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

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quantifies maximum speed growth, k2 represents <strong>the</strong> speed at which<br />

inhibition is released with time. All parameters in this <strong>model</strong> carry a clear<br />

biological meaning, considering hypo<strong>the</strong>ses that were formulated to build<br />

it.<br />

Usually, a fuzzy <strong>model</strong> is treated with fuzzy arithmetic. The output is<br />

<strong>the</strong>n "defuzzified" by one <strong>of</strong> several methods (Cox, 1999) to provide a<br />

crisp number (<strong>the</strong> most probable size <strong>of</strong> an individual at a determined age).<br />

Being simple enough, <strong>the</strong> current <strong>model</strong> can also be transformed into a<br />

classical analytic equation:<br />

D( t') = M ( t') ⋅ S( t') + M ( t') ⋅ L( t')<br />

(28)<br />

S L<br />

which gives, after combination <strong>of</strong> eqs 24-28 and simplification:<br />

Dt' ( ) = D +∆D<br />

Part IV: A growth <strong>model</strong> with intraspecific competition<br />

0<br />

1−e 1+ l ⋅e<br />

−k1⋅t' ∞ −k2⋅t' (29)<br />

This way <strong>the</strong> <strong>model</strong> can be treated with classical (crisp) arithmetic that<br />

<strong>of</strong>fers a larger panel <strong>of</strong> statistical tools than fuzzy arithmetic.<br />

Fitting <strong>the</strong> dataset<br />

Since echinoids are not tagged individually, it is not possible to track<br />

animals across measurement sets. Consequently, one will consider virtual<br />

individuals according to <strong>the</strong>ir relative position in <strong>the</strong> entire size<br />

distribution at each sampled time, that is, virtual individuals corresponding<br />

to fixed quantiles (or percentiles) in each size distribution. It should be<br />

noted also that, if mortality is not randomly distributed among individuals,<br />

actual growth speed could be different from <strong>the</strong> one calculated on virtual<br />

individuals. This means that if mortality preferably affects small<br />

individuals, growth speed is overestimated; conversely, if mortality affects<br />

ra<strong>the</strong>r larger animals, growth speed is underevaluated. In absence <strong>of</strong><br />

individual tagging, we will thus consider <strong>the</strong> apparent growth speed <strong>of</strong> <strong>the</strong><br />

virtual individuals as defined here above.<br />

152

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