Blue crabs connect North Carolina’s fisheries, estuaries, submerged aquatic vegetation, water quality, and coastal communities. Coastal Carolina Riverwatch is working with researchers at NC State University’s Center for Marine Sciences and Technology (CMAST), the North Carolina Division of Marine Fisheries, and North Carolina Sea Grant to better understand how disease, nursery habitat, and environmental conditions interact across coastal North Carolina.
This project focuses on Hematodinium, a parasite that can cause serious disease in blue crabs, and asks how infection patterns may relate to crab life stage, seagrass habitat, water quality, surrounding land use, and changing environmental conditions.
Blue crab health and nursery habitat are both management concerns
Blue crab (Callinectes sapidus) is one of North Carolina’s most economically important coastal fisheries. The state’s 2018 benchmark stock assessment found the stock was overfished and that overfishing was occurring. In September 2026, the North Carolina Division of Marine Fisheries reported that commercial landings, fishing trips, and fishery-independent indices of juvenile abundance continue to show declining trends. A new benchmark stock assessment is underway, with results expected in late 2026.
Habitat is part of the story. A 2026 analysis from the Albemarle-Pamlico National Estuary Partnership found that high-salinity seagrass habitat declined by at least 16,129 acres, or 16.2%, between 2006 and 2020. North Carolina still supports an estimated 88,531 acres of seagrass, the largest acreage along the East Coast, making protection of these underwater meadows especially important.
Blue crab juvenile recruitment and other fishery indicators remain a concern.
Documented decline in comparable high-salinity seagrass area, 2006–2020.
Samples represented in the current draft monthly Hematodinium summary.
162 of 597 samples were positive in the current draft dataset.
Juvenile blue crabs use submerged aquatic vegetation, or SAV, as nursery and refuge habitat. Prior research has linked Hematodinium disease risk and mortality with warm, higher-salinity conditions. This project asks how disease patterns may also relate to habitat availability, water quality, surrounding land use, and environmental conditions.
From field collection to disease detection and habitat analysis
This research was developed to address blue crab management and habitat questions identified in the North Carolina Blue Crab Fishery Management Plan. The study is designed to quantify Hematodinium infection in blue crabs and examine whether disease patterns are associated with nursery habitat, surrounding land use, and water-quality conditions.
Collect
Blue crabs are sampled through research field surveys and in coordination with North Carolina Division of Marine Fisheries monitoring. The design includes multiple life stages and coastal waterbodies.
Test
Samples are processed at NC State CMAST. Researchers extract DNA and use quantitative PCR, or qPCR, to detect and quantify Hematodinium.
Connect the data
Disease results can be compared with seagrass extent, salinity, temperature, dissolved oxygen, and other habitat, water-quality, and landscape variables to identify patterns that warrant further analysis.
What the expanded qPCR dataset is showing so far
The figures below summarize the research team’s current draft analysis. They should not be interpreted as a final statewide prevalence estimate or as evidence that life stage, sex, month, location, seagrass, or any environmental variable causes infection. Sampling effort and the mix of crab sizes and life stages differ across time and place. The research team is continuing analysis and interpretation.
An earlier 2025 Semester at CMAST student project reported an interim dataset of 104 blue crab samples, with 31 testing positive for Hematodinium (29.8%). The current draft analysis represents a much larger dataset: the monthly summary includes 597 samples, 162 of which were positive (27.1%). Because the dataset has expanded and its composition varies by life stage, month, sex, and location, these snapshots should not be treated as a direct year-to-year trend.
The strongest raw pattern is by life stage and crab size
The research team defined recruits as crabs with carapace width (CW) of 20 mm or less, juveniles as 21–120 mm, and adults as 121 mm or greater. Among the 587 samples with a recorded life stage, infection prevalence was highest in recruits and declined substantially in older/larger crabs.
Remaining samples were healthy
Carapace-width note: ten samples in the month-by-life-stage figure did not have carapace width recorded. The research team notes that these samples came from surveys in which only crabs smaller than 5 cm were retained, so they are expected to belong to the recruit or juvenile stage. Those ten samples included 7 infected and 3 healthy crabs.
View infection results by 20 mm size class
These are raw counts and percentages from the draft analysis. Several of the larger size classes contain relatively small sample sizes, so percentages should be interpreted with caution.
| Carapace width | Total n | Healthy n | Infected n | Infected % |
|---|---|---|---|---|
| 0–20 mm | 120 | 62 | 58 | 48.33% |
| >20–40 mm | 196 | 122 | 74 | 37.76% |
| >40–60 mm | 45 | 32 | 13 | 28.89% |
| >60–80 mm | 21 | 19 | 2 | 9.52% |
| >80–100 mm | 30 | 29 | 1 | 3.33% |
| >100–120 mm | 43 | 43 | 0 | 0.00% |
| >120–140 mm | 32 | 29 | 3 | 9.38% |
| >140–160 mm | 66 | 66 | 0 | 0.00% |
| >160–180 mm | 33 | 29 | 4 | 12.12% |
| >180–200 mm | 1 | 1 | 0 | 0.00% |
Infection also differed among crabs with recorded sex
Among samples with sex recorded, 64 of 196 females (32.65%) and 49 of 278 males (17.63%) were infected. Life-stage breakdowns show why the final analysis will need to consider multiple variables together rather than interpret the overall female–male difference by itself.
Females
32.65%
64 infected of 196 with sex recorded.
Males
17.63%
49 infected of 278 with sex recorded.
View infection by sex and life stage
| Sex | Life stage | Total n | Healthy n | Infected n | Infected % |
|---|---|---|---|---|---|
| Female | Recruit | 52 | 25 | 27 | 51.92% |
| Male | Recruit | 38 | 19 | 19 | 50.00% |
| Female | Juvenile | 130 | 93 | 37 | 28.46% |
| Male | Juvenile | 137 | 110 | 27 | 19.71% |
| Female | Adult | 14 | 14 | 0 | 0.00% |
| Male | Adult | 103 | 100 | 3 | 2.91% |
Raw infection prevalence changed across the sampling season
The monthly summary spans April through December. The highest raw monthly percentage occurred in November (64.71%), but that month included only 17 samples. June included the largest sample size (212) and had 35.85% infected. August had the lowest infection percentage among months with substantial sampling, at 6.10% of 82 samples. December included only one sample, which was healthy.
8 infected / 21 total
15 infected / 65 total
76 infected / 212 total
25 infected / 131 total
5 infected / 82 total
19 infected / 56 total
3 infected / 12 total
11 infected / 17 total
0 infected / 1 total
Why month and life stage need to be analyzed together: the mix of recruits, juveniles, and adults changed over the season, and infection prevalence differs strongly by life stage. The draft month-by-life-stage figure therefore reinforces the need to separate seasonal effects from changes in which sizes of crabs were sampled.
Healthy and infected crabs occur across coastal North Carolina
Draft statewide mapping shows both healthy and infected samples across the coastal study area. The research team also noted a strong preponderance of infected samples in the southern Outer Banks portion of the map.
That geographic pattern requires caution: the research team specifically notes that it may be linked to the much larger number of recruit-sized individuals sampled within that region. Because recruits had the highest infection prevalence in the draft dataset, differences in sampling composition can make a region appear to have a stronger disease signal. The final spatial analysis will need to account for life stage, sampling effort, habitat, and environmental conditions before drawing conclusions about location.
What these results mean — and what they do not yet mean
What the draft data support
The expanded dataset shows a clear raw association between smaller/younger crabs and higher Hematodinium infection prevalence. It also identifies differences across recorded sex, sampling month, and location that warrant further analysis.
What still needs to be tested
The current figures do not by themselves show whether SAV availability, salinity, temperature, dissolved oxygen, land use, sex, season, or geography independently changes infection risk. Those relationships require the project’s final multivariable and spatial analyses.
What is Hematodinium?
Hematodinium perezi is a parasitic dinoflagellate associated with “bitter crab disease.” It was first documented in blue crabs collected in Beaufort, North Carolina, in 1968. The parasite can infect multiple tissues and can cause high mortality in infected crabs, especially juveniles. Research from other Atlantic coast systems has found that warm, higher-salinity conditions can increase disease risk and mortality.
Public-health note: Hematodinium is a crab disease and is not considered a threat to human health. The concern is its potential effect on blue crab survival, juvenile recruitment, and the sustainability of the fishery.
Why seagrass matters to blue crabs
Blue crabs have a complex life cycle connecting the open ocean and estuaries. Females spawn in high-salinity waters, larvae develop offshore, and later stages return to estuaries. Young blue crabs settle into structurally complex habitats such as seagrass beds, marsh edges, and oyster habitat, where they find refuge and food as they grow.
North Carolina’s high-salinity seagrass community includes eelgrass (Zostera marina), shoalgrass (Halodule wrightii), and widgeongrass (Ruppia maritima). These underwater meadows provide habitat for fish and invertebrates, recycle nutrients, trap sediment, support water quality, store carbon, and help protect shorelines. Because seagrass needs sunlight, it is especially sensitive to conditions that reduce water clarity.
The next phase of analysis is especially important because it connects two management questions: where disease is occurring and what the surrounding habitat and water-quality conditions look like.
Who is involved
Dr. Y. Stacy Zhang
Dr. Zhang is a marine community ecologist at NC State studying biodiversity, species interactions, environmental change, essential fish habitat, and coastal ecosystem restoration.
Dr. Tal Ben-Horin
Dr. Ben-Horin is an aquatic pathologist at NC State whose research focuses on infectious disease ecology, aquatic animal health, and disease processes affecting fisheries and aquaculture.
Students and project partners
NC State students have contributed through Semester at CMAST and related research experiences, including field sampling, laboratory processing, data analysis, and public outreach. Alden Mazo’s 2025 Semester at CMAST project, “Interconnected Estuaries: Blue Crabs, Seagrass, and Hematodinium perezi,” helped build the early project dataset. Alden is contributing to the project again in fall 2026, including development of a StoryMap that will be coordinated with CCRW’s outreach work.
Coastal Carolina Riverwatch works with NC State CMAST, the North Carolina Division of Marine Fisheries, and North Carolina Sea Grant to connect scientific research with fisheries management, public education, and coastal community engagement.
Connecting the science to the Blue Crab Fishery Management Plan
Coastal Carolina Riverwatch’s role is to help translate the science into information that fishing communities, coastal residents, decision-makers, and the broader public can use. This project builds on CCRW’s Water Quality for Fisheries program, which brings fishermen, researchers, managers, and community partners together around water-quality threats affecting fisheries.
The North Carolina Division of Marine Fisheries began comprehensive review of the Blue Crab Fishery Management Plan in 2026 while a new benchmark stock assessment is underway. The research team is coordinating with DMF partners to discuss the project results and how the findings may be relevant to that management process.
Public outreach is also part of the research plan. CCRW and the research team are developing materials that explain blue crab life history, nursery habitat, water quality, land use, and Hematodinium. CCRW’s 10-part Seagrass Saturday series has already introduced these connections in an accessible format, and new findings will continue to be added as the research advances.
The project’s ArcGIS StoryMap is in development as a collaborative effort between the research team, student contributors, and Coastal Carolina Riverwatch. It will help people explore blue crab life history, sampling locations, seagrass habitat, disease patterns, and environmental conditions such as salinity and temperature. As analyses are finalized, the StoryMap will become an interactive companion to this webpage.
Additional resources
These resources provide authoritative background on North Carolina blue crab management, submerged aquatic vegetation, current seagrass trends, and the student research connected to this project.
- NC Division of Marine Fisheries: Blue Crab Management
- 2026 Blue Crab / FMP Update
- 2026 North Carolina Seagrass Update
- APNEP: Submerged Aquatic Vegetation Monitoring
- APNEP: The Albemarle-Pamlico’s Underwater Meadows StoryMap
- NC DMF: Submerged Aquatic Vegetation
- NC State CMAST: 2025 Semester at CMAST Projects
- North Carolina Sea Grant
More results are coming
This page will continue to be updated as the research team completes analysis and releases new maps, photographs, StoryMap content, and outreach materials. Follow Coastal Carolina Riverwatch on Instagram and Facebook at @coastalcarolinariverwatch and join our mailing list for research updates, volunteer opportunities, and Water Quality for Fisheries news.
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Research-status note: The 2026 infection results on this page are from a draft analysis shared by the research team and will be updated as the broader project analysis is completed and approved for public release.