powell_wetherall {TropFishR}

Powell-Wetherall method

Description

A method to estimate the instantaneous total mortality rate (Z) and the infinite length of the von Bertalanffy growth equation (Powell, 1979; Wetherall et al., 1987).

Usage

powell_wetherall(
  param,
  catch_columns = NA,
  savePlots = FALSE,
  reg_int = NULL,
  main = "Powell-Wetherall plot"
)

Arguments

param

a list consisting of following parameters:

• midLengths: midpoints of the length groups,

• Linf: infinite length for investigated species [cm],

• K: growth coefficent for investigated species [1/year],

• t0: theoretical time zero, at which individuals of this species hatch,

• catch: catch as vector, or a matrix with catches of subsequent years;

catch_columns

optional; in case catch is a matrix or data.frame, a number indicating which column of the matrix should be analysed (Default: NA).

savePlots

logical; if TRUE the plot is recorded. Default is FALSE.

reg_int

instead of using the identity method a range can be determined, which is to be used for the regression analysis. If equal to NULL identity method is applied (default).

main

title of plot (Default is "Powell-Wetherall plot")

Details

The first length group or age class within the list object midLengths or age will be used as the Lprim or tprime (length of recruitment to fishery). This function includes the identify function, which asks you to choose two points from a graph manually. The two points which you choose by clicking on the plot in the graphical device represent the start and end of the data points, which should be used for the analysis. Based on these points the regression line is calculated. The Powell and Wetherall method only works with length-frequency data.

Value

A list with the input parameters and follwing objects:

• tmean or Lmean: mean age or length of fish,

• Z: total mortality;

and/or following objects when applying the Powell and Wetherall method:

• Lmean_Lprime: dependent variable for regression analysis,

• Lprime: some length for which all fish of that length and longer are under full exploitation,

• Linf_est: infinite length in [cm] (Linf),

• se_Linf: standard error of Linf,

• confidenceInt_Linf: confidence interval for Linf,

• ZK: total mortality divided by K (Z/K),

• se_ZK: standard error of Z/K,

• confidenceInt_ZK: confidence interval of Z/K;

References

Powell, D.G., 1979. Estimation of mortality and growth parameters from the length- frequency of a catch. Rapp.P.-v.Reun.CIEM, 175:167-169

Sparre, P., Venema, S.C., 1998. Introduction to tropical fish stock assessment. Part 1. Manual. FAO Fisheries Technical Paper, (306.1, Rev. 2). 407 p.

Wetherall, J.A., J.J. Polovina and S. Ralston, 1987. Estimating growth and mortality in steady-state fish stocks from length-frequency data. ICLARM Conf.Proc., (13):53-74

Examples

data(synLFQ3)
powell_wetherall(synLFQ3)

data(synLFQ5)
powell_wetherall(synLFQ5, catch_columns = 1:12)