1. The objective was to identify the factors driving spatial and temporal variation in annual production (P A ) and turnover (production ⁄ biomass) ratio (P ⁄ B A ) of resident brown trout Salmo trutta in tributaries of the Rio Esva (Cantabrian Mountains, Asturias, northwestern Spain). We examined annual production (total production of all age-classes over a year) (P A ) and turnover (P ⁄ B A ) ratios, in relation to year-class production (production over the entire life time of a year-class) (P T ) and turnover (P ⁄ B T ) ratio, over 14 years at a total of 12 sites along the length of four contrasting tributaries. In addition, we explored whether the importance of recruitment and site depth for spatial and temporal variations in year-class production (P T ), elucidated in previous studies, extends to annual production. 2. Large spatial (among sites) and temporal (among years) variation in annual production (range 1.9-40.3 g m )2 per year) and P ⁄ B A ratio (range 0.76-2.4 per year) typified these populations, values reported here including all the variation reported globally for salmonids streams inhabited by one or several species. 3. Despite substantial differences among streams and sites in all production attributes, when all data were pooled, annual (P A ) and year-class production (P T ) and annual (P ⁄ B A ) and year-class P ⁄ B T ratios were tightly linked. Annual (P A ) and year-class production (P T ) were similar but not identical, i.e. P T = 0.94 P A , whereas the P ⁄ B T ratios were 4 + P ⁄ B A ratios. 4. Recruitment (Rc) and mean annual density (N A ) were major density-dependent drivers of production and their relationships were described by simple mathematical models. While year-class production (P T ) was determined (R 2 = 70.1%) by recruitment (Rc), annual production (P A ) was determined (R 2 = 60.3%) by mean annual density (N A ). In turn, variation in recruitment explained R 2 = 55.2% of variation in year-class P ⁄ B T ratios, the latter attaining an asymptote at P ⁄ B T = 6 at progressively higher levels of recruitment. Similarly, variations in mean annual density (N A ) explained R 2 = 52.1% of variation in annual P ⁄ B A , the latter reaching an asymptote at P ⁄ B A = 2.1. This explained why P ⁄ B T is equal to P ⁄ B A plus the number of year-classes at high but not at low densities. 5. Site depth was a major determinant of spatial (among sites) variation in production attributes. All these attributes described two-phase trajectories with site depth, reaching a maximum at sites of intermediate depth and declining at shallower and deeper sites. As a consequence, at sites where recruitment and mean annual density reached minimum or maximum values, annual (P A ) and year-class production (P T ) and annual (P ⁄ B A ) and yearclass P ⁄ B T ratios also reached minimum and maximum values.

One of the movement barriers that fish populations must overcome for migration success in the upper basin of Tormes river (Salamanca, Spain) is a 20 m high dam. The design of its pool and weir fishway for potamodromous fishes (mostly Iberian barbel—Luciobarbus bocagei—and Northern straight-mouth nase—Pseudochondrostoma duriense) to overcome the obstacle was improved in 2013. The aim of this study was to assess the efficiency of the fishway using FDX passive integrated transponder (PIT)-Tags inserted into the fish and PIT-Tag detection antennas at the fishway. During several sampling events, 7113 barbel and nase individuals were tagged and released at the point of capture along the basin (2538 and 4575 of which were tagged downstream and upstream, respectively). PIT-Tag Detection Antennas close to the top and bottom of the fishway monitored tagged fish continuously for 10 months (from March to December 2017), to analyze the performance of the fishway. Upstream passage efficiency was greater for barbel (60% and 25% for barbel and nase, respectively). Differences in passage efficiency between species may be due to differences in their size. Mean length for barbels attempting to pass was 336 mm (±47 mm) while for nases was 143 mm (±26 mm). Moreover, both the number of attempts to pass and ascend time for nases were higher than for barbels. Entrance efficiency was low (3.5% and 10.8% for barbel and nase, respectively), although 2017 was a very dry year, thus these results are most likely influenced by flow rates. Therefore, the fishway has proved to be functional but is actually poor for efficiency purposes, especially for small fish.

Pike have been in Spain since 1949(Gutierrez-Calderon, 1954) and the first specimen in the Esla basin, in north-western Spain, was caught in 1964(Pena, 1988. Since then pike have altered the existing autochthonous fish communities in several ways.Agundez et al. (1987) reported the finding of three hermaphrodite specimens in 1986; their study was only macroscopic. The present study shows that the 18 hermaphrodite specimens caught by electrofishing between March 1986 and October 1987 in different rivers and localities in the Esla basin, comprising 094% of all pike collected, were functional hermaphrodites. Histological investigation revealed the existence of spermatogonic isles within functional ovaries.Biometric and gonad state data were taken for each specimen (Table I). Gonads were preserved in 8% formaldehyde. The gonads were cut into serial sections and stained with haematoxylin for light microscopic observation. Photographs were taken with a Nikon OPTIPHOT photomicroscope.Most of the hermaphrodites were collected in the spawning period for the species, when macroscopic detection is least troublesome. Sizes ranged between 134 and 494 mm, and all specimens were found mature for both sexes.Histological study revealed different kinds of gonad disposition: ovary and testis, ovotestis and ovary, or two ovotestes, although separation between the sexes in the gonad was not clear. TABLE I. Details of hermaphrodite pike from the R. Esla basin Date Locality Weight Length (9) (m)

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