srug {MQMF}

srug is the Schnute and Richards Unified Growth Curve

Description

srug implements the Schnute and Richards (1990) unified growth curve that can be used to describe fish growth, maturation, and survivorship data. It is a curve that generalizes the classical logistic model used for maturity as well the growth models by Gompertz (1825), von Bertalanffy (1938), Richards (1959), Chapman (1961), and Schnute (1981). As with any asymmetric, multi-parameter model, it can be hard to obtain a stable fit of this curve to data. Here the model is implemented to range between 0 - 1, if you want to use it to describe growth then re-cast the function and add a fifth parameter to replace the 1.0 on top of the divisor. The main point of the curve, however, was to demonstrate how the different equations were related to one another. In working situations it is most efficient to use the original, simpler, curve/equation.

Usage

srug(p, sizeage)

Arguments

p

a vector of four parameters begin Schnute and Richards' a, b, c, and alpha, in that order.

sizeage

the age or size data used to describe the maturity transition.

Value

A vector of predicted proportion mature (proportion of 1.0) for the given parameters and the sizeage data

References

Schnute, J.T. and L.J. Richards (1990) A unified approach to the analysis of fish growth, maturity, and survivorship data. Canadian Journal of Fisheries and Aquatic Science 47:24-40

Examples

 L <- seq(50,160,1)
 p <- c(a=0.07,b=0.2,c=1.0,alpha=100.0)
 predR <- srug(p=p,sizeage=L) # proportion of total
 oldpar <- par(no.readonly=TRUE)
 plot1(L,predR,xlab="Length",ylab="Prop of Recruitment")
 abline(h=0.5) #visually confirm asymmetry
 par(oldpar)