Piper, Cyrenea Bragdon (2017-12). Size and Density-Dependent Survival in a Brown Shrimp (Farfantepenaeus aztecus) Population. Master's Thesis. Thesis uri icon

abstract

  • Compensatory density dependence is required for sustainable harvest of any stock and is assumed in brown shrimp stock assessments. However, the strength and mechanisms of this density dependence and the developmental stage upon which it acts on has not been identified for this species. In general, detecting density dependence is complicated by the need for data at the appropriate temporal and spatial scales and the presence of environmental variability, observation error and other process error. Previous research indicated that compensatory processes might affect the juvenile stage of brown shrimp. Using Bayesian methods, I fitted state space matrix population models to 11 years of length-specific count data of juvenile brown shrimp sampled a maximum of eight times per month in Caranchua Cove Galveston Bay, Texas. I evaluated the influence of size-dependent survival, Beverton-Holt density-dependent survival and size-dependent growth on the juvenile stage. Using information criteria to evaluate model fit, I found that size-dependent survival was the most important factor acting on this population, followed by density-dependent survival. To avoid over-parameterization of the model, I omitted environmental factors such as temperature and salinity, that have a strong influence on juvenile brown shrimp growth and survival. As a result, models had poor fit and low predictive value; consequently, parameter estimates should be viewed with caution. Due to convergence issues, I was unable to test both size and density-dependent survival in a single model. The presence of density-dependent survival in the juvenile stage of this population suggests that this is the regulating stage in the brown shrimp lifecycle. This study provides a first step in identifying the stage and mechanisms regulating brown shrimp populations to better inform management.
  • Compensatory density dependence is required for sustainable harvest of any stock and is assumed in brown shrimp stock assessments. However, the strength and mechanisms of this density dependence and the developmental stage upon which it acts on has not been identified for this species. In general, detecting density dependence is complicated by the need for data at the appropriate temporal and spatial scales and the presence of environmental variability, observation error and other process error. Previous research indicated that compensatory processes might affect the juvenile stage of brown shrimp.
    Using Bayesian methods, I fitted state space matrix population models to 11 years of length-specific count data of juvenile brown shrimp sampled a maximum of eight times per month in Caranchua Cove Galveston Bay, Texas. I evaluated the influence of size-dependent survival, Beverton-Holt density-dependent survival and size-dependent growth on the juvenile stage. Using information criteria to evaluate model fit, I found that size-dependent survival was the most important factor acting on this population, followed by density-dependent survival. To avoid over-parameterization of the model, I omitted environmental factors such as temperature and salinity, that have a strong influence on juvenile brown shrimp growth and survival. As a result, models had poor fit and low predictive value; consequently, parameter estimates should be viewed with caution. Due to convergence issues, I was unable to test both size and density-dependent survival in a single model. The presence of density-dependent survival in the juvenile stage of this population suggests that this is the regulating stage in the brown shrimp lifecycle. This study provides a first step in identifying the stage and mechanisms regulating brown shrimp populations to better inform management.

publication date

  • December 2017