Water level influences migratory patterns of anadromous brown trout in small streams

Abstract

Habitats modify the population ecology of species. Here, we show how low water level influences abundance and size of adult anadromous brown trout (Salmo trutta) entering a small, South Norwegian stream for spawning. After smolting, the fish appear chiefly to feed within 10 km of the home stream. In the autumn, South Norwegian streams typically flood because of heavy rainfall, when the anadromous brown trout entered from the sea. Mean annual duration of the upstream migration period was 34 days and ended when the flooding ended and the water temperature dropped to below 4°C. During most of the migration period, on average two trout ascended the river per day. The sexes entered the spawning area concurrently, and the male:female ratio of the anadromous trout was 1.27. No fish entered when the water depth just downstream of the spawning area was below 5 cm, and mean number of fish increased with increasing water depth to ca. 30 cm, but not at higher flows when the ascent gradually decreased. Mean and maximum size of the entering spawners increased with water depth between 5 and 16 cm. Among those that had been to sea, most were recaptured in the home stream, 4% in other streams, but only two of the strays were caught close to spawning time. The present results illustrate that population traits of anadromous brown trout from a small stream differ from those in larger rivers, probably because of selection associated with water flow.

Environmental threats and conservation implications for Atlantic salmon and brown trout during their critical freshwater phases of spawning, egg development and juvenile emergence

“…Discharge and its dynamics are crucial for the reproduction of brown trout and Atlantic salmon in rivers and streams. For Atlantic salmon and sea trout, flow events have been described to stimulate the entry into their natal rivers, often coupled with other environmental factors (Banks, 1969; Clarke et al, 1991; Erkinaro et al, 1999; Jonsson, 1991; Jonsson et al, 2018; Smith et al, 1994; Svendsen et al, 2004; Thorstad et al, 1998). A rare exception to this observation is the study from Davidsen et al (2013) conducted in Norway, which could not confirm that river entry of Atlantic salmon was facilitated by increased water discharge and/or ebb tide.…”

Straying of brown trout in the catchment of a large New Zealand river evaluated by otolith microchemistry

“…Large fecund migrants returning to spawn in small streams potentially establish a positive feedback loop (Mikheev et al, 2020). However, the precision of natal homing at the catchment scale appears to be highly variable, with some studies indicating high (85%–99%) precision (Berg & Berg, 1987; Jonsson et al, 2004, 2018; King et al, 2016), but other studies indicate a high (54%–60%) proportion of straying individuals (Degerman et al, 2012; Jensen et al, 2015; Knutsen et al, 2001). At the scale of individual tributaries, the proportion of straying individuals is often high with published natal homing rates of 18%, 22% and 55% (Baglinière et al, 1987; Frank et al, 2012; Huet & Timmermans, 1979).…”

“…We predicted that a high proportion of migratory brown trout spawning in the focal tributary would have originated there, having returned to their natal stream. This prediction is based on published examples of natal homing of brown trout (Jonsson et al, 2004, 2018; King et al, 2016; Tilzey, 1977) and the accessibility of the focal stream for migratory trout based on its lowland location. However, an alternative hypothesis was also considered that a high proportion of fish would have strayed from their natal streams located elsewhere in the Taieri catchment—crucially fish migrating downstream from the upper Taieri catchment would be unable to return to their natal stream due to multiple barriers to migration, and would thus be forced to use Silverstream as an accessible alternative.…”

Genetic structuring across alternative life‐history tactics and small spatial scales in brown trout (Salmo trutta)

“…For the former, brown trout can either be resident within rivers (river-resident) or lakes (lake-resident) for their entire life cycle. In both instances, localised dispersal movements are common (Jonsson, Jonsson, & Jonsson, 2018;Vøllestad et al, 2012). Most brown trout populations, however, are migratory with individuals moving between spawning (rivers) and feeding grounds on a regular basis, following a temporally predictable pattern.…”