Species invasion is one of the leading mechanisms of global environmental change, particularly in freshwater ecosystems. We used the Food and Agriculture Organization's Database of Invasive Aquatic Species to study invasion rates and to analyze invasion pathways within Europe. Of the 123 aquatic species introduced into six contrasting European countries, the average percentage established is 63%, well above the 5%-20% suggested by Williamson's "tens" rule. The introduction and establishment transitions are independent of each other, and species that became widely established did so because their introduction was attempted in many countries, not because of a better establishment capability. The most frequently introduced aquatic species in Europe are freshwater fishes. We describe clear introduction pathways of aquatic species into Europe and three types of country are observed: "recipient and donor" (large, midlatitude European countries, such as France, the United Kingdom, and Germany, that give and receive the most introductions), "recipient" (most countries, but particularly southern countries, which give few species but receive many), and "neither recipient nor donor" (only two countries). A path analysis showed that the numbers of species given and received are mediated by the size (area) of the country and population density, but not gross domestic product per capitaFinancial support was provided by the Spanish Ministry of Science and Technology (REN2002-10059-E and REN2003-00477) and the Catalan Ministry of Universities, Research and Information Society (Catalan Government Distinction Award for University Research 2004 to E.G.B.

The pathogen Aphanomyces astaci Schikora 1906 is responsible for the decline of the native crayfish species of Europe, and their current endangered status. This pathogenic species is native to North America and only colonizes aquatic decapods. The North American crayfish species have a high resistance to this pathogen, while species from other regions are highly susceptible. However, recent field and laboratory observations indicate that there might exist some populations with resistance against this disease. The objective of this study was to test the susceptibility of 8 selected native European crayfish populations of Austropotamobius pallipes Lereboullet 1858 from the Pyrenees. We challenged them against the genome sequenced strain AP03 of A. astaci isolated from a North American red swamp crayfish, Procambarus clarkii Girard 1852, in the Garrotxa Natural Park, Girona. The results showed that there are significant differences (P<0,001) among populations, although most of them show high mortality rates after the zoospore challenge with A. astaci. However, one population from Girona exhibited a 100% survival during a four-month monitoring period under the experimental conditions tested. Histological analyses revealed a high immune reaction in tissues examined, i.e., encapsulation and melanization of hyphae, similar to that found in North American resistant crayfish species. These results represent the first observation of a native European crayfish population showing high resistance towards the most virulent genotype of this pathogen, i.e., genotype Pc. The identification of this population is of key importance for the management of these endangered species, and represents a crucial step forward towards the elucidation of the factors involved in the immune reaction against this devastating pathogen.

–  We present the first data on three‐spined stickleback (Gasterosteus aculeatus) life history and habitat use in seasonal Mediterranean streams, analysing populations from NE Iberian Peninsula. Stickleback populations were strictly annual, with reproduction events involving exclusively 1+ fish. Somatic growth was concentrated in two periods, one in spring and early summer, and another in autumn and winter. During summer there was a clear stop in the somatic growth, coupled with low values of somatic condition. This growth pattern contrasts both with that of other Mediterranean stream fish species and that of stickleback in other European locations. Stickleback populations occupied mainly intermediate locations along fluvial gradients. The presence of abundant aquatic vegetation was identified as one of the main factors related with both stickleback presence and the abundance of its populations, while the latter was also negatively related with that of invasive fish and crayfish species. Stickleback tended to occur in native‐dominated fish communities, being almost absent from lower stream reaches, which bear high abundances of invasive fish species.

Oriental weatherfish has become an invasive fish species in many temperate areas. In this work we report the presence of an established weatherfish population in northeastern Iberian Peninsula and describe, for the first time in Europe, a clear range expansion. The species was first located in the Ebro River Delta in 2001 and has since been detected in 31 UTM 1 9 1 km quadrates. The capture of over 1,000 weatherfish shows that its population is composed by both juvenile and adult individuals. Weatherfish occupies mostly the web of irrigation channels associated with rice culture, although it has also been detected in rice fields and in the Ebro River. The expansion of the species in the area seems to be limited by water conductivity. An additional location for the species in the Ter basin (some 300 km to the north) suggests that inter-basin expansion could be occurring. This new fish invasion reinforces the need to implement strict controls to the trade and culture of ornamental fish.

We report the first data on the use of occasionally inundated habitats in a Mediterranean coastal lagoon by the Spanish toothcarp (Aphanius iberus), a cyprinodontid fish in danger of extinction. During a flooding period, we sampled the fish population and the macroinvertebrate community in three contrasting habitats: mats of green algae, open water, and a habitat dominated by glasswort (Salicornia patula) that is occasionally inundated. The three habitats displayed strong variation in density and species composition of invertebrates in the water column and the benthos. In general, algal mats had higher invertebrate biomasses, but glasswort had higher diversity of organisms, in part of terrestrial origin. The density of toothcarp was very low in the open water. The habitat that is occasionally inundated (glasswort) significantly had the highest density of mature toothcarp, while immature fish were similarly abundant in the glasswort and algal mat habitats. Condition (weight-length relationship) and total food biomass in the gut contents of immature toothcarp was significantly higher in algal mats than in glasswort, whereas there were no such differences for mature fish. Therefore, the occasionally flooded habitat (glasswort) was positively selected by large mature toothcarp but seemed a disadvantageous habitat for immature individualsFinancial support was provided by the Spanish Ministry of Science and Technology (REN2003-00477), the Government of Catalonia (Catalan Government Distinction Award for university research 2004 to EGB) and a LIFE-Nature project (LIFE 99 NAT/E/00 6386