As diferenças observadas na estratégia de crescimento das coortes das populações de sardinha da costa Norte e Sul de Portugal são possivelmente devidas às diferentes temperaturas observadas nessas zonas. Na fase de crescimento pré-maturação a temperatura parece estimular o crescimento no Sul, o que possibilita um maior investimento de energia na reprodução após a maturação, associado também a uma maturação mais cedo, em relação às sardinhas do Norte, e a uma época de desova mais extensa. Na zona Norte o crescimento inicial é menor, o que poderá estar associado às temperaturas mais baixas verificadas nesta zona. Deste modo, após a maturação os indivíduos desta população ainda continuam a investir muita energia para o crescimento, aumentando também a sua fecundidade. Esse investimento no crescimento diminui progressivamente ao longo da vida, aumentando a alocação para a reprodução. Na fase adulta o crescimento das sardinhas do Norte é mais sazonal, o que estará relacionado com a disponibilidade de fitoplâncton.
Em cada uma das zonas também se verificam diferenças de crescimento entre coortes durante o início de vida e no Norte, onde a abundância das coortes é maior. Após o segundo ano de vida as diferenças entre coortes são menores, o que evidencia um crescimento compensatório, que permite reduzir alguma desvantagem inicial associada às condições ambientais durante o inicio de vida das coortes.
Uma possível explicação para essas diferenças poderá ser um crescimento dependente de densidade, que indica que a disponibilidade de alimento e competição por este serão também factores importantes na regulação do crescimento das sardinhas. O esclarecimento desta possível dependência de densidade, assim como a exploração da relação entre a sardinha e as suas presas na fase inicial de crescimento, será importante para a compreensão da variabilidade do crescimento desta espécie.
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Anexo I
Figura 11.1. Curva ajustada aos comprimentos médios à idade e respectivos resíduos do modelo base para a zona Norte. As linhas a tracejado correspondem ao limite de confiança de 95%.
Figura 11.2. Curva ajustada aos comprimentos médios à idade e respectivos resíduos do modelo base para a zona Sul. As linhas a tracejado correspondem ao limite de confiança de 95%.
Anexo II
Anexo III
Figura 13.1. Comprimentos médios à idade, preditos pelo modelo base (linha sólida) e respectivos erros padrão (linhas a tracejado). Os pontos correspondem aos valores observados e as barras verticais ao desvio padrão.
Anexo III
Figura 13.2. Comprimentos médios à idade, preditos pelo modelo base (linha sólida) e respectivos erros padrão (linhas a tracejado). Os pontos correspondem aos valores observados e as barras verticais correspondem ao desvio padrão.