Short Seminar on Fish Growth and Maturity Daniel Pauly Fisheries Centre, University of British Columbia Vancouver, Canada Institute of Marine Research December 19, 2002 Reykjavik, Iceland

A vast number of fish species have been studied in the last 100 years in terms of their growth. From these, we can infer that: The von Bertalanffy growth function (VBGF) gives a fair, first-order description of the shape of growth curves; The asymptotic size (L  ; W  ) of the VBGF is generally not far from the maximum size in the population in question; The curvature parameter (K; 1/time) is, if indirectly, related to the metabolism of individual fishes.

Hence the direct relationship of K with natural mortality (M), among other things….

However, growth curves are not very informative… Relative age (years) For comparative purposes, we can plot, instead, log(K) vs. log( L  ); this gives ‘auximetric plots.’

In ‘auximetric plots,’ the growth space occupied by all fishes is well defined. Families occupy an ellipsoid subset of that space… Yellow dots represent all fishes; red dots represent the Gadidae.

…while genera and species form smaller ellipsoids within their families… Using FishBase, we could check that this holds for thousands of species, within hundreds of families.

To make a long story short: the reason for these patterns is basically that gills (which supply O 2 to the fish body) cannot grow as fast as O 2 requirements (proportional to body weight). Note slope, which is less than 1.

As it so happens, there is a rather close relationship between length at first maturity, Lm and L . The reason for this is rather elaborate….

Now imagine the effect of fishing a population whose individuals all have different growth curves… One thing we can expect is that overfishing the individuals capable of reaching larger sizes will lead to the remaining stock consisting predominantly of individuals which reach maturity at smaller sizes…

Given these points, and their various implications, I find it not surprising at all that there should be a decline of L m in an exploited fish population, e.g. of Icelandic cod. Let’s now discuss this. And thanks for your attention