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

General introduction
Russell, 1987; Rowley, 1990; Kenner, 1992; Ebert & Russell, 1992, 1993;
Russell et al, 1998; Lamare & Mladenov, 2000). This way, seasonal
variation in growth is also partly eliminated. For sea urchins, it is the
skeleton that is tagged using tetracycline (Kobayashi & Taki, 1969; Taki,
1972). Size increase of the ossicles can then be determined because a band
of tetracycline-labeled skeleton, visible under ultraviolet light, indicates its
size at tagging time. An allometric relationship between the size of the
given ossicles and the body size allow estimating the latter. To fit such
data, growth models need to be reworked, using a so-called Ford-Walford
representation, or Walford plot (Ford, 1933; Walford, 1946; Ebert, 1999)
where size at time t + 1 year (at recapture) is expressed as a function of
size at time t (at capture). Ebert (1999) reviewed such transformations for
von Bertalanffy, Gompertz, logistic, Richards and Tanaka models.
Sainsbury (1980) demonstrated the strong biases that could occur with
such a method when there is individual variation in growth in a population.
In fact, relative growth does not take the age into account, by definition.
Hence, due to individual variations in growth rate, some fast-growing but
young individuals have same size as slower-growing but older ones at a
given time. The method mixes all animals having the same size, no matter
their age, and calculates a mean growth rate at that size. This mixing of
age-cohorts is troublesome when growth variation is high in the
population. Rejection of a particular growth model could result from these
biases as Sainsbury evidenced, using a theoretical analysis with the von
Bertalanffy 1 model. As individual variation in growth pattern is probable,
one should be very careful using this method. Among all authors using
tagged sea urchins, Gage (1992) was the only one that cited Sainsbury's
work and care about a possible bias due to individual variation in growth
rate.
Clearly, there is no fool-proof method for modelling individual growth
of sea urchins in the field. In such a context, one can rely on experiments
conducted in aquaria, although it is evident that growth patterns observed
in artificial conditions could be very different to what happens in the field.
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