2011 Stone Crab Stock Assessment
Executive Summary of the 2011 Stock Assessment Update for the Stone Crab, Menippe spp., Fishery in Florida
Full PDF available upon request.
- The stone crab fishery, which occurs primarily on Florida’s west coast, is unique among fisheries in the southeast United States (SE US) because harvesters do not harvest the crabs; but, rather, the harvesters remove the crabs’ legal-sized claws at sea and return the declawed crabs to the water alive.
- The stone crab fishery is managed in the SE US with a seven-month fishing season (15 October through 15 May), a minimum harvest claw (propodus) size of 2-3/4 in (70 mm), trap specifications, and a passive trap limitation program. The minimum claw size ensures that most female crabs have already spawned one or more years by the time their claws reach legal size.
- To provide a historical perspective for evaluating more recent landings for which we have information on effort, historical landings, in pounds of claws on a calendar-year basis, were extended back to 1895. The stone crab fishery was a developing fishery until the 1990s.
- This update includes commercial landings through the 2009-10 fishing year. Peak landings were 3.5 million pounds of claws statewide in the 2000-01 fishing year. Statewide landings in 2009-10 were 2.4 million pounds of claws.
- The landings at the beginning of the fishing year in October are good predictors of the landings for the entire year and, since 1985-86, the landings in this half-month have averaged 18% of the fishing year’s total.
- An average of 31% of the claws observed by FWC samplers in fish houses statewide (weighted by regional landings) showed evidence of forced breaks, which lowers the crab’s likelihood of surviving after being declawed.
- Approximately 13% of the claws observed by FWC samplers in fish houses statewide had the broken stridulatory pattern, indicating that they were regenerated claws and more than half (52%) of these regenerated claws exceeded 80 mm (3.15 in) in length. The presence of these larger regenerated claws indicates that some crabs survive the loss of their claws. Depending upon the size of the crab and when in the intermolt cycle the crab is declawed, it can take 1 to 2 years for a crab to regenerate a claw to legal size.
- Since the 1962-63 fishing year (the first year with an estimate of the number of traps in the fishery), the number of traps in the fishery has increased more than a hundred-fold -- from 15,000 traps in the 1962-63 year to 1.6 million traps in the 2001-02 year. In the 1998-99 fishing year, FWC employees conducted a physical count of the stone crab traps and found a total of 1.4 million traps, which was twice the number that was estimated in 1992-93. As a response to the rapidly increasing number of traps in the fishery, the Florida State Legislature approved a stone crab trap limitation program in 2000 that was implemented in October 2002. Another measure of effort, the number of commercial trips, also increased from 19,000 in the 1985-86 fishing year (the first year with trip information available) to 38,000 trips in the 1996-97 fishing year and then declined afterwards.
- Catch-per-trap-claimed has fluctuated widely over time, but it has shown a generally decreasing trend. Catch rates dropped rapidly from more than 20 pounds per trap in the 1960s to less than 10 pounds per trap by 1971 to less than 5 pounds per trap by 1983. The catch-per-trap-claimed has been so low that, it declined only slightly since 1983, even though the number of traps doubled.
- Catch-per-trip, available only since the 1985-86 fishing year, was standardized using a generalized linear model (GLIM) to remove confounding effects such as differences in numbers of traps, location or time of the year. The catch-per-trip data declined until the 2007-08 fishing year and then began to increase. As would be expected in a fishery with a closed season, the stone crab fishery has a strong pattern of declining catch-per-trip within each season.
- Catch per trap pulled, also available since the 1985-86 fishing year and standardized with a GLIM, decreased until 2000-01 and then has slowly increased.
- We used two models to evaluate the condition of the stock. First, we developed a surplus production model with landings, in pounds of claws, and the estimated numbers of traps claimed by harvesters from the 1962-63 through 2009-10 fishing years. As expected, the model indicated that the fishing mortality rate was too high, i.e., there were more traps in the fishery than were necessary. Because the crabs are released alive after the legal-sized claws have been removed, estimating a biomass-of-claws benchmark is not relevant. The second model was a modified DeLury model in which we estimated the recruitment that would be necessary to account for the removals from the fishery and from natural mortality. Fishing effort in the DeLury model was the monthly number of commercial trips (continuity model) or the monthly number of traps pulled from the 1985-86 through 2009-10 fishing years. Recruitment to the fishery estimated with the DeLury models varied without trend during this period.
- The status of the stone crab stock is best indicated by the lack of an increase in landings when the number of traps more than doubled. The lack of correspondence between landings and effort indicate that the current level of landings represents all that can be harvested under current environmental conditions, regulations, and fishery practices. The surplus production model concluded that the fishery is overfishing. These conclusions were the same as those from the 1997, 2001, and 2006 assessments. Recruitment into the fishery has varied without trend over this period. Stone crabs may be resilient because most female stone crabs spawn one or more times before their claws reach legal size, some crabs survive declawing, and the fishery is closed during the principal spawning season. However, because of the reproductive behavior of stone crabs, there is concern for adequate numbers of large, mature males.
- The numbers of traps in the fishery will remain high for some time. Specifically, if the decline in the issuance of trap certificates continues at the present rate (2.57% per year), it will take 37 years for the fishery to reach the Commission’s goal of 600,000 traps in the fishery.
- A shortcoming of this assessment, and of all the previous stone crab assessments, is the total lack of recreational data. We know that there is a recreational sector that can either fish five traps per person or catch the crabs while diving but we have no idea of the number of participants or the magnitude of their harvest. In spiny lobster, there is a Saltwater Fishing permit which allows scientists to develop a mail survey to identify recreational fishing pressure and harvest. A similar permit for the recreational harvest of stone crabs would provide similar information and remove a major source of uncertainty in the assessment process.