NOTE Francis: Do herring grow faster than orange roughy? 



785 



as the slowest-growing because it is the slow- 

 est of the three in approaching its asymptote; 

 with methods 1, 2, 3, and 5, the ranking is re- 

 versed because of the influence of L x (it is also 

 reversed for method 4, but not because of L x ). 

 So far, method 6 has been defined only 

 where growth follows the von Bertalanffy 

 equation with / = 0. This definition works 

 equally well even if t * (this parameter 

 simply determines the horizontal position of 

 the growth curve and is therefore irrelevant 

 in describing the speed of growth). The defi- 

 nition also extends naturally to the Gompertz 

 and logistic growth equations as long as 

 growth is considered only to the right of the 

 inflection point. Both these equations have 

 a rate parameter precisely analogous to k (this 

 is the parameter g in equations 3 and 4 of 

 Schnute, 1981). However, more complex growth 

 models (e.g. the four-parameter model of 

 Schnute, 1981 ) are so flexible that it seems dif- 

 ficult to rank growth curves along a single di- 

 mension, from slower to faster. A simple solu- 

 tion, which is consistent with method 6, would 

 be to determine the age at which a species 

 reaches 90%, say, of L x . The younger this is, 

 the faster-growing is the species. 



160 



50 0.5 



Age (yr) 



Figure 2 



Example of three von Bertalanffy growth curves whose ranking, from 

 slowest- to fastest-growth, is P, Q, and R for method 6 but the re- 

 verse for all other methods in Table 1. 



Conclusions 



The main conclusion of this paper is that statements 

 like "P grows faster than Q" are ambiguous unless 

 the method of growth comparison (e.g. one of the 

 methods in Table 1 ) is specified. To some extent it does 

 not matter which method is used as long as this is 

 clearly stated. However, I have given some reasons to 

 believe that method 6 is the most natural method. 

 Amongst these reasons is the observation that, for von 

 Bertalanffy growth, it is the parameter k that describes 

 growth rate; h s merely describes (eventual) size. 



In order to make a point, the title of this paper 

 refers to comparisons between disparate species. 

 However, my main conclusion applies whether the 

 growth comparison is made within or between spe- 

 cies. I am happy to agree with an anonymous ref- 

 eree who asserted (forcefully) that some authors may 

 not share my preference for method 6 (particularly 

 for within-species comparisons). 



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