Fresh data on the timing and speed of the oceanic spawning migration of European eels suggest a new paradigm for spawning ecology.

Summary1. Several studies have offered evidence for the occurrence of density-dependent growth in stream-living Brown Trout. However, such evidence has been gleaned for low-density populations, whereas studies on persistently high-density populations have claimed that growth is density-independent. Such a paradoxical observation is shared with other salmonids and has been assumed by several authors to suggest that stream salmonid populations may be regulated by two different mechanisms: density-dependent growth at low densities and density-dependent mortality, in the absence of densitydependent growth, at high densities. 2. This comparative long-term study explored the occurrence of density-dependent growth by examining growth during the lifetime across cohorts in three stream-living Brown Trout populations representing the opposite extremes of growth and density documented throughout the species' distributional range. 3. This comparison highlighted identical growth-recruitment patterns in a highdensity population with low potential for growth, in a low-density population with high potential for growth and in a population with intermediate traits. In the three populations, growth declined with increased recruitment describing negative power trajectories. These observations are consistent with there being a single, negative power relationship between growth and density where the effects of density dependence are stronger at low densities and become negligibly low at high densities. 4. Stream-living Brown Trout populations may be regulated by the continuous operation of density dependence on growth and mortality. In poorly recruited cohorts density dependence may operate on growth but not on mortality during a time period after which density dependence operates on both growth and mortality. In highly recruited cohorts, density dependence operates simultaneously on growth and mortality from the youngest life stages.

Lobó n-Cerviá, J. and Rincó n, P. A. 2004. Environmental determinants of recruitment and their influence on the population dynamics of stream-living brown trout Salmo trutta . Á/ Oikos 105: 641 Á/646.The relative importance of endogenous feedback mechanism vs environmental factors in the dynamics of animal populations is a long-standing, but not fully resolved yet, issue in ecology. We have addressed this subject by examining the dynamics of a streamresident population of Salmo trutta in a northwestern Spain stream. Recruitment was the major determinant of population size and the abundance of recruits resulted from a combination of regional and local environmental factors. Stream discharge in March determined the amount of stream area suitable for newly emerged trout (r 2 0/0.59 Á/ 0.79%), that in turn determined the abundance of recruits at each site (r 2 0/0.51 Á/ 0.77%). Stream discharge determines the overall strength of annual recruitment. Discharge, however, combines with stream morphology at the site scale to result in a site-specific area suitable for juveniles and, hence, site-specific recruitment. Thus, our study exemplifies how an environmentally driven animal population may persist on time with little or no operation of endogenous regulatory mechanisms.

1. Resident brown trout Salmo trutta in the Esva River basin (north Spain) live in a patchy environment with tracts of riparian forest or meadow along stream banks. This study assessed whether the reproductive traits of brown trout from four contrasting sites reflected site‐specific factors. 2. Length at maturity (10.5–11 cm of 1 + individuals) was the same in the four sites examined but slowest growers in slow‐growing sub‐populations delayed maturity for 1 year relative to fast‐growing fish. The analysis of monthly variations in egg size and number suggest that two ‘decisions’ in two consecutive years are required to complete spawning. The first concerns the number of eggs, determined when trout are still 0 +, and the second concerns egg size. 3. At three sites, egg size and number did not differ significantly between years but highly significant interannual variations were apparent at another site. Fish length was the major determinant of egg size and number at all sites but for any given length, brown trout at sites where the fish exhibited higher growth rates spawned more, but smaller, eggs than those at slow‐growing sites. This spatial pattern was identical to the temporal pattern exhibited by trout at another site. The combination of temporal (year‐to‐year) and spatial (between rivers) variations in egg size and number showed a significant negative correlation, supporting the operation of a trade‐off between these two traits. 4. The trade‐off between egg size and number seems to be determined by site‐specific factors, with slow‐growing trout at sites which are fully covered by canopy spawning fewer, but larger, eggs than fast‐growers in unshaded sites.

The objective was to uncover the roles of environmental, density-independent, and intrinsic, density-dependent factors as drivers of population size in stream-resident brown trout (Salmo trutta) across space and time. The study was based on census data and environmental variables collected every fourth month over 1986–2006 at 12 sites selected along four Rio Esva tributaries (northwestern Spain). Density-dependent hypotheses involving the operation of current and lagged densities on recruitment and subsequent spawner density were analyzed at three increasingly larger geographical scales (site, stream, and river drainage). No evidence supported these hypotheses, suggesting an overall lack of negative feedbacks among year classes. Stream discharge in March when trout emerge and site depth explained substantial amounts of spatiotemporal variation in recruitment. The effects of these two factors on recruitment were sufficiently strong to be detected in year-class mortality and subsequent spawner density. Discharge and site depth determine the amount of spatial habitat suitable for sized individuals during their lifetime and, hence, the maximum number of individuals across space and time. Recruitment dependence, as a delayed density-dependent factor, adjusts the numbers in excess to the numbers established by spatial habitat. These findings differ markedly from the prediction of the most recent theories of population dynamics and are discussed in this context.