Fisheries and Resources Monitoring System

Blue crab - USA Louisiana
Marine Resource  Fact Sheet
Status of stocks and resources 2019
Blue crab - USA Louisiana
Fact Sheet Citation  
Owned byFood and Agriculture Organization (FAO) – More
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FAO Names: en - Blue crab, fr - Crabe bleu, es - Cangrejo azul
Geographic extent of Blue crab - USA Louisiana
Main Descriptors
Considered a single stock: Yes        Spatial Scale: National
Management unit: Yes        Reference year: 2017
Biological State and Trend
State & Trend Descriptors
Exploitation rateF(2016)/Flimit = 0.82Not applicable
Abundance levelSSB(2017)/SSBlimit = 1.36Not applicable
FAO Categories
Exploitation stateMaximally sustainably fished

An assessment of blue crab in Louisiana waters, where most of the landings of the Western Gulf of Mexico stock come from, indicated that in 2015 the stock was overfished and experiencing overfishing (West et al., 2016b). An update assessment with data up to 2017 estimated that the stock was no longer overfished nor experiencing overfishing (West et al., 2018). 
Habitat and Biology
Climatic zone: Tropical.   Bottom type: Soft bottom.   Depth zone: Coastal (0 m - 50 m).   Horizontal distribution: Estuarine.   Vertical distribution: Demersal/Benthic.  

Water Area Overview
Spatial Scale: National

Geo References
Resource Structure
Considered a single stock: Yes
Level of uncertainty: Intermediate


Data up to 2017. Blue crab landings from the state of Louisiana represent approximately 37% of total landings in FAO Area 31. Data requirements include a time-series of observed landings and corresponding abundance indices for juvenile and exploitable life stages. Louisiana blue crab commercial harvest is derived from NMFS (National Marine Fisheries Service) statistical records, 1968-1998, and the LDWF (Louisiana Department of Wildlife and Fisheries) Trip Ticket program, 1999-2017 (Table 1). Landings are taken from NMFS statistical records, 1968-1998, and the LDWF Trip Ticket Program, 1999-2017. A time-series of recreational harvest records currently does not exist. Blue crab abundance indices are derived from the LDWF fishery-independent marine inshore 16-foot trawl survey.
Assessment Model
Type:  Others
Collie-Sissenwine Analysis

A catch-survey (CS) or Collie-Sissenwine analysis (Collie and Sissenwine, 1983) is used in this assessment to describe the dynamics of the Louisiana blue crab stock. The CS modeling approach is intended for data moderate situations where a full age structure is lacking. Model requirements include: 1) annual abundance indices for juvenile and adult life stages, 2) annual harvest estimates as individuals, 3) an estimate of instantaneous natural mortality, and 4) the relative selectivity of the juvenile and adult life stages to the survey gear.
Overall Assessment Results

Model Results - The assessment model provides reasonable fits to the adult abundance index (Figures 2-3). However, the fit to the juvenile abundance index (Figure 4) is consistently overestimated in later years of the time-series. The juvenile index suggests a considerable decline over the latter half of the time-series examined. The assessment model tracks this decline, but underestimates its magnitude suggesting additional processes aren’t captured by the assessment model (e.g. temporal, spatial, and/or environmental; see Research and Data Needs Section). The catchability coefficient is estimated as qn=0.0048 in this assessment. Annual exploitable (adult) biomass estimates range from 15 to 92 million pounds (Table 10, Figure 5). The 2017 exploitable biomass estimate is 25 million pounds. Exploitable biomass levels generally decline after 1990, where previous years estimates were rarely below 40 million pounds. Increases in exploitation during the 1990s coincide with this decline (Figure 6). A large population response is evident in the years following the passages of Hurricane Katrina and Rita which caused a substantial reduction in the directed effort and supporting infrastructure of the Louisiana commercial blue crab fleet. These storms also provided optimum environmental conditions for settlement of megalopae and young crabs into Louisiana estuaries via storm surge and likely enhanced recruitment. Juvenile abundance estimates range from 152 to 574 million individuals (Table 10, Figure 5) and exhibit a considerable decline over the latter half of the time-series examined. The eight most recent juvenile abundance estimates are the lowest on record with the exception of the 1976 estimate. The 2017 juvenile abundance estimate (152 million individuals) is the lowest on record. Additionally, in the two most recent decades only two juvenile abundance estimates are above the time-series average and in the most recent decade no estimates are above the time-series average (Figure 7). It’s important to point out here the consequence of this decline on management reference point estimation. Because equilibrium conditions (i.e., average recruitment) are assumed in reference point estimation, management benchmarks will generally be biased when below average conditions persist for extended time periods. Annual instantaneous fishing mortality estimates range from 0.07-0.92, with peaks in exploitation occurring in 2002, 2012 and 2014 (Table 10, Figure 8). Trends in fishing mortality estimates, 1999-2016, are generally consistent with fishing effort (i.e., trap fisher trips) derived from the LDWF Trip Ticket Program (Figure 8). A large reduction in fishing mortality/effort was observed in the years following the passages of Hurricane Katrina and Rita. Fishing effort is not used in the assessment model but is presented here to validate trends in fishing mortality. However, the number of trap fisher trips may not be a suitable measure of fishing effort (specifically for catch per unit effort data) if the number of traps fished per trip increases (or decreases) through time and should be considered with caution. No discernable relationship is observed between exploitable biomass and subsequent recruitment. However, the most recent half decade of data pairs are all below the juvenile abundance time-series average and are some of the lowest adult biomasses observed (Figure 9). - Stock Status - The history of the Louisiana blue crab stock relative to the reference points described above is illustrated in Figures 11 and 12. Fishing mortality rates exceeding Flimit (or ratios of F/Flimit>1.0) indicate overfishing; stock biomasses below SSBlimit (or ratios of SSB/SSBlimit<1.0) indicate an overfished condition. - Overfishing Status - The 2016 estimate of F/Flimit is 0.82, suggesting the stock is not currently experiencing overfishing. However, the 2014, 2015, and 2016 fishing mortality rate estimates are past their target and the 2014 value was very close to the overfishing threshold. These estimates should be considered from a precautionary viewpoint given the upward trend in fishing mortality coupled with the decrease in exploitable biomass observed in the most recent decade. Again, the overfishing reference point estimate in this assessment (i.e., the Flimit) is calculated assuming equilibrium conditions and maybe overestimated given the below average recruitment observed in the past 20 years. Estimates of fishing mortality are not available for the terminal year of the assessment. - Overfished Status - The 2017 estimate of SSB/SSBlimit is 1.36, suggesting the stock is currently not in an overfished condition. The stock was considered overfished, however, in 1995, 2013, and 2015.
Scientific Advice

• Research emphasis on the Louisiana blue crab fishery is lacking, particularly in consideration of the value and size of the fishery (Guillory et al. 1996). The authors suggest that blue crab research done on the Atlantic coast may not be applicable to Gulf of Mexico populations. Based on this assessment, the following research and data needs are identified as priorities for future assessment of the Louisiana blue crab stock. • Due to the rapid growth and short life span of blue crab an annual time-step in the assessment model may not adequately describe the population dynamics of blue crab. Future assessment modeling efforts should explore finer temporal scales. • Environmental factors influencing year-class strength and the survival of recruits to exploitable life stages are not well understood. Further analysis of these factors could elucidate the link between the environment and blue crab productivity. Contributing factors could also be used in development of predictive models allowing for short-term forecasts for resource managers and industry. • In addition to research specific to the Louisiana blue crab stock, continuous fishery dependent monitoring programs, as part of a comprehensive monitoring plan, are needed. Differences in exploitation rates of male and female blue crabs likely exist. Continuous information on size and sex distributions of the commercial and recreational harvest are not available. Continuous harvest data for the recreational sector is also lacking. These data would reduce the number of assumptions required in future assessments. • The magnitude of blue crab catch in shrimp trawls and the associated mortality is currently unknown. In this assessment, this mortality was assumed negligible. Future estimates of blue crab catches in shrimp trawls and the mortality induced would reduce the number of assumptions required in future assessments. • Commercial effort data is currently only available in terms of the number of trips taken. A more useful measure of effort that could improve future blue crab stock assessment and management is the number of traps fished by basin/season/region. • Estimates of natural mortality in this assessment are based on assumptions of longevity. Without the ability to directly age blue crabs with conventional methods (i.e., calcified hard parts), growth estimation and resulting longevity estimates remain difficult to quantify. Estimates of these life history parameters for the Louisiana blue crab stock, perhaps from tagging or pond studies, would aid in refining life history assumptions in future assessments. • Assessment of regional or basin-specific sub-populations could differentiate exploitation rates and stock status within the state. If available data is adequate for regional assessment, results could be used to determine if regional management is an effective alternative to optimize yield within the state. • The relationship between wetlands losses and the continuation of fishery production within Louisiana has been discussed by numerous authors. Understanding this relationship as it applies to the Louisiana blue crab stock should be an ongoing priority. • With the recent trend toward ecosystem-based assessment models, more data is needed linking blue crab population dynamics to environmental conditions. The addition of environmental data coupled with food web data may lead to a better understanding of the blue crab stock and its habitat. • Fishery dependent data alone is not sufficient to accurately assess stock status and trends in abundance. Consistent fishery independent monitoring, in addition to fishery dependent monitoring, are integral components of this ability. Present monitoring programs should be assessed for adequacy with respect to their ability to evaluate stock status and should be modified or enhanced to optimize their capabilities.
Management unit: Yes
Source of information
FAO. Western Central Atlantic Fishery Commission. 2019. Review of the state of fisheries and fisheries resources in the WECAFC region. Meeting document WECAFC/SAG/IX/2018/3 of the ninth session of the Scientific Advisory Group, Christ Church, Barbados, 19-20 November 2018.  Click to open
West, J. Blanchet, H. &amp;amp; Cagle, P. 2018. Update Assessment of Blue Crab in Louisiana Waters. 2018 Report. Louisiana Department of Wildlife and Fisheries. 33 pp.  Click to open
FAO. Western Central Atlantic Fishery Commission/FAO Commission des pêches pour l’Atlantique Centre-Ouest/FAO Comisión Central de Pesca para el Atlántico CentroOccidental. 2019. Report of the ninth session of the Scientific Advisory Group, Christ Church, Barbados, 19-20 November 2018. Rapport de la neuvième session du Groupe scientifique consultatif, Christ Church, Barbade, 19-20 Novembre 2018. Informe de la octava sesión del Grupo Asesor Científico, Christ Church, Barbados, 19-20 de Noviembre de 2018. FAO Fisheries and Aquaculture Report/Rapport sur les pêches et l’aquaculture/Informe de Pesca y Acuicultura. No. 1266. Bridgetown, 156 pp.  Click to open
All references to figures, tables and bibliography in the text are found within the source of information.
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