Growth model of the reared sea urchin Paracentrotus ... - SciViews

General conclusions
individuals, whose inhibition is suddenly eliminated, could quickly replace
missing adults. Two conditions should be met, however, to obtain this
result on the long term. First, mortality of small and mid-sized individuals
should not increase when large adults are removed (indeed, when
removing large adults, the passive protection of small individuals against
predators disappears). If necessary, part of the adults should be left in
place during harvesting. Second, recruitment should not be a limiting
factor. By harvesting large adults before they spawn, recruitment is de
facto lowered. An artificial production of a large amount of seed in
hatcheries is one way to maintain recruitment levels. Beyond these general
considerations, it is difficult to define rules for sustainable fishery
practices. If the growth model with intraspecific competition were
calibrated against field population data, it would be possible to quantify
the impact of various fishery methods, and to provide objective criteria for
sustainable sea urchin fisheries (Grosjean & Jangoux, 2000).
From a theoretical point of view, the new growth model rehabilitates a
60-year old theory of growth elaborated by von Bertalanffy (1938). Since
then, several authors have questioned its validity (Knight, 1968; Roff,
1980; Frontier & Pichot-Viale, 1993). Many other works have indicated
that the von Bertalanffy 1 model is probably not acceptable to describe the
growth of sea urchins (Gage & Tyler, 1985; Gage et al, 1986; Gage, 1987;
Dafni, 1992; Ebert, 1980a; Ebert & Russell, 1993; Lamare & Mladenov,
2000), including P. lividus (Cellario & Fenaux, 1990; Turon et al, 1995).
Here we have shown that sigmoidal growth does not necessarily means
that von Bertalanffy's theory is invalid. An S-shape could result from
inhibition of growth at small sizes/ages. P. lividus follows the von
Bertalanffy's law for its somatic growth when it is not inhibited. In a
cohort, the non-inhibited fraction amounts for less than 10% of all the
individuals. Using least-square regression leads to the rejection of the von
Bertalanffy 1 model. Using quantile regression validates it for the largest
fraction. Using an envelope model with intraspecific competition
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