We studied age, growth and reproduction of an introduced poeciliid, Gambusia holbrooki (Agassiz, 1859)

Implementation of the Water Framework Directive requires tools for measuring and monitoring the ecological status of aquatic ecosystems. Several indices are in use in the Iberian Peninsula, although there has been little comparison among them. We sampled the fish assemblage and limnological features of the Tordera stream (NE Spain) quarterly from September 2001 to May 2003 to evaluate the usefulness of several fish metrics and to compare habitat quality and biotic indices currently in use. Data for eight biotic and abiotic indices for this and three other Catalan river basins were also compiled in order to analyse the relationships among indices. In the Tordera stream, fish abundance and richness increased with stream order except in the last sampling site that had the lowest fish abundance owing to the effects of drought and water abstraction. Although most indices were positively correlated, some displayed low or null correlations particularly for the Tordera basin which is more affected by water abstraction and less by pollution; a commonly used physico-chemical index (ISQA) was the least correlated. In a regional fish index (IBICAT) under development, the brown trout (Salmo trutta) has been previously considered as introduced in the Tordera basin. Here, we report an old published record that demonstrates that trout was present before 1845 and we argue that its status should be considered as uncertain given the current information available. Whether brown trout is treated as native or introduced to this river basin has profound effects on the results of fish metrics because of its dominance in the upper reaches. We briefly discuss the role of introduced species, particularly in headwater streams, in the development of fish indices. Our study exemplifies the need for careful, basin-specific assessment of native/introduced status in the development of fish metrics.

The life-history pattern was studied of an endangered endemic cyprinodontid from the Iberian peninsula, Aphanius iberus (Cuvier & Valenciennes, 1846), found in a Mediterranean estuary, the Ebro delta. Maximum ages observed were 1+ in both sexes. The seasonal growth period started in March and continued until September. The females reached larger sizes than the males, as they have a higher rate of growth and live longer. The largest sizes found were 45 mm versus 38 mm. A. iberus is a multiple spawner that releases batches of eggs between May and August but with few eggs each time (varying from 10 to 30). Despite this continuous release of eggs two moments appeared to stand out as more important. After the reproduction period the parental cohort reduced drastically. The relationship between fecundity (F) and total length (mm) was represented by the formula: F = 8.358 x 10-9 x Lt6.343. The biology of the Iberian toothcarp is characterized by fast growth, early maturity, high reproductive effort, multiple spawning and reduced longevity. This may be a good life strategy for small fish species in unstable environments, such as estuaries, where adult mortality is high, variable or unpredictable. It allows them to exploit favourable environmental conditions, which occur in a specific, reduced period; practically all the population is renewed during this period.

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