Javier Lobón-Cerviá

Composition and trophic structure of a fish community of a clear water Atlantic rainforest stream in southeastern Brazil

As part of a larger project that aimed to determine the factors that regulate fish productivity in a clear water river of the Atlantic rainforest, São Paulo State, Brazil, composition and the trophic structure of the fish community of the middle course of the river were studied from January 1995 to May 1996. Of the 17 species collected, Mimagoniates microlepis, Schizolecis guentheri, Phalloceros caudimaculatus and Kronichthys heylandi accounted for 83.2% of the total fish abundance. Most of the species were insectivorous (35.7%), followed by detritivores (21.4%), benthivores (14.2%), omnivores (14.2%), herbivores (7.1%), and piscivores (7.1%). Results of food overlap analysis as calculated by the Index of Morisita showed that 17.5% of the species pairs overlapped. Habitat segregation, however was observed among most of the species, suggesting some degree of food partitioning. Despite the characteristics of this escarpment river, which produces spates year round, no seasonal variation in diet was observed, suggesting that although food abundance may fluctuate throughout the year, most items are in constant supply. The importance of allochthonous food was considered and it appears that, though only a few species feed on this source (including detritivores), they make up 87.2% of the total fish abundance. The low abundance of most of the fishes, the high number of endemic species and the strong dependence of the species on a few food resources suggest that these systems are sensitive to anthropogenic impacts and require future studies.

Microhabitat Use by Stream-Resident Brown Trout: Bioenergetic Consequences

Abstract Seasonal shifts in the microhabitats used by feeding brown trout Salmo trutta in a northern Spanish river were related to changes in habitat availability. At higher flows, mean water column velocities, focal velocities, and depths occupied by brown trout were greater and focal elevations above the substratum were less, but no changes in substratum use were detected. Cover use differed between dates, apparently according to the presence of suitable materials. In May, a lower proportion of the observed fish were holding feeding positions than in July, October, or January. Total metabolic costs associated with holding feeding stations were primarily determined by swimming costs, and both costs varied substantially between dates, mainly as a consequence of the different swimming speeds of brown trout on the different dates. Standard metabolism was less important than swimming cost in total energy expenditure and showed no obvious relationship with total metabolic rate. Brown trout exhibited considera...

Field Assessment of the Influence of Temperature on Growth Rate in a Brown Trout Population

Abstract Growth rates of age-0 brown trout Salmo trutta during the growing period (May–September) were estimated in nine consecutive year-classes in three sites of Arroyo Chabatchos (Esva River basin, northern Spain). Observed growth rates were highly correlated with average water temperature, which explained 48, 68 (90% excluding the severe drought of 1989), and 89% of the interannual variation in respective growth rates at the three sites studied. Observed growth rates varied from those predicted by a model by Elliott and others, which predicted higher-than-observed growth rates in winter and negligible growth in summer. Observed growth rates were consistently higher than those predicted in two sites, and the only four instances (out of 27 possible) of lower-than-predicted growth rates occurred in fish at the site at which observed growth was slowest. As expected from the model, the predicted growth rates were also correlated with water temperature, but the slopes of the observed and predicted regressio...

NONLINEAR SELF-THINNING IN A STREAM-RESIDENT POPULATION OF BROWN TROUT (SALMO TRUTTA)

Self-thinning, the reduction in abundance of a cohort of growing organisms due to competition for limiting resources, has been observed in mobile animals, including many stream-dwelling salmonid fishes. Identifying self-thinning and hypothesizing its caus- al mechanisms rely on inspection of the slope of the relationship between log body mass and log density. We have examined such relationships for 42 brown trout cohorts (11 yr classes at four sites) sampled bimonthly throughout their complete life span. Trajectories displayed two distinct phases whose transition occurred at a body mass of 30-40 g (14- 15 cm). Split lines provided a significantly better fit than straight lines in 32 of 42 cases and explained an average of 81.3% of the variation in trout abundance. The slopes of the first phase typically were not significantly different from zero and were shallower than predictions from either energetic or territorial allometry, whereas those of the second phase were steeper. This pattern strongly suggested that resource availability for brown trout was size-dependent in our study sites. Cohorts did not appear to be self-thinning initially, but resources (presumably pool habitat) seemed to become progressively more limiting as trout grew to lengths exceeding 14-15 cm. Thus, larger size may allow mobile animals to exploit previously inaccessible resources but also may render previously used resources unavailable. Therefore, the relationship between per capita resource availability and body size is likely to depend on local conditions and may not be directly predictable only from scaling the needed amount of food or space with body size.

Microhabitat use in a mediterranean riverine fish assemblage

SummaryOver a 22 month period Barbus graellsii, Chondrostoma toxostoma, Cyprinus carpio, Esox lucius, Gobio gobio, and Leuciscus cephalus displayed non-random microhabitat use in the Rio Mattarraña, Spain and generally were overrepresented in deep microhabitats with low or undetectable flow velocities. Substrate composition did not strongly affect microhabitat use outside of its covariation with depth and velocity. Most seasonal differences in microhabitat use were attributable to seasonal changes in microhabitat availability, although all species selectively occupied deeper microhabitats during Spring 1984, 1985, and Early Summer 1984. Smaller specimens of B. graellsii, Ch. toxostoma, G. gobio, and L. cephalus all occurred closer to the substrate than larger specimens. Smaller specimens also tended to occupy shallower areas with greater amounts of erosional substrates (except for Ch. toxostoma). Assemblage members occupied statistically distinct microhabitats and could be classified as: 1) upper water column (L. cephalus), 2) mid-water column (Ch. toxostoma, C. carpio), 3) lower water column (B. graellsii), or 4) benthic (G. gobio, E. lucius). We hypothesize that the observed pattern of vertical segregation was produced by a combination of predator avoidance and differential evolutionary adaptation rather than by interspecific competition for space.

Response of a Stream Fish Assemblage to a Severe Spate in Northern Spain

Abstract I assessed the effects of a devastating spate upon the populations of brown trout Salmo trutta, Atlantic salmon S. salar, and European eel Anguilla anguilla of the Esva River basin in northern Spain. Numbers and lengths were determined for fish sampled with electrofishing techniques at nine sites along three streams before and after the spate. In addition, brown trout and Atlantic salmon that had been marked in two streams prior to the spate permitted a direct evaluation of the immediate effects. Because the spate occurred at the spawning time and destroyed reproductive habitats, I also determined its effects upon the recruitment of that year-class of brown trout. There was no evidence of negative effects of the spate upon the variables examined. The persistence of the site-specific populations after the spate was independent of site characteristics and the corresponding numbers and sizes of fish. The recruitment of brown trout was successful and similar to that of previous years. I hypothesize t...

Spatiotemporal Dynamics of Brown Trout Production in a Cantabrian Stream: Effects of Density and Habitat Quality

Abstract Spatiotemporal patterns of growth, survival, biomass, and production were investigated in seven cohorts of a resident population of brown trout Salmo trutta in Rio Chabatchos, northwestern Spain. Seasonal growth matched the sinusoidal pattern that typifies fish growth in temperate climates. However, all of the population traits examined differed among sites, markedly at one site. Cohort (40.3 g/m2) and annual production (41.0 g/m2) were greater than reported for other stream salmonids but varied dramatically across sites and years. Stream discharge in March appeared to be the major determinant of annual recruitment, whereas site characteristics related to stream channel structure constrained recruitment at the site scale. Cohort survival showed two-phase trajectories characterized by a long period with low or no mortality followed by increased mortality after a certain point. The point of inflection differed by more than 150 d among sites but occurred at similar trout sizes (15 cm). Recruitment e...

Empirical observations of the spawning migration of European eels: The long and dangerous road to the Sargasso Sea

Fresh data on the timing and speed of the oceanic spawning migration of European eels suggest a new paradigm for spawning ecology. The spawning migration of the European eel (Anguilla anguilla L.) to the Sargasso Sea is one of the greatest animal migrations. However, the duration and route of the migration remain uncertain. Using fishery data from 20 rivers across Europe, we show that most eels begin their oceanic migration between August and December. We used electronic tagging techniques to map the oceanic migration from eels released from four regions in Europe. Of 707 eels tagged, we received 206 data sets. Many migrations ended soon after release because of predation events, but we were able to reconstruct in detail the migration routes of >80 eels. The route extended from western mainland Europe to the Azores region, more than 5000 km toward the Sargasso Sea. All eels exhibited diel vertical migrations, moving from deeper water during the day into shallower water at night. The range of migration speeds was 3 to 47 km day−1. Using data from larval surveys in the Sargasso Sea, we show that spawning likely begins in December and peaks in February. Synthesizing these results, we show that the timing of autumn escapement and the rate of migration are inconsistent with the century-long held assumption that eels spawn as a single reproductive cohort in the springtime following their escapement. Instead, we suggest that European eels adopt a mixed migratory strategy, with some individuals able to achieve a rapid migration, whereas others arrive only in time for the following spawning season. Our results have consequences for eel management.

A simple model to determine stream trout (Salmo trutta L.) densities based on one removal with electrofishing

Abstract We have investigated the relationships between three successive removals with electrofishing and the subsequent estimated numbers for two and three catches (Zippin method) in four contrasting trout streams (166 samples). Significant correlations were found for all these parameters. For three size-age groups (O+, > O+ and total population), prediction of fish numbers from the first capture ( C 1) can be obtained from linear regressions. For any of these groups, catchability ( p ) is rather unpredictable.

Reproductive ecology and growth of a population of brown trout (Salmo trutta L.) in an aquifer-fed stream of Old Castile (Spain)

In the River Lobos-Ucero and its tributary the River Avión-Milanos (Duero basin, Old Castile, Central Spain), two limestone streams fed by aquifers, the population of brown trout, as compared with the populations of other European streams, shows a high growth rate, high condition coefficients, short life-span and early age at first maturity. Gonad cycle was also studied. Size distributions of unshed eggs exhibit a dynamic activity with a bi-modal distribution from June onwards, spawning occurred in the last days of November. Fecundity (F) can be predicted from trout length (L, mm) according to the equation: F= −646.47+5.6167 · L. Numbers and standing crop of trout range from 18 to 3903 ind. ha−1 and 3.6 to 452.9 Kg ha−1, reaching higher values in the sites close to the aquifers. Egg production had values of 22.4 and 18.0 eggs m−2 in the Rivers Ucero and Avión-Milanos respectively. Some factors suggested as regulators of these demographical characteristics are discussed in the light of recent literature.