Using fish as an example, if you have total length and age and compute a growth rate by dividing total length by age would this "age-specific" growth rate, "absolute" growth rate, etc.?
It depends on how you estimated your growth rate. Typically, I use the von Bertalanffy Growth Functions (VBGF). There are several papers on this topic out there.
Here is a good paper about a few theories for growth rates (sorry I don't have a copy to share):
http://onlinelibrary.wiley.com/doi/10.1111/j.1749-7345.1992.tb00766.x/abstract
Commonly, Specific Growth Rate (SGR) is used to adress fish growth. This is done using the followning equation: SGR = (ln(final weight in grams) - ln(initial weight in grams) x100) / t (in days)
This is usually done in a lab setting with different treatments and a start and stop. From field collections, it might be more difficult. Perhaps there is data on their weight/length/size correlated to days post hatch that you can extrapolate from to get a starting value? Growth is often seasonal though, and different at different life stages.
Also, please rephrase your question, I think a "be" is missing there somewhere.
Best Regards,
the rate (or speed) of absolute annual growth is calculated from the formula of Hagborg & Welcomme (1979): TCA = ln (LT (t +1) / LT (t))
with LT = total length of fish and t = age in years LT (t) = initial length at age t
and LT (t +1) = next length at age (t +1)
We can also express this rate of growth in weight using the same formula and turning lengths by weight, from the length-weight relationship: WT = a (LT) b
or log WT = b (logLT) + loga
Paul,
To answer your question directly, dividing the total length by the age gives you a number that probably isn't very useful. You could think of it as the average growth per year, but it is equivalent to plotting growth as a straight line. Many fish, if not most, grow more quickly when young. Growth typically slows once fish become sexually mature and put more energy into reproduction and less into growth.
There are many other growth models, such as the von Bertalanffy, and seasonal growth models, as mentioned in other responses. These other models typically contain a growth rate parameter (e.g. K in von Bertalanffy) that can be compared between or within species. The specific growth rate or annual growth rate formulas mentioned can be used to compare growth within a species for specific time periods.
There is a good chapter on fish growth models in Quinn & Deriso (1999), Quantitative Fish Dynamics. It discusses many of the most often used models and how to select the best model. Good luck.
Estimating growth rate is not independent from the experimental set up. You talk about length and age. I do not know whether you have a set of individuals that have been followed along time as a group (which I suppose) or you have individual measurements for each individual. In any case, growth rate does not usually relate to age in a linear way, which is assumed under your treatment, most often we would be having exponential increase according to different equations which basically depend on the length of the time you have followed growth evolution: if the title covers time to maturity when energy is going derived to reproduction a sharp decrease in growth rate tends to occur, and if you are covering only early growth stages you may be obtaining a clear exponential increase. Other responses have gone intro detail and of course Von Bertalanffy literature is very clarifying.
Using the von Bertallanff's Growth Function estimation of the growth is easy provided the asymptotic length and the Growth coefficient has been properly estimated. There are several papers from the tropical countries where length based methods have been used for the estimation of growth . If need be the author can refer to many of the papers on croakers by this author
Sorry to say but the formula is not correct; it has to be: SGR = (ln(final weight in grams) - ln(initial weight in grams)) x100 / t (in days).
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