Brain weight $B$ as a function of body weight $W$ in fish has been modeled by the power function $B=.007W^{2/3}$, where $B$ and $W$ are measured in grams. A model for body weight as a function of body length $L$ (measured in $cm$) is $W=.12L^{2.53}$. If, over $10$ million years, the average length of a certain species of fish evolved from $15\,cm$ to $20\,cm$ at a constant rate, how fast was the species' brain growing when the average length was $18\,cm$? Round your answer to the nearest hundredth.

$nanograms/yr$