R/lh-m-lorenzen.R

In laurieKell/lh: Life History Relationships

#' lorenzen
#'
#' Lorenzen natural mortality equation provides an estimate of M
#' as a function of weight. M=a*wt^b; 
#' 
#' @param wt  mass at which M is to be predicted
#' @param par an FLPar with two values; i.e. a equal to M at unit mass 
#' and b a power term; defaults are a=0.3 and b=-0.288
#' 
#' #' @export
#' @docType methods
#' @rdname lorenzen
#' 
#' @seealso \code{\link{chenWatanabe},\link{gislason}}
#'  
#' @examples
#' \dontrun{
#' mass=FLQuant(c( 1.90, 4.23, 7.47,11.48,16.04,20.96,26.07,31.22,
#'                36.28,41.17,45.83,50.20,54.27,58.03,61.48,64.62),
#'              dimnames=list(age=1:16))
#' lorenzen(mass)
#' }

lorenzenFn<-function(wt,a=.3,b=-0.288) {
  if ("FLPar"%in%is(b)) res=wt%^%b  else res=wt^b
  if ("FLPar"%in%is(a)) res=a%*%res else res=a*res
  res}

setGeneric('lorenzen', function(wt,par,...)
  standardGeneric('lorenzen'))

setMethod('lorenzen', signature(wt='FLQuant',par='missing'),
      function(wt,...) { 
          res=lorenzenFn(wt)
          res@units='yr^-1'
          res})
setMethod('lorenzen', signature(wt='FLQuant',par='numeric'),
      function(wt,par,...) { 
          res=par[1]*wt^par[2]
          res@units='yr^-1'
          res})
setMethod('lorenzen', signature(wt='FLQuant',par='FLPar'),
      function(wt,par,...){   
          res=par[1]%*%(wt%^%par[2])
          res@units='yr^-1'
          res})