all the sample types to examine the diversity and distribution of marine parasites associated with the different habitats.

To collect the biofilm samples, we are using microscope slides, which were secured into a slide holder and suspended off of a local dock. The plan is to them scrape the biofilm off the microscope slides at scheduled intervals during our trip. I can’t wait to see what’s growing on them in a few days!

Read more on parasites surviving in disease reservoirs >>

From the Field: Parasite Hunting in Florida

Friday, May 31st, 2013

by Katrina Lohan

Jack's Island oyster reef in Indian River Lagoon, Fla. (Katrina Lohan)

Jack’s Island oyster reef in Indian River Lagoon, Fla. (Katrina Lohan)

For our next parasite hunting adventure, Kristy and I will be spending two weeks on Florida’s Atlantic Coast at the Smithsonian Marine Station at Fort Pierce. The marine station is located on a beautiful lagoon. We arrived on Monday and got a warm welcome from the staff, a thorough tour of the facilities and unpacked all of our gear, all four boxes of it. (No, we don’t travel light!) While here, we are staying on campus at the Taylor house, which is a residence for visiting scientists complete with kitchen, full bathroom, and a beautiful wrap-around porch that faces the water. We have long days and nights on these field trips, so it’s nice to have close accommodations that allow us to easily get back and forth to the lab, along with a full kitchen so we can cook quick and easy meals before returning to processing oysters. So far, I’m impressed!

On Tuesday, we headed out to our first field location. We hopped in a small boat owned by the Smithsonian and went to Jack’s Island, an oyster reef surrounded by mangrove trees. We were able to easily find Crassostrea virginica, as it is the species that makes up the reef, and then had to spend a little more time searching for Isognomon sp. at each site to get the numbers we needed. Also, it’s virtually impossible to tell which oysters are Ostrea sp. without opening the shells, so hopefully we have enough of those….

We also had a new first at this site—this is the first time that we have had a tour boat pass us while we were collecting!

Read accounts of marine biologists hunting for oyster parasites in Panama >>

From the Field: Insects Behind the Mangrove Invasion

Thursday, May 30th, 2013

by Mayda Nathan

Many insects visit black mangrove flowers, including bumblebees (left) and Pseudomyrmex ants (right). But which pollinators are the most important? (Mayda Nathan)

Many insects visit black mangrove flowers, including bumblebees (left) and Pseudomyrmex ants (right). But which pollinators are the most important? (Mayda Nathan)

Introduced species have a bad—and sometimes well-earned—reputation. Brown tree snakes in Guam, mosquitoes in Hawaii, cheatgrass in the intermountain west, and many more invasive organisms have turned native ecosystems upside-down, changing fundamental ecosystem properties like species diversity, nutrient availability, and the size and shape of food webs. Biologists are hard at work learning how to tell when, where, and how a species becomes a successful invader and driver of ecosystem change. (See a recent post on how tricky this can be.)

But how can we make predictions about invaders that are…native?

In other words, what happens when an organism starts to spread out from its native range into adjacent territory—without hitchhiking along with humans? And why does this happen in the first place?

Click to continue »

America’s Endangered Orchids

Friday, May 17th, 2013

North America is home to over 200 species of orchids. More than half are endangered or threatened somewhere in their territories. Some fall prey to poaching. Others fall to habitat loss. But much of their survival depends on something smaller: microscopic fungi in the soil. In the early–and sometimes later–stages of their lives, orchids depend on the symbiotic relationships they form with these fungi to obtain nutrients. If the soil is altered the fungi can disappear, and the orchids soon follow.

May 17 is Endangered Species Day. This year we’re highlighting some of the silent victims in the orchid gallery below.

Ghost Orchid (Dendrophylax lindenii)
This ethereal leafless orchid haunts the swamps of Florida’s deep south, the only state where it can be found. It is a frequent target of poaching, and generally dies within a year of being taken out of the wild.
Status: Endangered in Florida.

Ghost orchid (NC Orchid)

Ghost orchid (NC Orchid)

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