1. David A. Dippold1, Robert T. Leaf1, and J. Read Hendon2
Using relative stock density as a management tool for Spotted Seatrout in Mississippi
David A. Dippold1, Robert T. Leaf1, and J. Read Hendon2
1University of Southern Mississippi, Department of Coastal Sciences, Gulf Coast Research Laboratory, Ocean Springs, MS 39564
2Center for Fisheries Research and Development, Gulf Coast Research Laboratory, Ocean Springs, MS 39564
Introduction
Spotted Seatrout are the most popular recreational species in Mississippi inshore waters.
Relative stock density (RSD) is a metric used to quantify the length composition of a stock and is defined as the percentage of fish greater than stock length that are also greater than a specified length.
RSD is not widely used in marine fish stocks.
Spotted Seatrout exist in spatially-distinct sub stocks and are an ideal candidate for using RSD to assess the length-frequency of the stock.
Understanding how RSD changes under different management regimes, defined as a specific minimum length limit and level of fishing mortality (F, y-1), is useful in considering how different regulations may affect the length composition of the stock.
In this study we investigated how measures of RSD change under different management regimes.
Results
Conclusions
Because of the growth characteristics of individuals in the Mississippi stock, the percentage of “Trophy” individuals in the stock was less than 0.6% across all management regimes.
At the current estimate of F (0.65 y-1), the RSD-Quality values ranged from 36% to 52% depending on the specific minimum length limit.
Greater minimum length limits resulted in greater mean RSD values.
Minimum length limits and different levels of fishing mortality can alter the length composition of a stock.
We recommend the use of RSD as an additional assessment tool for Spotted Seatrout in Mississippi because it is easily calculated, uses data that are readily available, and provides information on the stock length-composition.
RSD values were calculated for the Quality (Q), Preferred (P), Memorable (M), and Trophy (T) categories (Table 1.)
RSD values decrease with increasing fishing mortality and increase with increasing minimum length limits (Figures 3-6.).
The standard deviation values for did not exceed 2.0% for any of the RSD values in the simulation.
The greatest difference in magnitude between RSD values occurred between the 16” and 18” length limits for the RSD-Quality length category.
Figure 2. Histograms of the logistic length-at-age model parameters resulting from the resampling analysis.
Simulation Procedure
Generate frequency distributions for length-at-age parameters using residual bootstrapping techniques. (n = 1,000, Figure 2.)
Calculate mean length-at-age, age-specific selectivity, and natural mortality, for every set of growth parameters.
Using the exponential growth model, calculate the number of individuals-at-age for every management regime
Calculate the RSD value for every length category under every management regime (minimum length limit and level of F.)
Table 2. Ranges of RSD values at the current estimated level of fishing mortality (F, y-1) and the simulated minimum length limits.
Length Category
RSD Value (%)
Quality
36.0 to 52.0
Preferred
13.0 to 20.0
Memorable
1.0 to 2.5
Trophy
<0.05
Figure 3. Mean RSD-Q values for each management regime.
Figure 4. Mean RSD-P values for each management regime.
Figure 1. Mean predicted length-at-age and plot of residuals from fitting the logistic growth model.
Copyright Colin Purrington (
Acknowledgments
Funding provided by the Mississippi
Department of Marine Resources,
Mississippi Tidelands Trust Fund Program.
Table 1. Total length values for RSD length categories
Length Category
Total Length (≥)
(mm)
(in)
Stock
261
10.3
Quality
361
14.2
Preferred
451
17.8
Memorable
592
23.3
Trophy
686
27.0
Figure 5. Mean RSD-M values for each management regime.
Figure 6. Mean RSD-T values for each management regime.