Irene Parra

Latitudinal and altitudinal growth patterns of brown trout Salmo trutta at different spatial scales.

Spatial variation in growth of stream-dwelling brown trout Salmo trutta was explored in 13 populations using a long-term study (1993-2004) in the Bay of Biscay drainage, northern Spain. The high variability in fork length (L(F)) of S. trutta in the study area was similar to the body-size range found in the entire European distribution of the species. Mean L(F) at age varied: 0+ years, 57.4-100.7 mm; 1+ years, 111.6-176.0 mm; 2+ years, 155.6-248.4 mm and 3+ years, 194.3-290.9 mm. Average L(F) at age was higher in main courses and lower reaches compared with small tributaries and upper reaches. Annual specific growth rates (G(L)) were: 0+ to 1+ years, 0.634-0.825 mm mm(-1) year(-1); 1+ to 2+ years, 0.243-0.342 mm mm(-1) year(-1); 2+ to 3+ years, 0.166-0.222 mm mm(-1) year(-1), showing a great homogeneity. Regression models showed that water temperature and altitude were the major determinants of L(F) at age variability within the study area. A broader spatial analysis using available data from stream-dwelling S. trutta populations throughout Europe indicated a negative relationship between latitude and L(F) of individuals and a negative interaction between latitude and altitude. These findings support previous evidence of the pervasive role of water temperature on the L(F) of this species. Altitude appeared as the overall factor that includes the local variation of other variables, such as water temperature or food availability. At a larger scale, latitude was the factor that encompassed these environmental gradients and explained the differences in L(F) of S. trutta. In summary, L(F) at age in stream-dwelling S. trutta decreases with latitude in Europe, the converse of Bergmanns rule.

Thermal Carrying Capacity for a Thermally-Sensitive Species at the Warmest Edge of Its Range

Anthropogenic environmental change is causing unprecedented rates of population extirpation and altering the setting of range limits for many species. Significant population declines may occur however before any reduction in range is observed. Determining and modelling the factors driving population size and trends is consequently critical to predict trajectories of change and future extinction risk. We tracked during 12 years 51 populations of a cold-water fish species (brown trout Salmo trutta) living along a temperature gradient at the warmest thermal edge of its range. We developed a carrying capacity model in which maximum population size is limited by physical habitat conditions and regulated through territoriality. We first tested whether population numbers were driven by carrying capacity dynamics and then targeted on establishing (1) the temperature thresholds beyond which population numbers switch from being physical habitat- to temperature-limited; and (2) the rate at which carrying capacity declines with temperature within limiting thermal ranges. Carrying capacity along with emergent density-dependent responses explained up to 76% of spatio-temporal density variability of juveniles and adults but only 50% of young-of-the-years. By contrast, young-of-the-year trout were highly sensitive to thermal conditions, their performance declining with temperature at a higher rate than older life stages, and disruptions being triggered at lower temperature thresholds. Results suggest that limiting temperature effects were progressively stronger with increasing anthropogenic disturbance. There was however a critical threshold, matching the incipient thermal limit for survival, beyond which realized density was always below potential numbers irrespective of disturbance intensity. We additionally found a lower threshold, matching the thermal limit for feeding, beyond which even unaltered populations declined. We predict that most of our study populations may become extinct by 2100, depicting the gloomy fate of thermally-sensitive species occurring at thermal range margins under limited potential for adaptation and dispersal.

Evaluation of PCBs and DDTs in endemic Iberian barbel Barbus bocagei (Steindachner, 1864) populations.

PCB and DDT levels were evaluated in populations of endemic Iberian barbel (Barbus bocagei) in the Jarama River in Spain via a pollution gradient from well-preserved areas upstream to contaminated downstream areas. Age structure, abundance, recruitment and levels of morphological abnormalities and ectoparasites were assessed. Upstream to downstream PCB concentrations ranged from 3.4 to 101.4 ng/g (ww) and from 0.9 to 19.6 ng/g ww for DDTs. The PCB pattern was dominated by the PCB 153, 138 and 180 congeners, and the less chlorinated ones had a relatively high contribution upstream. Barbels exposed to low PCB and DDT levels had a well-balanced population with a predominant cohort of young fish, indicating good recruitment. The most contaminated sites displayed a disrupted age distribution, where the proportion of young fish was clearly under-represented. Recruitment and total density of barbel populations decreased downstream where the highest PCB and DDT levels were found. In addition, a higher incidence of abnormalities and ectoparasites was observed at these sites. High concentrations of PCBs and DDTs most likely contribute to the reduction of Iberian barbel reproductive performance in the most contaminated sites, as shown by the disrupted age-distribution found in our study.