162 R. J. H. Beverton and S. J. Holt 



In four of the main groups of fish there is a sufficient range 

 of values to enable each group to be examined separately. 

 These are the herring and related species (Clupeoidei), the cod 

 family (Gadiformes), the salmonoids (Salmonoidei) and the 

 flatfishes (Pleuronectoidei). Fig. 4 shows the relations between 

 L^ and T^^^ax i^ these groups. In each case there is a well- 

 defined trend, especially so in the Clupeoidei which are per- 

 haps a more homogeneous group than the others. The lines 

 drawn through the points have no statistical significance, 

 since the precise accuracy of the individual points is largely 

 unknown, and in some cases the values recorded are certainly 

 over- or under-estimates of the true values ; this is particularly 

 so with the parameters Tj^^ax ^^^ ^'> which are more difficult 

 to determine accurately than are the growth parameters L^ 

 and K, and the lines have been drawn with these considera- 

 tions in mind. 



Despite these qualifications, it does seem that the line for 

 the Clupeoidei differs from that for the other groups, the larger 

 members of the herring family appearing to live to a greater 

 age than do fish of the other groups of a comparable size, the 

 contrast being most noticeable with the gadoids. The scatter 

 of the points is most marked in the salmonoids, which may be 

 a reflection of the heterogeneity of this group and of the varied 

 environments in which members of it are found, since they 

 include marine, freshwater and anadromous species. The 

 true salmon, ringed by a broken line, fall outside even the 

 considerable variation of the rest of the salmonoids, since for 

 their size they have a very short lifespan indeed. The pleuro- 

 nectoids form a compact group, with a closely linear relation 

 between L^ and T^^^^ with the exception of the halibut ; the 

 maximum age recorded for this species (30 years) may, how- 

 ever, be somewhat below the real maximum owing to diffi- 

 culties of determining the true age of the oldest fish, and the 

 fact that there was some fishing on the populations in question. 



The lines drawn for the four groups shown in Fig. 4 have 

 been reproduced in Fig. 5, together with the data for all other 



