Is the duration of the smolt window related to migration distance in coho salmon Oncorhynchus kisutch?

Physiological condition infers habitat choice in juvenile sockeye salmon

The amount of time that juvenile salmon remain in an estuary varies among and within populations, with some individuals passing through their estuary in hours while others remain in the estuary for several months. Underlying differences in individual physiological condition, such as body size, stored energy and osmoregulatory function, could drive individual variation in the selection of estuary habitat. Here we investigated the role of variation in physiological condition on the selection of estuarine and ocean habitat by sockeye salmon (Oncorhynchus nerka) smolts intercepted at the initiation of their 650-km downstream migration from Chilko Lake, Fraser River, British Columbia (BC). Behavioural salinity preference experiments were conducted on unfed smolts held in fresh water at three time intervals during their downstream migration period, representing the stage of migration at lake-exit, and the expected timing for estuary-entry and ocean-entry (0, 1 and 3 weeks after lake-exit, respectively). In general, salinity preference behaviour varied across the three time periods consistent with expected transition from river to estuary to ocean. Further, individual physiological condition did influence habitat choice. Smolt condition factor (K) and energy density were positively correlated with salinity preference behaviour in the estuary and ocean outmigration stages, but not at lake-exit. Our results suggest that smolt physiological condition upon reaching the estuary could influence migratory behaviour and habitat selection. This provides evidence on the temporally dependent interplay of physiology, behaviour and migration in wild juvenile Pacific salmon, with juvenile rearing conditions influencing smolt energetic status, which in turn influences habitat choice during downstream migration. The implication for the conservation of migratory species is that the relative importance of stopover habitats may vary as a function of initial condition.

A temperature shift on the migratory route similarly impairs hypo-osmoregulatory capacities in two strains of Atlantic salmon (Salmo salar L.) smolts

Anthropogenic use of water systems may cause temperature fluctuations between tributaries and large rivers for which physiological population related-effects on osmoregulatory capacity of Atlantic salmon are not well described. We simulated the downstream route in the case of the River Meuse basin to investigate the impact of a 5 °C temperature shift during smoltification on hypo-osmoregulatory capacities of smolts. Three temperature regimes were tested: control temperature-treatment (T1) without temperature shift, early (T2) or late (T3) temperature shift-treatment. Moreover, fish were subjected to seawater challenge during and after the downstream migration peak time. Two allochtonous strains were used: Loire-Allier (LA) and Cong (CG). Without temperature shift (T1), significant differences between the strains were noticed in the peak date and maximum activity of gill Na⁺/K⁺ATPase as well as in plasma sodium and potassium concentrations. For early (T2) and late (T3) temperature shift-treatments, gill Na⁺/K⁺ATPase activity, plasma osmolality and ion concentrations were negatively influenced in both strains. After salinity challenge, the highest osmolality was measured in smolts subjected to the temperature shift. Predictably circulating levels of GH and IGF-1 changed over the smolting period but they did not explain the observed modifications in hypo-osmoregulatory abilities whatever the population. The results show a negative impact of a temperature shift on hypo-osmoregulatory capacities of smolts regardless of population differences in smoltification timing under conditions without temperature shift. The resilience of such physiological impact was sustained at least for 1 week, comforting the role of high temperature in influencing the rate of changes occurring during smoltification. Therefore, favouring the downstream migration to help smolts reach the sea faster may mitigate the impact of a rapid temperature increase.