Geology topics

Leif Asbjørn Vøllestad

Publications and source records attributed to Leif Asbjørn Vøllestad.

3 recordsLinked to original sources

USGS2020

A call for global action to conserve native trout in the 21st century and beyond

Trout and char (hereafter, trout ) represent some of the more culturally, economically and ecologically important taxa of freshwater fishes worldwide (Kershner, Williams, Gresswell, & Lobón‐Cerviá, 2019a ). Native to all continents in the Northern Hemisphere (as well as western Mediterranean Africa), trout belong to seven genera ( Oncorhynchus , Salvelinus, Salmo , Hucho, Parahucho, Brachymystax and Salvethymus ), which are distributed across more than 60 countries (Muhlfeld et al., 2019 ). Despite their broad importance as indicators of biodiversity in cold‐water ecosystems (Haak & Williams, 2013 ), as well as cultural icons for food and recreation, nearly half of the world's recognised trout species (IUCN, 2018 ) are imperilled or at risk of global extinction (Muhlfeld et al., 2018 , 2019 ). The root causes of their vulnerability include broad‐scale alteration of landscapes and watersheds, dams, overharvest, pollution, interactions with hatchery‐bred conspecifics and non‐native species. However, emerging threats such as climate change and related problems such as the spread of diseases and parasites pose significant challenges and uncertainties to native trout and their habitats (Kovach et al., 2016 ; Muhlfeld et al., 2018 ). Ultimately, conservation of native trout depends on understanding their diversity, a willingness to address threats at their root causes and implementing progressive conservation solutions that promote persistence of these iconic species in the face of growing human pressures.

Ecology of Freshwater Fish↗

USGS2020

Global status of trout and char: Conservation challenges in the twenty-first century

Freshwater ecosystems are among the most threatened ecosystems in the world (Richter et al. 1997; Strayer and Dudgeon 2010), and freshwater fishes may now be the most threatened group of vertebrates (Ricciardi and Rasmussen 1999; Vorosmarty et al. 2010; Darwall and Freyhof 2016). Of the 7,300 freshwater fish species globally assessed by the International Union for Conservation of Nature (IUCN, www.iucnredlist.org) in 2013, nearly one of every three species was threatened with extinction (Darwall and Freyhof 2016). Growing pressures from a multitude of direct and indirect human stressors (e.g., habitat loss and degradation, pollution, invasive species, overexploitation, diversion or alteration of biological flows, climate change, and others) threaten the persistence of many freshwater fish species and entire aquatic communities around the globe (Limburg et al. 2011). This pattern is particularly true for salmonid fishes (family Salmonidae, subfamily Salmoninae, belonging to the genera Oncorhynchus, Salvelinus, Salmo, Hucho, Parahucho, Brachymystax, and Salvethymus). Salmonids are globally-distributed, coldwater taxa with life-cycles restricted entirely to freshwater ecosystems (typically referred to as trout and char), but also Atlantic and Pacific salmon with more complex anadromous life-histories.

Book chapter↗

USGS2013

Basin-scale phenology and effects of climate variability on global timing of initial seaward migration of Atlantic salmon (Salmo salar)

Migrations between different habitats are key events in the lives of many organisms. Such movements involve annually recurring travel over long distances usually triggered by seasonal changes in the environment. Often, the migration is associated with travel to or from reproduction areas to regions of growth. Young anadromous Atlantic salmon ( Salmo salar ) emigrate from freshwater nursery areas during spring and early summer to feed and grow in the North Atlantic Ocean. The transition from the freshwater (‘parr’) stage to the migratory stage where they descend streams and enter salt water (‘smolt’) is characterized by morphological, physiological and behavioural changes where the timing of this parr-smolt transition is cued by photoperiod and water temperature. Environmental conditions in the freshwater habitat control the downstream migration and contribute to within- and among-river variation in migratory timing. Moreover, the timing of the freshwater emigration has likely evolved to meet environmental conditions in the ocean as these affect growth and survival of the post-smolts. Using generalized additive mixed-effects modelling, we analysed spatio-temporal variations in the dates of downstream smolt migration in 67 rivers throughout the North Atlantic during the last five decades and found that migrations were earlier in populations in the east than the west. After accounting for this spatial effect, the initiation of the downstream migration among rivers was positively associated with freshwater temperatures, up to about 10 °C and levelling off at higher values, and with sea-surface temperatures. Earlier migration occurred when river discharge levels were low but increasing. On average, the initiation of the smolt seaward migration has occurred 2.5 days earlier per decade throughout the basin of the North Atlantic. This shift in phenology matches changes in air, river, and ocean temperatures, suggesting that Atlantic salmon emigration is responding to the current global climate changes.

Global Change Biology↗