Anthropogenic and natural size-related selection act in concert during brown trout (Salmo trutta) smolt river descent

Hydrobiologia - By hindering migration and inducing direct turbine mortality during downstream migration, hydropower is regarded as one of the most serious threats to anadromous salmonids. Yet,...     

A survey and GIS-based estimate of the breeding population of Great Snipe Gallinago media in Central Norway

Capsule By using a GIS-model to identify suitable breeding habitats for Great Snipe in Central Norway, we estimated a total of 276 leks holding approximately 2700 males.

Aim To estimate the size of the Great Snipe population in central parts of the species remaining breeding areas in Western Europe.

Methods GIS-analysis identified an area of 528?km² as suitable habitat for Great Snipe in the total study area (22?000?km²). Complete surveys were made in 8% of these habitats by using a subset of 53 sampling areas.

Results A total of 28 Great Snipe leks were found within the sampling areas. Ten of these were found in previously known lek areas, while 18 leks were found in areas with no previous knowledge of leks. Extrapolating the lek density and the lek size found in the surveyed areas, resulted in a total estimate of 276 Great Snipe leks holding approximately 2700 lekking males. The leks were found on open fens along the forest edge and were mainly situated on base-rich bedrocks. Mean altitude of the leks was 570?m.

Conclusion The total population of Great Snipe in Norway was previously assumed to comprise 5000–15?000 lekking males or ‘pairs'. By extrapolating the densities of leks found in Central Norway to a national scale, we expect the true breeding population of Great Snipe to be approximately 13?500 males. Changes in the elevation of the tree limit and increased overgrowth, as a result of reduced grazing pressure and/or global warming, are possible threats that may reduce the availability of preferred Great Snipe habitats and increase population fragmentation.     

Does increased abundance of sea lice influence survival of wild Atlantic salmon post‐smolt?

A synthesis of results from two projects was assessed to analyse possible influence of sea lice Lepeophtheirus salmonis on marine Atlantic salmon Salmo salar survival. During the years 1992–2004, trawling for wild migrating post-smolts was performed in Trondheimsfjord, a fjord in which no Atlantic salmon aquaculture activity is permitted. Prevalence and intensity of sea lice infections on migrating wild post-smolts differed between years. A correlation analysis between 1 sea-winter (SW) Atlantic salmon catch statistics from the River Orkla (a Trondheimsfjord river) and sea lice infections on the migrating smolts in the Trondheimsfjord was not significant. Up to 2% reduction in adult returns due to sea-lice infection was expected. In addition, experimental releases from 1996 to 1998 with individually tagged groups of hatchery-reared Atlantic salmon smolts given protection against sea-lice infection was performed. Higher recaptures of adult Atlantic salmon from 1998 treated smolts compared to the control group may correspond to high abundance of sea lice found on the wild smolt, and may indicate influence on post-smolt mortality. These studies indicate that post-smolt mortality in Trondheimsfjord is marginally influenced by sea lice infection; however, the methods for assessing wild smolt mortality might be insufficient. Higher infections of sea lice farther out in the fjord may indicate more loss in Atlantic salmon returns in some years.     

The throat badge as a status signal in juvenile male willow titsParus montanus

Summary Status signalling in juvenile male Willow TitsParus montanus was studied by observing the outcome of interactions of birds with different size of their black throat badge, and by manipulating the size of their badge. The bird with the largest throat badge was the dominant one in all dyads studied (N=25), and the relative rank position of the males was positively correlated with the size of their badge. The badge size explained about 95% of the variation in the birds' dominance rank. When confronting juvenile males with a stuffed Willow Tit mounted close to a feeder placed in an aviary, there was found a negative correlation between the size of the badge of the males and the time from they were released till they visited the feeder. After subordinate birds from seven earlier dyads got their throat badge artificially increased, the originally dominant bird reacted submissively and tried to avoid the bird with a larger dyed badge, that temporary had increased its rank position. Our data support the status signalling hypothesis.

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Zusammenfassung Durch Beobachtung von Interaktionen zwischen jungen Männchen der Weidenmeise (Parus montanus) mit schwarzem Kehlfleck unterschiedlicher Größe und durch experimentelle Veränderung der Kehlfleckgröße wurde untersucht, ob durch den Kehlfleck der Dominanzstatus eines Vogels signalisiert wird. In allen untersuchten Paarvergleichen war der Vogel mit dem größten Kehlfleck dominant (N=25); die relative Rangposition eines Männchens war mit der Kehlfleckgröße positiv korreliert. Kehlfleckgröße erklärte 95% der Variation in der Rangstellung. Wurden junge Männchen mit einem Stopfpräparat konfrontiert, das in einem Käfig nahe der Futterstelle montiert war, ergab sich eine negative Korrelation zwischen der Kehlfleckgröße der Versuchsmännchen und dem Intervall zwischen ihrem Einsetzen in den Käfig und dem Besuch der Futterstelle. Nachdem die Kehlflecke von unterlegenen Männchen in sieben der Paarversuche künstlich vergößert worden waren, reagierte das vorher dominante Männchen unterwürfig und versuchte den Vogel mit dem durch Färbung vergrößerten Kehlfleck zu meiden, der dadurch vorübergehend seine Rangstellung verbesserte. Unsere Ergebnisse unterstützen die status signalling-Hypothese.