Smolt Migration Research Articles

Reducing Feed Levels during the Last Month of Rearing Enhances Emigration Rates of Hatchery-Reared Steelhead Smolts

Abstract Downstream migration of hatchery-raised smolts of steelhead Oncorhynchus mykiss maintained on a reduced-feed diet during the last month of rearing was compared with migration of fish fed to satiation (control diet) before release. Five releases were made in two streams over 3 years. Fish on the reduced diet averaged 7 mm shorter and 21 g lighter than control fish at release and had an average condition factor (K = 105 [weight,g]/[length,mm]3) of 0.92, versus 1.03 for control fish. An average of 57.5% of reduced-diet fish were recaptured, compared with 49.4% recapture of control fish; recoveries favored the test group for each release, and the difference was significant for three releases. Fish on the reduced diet generally arrived at traps 1–2 d sooner than control fish. Savings in fish feed costs were also realized with the reduced diet strategy.

The migratory behaviour of wild Atlantic salmon (Salmo salar) smolts in the estuary of the River Conwy, North Wales

Thirty-two wild Atlantic salmon (Salmo salar) smolts, tagged with miniature acoustic transmitters, were tracked in the River Conwy, North Wales, to describe the freshwater and estuarine patterns of migration. Migration in fresh water was predominantly nocturnal, although there was a seasonal change in this pattern with later run fish moving during both the day and night. Smolts tagged earlier in the study spent significantly longer in the river (mean 456 ± 43 h) before migrating into coastal waters than fish tagged later in the study (mean 201 ± 30 h). The movement of smolts through the estuary was indicative of a nocturnal selective ebb tide transport pattern of migration. All of the smolts migrated seawards on an ebb tide close to the surface and within the fastest moving section of the water column. The nocturnal pattern of migration would appear to be the result of an endogenous rhythm of swimming activity that results in the smolts moving up into the water column after dusk and migrating seawards. Smolt migration in the lower portion of the estuary was indicative of active directed swimming and there was no apparent period of acclimation required when moving from fresh to salt water.

Does Temporal Stratification Improve the Accuracy of Mark–Recapture Estimates of Smolt Production? A Case Study Based on the Conne River, Newfoundland

Based on an actual mark–recapture experiment with Atlantic salmon Salmo salar in the Conne River, a method was developed for simulating smolt migration when initial capture and tagging occur upstream and recapture occurs downstream at times that vary with the individual fish. The properties of Darroch's maximum-likelihood estimator of population size under time interval stratification were explored by Monte Carlo methods. In particular, the effects of population size and the width of the stratum interval on the bias and precision of the estimate of population size were examined under models of constant and variable probability of recapture. The viability of the asymptotic approximation of the variance of the estimate of population size was likewise examined. In addition to temporal strata, the case of no stratification was considered as well as a new estimator based on day-of-departure–day-of-recapture strata, from which daily estimates of the numbers leaving the upper site can be obtained. The study suggests that there is little advantage to be gained from temporal stratification for the situations considered. Standard errors computed from the asymptotic formula appear adequate except for small populations (and, thus, small numbers of recaptures), for very short stratification intervals, or when there is a variable probability of recapture that does not steadily increase or decrease, The study also confirms that previous standard error estimates of 3–4% for this population of approximately 70,000 fish are realistic. Although developed for a specific case, the methodology is modular and can be readily adapted to other scenarios.

Simulation of juvenile sockeye salmon (Oncorhynchus nerka) migrations in the Strait of Georgia, British Columbia

ABSTRACTPrevious research has documented two main migratory routes of juvenile sockeye salmon (Oncorhynchus nerka) through the Strait of Georgia, British Columbia, Canada, and large interannual variability in marine survival rates of the Chilko Lake stock. Simulation models were used to explore the influence of surface currents on the migratory route of juvenile sockeye salmon (smolts) through the Strait of Georgia. We used a model of downstream migration to generate daily numbers of Chilko Lake sockeye salmon smolts entering the Strait of Georgia, based on daily counts of smolts leaving the rearing lake. A numerical hydrodynamic model (driven by surface wind, tide, and Fraser River discharge) hindcasted surface currents in the Strait of Georgia on a 2 km × 2 km grid. A smolt migration model simulated fish moving through the Strait with different compass‐oriented migratory behaviours (i.e. swimming speed and directional orientation) within the time‐varying surface advection field. Results showed that surface currents within the Strait of Georgia can affect the migratory route of sockeye salmon smolts in spite of their large size (8 cm). Wind is the forcing mechanism primarily responsible for determining which migratory route would be used. Under prevailing wind conditions (i.e. toward the north‐west), most sockeye salmon smolts would use the eastern migratory route; however, relatively brief south‐eastward wind events (lasting about 2 days) would force most smolts into the western migratory route. Given the heterogeneity of food for salmon within the Strait, we hypothesize that wind‐driven variability in the annual proportion of smolts that use the western and eastern migratory routes in the Strait of Georgia affects early marine survival rates of Fraser River sockeye salmon.

Period of downstream migration of sea trout (Salmo trutta L.) smolt grown in Gowienica River

The period and intensification of downstream migration of sea trout smolts (Salmo trutta L.) grown in natural condition in the drainage basin of (Gowienica River, being a part of lower Odra system (in 5 consecutive years: 1980-1984), as well as water temperature in the lower section of Gowienica in this period is demonstrated in the present paper.

Downstream migration of immature salmon ( Salmo salar) smolts blocked by implantation of the androgen 11-ketoandrostenedione

The effect of androgen treatment on individual downstream migratory activity of 2-year-old Baltic salmon ( Salmo salar) smolts was studied in a circular stream channel during the natural smolt migration period. Fish were tagged with passive integrated transponder (PIT) tags and implanted with silastic capsules containing the androgen 11-ketoandrostenedione or empty capsules. 11-ketoandrostenedione gives increased blood plasma levels of 11-ketotestosterone through peripheral conversion. The migratory activity of individual fish was monitored automatically by means of a PIT-tag system. In androgen-treated fish, the number of downstream passages per day during peak migration was reduced to about 10% of that of control fish. It is concluded that androgens seem to prevent downstream migration, the development of hypo-osmoregulatory ability, silvering, and the decrease in condition factor associated with smolting. Rather, androgens seem to promote retention of freshwater adaptations and increased growth in weight during spring.

Predation and dietary habits as possible explanations for poor return rates of stocked sea trout from the river Aurland

The river Aurland was earlier known to be the best Norwegian river for sea trout fishing. Today, after regulation for hydroelectric purposes, the catch has fallen dramatically despite releases of hatcheryreared sea trout smolts and a remaining natural production. A field experiment in May–June 1991 indicated heavy predation on smolts leaving the river. In the river Aurland the water discharge during the smolt migration period is greatly reduced as compared to the situation prior to regulations. Other authors have shown that large water discharge during smolt migration correlates with a high return rate of adult salmon, presumably because of reduced predation pressure on the smolts. Results are also presented indicating poor feeding success of reared smolts compared to wild smolts, which may affect marine survival.

A model for the processes regulating recruitment for a sockeye salmon stock

Recruitment relationships are a central issue in the management of sockeye salmon ( Oncorhynchus nerka ) but when these relationships are fitted, they have almost never been statistically significant and their underlying mechanisms have never been demonstrated. In contrast, the model presented here is significant at least at the α = 0.05 level and is based on mechanisms which have been described in the literature. The Chilko Lake stocks (Fraser River system, British Columbia) exhibit resource limitation during fry migration and a Ricker-type compensatory relationship due to density-dependent growth and subsequent size-dependent mortality. Modeling resource limitation and Ricker's curve accounts for 79.5% of the total variation in the number of smolts emigrating from the nursery lake and the number of emigrating smolts accounts for 71.0% of the total variation in the number of adults recruiting to the fishable population. Depensatory mortality during smolt migration together with weak compensation account for an additional 9.1% of the variation. Thus, 80.1% of the total variation is accounted for. The remaining 19.9% is probably mostly measurement error.

A diffusion model for the downstream migration of sockeye salmon smolts

Abstract The smolt migration (the migration from their nursery lake to the sea) of sockeye salmon ( Oncorhynchus nerka ) is modeled as one-dimensional diffusion with drift. The average downstream velocity of smolts is assumed proportional to the water velocity and water velocity is modeled based on Manning's roughness formula. This leads to the approximation that the expected transit time for a smolt is proportional to the minus two-fifths power of the river's discharge. The smolts disperse as they move downstream and their spatial positions at a given time after emigration are modeled as normally distributed with the variance proportional to the time after emigration. Mark-recapture data on smolts from Chilko Lake (Fraser River system, British Columbia) are fitted to this model by maximizing the conditional likelihood. Smolts from Chilko Lake emigrate daily at dusk during three distinct emigration events of a few days each between late April and mid May. By the time these smolts reach the mouth of the Fraser Estuary, the fitted model predicts that smolts from these three emigration events will have merged, so that their arrival times form a relatively smooth curve spanning five or six weeks.

Increased Recapture Rate of Adult Atlantic Salmon Released as Smolts into Large Shoals of Wild Smolts in the River Orkla, Norway

Abstract Carlin-tagged, hatchery-reared smolts of Atlantic salmon Salmo salar were released in the regulated River Orkla at the same time of year and same site from 1984 through 1988. Recapture rates of adult Atlantic salmon varied considerably (from 0.7 to 6.8% of number of smolts released). Survival of released hatchery-reared smolts increased when they were released into large shoals of migrating wild smolts. Wild Atlantic salmon smolts generally aggregate into large schools when migrating out of a river, although the size of schools varies between years. When the hatchery-reared smolts were stocked within lesser shoals, survival was lower than in years with larger shoals of wild smolts. The magnitude of water discharge, as well as the direction of change in the magnitude, greatly influences the initiation of smolt migration and shoal formation in the Orkla. Artificial manipulation of water discharges throughout May can influence formation of large shoals of wild smolts and is a means of achieving opti...

Monitoring of the in-river migration of smolts from two groups of spring chinook salmon, Oncorhynchus tshawytscha (Walbaum), with different profiles of Renibacterium salmoninarum infection

Broodstock segregation based on the measurement of maternal Renibacterium salmoninarum infection levels by the enzyme-linked immunosorbent assay (ELISA) and the membrane filtration-fluorescent antibody technique (MF-FAT) was previously shown to affect the prevalence and levels of bacterial kidney disease (BKD) in progeny of chinook salmon, Oncorhynchus tshawytscha (Walbaum), during hatchery rearing. Subgroups of fish from that study were marked with passive integrated transponder (PIT) tags, and monitored by PIT-tag detectors during the first 342km of their migration to the Pacific Ocean. Differences in the recovery of tagged fish were significant (P≤ 0·01) at each detection point and became more pronounced as the fish moved downstream. Cumulative recoveries of fish from the low-BKD group and the high-BKD group, respectively, were 31% and 28% after 116km, 44% and 37% after 176km, and 51% and 42% after 342km. There were no apparent differences in the migration timing of the two groups to the first detection point. The data suggested that in-river survival was higher in the progeny group from parents that had low R. salmoninarum infection levels or tested negative for R. salmoninarum (low-BKD group) than in the group female parents with high infection levels (high-BKD group).

Optimal Time and Size for Smolt Migration in Wild Sea Trout (Salmo trutta)

A tagging programme, in which wild sea trout (Salmo trutta) were marked at the parr stage with internal tags in winter and recaptured during smolt migration in a trap in the following spring, was conducted for three seasons in a small stream in southwestern Sweden, individuals that were initially smaller migrated later, grew more in length, and were smaller at migration than those initially bigger. Body length at migration decreased with migration time during the season for tagged as well as nontagged trout. A model for optimal time of and size at migration is proposed, based on the assumption that the expression (body length at the end of the season) × (survival over the season) is maximized and that migratory and postmigratory survival is positively size dependent. The qualitative predictions on the relationship between initial size and time of and size at migration are supported by the tagging data. Pre- and postmigratory growth rates are predicted to have opposite effects on migration time, so the migration timing is expected to depend strongly on the degree of dependence between these.

Feeding behaviour and diet of goosanders(Mergus merganser)in relation to salmonid seaward migration

Foraging activities of goosanders (Anatidae, Mergus merganser) were studied in relation to seaward migration of smolt in the River Halselva in northern Norway, 1987-1989. Birds aggregated in the estuary in June, during mass migration of smolts. When present in the estuary males and females spent 28.7 and 35.6% of their time feeding. Only 15% of dives resulted in birds' bringing prey to the surface and about 25% of prey found in goosander stomachs were salmonids. For hatchery-reared smolts released in the estuary, average length taken did not differ from that available; for wild smolts of Arctic char, however, smaller fishes than those available were taken. For 1989, we estimate that goosanders took 1% of the hatchery-reared smolts released in the estuary, and 2% of the wild smolts when smolts passed the estuary during seaward migration. As it is likely that less fit smolts (e.g. sick, injured, small) are most prone to predation, we argue that such a level of predation on migrating smolts from goosanders has only a minor impact on salmonid production. Suggestions to reduce predation are discussed.

Migration of hatchery reared juvenile Atlantic salmon, Salmo salar L., down a release ladder. 2. Effect of fish developmental strategy on speed and pattern of movement

The behaviour of different types of Atlantic salmon smolt migrating down a release ladder were compared. Immature S2 smolts were faster through a release ladder, and their journey times less variable than S1 and mature male S2 fish. Journey time variability was less among stripped than among unstripped mature male smolts. The mean residence time of passive objects (oranges) in each pool of the ladder was correlated with pool surface area, whilst that of fish was correlated with pool volume. The general pattern of movement of smolts through the ladder was consistent with the hypothesis of non-directed passive displacement of fish near the water surface. In fact, fish spent most of their time nearer the bottom of the pools with the result that their migration speed was slower than that of passive objects. Pools towards the top of the ladder had the highest volume and surface area and therefore offered the greatest hindrance to passive displacement. Immature S2 smolts may have been more active in seeking a passage through this barrier. In 1986 and 1987 no significant change in ladder travel time or fish condition factor was detected over the period May to July. In 1986 fish condition factors and variance in ladder travel time increased significantly in August. Immature S2 smolt migration may have been less nocturnal. Mature male smolts were affected more by high daytime water temperatures than immature S2 and S1 smolts.

Migration of hatchery reared juvenile Atlantic salmon, Salmo salar L., smolts down a release ladder. 1. Environmental effects on migratory activity

The effects of light intensity, water temperature and river spate conditions on the rate of migration of hatchery‐reared Atlantic salmon smolts down a release ladder were examined. Low light intensity and high day time water temperatures raised smolt migration rates: water temperature had littleor no effect at night. Thediel patternofdown‐ladder movement wasdetermined by these two environmental stimuli. Smolts demonstrated a threshold response to both light and water temperature: a fall in light intensity, or increase in water temperature, below or above their respective thresholds elicited no further response. As fish progressed through the ladder their movement became increasingly nocturnal, and most entry into the estuary occurred at night. Heavy rainhll caused rapid migration of smolts through the ladder. As ladder discharge remained constant, smolts must have been responding to some other change in the water conditions. Smolts responded to spate conditions irrespective of water temperature and light intensity, and they responded to light intensity irrespective of water temperature. A hierarchy of environmental cues, responsible for triggering migratory behaviour on a day to day basis during the smolt run, of spate> light intensity > water temperature is therefore suggested.

Bimodal size distributions in wild juvenile Atlantic salmon populations and their relationship with age at smolt migration

A bimodal length distribution developed in an Atlantic salmon population planted as start‐fed fry in a Norwegian stream. The earliest fish to smolt were from the upper modal group.

Evaluation of Carlin tagging as a mark-recapture technique for estimating total river runs of salmon smolts, Salmo salar L.

. Mark-recapture experiments using Carlin tags on migrating wild smolts in the River Bush indicated that 16·4% were not recovered under total trapping conditions. The experimental procedure was also found to delay the migration of the smolts by varying amounts. The implications for the use of this technique as a means of assessing both smott run sizes in partially trapped rivers and smolt migration speeds are discussed.

Predictability in a small commercial Atlantic salmon fishery in western Newfoundland

This study examined the dynamics of a small Atlantic salmon ( Salmo salar L.) stock in relation to its home water commercial fishery on the west coast of Newfoundland, 1981–1988. Catch rates in the fishery were estimated from daily logbooks. Counts of adults and smolts were taken at a counting fence in the river. Smolt migrations were used to predict stock abundance. Season and year effects of catch rates and run timing were compared between the fishery and the river using a multiplicative model. There was no significant annual variation in run timing for fishery or river, although the median date in the fishery occurred ∼ 2 weeks earlier than in the river. Catchability in the fishery was constant over a 2-fold change in stock abundance. Standardized return rates of adults were a parabolic function of smolt abundance and were used to estimate optimum smolt yield for the river. The commercial fishery selected significantly larger salmon, leaving only small virgin grilse to spawn.

Does juvenile experience affect migration and spawning of adult Atlantic salmon?

This paper documents differences in seasonal time of river ascent and descent, and instream behavior of adult wild and sea-ranched Atlantic salmon (Salmo salar) of the Norwegian River Imsa stock during the period 1981–1989. Wild fish use River Imsa as a nursery, and at an age of 2 years most of them migrate to the sea as smolts. The sea ranched fish are hatchery reared offspring of the River Imsa stock and are released as smolts at the mouth of the river. They are thus deprived of juvenile river life and a downstream smolt migration. Wild and sea ranched salmon feed for 1 or more years in the Norwegian Sea before homing as spawners. Both groups returned simultaneously to coastal Norway, but sea ranched fish ascended the river later and descended sooner after spawning than wild fish. All wild females and almost all wild males (96.2%) spawned in the river, whereas 13.5% and 36.7%, respectively, of the mature sea-ranched females and males left the river unspawned. The annual number, but not the proportion, of unspawned fish increased with increasing density of adult salmon in the river. Unspawned females were medium sized and small (45–70 cm); unspawned males were medium sized and large (50–90 cm). Independent of the density of spawners in the river, sea ranched fish moved up- and downstream the river more often than wild fish. More than 20% of the sea-ranched salmon and less than 1% of the wild salmon passed a trap 100 m above the river outlet more than once in each direction during the same spawning reason. Moreover, sea-ranched salmon were about twice as often seriously injured during spawning as wild fish. Lack of juvenile experience from the river may be the main reason for the behavioral differences between sea-ranched and wild fish.

Salmon ranching experiments in the River Imsa: Effect of timing of Atlantic salmon ( Salmo salar) smolt migration on survival to adults

We tested the hypothesis that survival of Atlantic salmon adapted to seawater in the spring does not change with time of release. This was done through successive releases of tagged smolts and post-smolts in the estuary of the River Imsa and at a sea locality 4 km outside the Research Station for Freshwater Fish at Ims, S.W. Norway, where the fish were reared. Based on tag returns it was demonstrated that survival was highest for fish released in the spring, at the same time natural smolts leave the River Imsa. In autumn and winter the survival to adults was poor. Our results suggest that there is a window of migration which gradually closes in late summer. We hypothesise that behaviour of the fish will change with time of release and that this will result in heavy predation at times other than spring.