Graduation Year

2017

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Marine Science

Major Professor

Kendra L. Daly, Ph.D.

Committee Member

Ryan L. Gandy, Ph.D.

Committee Member

David Chagaris, Ph.D.

Committee Member

Cameron Ainsworth, Ph.D.

Committee Member

Christopher Stallings, Ph.D.

Keywords

stone crab, size at maturity, histology, fecundity, spawning potential ratios

Abstract

The stone crab, Menippe mercenaria, supported the third most valuable fishery in Florida in 2016. Declining catch per unit effort (CPUE) and overall negative trend in landings since 2000 have raised concerns among fisherman, researchers, and fishery managers about the resiliency of the fishery. The Florida Fish and Wildlife Research Institute estimated that the Florida stone crab fishery has been overexploited since 1997. Overexploited fisheries, such as the commercial stone crab fishery, risk induced changes in life history parameters, such as age at maturation and reproductive output. In turn, these population-level changes have the potential to reduce the spawning stock biomass and future recruitment to the fishery. The research presented in this dissertation was intended to assess the Florida stone crab fishery stock for: 1) size at sexual maturation in females, 2) size at maturation in males, 3) potential fecundity, modeled as a function of spatial and temporal variables, 4) spawning potential ratios, an index that reveals potential recruitment overfishing, by formulating a per recruit model using the reproductive indices developed in this research, and 5) evaluating the impact of management changes on the spawning stock biomass.

The physiological, behavioral, morphological, and functional maturity of female stone crabs were assessed in individuals collected throughout the fished range of the population on the west Florida shelf (Cedar Key – Sawyer Key). Histology was used to determine oocyte maturity, presence of spermatophores, and presence of postovulatory follicles. Decision-tree analysis suggested that 50% of female stone crabs were mature at 43 mm carapace width, when using a combined variable of physiological and behavioral maturity. The decision tree accurately classified 98% of mature females and 85% of immature females. Morphological maturity was assessed using piecewise regression to determine where allometric changes in growth occurred between carapace width and crusher claw length. The transition point between mature and immature females occurred at a carapace width of 66.3 mm. Differences in estimates of size at sexual maturity between morphological and physiological, behavioral, and functional maturity indicate that relative growth of body parts is not a reliable estimate of sexual maturity in stone crabs. Histological analysis revealed that smaller females are contributing to the spawning population more than previously estimated. The corresponding carapace width at which female stone crab claws are vulnerable to harvest, occurs at 78 mm. The new estimates indicated that females contribute to the spawning population 1-2 times prior to vulnerability to the fishery. Continued monitoring of sexual maturity using these methods is recommended to strengthen understanding of the fishery and of interactions between stone crab populations.

Two methods were used to assess sexual maturity physiologically and morphologically in male stone crabs collected throughout the range of the fishable population. Histology was used to identify the production of spermatophores in the anterior vas deferens, indicating physiological maturity, and each crab was assigned a maturity stage. Decision-tree analysis suggested that physiological maturity occurred at a carapace width of 34.8 mm, accurately classifying 98.9% of mature males accurately and 72.5% of immature males. Piecewise regression estimated that the allometric relationship between carapace width and crusher claw length changed at a carapace width of 63.1 mm. The difference between physiological and morphological estimates of size at sexual maturity indicates that, like females, changes in male allometric relationships between body parts may not be a reliable indicator of sexual maturity. Large morphometrically mature males may be more successful in mating encounters, but in times of flux in population size structure, morphologically immature, but physiologically mature, males may have more opportunities for successful mating encounters. Carapace width at which male stone crab claws are vulnerable to the fishery occurs at 88 mm, indicating that they are capable of contributing to the reproductive population multiple times prior to vulnerability to the fishery.

Multiple factors influencing variability in stone crab batch fecundity were quantified across Florida fishery sites, by implementing a negative binomial model. Fecundity was strongly and positively related to carapace width, and parameter estimates indicated that batch fecundity was on average higher at Pavilion Key (southwest Florida) and Tampa Bay (central Florida), relative to Sawyer Key (south Florida) and Cedar Key (northwest Florida) sites at the northern and southern ends of the study area. Fecundity varied among 2013-2016 and was highly variable among months. In general, the lowest batch fecundity occurred in winter and spring months, while peak batch fecundity occurred in July and August, highlighting the common seasonal trend in crustacean reproductive cycles. Comparison to a 1993-1995 study of fecundity estimates in Tampa Bay, Florida indicated that there was little change in stone crab fecundity between individuals collected in 1993 and 2013. However, this temporal relationship accounts for a small portion of the fished population and may not be reflective of the changes over time across the entire fishery. Although sea surface temperature and ocean color data do not explain the variability in fecundity between sites, I hypothesize that food quality and quantity and fishing effort may play critical role in regionally variable individual reproductive output. I suggest that given the variability in reproductive output and regional fishing practices, that further work be dedicated to identifying critical spawning and recruitment areas to ensure resiliency of the stone crab population.

Spawning potential ratios were used to evaluate the impact of alternative biological and management scenarios to the reproductive output in a theoretical stone crab population. The model incorporated the findings of size at maturation and fecundity established in previous chapters, as well as contemporary temperature-dependent declaw mortalities. Spawning potential ratios increased with decreasing size at sexual maturity and were sensitive to past and present estimates of size at sexual maturity, and increasing natural mortality. However, the largest contributor to changes in the fished stock appears to be mortality associated with declawing and releasing stone crabs. We assessed various management scenarios including declaw lengths, vulnerability to capture, and variable season timing and length. Increases in declaw length and size at vulnerability to capture provided moderate protection to the spawning stock. The model estimated that adaptive management regulations which considered the timing and duration of seasonal fishing, based on mean monthly temperatures below 24°C (temperature at which declaw mortality increases), had the potential to greatly reduce mortality, and provided protection to the spawning population. The model was able to examine a wide range of biological and management scenarios; however, more research should be conducted to determine the socioeconomic impact of regulation changes. The Florida stone crab fishery is an amalgamation of unique fishing practices, rich culture, and both regional and economic influence. Evolving fishing practices, conservation initiatives, and environmental conditions have the potential to transform the fishery. Therefore, fisheries managers, scientists, and stakeholders must work cooperatively to ensure the resiliency of the stone crab fishery.

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