I'm not familiar with the models for fish, but isn't the standard form of the allometric equation such that the slope ("b" in the model above) is an exponent? Otherwise the equation says that weight is directly proportional to length with only a constant multiplier used to determine the relationship?

Expressed in different terms, the equation as presented would say that the weight of a fish is always equal to a constant multiplier times the length? That does not match my experience, but I've been wrong before...and probably will be again. ;\)