Jan Henning L'Abée-Lund

Spatial segregation by age and size in Arctic charr: A trade-off between feeding possibility and risk of predation

1. Arctic charr Salvelinus alpinus underwent phenological and ontogenetic habitat shifts between the epibenthic and pelagic zones in five Norwegian study lakes. Juveniles exploited epibenthic areas until they reached a body length of 13-18 cm when many moved into pelagic waters during summer. 2. Pelagic juvenile Arctic charr had larger age-specific lengths than their epibenthic counterparts, and few juvenile Arctic charr moved from epibenthic to pelagic areas in the presence of large brown trout, the main potential fish predator present

Evolution of the life history of Arctic charr Salvelinus alpinus

SummaryArctic charrSalvelinus alpinus life histories are very variable, both within and between localities. In some lakes we find more than one phenotype (in Lake Thingvallavatn there are four), each phenotype having a characteristic life history. In spite of this, the life histories of 44 populations of Arctic charr from throughout its range of distribution can be described accurately by a number of dimensionless indices made up from some common life-history descriptors (age and length at maturity, theoretical maximal length, instantaneous rate of natural mortality). This is contrary to what we previously have found for Brown troutSalmo trutta and we discuss this difference. We also test a new model which seeks to predict reproductive effort based on information on relative length at maturity (Lα)/Linf) and age at maturity. The model did not fit the observed reproductive investment data.

Divergent trends in anadromous salmonid populations in Norwegian and Scottish rivers.

The Atlantic salmon (Salmo salar) is a charismatic anadromous fish of high conservation and economic value. Concerns have been expressed regarding the long-term viability of fisheries throughout the speciess distributional range because of abundance variations that cannot currently be explained or predicted. Here, we analyse long-term catch data obtained over a wide geographical range and across a range of spatial subscales to understand more fully the factors that drive population abundance. We use rod catch data from 84 Norwegian rivers over 125 years (1876–2000) and 48 Scottish rivers over 51 years (1952–2002). The temporal correlation in catches is very long-term, with trends persisting over several decades. The spatial correlation is relatively short-range, indicating strong local-scale effects on catch. Furthermore, Scottish salmon populations exhibit recent negative trends in contrast to some more positive trends in Norway—especially in the north.

Contemporary ocean warming and freshwater conditions are related to later sea age at maturity in Atlantic salmon spawning in Norwegian rivers

Atlantic salmon populations are reported to be declining throughout its range, raising major management concerns. Variation in adult fish abundance may be due to variation in survival, growth, and timing of life history decisions. Given the complex life history, utilizing highly divergent habitats, the reasons for declines may be multiple and difficult to disentangle. Using recreational angling data of two sea age groups, one-sea-winter (1SW) and two-sea-winter (2SW) fish originated from the same smolt year class, we show that sea age at maturity of the returns has increased in 59 Norwegian rivers over the cohorts 1991–2005. By means of linear mixed-effects models we found that the proportion of 1SW fish spawning in Norway has decreased concomitant with the increasing sea surface temperature experienced by the fish in autumn during their first year at sea. Furthermore, the decrease in the proportion of 1SW fish was influenced by freshwater conditions as measured by water discharge during summer months 1 year ahead of seaward migration. These results suggest that part of the variability in age at maturity can be explained by the large-scale changes occurring in the north-eastern Atlantic pelagic food web affecting postsmolt growth, and by differences in river conditions influencing presmolt growth rate and later upstream migration.

Rivers of the Boreal Uplands

In Europe, the Boreal forest ecosystem centers around 60°N. The northern limit is roughly along the July 13 °C isotherm and the southern limit along the July 18 °C isotherm. This chapter describes the rivers within this region along the western coast of the Scandinavian Peninsula that drain into the Skagerrak or North Sea. A total of 173 major rivers belong to this region, of which 10 rivers have been selected to illustrate major characteristics. Although a general climate regime for the Boreal Uplands exists, a great variety of micro-climates occurs that significantly affects local vegetation. The south-eastern and northern watersheds are dominated by birch, pine and spruce, whereas western watersheds are dominated by birch. Watersheds in the Boreal Uplands can be divided into three main groups. Most watersheds contain rivers having considerable gradients, intermediate discharge, lakes in the headwaters in mountain regions, and some are even glacier-fed. The second group consists of large watersheds mainly situated in the southeast. These rivers are characterized by relative high discharge and long stretches with low gradients; large, deep lakes are common. The third group comprises of small watersheds in coastal areas. Here, rivers have low discharge, respond quickly to fluctuations in precipitation, and have intermediate gradients. Rivers of the Boreal Uplands have been of significant importance to humans for centuries as reproductive areas for Atlantic salmon, as a transport medium for timber, and for hydropower.

The potential influence of Atlantic salmon Salmo salar and brown trout Salmo trutta on density and breeding of the white-throated dipper Cinclus cinclus

Abstract Interactions between birds and fish are often overlooked in aquatic ecosystems. We studied the influence of Atlantic salmon and brown trout on the breeding population size and reproductive output of the white‐throated dipper in a Norwegian river. Acidic precipitation led to the extinction of salmon, but salmon recolonized after liming was initiated in 1991. We compared the dipper population size and reproductive output before (1978–1992) and after (1993–2014) salmon recolonization. Despite a rapid and substantial increase in juvenile salmon, the breeding dipper population size and reproductive output were not influenced by juvenile salmon, trout, or total salmonid density. This might be due to different feeding strategies in salmonids and dippers, where salmonids are mainly feeding on drift, while the dipper is a benthic feeder. The correlation between the size of the dipper population upstream and downstream of a salmonid migratory barrier was similar before and after recolonization, indicating that the downstream territories were not less attractive after the recolonization of salmon. Upstream dipper breeding success rates declined before the recolonization event and increased after, indicating improved water quality due to liming, and increasing invertebrate prey abundances and biodiversity. Surprisingly, upstream the migratory barrier, juvenile trout had a weak positive effect on the dipper population size, indicating that dippers may prey upon small trout. It is possible that wider downstream reaches might have higher abundances of alternative food, rending juvenile trout unimportant as prey. Abiotic factors such as winter temperatures and acidic precipitation with subsequent liming, potentially mediated by prey abundance, seem to play the most important role in the life history of the dipper.