Large benthic charr on the spawning grounds, picture took Kalina H. Kapralova and Quentin Horta.

Now published in the Icelandic Naturalist. Areas used by Salmonid juveniles in Lake Þingvallavatn and connected rivers. Uppeldissvæði laxfiska í Þingvallavatni og tengdum ám (in icelandic, w. English abstract). Guðbjörg Ósk Jónsdóttir, Benóný Jónsson, Magnús Jóhannsson og Arnar Pálsson.  (Náttúrufræðingurinn).

Nursery grounds of juvenile fish provide them shelter and food making them essential for the life of an individual, the population and species as a whole. In Lake Þingvallavatn, Iceland, two species of salmonids inhabit the lake, Brown trout (Salmo trutta) and Arctic charr (Salvelinus alpinus). Extensive biological studies have been conducted on adult fish of both species in the lake and its catchment area, but much less is known about the biology of age-0 fish and the ecology of their first 1−2 years in the life. Given the critical importance of nursery areas for the existence of fish populations and their contribution to biological diversity, research was undertaken to examine the distribution of charr and trout juveniles in the surf zone of Lake Þingvallavatn and in streams connected to the lake. The research questions were,

1) Where in the lake and connected streams are juvenile fish found?

2) Has the density of charr and trout juveniles changed in the last twenty years?

3) Is there a connection between the environmental conditions and the presence of juvenile fish?

Data from Veiðimálastofnun (now Hafrannsóknastofun) surveys of salmonid juveniles in Þingvallavatn and adjacent rivers, spanning 2000 to 2021, were analyzed. In the summer of 2022, ten locations in Þingvallavatn were surveyed, fish were caught and measured, and several environmental factors were assessed using electrofishing. Fish were counted, identified by species, and measured for length and weight. Additionally, environmental factors, including the lake bottom composition and shoreline vegetation, were documented. The results show that the two species utilize different juvenile habitats, trout predominantly occupy river habitats, whereas charr are more abundant in the surf zone of the lake. There was a significant increase in density of the Brown trout over the 19-year period, but density of Arctic charr remained relatively stable with a sign of subtle decline. In 2022, six possible nursery sites were documented in the surf zone, four with majority of Arctic charr and two with mainly Brown trout. There was little overlap in the species distribution. Vegetation on the shoreline was the only environmental factor significantly correlated with the presence or absence of juvenile fish on a given site. Further studies on juvenile fish biology in the lake and adjacent streams and river might explore the interplay of environmental factors and genetics in shaping the size and morphology of adult fish in both species´ populations. This is particularly relevant for Arctic charr, which features four distinct sub-populations (or morphs) differing in size, shape, trophic morphology and ecology.