Migratory timing, marine survival and growth of anadromous brown troutSalmo truttain the River Imsa, Norway

Abstract

The aim of the paper was to study sea migration, growth and survival of brown trout Salmo trutta of the River Imsa, 1976-2005. The migratory S. trutta were individually tagged and fish leaving or entering the river were monitored daily in traps located near the river mouth. The mean annual duration of the sea sojourn was 6-9 months for first-time migrants moving to sea between January and June. It was 8-18 months for those migrating to sea between July and December. Veteran migrants stayed 12 months or less at sea and most returned to the river in August. Early ascending fish stayed the longest in fresh water because most returned to sea in April to May. The day number of 50% cumulative smolt descent correlated negatively with mean water temperature in February to March and the February North Atlantic Oscillation index (NAOI). Mean annual sea growth during the first 2 years after smolting was higher for S. trutta spending the winter at sea than those wintering in the River Imsa. First year's sea growth was lower for S. trutta descending in spring than autumn. For first-time migrants, it correlated negatively with the February NAOI of the smolt year. Sea survival was higher for spring than autumn descending first-time migratory S. trutta with a maximum in May (14.9%). Number of anadromous S. trutta returning to the river increased linearly with the size of the cohort moving to sea, with no evidence of density-dependent sea mortality. Sea survival of S. trutta smolts moving to sea between January and June correlated positively both with the annual number of Atlantic Salmo salar smolts, the specific growth rate at sea, and time of seaward migration in spring. This is the first study indicating how environmental factors at the time of seaward migration influence the sea survival of S. trutta.

Marine migrations in anadromous brown trout (Salmo trutta). Fjord residency as a possible alternative in the continuum of migration to the open sea

“…Furthermore, a fjord-resident fish recaptured in the fjord after approximately 1.4 years showed a growth of 2.4 cm·month -1 . This growth rate is comparable to the growth of the first-time sea migrants between 1 and 2 years at sea (Jonsson and Jonsson 2009), suggesting normal growth rates for the FR fish.…”

“…This return 21/28 rate is comparable to other studies of salmonids suggesting return rates in the natal rivers from 1 % to 20 % (Hvidsten et al, 2004). This percentage appears adequate to maintain the sea trout population in the river of origin as shown by Jonsson and Jonsson (2009). Furthermore, a fjord-resident fish recaptured in the fjord after approximately 1.4 years showed a growth of 2.4 cm·month -1 .…”

Differential changes in growth patterns of anadromous brown trout and Atlantic salmon from the River Etneelva over a 25-year period

“…Thus, it is most likely that the observed reduction in growth has caused reduced survival and recruitment in anadromous brown trout from the River Etneelva as well. Similar to the present study, Jonsson & Jonsson (2009a) failed to find a correlation between sea temperature and post-smolt growth, which they explained by the ability of anadromous brown trout to partly select its thermal habitat in the fjord. Our data suggest that other factors may override the effect of temperature on growth.…”

“…A reduction in length of about 4 cm after the first summer in the sea may correspond to a reduction in weight of about 100Á200 g or more, equivalent to a weight reduction of about 20Á40%. A long-term study of anadromous brown trout from the Norwegian river Imsa has revealed a positive correlation between sea growth and survival (Jonsson & Jonsson 2009a). Thus, it is most likely that the observed reduction in growth has caused reduced survival and recruitment in anadromous brown trout from the River Etneelva as well.…”

Evidence of salmon lice-induced mortality of anadromous brown trout (Salmo trutta) in the Hardangerfjord, Norway

“…Similar findings on historically high return rates are reported from the River Vardnes on Senja, where the recapture of first-time migrants was 37% Salmon lice induced mortality in anadromous brown trout 283 (Berg & Jonsson 1990). With a sea survival rate of only 0.58Á3.41% for marked or tagged smolts from Guddalselva, the sea survival of trout from this river is extremely low compared with what is recognized as normal marine survival of anadromous brown trout, even when tagging-induced mortality is taken into account (Jonsson & Jonsson 2009). Since the highest return rates for the smolt cohorts from Guddalselva during 2002Á2010 are concurrent with years in which the lowest infection levels of premature, returning trout were recorded in the same area, this suggests a connection between salmon louse infection and marine survival of trout, as has also been reported from the west coast of Scotland (Middlemas et al 2010).…”