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Life-history theory predicts that the optimal strategy in the trade-off between egg size and number varies in relation to resource availability and environmental disturbance. We assessed interspecific differences in egg size, fecundity and other life-history traits in a species complex of stream-resident galaxiid fish, which are distributed across a range of contrasting habitat types on the South Island, New Zealand. Oocyte size, fecundity and reproductive effort were measured from gravid females collected immediately before spawning. Proxy measures of stream productivity, flow variability and predation pressure were extracted from modelled data sets. A suite of different egg sizes were identified across species within the Galaxias vulgaris complex, with mean oocyte volume differing by up to 133% between species. The species with the smallest eggs showed mean size-relative fecundities 246% higher than the species with the largest eggs. A significant negative relationship was found between species’ mean egg size and size-relative fecundity, suggesting a trade-off between these traits. Species with larger eggs had larger maternal body size, lower reproductive effort and delayed maturity compared to ‘small-egg’ species. Consistent with the predictions of life-history theory, species with larger eggs, lower size-relative fecundity, lower reproductive effort and delayed maturity were associated with low productivity, stable streams, whereas species exhibiting the opposite set of traits occurred in relatively productive but disturbed systems. |
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