National Oceanic and Atmospheric Administration

Looking for blue crab preferences in a changing coastal environment

Linking blue crab abundance and mortality to marsh fragmentation and submerged aquatic vegetation cover

Other Author(s): Co-PIs: Kelly Darnell and Zack Darnell (University of Southern Mississippi)

The Challenge

Despite coastal land loss, Louisiana’s coastline and estuaries support some of the most productive and economically important fisheries in the Gulf of Mexico including the blue crab. In 2015, more than 41 million pounds of blue crab – both hard and soft shell – were harvested in Louisiana for a whole sale value of more than $58 million, according to the Louisiana Blue Crab Assessment in 2016.

Land loss not only includes shoreline erosion, but also result in fragmented marsh, small patches of land dotted in open water areas, in areas where it used to be continuous marsh. What this fragmentation means to crab populations now and into the future is important for blue crab management. 

The Approach

Institute researchers selected three sites in Terrebonne Basin near Coastwide Reference Monitoring System stations. At each of the locations, researchers chose a 500 x 500 m areas that represented low, intermediate, and high marsh fragmentation, as well as the three habitat types – marsh, submerged aquatic vegetation, and bare sediment for a total of 27 test sites. The cover of submerged aquatic grass at each location was recorded. The level of fragmentation within each site was determined using a Geographic Information System, aerial imagery, and by calculating the marsh edge perimeter to area ratio. Blue crabs, adults and large juveniles, were sampled using commercial crab traps that were set up in each of the 27 sites Each crab caught was measured for carapace width, sexed, counted, and immediately released. Overall, three traps were deployed per habitat, three per fragmentation level, at each of the three sites. During the six months of sampling, the crab traps were set out 486 times.

Juvenile crabs were sampled in each habitat with 1 m throw trap – an aluminum cube with mesh netting on the sides, but not the top or bottom. The plant material was removed and a 1 m bar sein was used to remove all the crabs. Used twice per each habitat and each fragmentation level during the six-month sampling period, the throw trap samples were used 972 times between May and October. Researchers looked at how the population of juvenile and adult blue crabs might vary across different levels of marsh fragmentation and submerged aquatic grass. . A better understanding of blue crab habitat use will be essential for predicting future changes in fishery production and ensuring the sustainability of the fishery.