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Fisheries

How important are claws to stone crabs?

The Paper: Duermit, E., Kingsley-Smith, P. R., & Wilber, D. H. (2015). The Consequences of Claw Removal on Stone Crabs Menippe spp. and the Ecological and Fishery Implications. North American Journal of Fisheries Management, 35(5), 895–905. doi:10.1080/02755947.2015.1064836

Introduction

We dine on crabs in a variety of ways. The meat formed into crab cakes or boiled whole with a good seasoning are my personal favorites. Often the meat of the body is overlooked in favor of the tasty crab legs and claws. People around the world eat Alaskan king crab legs, snow crab legs or, particularly in Florida, stone crab claws. Stone crabs (Menippe spp.) have small bodies but huge, meaty claws for breaking open oyster shells. The claws make up half of the weight of the adult of crab!

 

Florida stone crab. credit: Florida Fish and Wildlife

Florida stone crab. credit: Florida Fish and Wildlife

Stone crabs are caught much like other crabs in a regulated fishery, but their claws are removed and they are returned live (but defenseless) to the wild. Crabs are able to regenerate limbs (sometimes helpful for predator escape) and after they undergo a molt may emerge from their old shell with a new set of meaty (and tasty) claws.

There are a handful of crab fisheries in the world that operate with a similar claw-only policy and it has been thought that such fisheries may impact the crab populations to a lesser degree because the caught crabs are not killed and removed from the population. On the other hand, the sublethal effects of claw removal are not well understood, such as how it may change behaviors related to feeding, fighting or otherwise being a successful crab. This study investigates how clean and forced (leaving a large wound), 1 or 2-claw removal affects the ability of the stone crab to survive and to effectively consume prey.

Methods

Stone crabs have two asymmetrical claws: one for crushing and one for more precise manipulations called a pincer claw. Bivalves like oysters with difficult to open shells are a primary source of food making these claws an important part of typical feeding behavior. The investigators hypothesized that removing a crushing claw would impede or limit the ability of the crab to eat oysters by destroying their shell to reach the meat inside.

Florida is responsible for 98% of the stone crab harvest in the United States. Fishery restrictions allow removal of two claws from each crab as long as the claws are at least 7 cm long. South Carolina allows only one claw to be removed per crab.

The researchers removed one or two claws then monitored the crab for mortality and prey consumption over the next two weeks. They also followed the crabs over 11 months to document growth (including molting, perhaps with brand new claws) and survival.

After claw removal crabs were kept in a tank and given three types of food that represent three levels of difficulty to eat: fish (easy), mussel (medium), oyster (difficult). The amount of each consumed by the crab was measured each day. After two weeks the crabs were put in a communal tank and prey consumption was no longer measured.

On top of the lab experiments this paper covers a field study observing the relative frequencies of different types of breaks that occur in the removal of claws such as a clean break or a forced break. A clean break occurs when the crab drops its leg easily, leaving a smaller wound. A forced break occurs with more pulling of the leg. In this study to type of break and the location and size of the wound were recorded.

Results and Significance

Mortality and wound size

Crabs that did not suffer claw removal showed the fewest deaths. Wound size was an important factor in mortality and if the claw removal left a wound larger than 7 mm the crabs did not survive. The large wounds were more likely to come from unclean breaks where the removal was forced

Feeding habits

Crabs with no claws did not eat oysters or mussels, only fish. Crabs with a remaining pincer crab sometimes used other legs to stabilize mussels or oysters and crushed them with the pincer claw. In the wild if clawless crabs did not find readily available food that did not need to be crushed they may end up hungry.

FIGURE 1. Influence of wound width on survival probability for stone crabs in laboratory experiments I and II (n = 90). The graduated circles depict survival (0.0) and mortality (1.0), with the circle size proportional to the number of stone crabs represented (1−10). The curve is predicted from the logistic regression model (Wald χ2 = 2.10, P = 0.035).

Influence of wound width on survival probability for stone crabs in laboratory.  The graduated circles depict survival (0.0) and mortality (1.0), with the circle size proportional to the number of stone crabs represented (1−10).

FIGURE 3. Chesson's α (mean + SE) of each prey type for control stone crabs (n = 9) and stone crabs with one claw removed (n = 4) in experiment II. The horizontal line indicates α with equal consumption of all prey types.

Measure  of each prey type consumed for control stone crabs and stone crabs with one claw removed.

Long term observations

Large crabs did not molt, even after 11 months (In crabs the time between molts increases as they get larger). All crusher claws that were regenerated regrew as pincer claws. If two claws were removed, then two intermediate sized claws were grown. No new claws were of legal size for harvesting.

Conclusions and Fishery implications

This study shows there are consequences for claw removal in the stone crab. Throwing back a live crab does not guarantee that it will continue as a functioning member of the population. Crabs with large wounds will die and losing limbs appears to impact the ability of crabs to consume a common food source- the bivalve. Larger crabs are more likely to have above legal length claws for harvesting, but will be without claws for the longest due to increased time between molting as a crab ages.

 

This study helps us understand what happens when the crab is returned to the water. When making fishery decisions it is important to understand how the crabs are functioning after claw removal and not count all crabs in area as equal contributors to the population. This work opens more questions about how other aspects of the crab’s survival could be impacted by claw removal such as defense or reproduction. Is claw removal really that much less harmful than whole crab removal?

Sarah Giltz
I am a doctoral candidate in Ecology and Evolutionary Biology at Tulane University. My research focuses on the larval dispersal and development of the blue crab in the Gulf of Mexico.

When not concerning myself with the plight of tiny crustaceans I can be found enjoying life in New Orleans with all the costumes, food, and music that entails.

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