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).
a list consisting of following parameters:
optional; in case catch is a matrix or data.frame, a number
indicating which column of the matrix should be analysed (Default:
logical; if TRUE the plot is recorded. Default is FALSE.
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).
title of plot (Default is "Powell-Wetherall plot")
The first length group or age class within the list object
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.
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;
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
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