vblrt {fishmethods} | R Documentation |
Likelihood ratio tests for comparison of two von Bertalanffy growth curves following Kimura (1980).
vblrt(len = NULL, age = NULL, group = NULL, error = 1, select = 1, Linf = NULL, K = NULL, t0 = NULL)
len |
the vector of lengths of individual fish. |
age |
the vector of ages associated with the length vector. |
group |
the vector of character or numeric codes specifying group association. |
error |
the error variance assumption. 1= constant variance for all lijs; 2= constant variance for all mean lengths at age; 3=var of lij varies with age. See methods a-c in Kimura (1980: pp. 766). The required statistics for each type of error are calculated from the individual length-age observations. |
select |
the selection of starting values of L-infinity, K, and t0. 1=automatic selection, 2=user-specified. If select=1, initial starting values of L-infinity, K, and t0 are calculated from Walford lines (Everhart et al. 1975), and ages represented as decimal values are truncated to the integer before linear regression is applied. If select=2, the user must specified the values of L-infinity, K, and t0. |
Linf |
if select=2, the starting value for L-infinity of the von Bertalanffy equation. |
K |
if select=2, the starting value for K of the von Bertalanffy equation. |
t0 |
if select=2, the starting value for t0 of the von Bertalanffy equation. |
Following Kimura (1980), the general model (6 parameters; one L-infinity, K, and t0 for each group) and four sub models are fitted to the length and age data using function nls (nonlinear least squares). For each general model-sub model comparison, likelihood ratios are calculated by using the residual sum-of-squares and are tested against chi-square statistics with the appropriate degrees of freedom. Individual observations of lengths-at-age are required. If error variance assumptions 2 or 3, mean lengths and required statistics are calculated. A dummy vector called cat, containing 0s for the first group with lower alpha-numeric order and 1s for the second group, is used in the estimation of group parameters.
results |
list element with the likelihood ratio tests comparing von Bertalanffy models. |
model Ho |
list element with the nls fit for the general model. |
model H1 |
list element with the nls for model H1 (Linf1=Linf2). |
model H2 |
list element with the nls fit for model H2 (K1=K2). |
model H3 |
list element with the nls fit for model H3 (t01=t02). |
model H4 |
list element with the nls fit for model H4 (Linf1=Linf2, K1=K2, t01=t02). |
rss |
list element with the residual sum-of-squares from each model. |
residuals |
list element with the residuals from each model. |
Gary A. Nelson, Massachusetts Division of Marine Fisheries gary.nelson@state.ma.us
Everhart, W. H., A. W. Eipper, and W. D. Youngs. 1975. Principles of Fishery Science. Cornell University Press.
Kimura, D. K. 1980. Likelihood methods for the von Bertalanffy growth curve. U. S. Fish. Bull. 77(4): 765-776.
## Normally, the length and age data will represent data for individuals. ## Kimura's data are mean lengths-at-age but are usable because error=2 ## will calculate mean lengths-at-age from individual data. Since only ## one value is present for each age,the mean length will be calculated ## as the same value. data(Kimura) vblrt(len=Kimura$length,age=Kimura$age,group=Kimura$sex,error=2,select=1)