Getting to the Bottom of Stone Crab Population Trends

Researchers are conducting a long-term fishery-independent monitoring program to better understand stone crab population dynamics in Florida.
biologists measuring carapce, caption below
Biologists measure a stone crab's carapace (shell)
width while out on a sampling trip.

Florida’s commercial stone crab fishery averages between 2 and 3 million pounds of claws per season and provides 99 percent of all landings in the U.S. The fishery, which includes the Florida stone crab (Menippe mercenaria) and Gulf stone crab (M. adina), is unique in that only the claws are harvested. Animals are returned to the water alive after claw removal, unlike most other fisheries. The drawback of this harvesting method is biologists cannot obtain stone crab population data directly from the harvest because the crabs are returned to the water, preventing researchers from recording important details about the animals such as gender, size, health, and number of crabs harvested. Without these data, it is difficult for fisheries managers to develop strategies that address biological trends in this valuable commercial fishery. To address this information need, Fish and Wildlife Research Institute (FWRI) researchers are conducting a long-term fishery-independent monitoring program to better understand the factors that lead to annual fluctuations in stone crab catch.

FWRI biologists monitor eight locations in two regions along Florida’s west coast, as most of the state’s stone crab landings occur on this coast. The southwest region, found offshore of the Everglades and Florida Keys, includes Pavilion Key, Oxfoot Bank, Harbor Keys and Sawyer Key. The northwest region, stretching from Tampa Bay north to the southern edge of Florida’s Big Bend, comprises Steinhatchee, Cedar Key, Homosassa and Tampa Bay. At each location, researchers set traps at four sites arranged inshore to offshore and service them every two weeks, year round. Researchers count the catch, determine each crab’s sex and assess individuals for injuries. They measure carapace (shell) width and claw length; determine claw type and whether they are original or regenerated; and collect a genetic sample. Researchers also record water characteristics at each trap site. Each month, traps are scraped and cleaned, and biologists count and measure juveniles that have settled on the traps. During stone crab season, Oct. 15 to May 15, researchers also visually scan the area around FWRI traps and count buoys to estimate fishing pressure.

Researchers compare current observations to data collected in prior years of the monitoring program, which began in the southwest region in 2005 and northwest in 2006. Through comparison, staff can track recruitment areas where juveniles develop, juvenile settlement timing, how well crabs survive after declawing, reproductive development, population genetics and effects of various diseases. So far, they have noticed the size composition of claws in the fishery is getting smaller. Because landings consist mostly of small- and medium-size claws, researchers suspect there may be fewer large crabs in the fishery. Observations during this monitoring program also reinforce data from other studies showing most landings occur early and late in the season. Biologists are investigating the causes of these trends, as well as the possibility that mortality rates for declawed crabs may be higher than previously reported.



FWC Facts:
Scientists can determine the age of a fish by counting growth rings, similar to growth rings of a tree, on otoliths, the “inner ear bones” of fish.

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