gam

Dear all

i have done GAM for data ( they are non- normal and i find regression
between Sillaginidae with several hydrological factors),i done :

[1] "depth"        "temperature"  "salinity"     "Sillaginidae"

 > pairs(sc,panel=function(x,y){points(x,y);lines(lowess(x,y))})

model<-gam(Sillaginidae~s(temperature)+s(salinity)+s(depth))

but i take this message:

Error in smooth.construct.tp.smooth.spec(object, dk$data, dk$knots) :
  A term has fewer unique covariate combinations than specified
maximum degrees of freedom


please help me what can i do? or what is meaning this sentence?

thanks


Mahnaz