José Prenda

THE RELATIVE INFLUENCE OF RIPARIAN HABITAT STRUCTURE AND FISH AVAILABILITY ON OTTER Lutra lutra L. SPRAINTING ACTIVITY IN A SMALL MEDITERRANEAN CATCHMENT

The influence of riparian habitat structure and fish availability on otter Lutra lutra marking activity was studied at the head of the Guadalete river (southern Spain). Otter habitat in the study area could be ordered along two gradients in structural properties (altitude, stream order, channel form, water current, bank vegetation), water quality and bankside plant species composition. Generally, higher altitude sites had more dense bankside vegetation cover and better water quality. The dominant upstream bankside plant assemblage consisted of species such as Rubus sp., Salix spp., and Arundo donax, which provided secure shelter for otters. In contrast, at downstream sites the typical plant assemblage was Tamarix africana, Nerium oleander and helophytes, overall bankside cover was lower but wider and the water was more eutrophic. Otter sprainting activity was correlated with this gradient, being higher in downstream sites, especially in areas dominated by short vegetation and scarce human presence. Similarly, fish biomass was greater downstream. A strong positive correlation was observed between otter sprainting activity and larger fish size. Thus, the variability in otter marking intensity along the upper Guadalete seemed to be more dependent on fish availability, and secondly on human disturbance, than on particular riparian habitat features, including the species composition of vegetation growing on the river banks. Implications for conservation management of otters in Mediterranean areas are discussed.

Freshwater ostracods as environmental tracers

This paper revises the response of freshwater ostracods to different environmental conditions and anthropogenic impacts, with a worldwide overview of the potential use of these microcrustaceans as bioindicators and several examples of applications in different scenarios. The development of either a single species or an ostracod assemblage is influenced by physical–chemical properties of waters (salinity, temperature, pH, dissolved oxygen), hydraulic conditions, bottom grain sizes or sedimentation rates. In addition to population and community changes, morphological and geochemical changes can also be detected in the ostracod carapace, which serves as a tracer of the water quality. All these features permit to delimit the spatial effects of urban sewages, mining effluents, agricultural wastes, watershed deforestation or road building. These data are the basis for the palaeoenvironmental reconstruction of cores, with an interesting application to archaeology. In addition, favourable results of recently developed bioassays, coupled with an important variability of local assemblages under changing conditions in both waters and sediments, suggest that these microcrustaceans may included between the most promising sentinels groups in freshwater areas. These microcrustaceans show high sensitivity to pesticides, herbicides, heavy metal pollution and oil inputs.

Identifying priority sites for the conservation of freshwater fish biodiversity in a Mediterranean basin with a high degree of threatened endemics

The Guadiana River basin’s freshwater fish species richness, endemicity and threatened status (92% of native species are threatened) highlight the need for a large-scale study to identify priority areas for their conservation. One of the most common problems in conservation planning is the assessment of a site’s relative value for the conservation of regional biodiversity. Here we used a two-tiered approach, which integrates an assessment of biodiversity loss and the evaluation of conservation value through site-specific measures. These measures based on the reference condition approach introduce the ability to make objective comparisons throughout the Guadiana River basin, thus avoiding a priori target areas. We identified a set of biodiversity priority areas of special conservation significance—which contain rare taxa as well as intact fish communities—because of their outstanding contribution to the basin’s biodiversity. The inclusion of complete sub-basins in these priority areas might guarantee an optimal solution in terms of spatial aggregation and connectivity. However, the high spatial fragmentation to which the Guadiana River basin is submitted due to river regulation highlights the necessity of a systematic approach to evaluate the capability of the identified priority areas to maintain the Guadiana’s freshwater fish biodiversity.

Potential impact of an exotic mammal on rocky intertidal communities of northwestern Spain

Being the interface of sea and land, the coast can be invaded by introduced species coming from either of these two worlds. Recent reviews of coastal invasions emphasize the human-mediated transport of non-indigenous marine plants and invertebrates, forgetting the potential role of invaders of terrestrial origin. By studying the diet of the introduced American mink (Mustela vison) on a rocky shore of southwestern Europe, we draw attention to the potential impact on intertidal communities of exotic species coming from inland. We analysed 199 mink faeces collected in August 1997 and August 1999 in Baiona, a coastal and urban area of northern Spain recently invaded by minks. The diet of the species was based almost exclusively on crabs (45.4% of individual prey) and fish (53.3%). Most crabs were marbled crabs (Pachygrapsus marmoratus) and most fish were adult blennies (Coryphoblennius galerita and Lipophrys pholis). Given its energy requirements (about 1250 kJ/day), a single mink will consume during the month of August approximately 945 blennies and 496 crabs. Although we lack accurate data on mink abundance, a cautious estimation (4 mink/km before dispersal), supported by field observations, suggests that predation in August may reach 3780 blennies and 1984 crabs per km of shoreline. This predation pressure could affect the numbers of blennies and (less probably) crabs, indirectly benefiting the populations of their prey, that is, sessile invertebrates and snails. More field research is needed, but our results suggest that an exotic non-marine top predator such as the American mink could affect intertidal communities in Eurasia.

Population and microhabitat effects of interspecific interactions on the endangered Andalusian toothcarp (Aphanius baeticus)

The Andalusian toothcarp, Aphanius baeticus, is a critically endangered cyprinodontid species, with only nine known extant populations. Although not yet studied in the field, the distribution and abundance of Andalusian toothcarp are thought to be strongly influenced by interspecific interactions. We analysed the abundance and microhabitat use of Andalusian toothcarp in two water courses, one in which several other fish species occurred (sympatric site) and one hypersaline stream in which toothcarp was the only species present (allopatric site). Fish were sampled using plastic minnow traps and results were analysed separately for three size categories. Toothcarps were clearly more abundant in the allopatric population than in the sympatric one, though the difference was less apparent in the smallest size category. In coexistence with other species, toothcarp occupied shallower microhabitats, but in both sites in the absence of shelter fish selected deeper positions than in its absence. While in the sympatric site sheltered microhabitats were used predominately by small individuals, in the allopatric ones they were used by larger ones. Observed patterns strongly suggest that predation is the main mechanism involved in the differences in abundance and microhabitat use between sites. Our results confirm that the presence or absence of coexisting species is an important habitat feature for Andalusian toothcarp populations.

Hydrological stability and otter trophic diversity: a scale-insensitive pattern?

Two recent works related Eurasian otter (Lutra lutra (L., 1758)) trophic patterns over large areas with the stabil- ity of aquatic ecosystems. Higher levels of instability lead to reduced availability and (or) predictability of fish, and conse- quently, to a decrease in fish consumption by otters. The aim of the present study is to test these macrogeographical patterns in otter diet at regional and local scales. We analysed otter diet in Mediterranean streams in southwestern Iberian Peninsula where clear hydrological stability gradients (related to drainage area or distance to the sea) could be defined. Hydrological stability was directly related to fish consumption and inversely to otter diet diversity in terms of occurrence and biomass, both at regional and local scales. The level of stability of aquatic ecosystems appears to be a critical indirect factor that modulates otter diet through its effects on fish populations. The resulting trophic patterns are maintained from local to macrogeographical scales.