The 2006 Stock Assessment Update for the Stone Crab, Menippe spp., Fishery in Florida

This assessment provides an update on Florida's stone crab fishery

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Executive Summary

The stone crab fishery is unique in that fishers do not harvest the crabs; rather, the fishers remove legal-sized claws from the animals and then return the crabs alive to the water. Most female crabs have already spawned one or more seasons by the time their claws reach legal size.

The stone crab fishery is managed with a seven-month fishing season (October 15 through May 15), a claw (propodus) minimum harvest size of 2-3/4 inches (70 mm), trap specifications, and a passive trap limitation program.

An average of 34% of the claws (weighted by regional landings) observed by Florida Fish and Wildlife Conservation Commission (FWC) samplers in fish houses statewide showed evidence of forced breaks. Approximately 13% of the claws were regenerated claws, or claws that grew back, providing evidence that some declawed crabs survive the loss of their claws.

Historical landings, in pounds of claws on a calendar-year basis, were extended back to 1902 to provide a context for evaluating the more recent landings for which we have information on effort. Landings generally increased until 1998.

This update includes commercial landings through the 2004-05 fishing season because the information from the 2005-06 season is not yet available. Landings, in pounds of claws on a fishing-season basis, have varied without trend since 1989-90. Peak landings were 3.5 million pounds statewide in the 1997-98 fishing season. Statewide landings for 2004-05 were 3.0 million lb of claws.

The landings in October are good predictors of the landings for the entire season. However, the October 2005 landings are not expected to be a good indicator of the 2005-06 fishing season landings due the effects of Hurricane Wilma, which reached the Florida Keys on October 24 ,2005. Both the number of trips and the pounds of claws landed in October 2005 were 56% of the 1985-2004 October averages.

Since the 1962-63 fishing season (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 season to 1.6 million traps in the 2001-02 season. In a physical count of traps conducted in the 1998-99 fishing season, FWC employees found 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 legislature in 2000 approved the stone crab trap limitation program, which was implemented in October 2002. The number of commercial trips also increased from 19,000 in the 1985-86 season (the first season with trip information available) to a maximum of 38,000 trips in the 1996-97 season and then declined afterwards.

Catch-per-trap has fluctuated widely, but it has shown a generally decreasing trend over time. 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 five pounds per trap by 1983. Catch rates continued to decline as the number of traps increased. The catch-per-trap since 1983 has been so low that it declined only slightly with the further doubling of traps. Catch-per-trip was standardized using a generalized linear model to remove confounding effects such as differences in location or time of the year. Most of the stone crab landings come from Florida's gulf coast and the Florida Keys. As would be expected in a fishery with a closed season, the stone crab fishery has a strong pattern of declining catch-per-trip during each season. The catch-per-trip data, available only since the 1985-86 season, also showed that the catch-per-trip has been declining over the same time period.

We used two models to evaluate the condition of the stock. First, we used the landings, in pounds of claws, and the estimated numbers of traps in the fishery from the 1962-63 through 2004-05 fishing seasons in a surplus production model. As expected, the fishing mortality rate compared to its benchmark was too high. Because of the nature of this fishery, biomass of claws is not directly relevant. Second, in a modified DeLury model, we used the monthly landings, expressed as numbers of claws, and the commercial trips from the 1985-86 through 2004-05 fishing seasons to estimate the October recruitment that would be necessary to account for harvest and natural mortality (the DeLury continuity model). We found that recruitment has varied without trend during this period.

The status of the stock is best indicated by the stable landings after 1989-90. The three-fold increase in the number of traps since then suggests that the current level of landings is all that can be harvested under current environmental conditions, regulations, and fishery practices and that the fishery is overfishing. Recruitment does not show any decline over the time series. These conclusions were the same as those from the 1997 and 2001 assessments. The stone crab fishery may be resilient because most female stone crabs spawn one or more times before their claws reach legal size, because some crabs survive declawing, and because the fishing season is closed during the principal spawning season. However, the fishery continues to have too many traps in the water. Further evidence of excess traps is the low catch-per-trap level over a very wide range of numbers of traps.

In earlier assessments, we concentrated only on the harvested claws; but in this assessment, we began to investigate the biological basis of the fishery, i.e., the number of crabs affected by the fishery. There is no direct measure of this number. Therefore, we used the average weight of claws in the commercial claw-size categories to estimate the number of claws harvested and the monthly estimates of the average number of legal-sized claws per crab from a fishery-independent trapping study in Tampa Bay to estimate the number of crabs with legal-sized claws. Fishery-independent sampling has been conducted in the Tampa Bay region since 1988. To add some credence to applying information from Tampa Bay to the entire gulf coast of Florida, we compared the claws per crab from Tampa Bay from February through May 2005 with those from the Florida Keys (the only other area and time with comparable information). There was no significant difference between the number of claws per crab (1.21 claws per crab in the Keys and 1.23 claws per crab in Tampa Bay).

For the past decade (1995-96 to 2004-05 fishing seasons), the gulf coast fishers have declawed approximately 10.5 million crabs during each seven-month fishing season.

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