Sea trout in North Argyll Sea lochs, population, distribution and movements

Sea trout were sampled by shore seining in the sea lochs of the west coast of Scotland between 1970 and 1974, This area is of special interest because of the indentation of the coastline and the varying degree of freshwater and marine influence in the sea lochs. The majority of fish caught were in their first year after smolt migration. These post-smolts were caught mainly in May and June, after which the number of trout present was low until fish at the 'whirling' stage appeared in the catches at the end of August, continuing through to the following spring. An additional recruitment of mainly unsilvered young trout from the rivers to the sea lochs was found in the autumn. Age and sex composition of both spring and autumn recruits, and of mature fish, were investigated and compared.
A total of 3228 sea trout were tagged, with 311 recaptures. These recaptures supported the evidence from smolt trapping and beach seine catches that, in the first post-migration year, the smolts migrate from the rivers from late March to early May, they then migrate from the sea lochs in May and June and return in late summer and autumn.     

Effect of origin, sex and sea age of Atlantic salmon on their recapture rate after river ascent

The recapture rate of Atlantic salmon (Salmo salar L.) after river ascent was examined by the trapping and tagging of ascending spawners in the lower reaches of the Simojoki River, which flows into the northern Baltic Sea. In 1997 and 1998, altogether 825 Carlin‐tagged salmon were released to continue their upstream migration. Of these, 800 could be sexed and categorized as reared (91%) or wild (9%) salmon. In 1997, most of the ascending salmon were multi‐sea‐winter (MSW) fish, whereas in 1998 almost all were one‐sea‐winter (1SW) male grilse due to the late trapping season. About 10% of all tagged fish were recaptured, two‐thirds of which were caught in the river before their descent to the sea. There was no difference in the recapture rate between salmon of wild (8.5%) or reared (9.5%) origin, or between females (11.6%) and males (9.3%). Generalized linear models for data from 1997 showed that the recapture rate increased with length and age of females, but that the opposite was true for males. River fishing did not seem to remove proportionally more early ascending salmon than fish that ascended later.     

The influence of precocious sexual maturation on survival to adulthood of river stocked Baltic salmon, Salmo salar, smolts

During three consequtive years, 1975–1977, Individually tagged Baltic salmon Salmo salar smolts of sexually immature male and female fish (n = 35027, mean size: 15.2 cm) and precocious males (n = 6518, mean size: 14.2 cm) were released into Umeälven (Ume river), northern Sweden. Rate of survival (% captured adults) based on 3714 recoveries was significantly higher (p < 0.01) for smolts from immature fish (10.2%) than those from smolts of early maturing males, i.e. precocious males (2.2%). corresponding to an average yield of 474 and 85 kg per KHX) smolts released, respectively. Gain in survival was on average 2.5% and 1.4% per cm increase in smolt size for immature smolts and smolts from precocious males, respectively. The poor survival among smolts of precocious males is suggested to he related to an interaction between sexual maturation and smolting linked to incompletely resorbed gonads leading to a non migratory behaviour. These non migratory males are then suggested to suffer heavily by predation in the river.
The two smolt categories had a similar growth pattern in sea. Smolts from precocious males did not mature early in sea indicating no relation to grisling, i.e. sexually maturing fish returning after first winter in sea. Adult weight of fish returning the fourth summer after release was related to smolt size (P < 0.05). Our Response Surface Model (RSA) predicted that large smolts (19.0 cm) had a higher specific growth rate over their life-span compared to small smolts (<15.0 cm), 0.86% d⁻¹ and 0.46% d⁻¹, respectively. Large smolts (19.0 cm) attained a size of 3.0 kg during their second winter in sea about six months earlier than small smolts (13.0 cm). The paper discusses alternative release strategies that can be employed if the ultimate goal of salmon stocking is maximizing yield.     

Net ground speed of downstream migrating radio-tagged Atlantic salmon (Salmo salar L.) and brown trout (Salmo trutta L.) smolts in relation to environmental factors

The downstream migration of Atlantic salmon (Salmo salarL.) and sea trout smolt (S. trutta L.) was investigated using radio telemetry in the spring of 1999 and 2000. Forty wild sea trout smolts, 20 F1 sea trout smolts, 20 hatchery salmon smolts and 20 salmon smolts from river stockings were radio tagged and released in the Danish River Lilleaa. The downstream migration of the different groups of fish was monitored by manual tracking and by three automatic listening stations. The downstream migration of radio tagged smolts of both species occurred concurrently with their untagged counterparts. The diel migration pattern of the radio tagged smolts was predominantly nocturnal in both species. Wild sea trout smolt migrated significantly faster than both the F1 trout and the introduced salmon. There was no correlation between net ground speed, gill Na⁺,K⁺-ATPase activity or fish length in any of the different groups. The migration speed of wild sea trout smolts was positively correlated with water discharge in both years. In F1 sea trout smolts, migration speed was positively correlated with temperature in 1999. The migration speed of salmon smolts did not correlate to any of the investigated parameters.     

Seaward migrating Atlantic salmon smolts with low levels of gill Na+, K+ -ATPase activity; is sea entry delayed?

Two groups of migrating wild Atlantic salmon (Salmo salar) smolts caught within a 1 week interval in the River Alta, northern Norway, were tagged with acoustic transmitters and measured for gill Na⁺, K⁺ -ATPase activity in order to compare their smolt status with timing of sea entry. The first group of smolts had low levels of gill Na⁺, K⁺ -ATPase activity and resided in the lower part of the river twice as long as the second group that had high levels of gill Na⁺, K⁺ -ATPase activity. This indicates that early migrating smolts may not be completely physiologically adapted for salt water and delay their sea entry, thereby also synchronizing their seaward migration with the later migrating smolts.     

Does increased abundance of sea lice influence survival of wild Atlantic salmon post‐smolt?

A synthesis of results from two projects was assessed to analyse possible influence of sea lice Lepeophtheirus salmonis on marine Atlantic salmon Salmo salar survival. During the years 1992–2004, trawling for wild migrating post-smolts was performed in Trondheimsfjord, a fjord in which no Atlantic salmon aquaculture activity is permitted. Prevalence and intensity of sea lice infections on migrating wild post-smolts differed between years. A correlation analysis between 1 sea-winter (SW) Atlantic salmon catch statistics from the River Orkla (a Trondheimsfjord river) and sea lice infections on the migrating smolts in the Trondheimsfjord was not significant. Up to 2% reduction in adult returns due to sea-lice infection was expected. In addition, experimental releases from 1996 to 1998 with individually tagged groups of hatchery-reared Atlantic salmon smolts given protection against sea-lice infection was performed. Higher recaptures of adult Atlantic salmon from 1998 treated smolts compared to the control group may correspond to high abundance of sea lice found on the wild smolt, and may indicate influence on post-smolt mortality. These studies indicate that post-smolt mortality in Trondheimsfjord is marginally influenced by sea lice infection; however, the methods for assessing wild smolt mortality might be insufficient. Higher infections of sea lice farther out in the fjord may indicate more loss in Atlantic salmon returns in some years.     

Sea growth, smolt age and age at sexual maturation in Atlantic salmon

Relationships between growth at sea, smolt size and age at sexual maturation of Atlantic salmon Salmo salar were tested. The fish were offspring of brood stocks sampled in eight Norwegian rivers at latitudes between 59° and 70° N, hatchery reared and released at smolting at the mouth of the River Imsa (59° N). Smolt size influenced the subsequent growth rate of Atlantic salmon. The larger the fish were at release, the slower the yearly length increment at sea. Mean sea age at sexual maturity, measured as proportion of the returning adults attaining sexual maturity at sea age 2 years, was significantly correlated with mean growth rate during the first year at sea and mean smolt size ( r ²= 0·74, P < 0·001). Fish attaining maturity at a relatively high sea age were more fast growing during their first year at sea than those maturing at a younger age. The results indicate that high sea age at sexual maturation is a population-specific characteristic and associated with high early growth rate at sea.     

Population and size-specific distribution of Atlantic salmon Salmo salar in the Baltic Sea over five decades

Population-specific assessment and management of anadromous fish at sea requires detailed information about the distribution at sea over ontogeny for each population. However, despite a long history of mixed-stock sea fisheries on Atlantic salmon, Salmo salar, migration studies showing that some salmon populations feed in different regions of the Baltic Sea and variation in dynamics occurs among populations feeding in the Baltic Sea, such information is often lacking. Also, current assessment of Baltic salmon assumes equal distribution at sea and therefore equal responses to changes in off-shore sea fisheries. Here, we test for differences in distribution at sea among and within ten Atlantic salmon Salmo salar populations originating from ten river-specific hatcheries along the Swedish Baltic Sea coast, using individual data from >125,000 tagged salmon, recaptured over five decades. We show strong population and size-specific differences in distribution at sea, varying between year classes and between individuals within year classes. This suggests that Atlantic salmon in the Baltic Sea experience great variation in environmental conditions and exploitation rates over ontogeny depending on origin and that current assessment assumptions about equal exploitation rates in the offshore fisheries and a shared environment at sea are not valid. Thus, our results provide additional arguments and necessary information for implementing population-specific management of salmon, also when targeting life stages at sea.     

Increasing temperature associated with increasing grilse proportion and smaller grilse size of Atlantic salmon

Effects of temperature on Atlantic salmon (Salmo salar) were analysed using Carlin tag recovery data (1985–2014), and mixed-stock catch data (smolt years from 2001 to 2012) in northern parts of the Baltic Sea. During warmer summers, the mean smolt length of the recaptured salmon tended to be smaller, and salmon were recaptured more frequently in feeding grounds closer to the home rivers in the Gulf of Bothnia, while colder summers were associated with more recaptures further south, in the Baltic Main Basin. Moreover, a warmer spring in the smolt year was associated with decreased weight of male grilses in mixed stock data. Further, warmer spring temperatures during the smolt year were associated with a higher proportion of one-sea-winter (1SW) males during the return migration in mixed stock data. These results suggest that the increasing global temperature may affect Atlantic salmon life history demographics.     

Divergence in smolt production from the stocking of 1-summer-old and 1-year-old Atlantic salmon parr in a northern Baltic river

One‐year‐old and 1‐summer‐old salmon parr have been stocked in the Simojoki, a northern Baltic river, to protect the natural salmon stock from extinction. This enhancement practice was studied during a 5‐year stocking period, when an average of 14.3 1‐summer‐old parr and 5.4 1‐year‐old parr was needed to produce one sea running smolt. The mean yield from stocking was 70 smolts per 1000 stocked 1‐summer‐old parr and 186 smolts per 1000 stocked 1‐year‐old parr. The mean cost of smolts produced by stocking 1‐year‐old and 1‐summer‐old parr was 1.2 and 2.6 times the cost of producing one 50 g smolt in the hatchery, respectively. There was a negative relationship between density of wild parr and length of smolts stocked as 1‐summer‐old parr in the river, but no relationship between wild parr density and length of smolts stocked as 1‐year‐old parr. This indicated that 1‐summer‐old parr were more susceptible than the older parr to competition with wild parr. Considering production costs and smolt yield, stocking of 1‐year‐old parr appeared to be more profitable than that of 1‐summer‐old parr in the enhancement of the endangered salmon stock.     

Association between environmental factors, smolt size and the survival of wild and reared Atlantic salmon from the Simojoki River in the Baltic Sea

The survival of Atlantic salmon Salmo salar in the Baltic Sea was examined in relation to smolt traits (length and origin) and annual environmental factors [sea surface temperature (SST) and seasonal North Atlantic Oscillation (NAO) index], and prey fish abundance (herring Clupea harengus and sprat Sprattus sprattus) in the main basin and the southern Gulf of Bothnia. The study was based on recapture data for Carlin‐tagged hatchery‐reared and wild smolts from the Simojoki, a river flowing into the northern Gulf of Bothnia. The survival of the wild and reared groups was analysed using an ANOVA model and a stepwise regression model, with the arcsin‐transformed proportion of recaptured fish as the response variable. The results demonstrated a combined influence of smolt traits and environmental factors on survival. For the reared Atlantic salmon released in 1986–1998 (28 groups), the increasing annual mean SST in July in the southern Gulf of Bothnia and increasing mean smolt size improved survival. If the SST in July was excluded from the model, the NAO index in May to July also had a positive effect on survival (P < 0·10). The log₁₀‐transformed abundance of 0+ year herring in the southern Gulf of Bothnia entered the model (P < 0·15) if the SST and NAO index were excluded. For the wild Atlantic salmon released in 1972–1993 (21 groups), only the increasing SST in July showed a significant association with improved survival (P = 0·004). Prey fish abundance in the main basin of the Baltic Sea had no influence on the survival of reared or wild smolt groups. The interaction between smolt size and the SST in July was not significant. The origin was a better, but not a significant, predictor of marine survival compared to the smolt size or the SST in July. The mean recapture rate of the wild groups was twice that of the reared groups in the whole data. The results suggest that cold summers in the Gulf of Bothnia reduce the survival of young Atlantic salmon in both wild and reared groups. The larger smolt size of the reared groups compared with the wild groups to some extent compensated for their lower ability to live in the wild.     

Temporal variation in abundance, return rate and life histories of previously spawned Atlantic salmon in a large subarctic river

The 30 year time series analyses revealed large temporal variation in the return rates and a recent increase in abundance of previous spawning Atlantic salmon Salmo salar in the River Teno, northern Scandinavia. The mean proportion of repeat spawners was 7 and 4% in the total Atlantic salmon catch and 9 and 22% in multi‐sea‐winter (MSW) catch component for females and males, respectively. Previous spawners constituted on the average 7% of the catch in mass but up to 20%(31 t) and 30%(19 t) in 2003 and in 2004, respectively. In 1975–2000, the proportion of previous spawners varied between 1 and 6%(3–12% of MSW Atlantic salmon), whereas in 2001–2004, they accounted for 8–21%(16–35% of MSW Atlantic salmon) of the total Atlantic salmon catch. The number of previous spawners in the catch correlated significantly with the preceding numbers of respective 1–3 sea‐winter (SW) maiden Atlantic salmon 2 years earlier. The recent increase in the numbers of 1S1 and 2S1 (1 or 2 years at sea followed by first spawning and 1 year reconditioning period at sea) alternate spawning Atlantic salmon was a consequence of higher numbers of maiden 1SW and 2SW Atlantic salmon in the catches and increased sea temperatures. Similarly, the return rate of 1SW Atlantic salmon to second spawning has improved in recent years. Most previous spawners ascended and were captured early in the fishing season. The smolt and sea‐age combinations of repeat spawners comprised 68 age groups contributing with the annual mean of 15 age groups to the great diversity of the River Teno Atlantic salmon population complex.     

Production of salmon and trout in a stream in Scotland

For three years the population size, rates of growth, standing stock, production and yield of all year classes of salmon and trout within three sections of a stream in Scotland were studied. Total salmon production as fresh weight per m² was 6.5 g in 1966, 10.6 g in 1967 and 11.1 g in 1968, and total trout production was 10.3g in 1966, 12.3 g in 1967 and 7.7g in 1968. Fish of 0+ and 1+ year old provided usually more than 90% of the total annual salmon production and 80 % of the annual trout production. Yield of salmon smolts (about 9 cm or longer after 2 years growth) per m² was 0.10 in 1966, 0.22 in 1967 and 0.15 in 1968. The smolt yield by weight was 29 % of the production of the 1966 year class of salmon and 16% of the 1967 year class. Numbers of trout of 9 cm or longer produced in the same time were higher and their weight was 60% of the total production of the 1966 year class and 32% of the 1967 year class.     

Age, sex ratio and timing of the catch of kelts and ascending Atlantic salmon in the subarctic River Teno

By 15 June, 82% of the catch of Atlantic salmon Salmo salar kelts had been taken from the middle part of River Teno, northern Scandinavia. The median date of capture was 4 June for males and 8 June for females. Salmon of 1–4 sea–winters (SW) of both sexes survived spawning to return to sea as kelts. Among males, 1 SW kelts were caught earliest in the spring and 3 SW latest, but among females 4 SW were earliest, then 3 SW and finally 1 and 2 SW. There were 17 river and sea–age combinations among the kelts compared with 23 among the ascending salmon. The smolt age distribution and the mean smolt age differed significantly only between female 2 SW ascending salmon (3·97 years) and kelts (4·14 years). The proportion of 1 SW females was higher and that of 3 SW males lower among kelts than among ascending salmon. The proportion of males among 1 SW ascending salmon was 80% but among kelts only 57%. Similarly, the proportion of males among 3 SW fish was 21% for ascending salmon but only 7% for kelts. Hence overwinter mortality was higher among males. Male and female kelts of 1 and female kelts of 2 SWhad a greater mean length than ascending salmon in corresponding groups indicating a better survival of larger fish within an age group. Grilse ascend rivers after most kelts have left, but the main catch of ascending 2–3 SW salmon takes place concurrently with kelts leaving the river, inadvertently targeting kelts in the fishery.     

Relationships between smolt size, postsmolt growth and sea age at maturity in Atlantic salmon ranched in the Baltic Sea

Tagging data were used to examine the relationships between smolt size, post-smolt growth and sea age at first maturity for the short-migrating Neva strain of Atlantic salmon ( Salmo salar L.) ranched in the Bothnian Sea and the Gulf of Finland. The results provided evidence that post-smolt growth was influenced by both relative and absolute smolt size. For both sea-areas, 2-year smolts of small relative size within a release group grew more rapidly in the sea than did smolts of higher relative, but equivalent absolute size. The negative influence of increasing relative smolt size on marine growth was, however, outweighed by the stronger positive influence of increasing absolute smolt size. A 160-mm increase in smolt size (140–300 mm) resulted in an overall growth advantage of about 1 year. In the Bothnian Sea, the predicted mean length after 1 year in the sea was 288 ± 25 mm for 140-mm smolts and 560 ± 16 mm for 300-mm smolts. Under the more favourable conditions of the Gulf of Finland, the respective mean lengths were 369 ± 15 mm and 613 ± 12 mm. The sea age at first maturity was inversely related to both freshwater and marine growth rates. For both sea areas, large smolts yielded proportionately more grilse than did small ones. Smolt years with good post-smolt growth rates yielded more grilse than did years with poor growth rates. The overall level of grilsing was higher in the Gulf of Finland than in the Bothnian Sea. These results suggest that the relationships between smolt size, post-smolt growth and age at first maturity in the sea are influenced by the environmental conditions of the respective sea area. A framework explaining the links between smolt size, marine growth, survival and sea age at maturity in Neva salmon is presented for the Gulf of Finland and the Bothnian Sea.     

Effect of maternal and paternal line on spatial and temporal marine distribution in Atlantic salmon

We examined the inheritance of the sea migration pattern of Atlantic salmon, Salmo salar, in a crossing and tagging experiment in the Baltic Sea. Individuals from the parental stocks, Neva and Iijoki, and their reciprocal hybrids were released as 2-year-old smolts, into the same estuary of the Bothnian Sea in 1994. Two thousand smolts from each of the four groups were marked with Carlin tags. The recapture rate of the tags was nearly 10%. We used log-linear models to analyse the marine distribution of the salmon groups from the tag recovery data. The pure stocks and their pooled hybrid groups all showed statistically significant differences between each other in spatial and temporal sea distribution. The Iijoki salmon were more frequently (9%) caught outside the Bothnian Sea than were the Neva salmon (2%). The majority of the Iijoki salmon (55%), but fewer Neva salmon (40%), were caught in the second sea year. In spatial distribution, the hybrids seemed to be intermediate between the parental stocks, with no differences between reciprocal female and male lines. In duration of sea migration and age at maturity, however, the hybrids were very similar to their maternal line, the effect of which was thus clearly stronger than that of the paternal line. Copyright 2000 The Association for the Study of Animal Behaviour.     

The effect of sexual maturation on the spatial distribution of Baltic salmon

This study examines the migratory habits of tagged mature and immature Atlantic salmon parr Salmo salar released from the Norrfors hatchery on the River Umeälven, Sweden from 1975 to 1977 and 1988 to 1990. Tags were recaptured in the Baltic Main Basin, the Gulf of Bothnia, and in the Umeälven. Ninety-three per cent of previously mature males were recovered in the Umeälven compared with 23% of the previously immature smolts during the calendar year of release. In the second year after release (grilse year), the proportion of early maturing males recovered in the Umeälven was significantly greater than the proportion of previously immature smolts recovered in the Umeälven. Likewise, the proportion of previously mature males recovered in the Main Basin was significantly less than the proportion of previously immature smolts recovered in the Main Basin in the second calendar year after release. Previously mature males rematured after fewer years at sea, on average, than the previously immature smolts. Following the second calendar year after release, the proportions of previously mature males and previously immature smolts were not significantly different throughout the Baltic Sea.     

Changes in smolt traits of Atlantic salmon (Salmo salar Linnaeus, 1758) and linkages to parr density and water temperature

Smolt traits (length, age) and timing of smolt migration of wild Atlantic salmon, Salmo salar L., were investigated in the Simojoki River, northern Baltic Sea. The aim was to determine whether they responded to changes in parr length, parr density and temperature from 2000 to 2014. Annual electrofishing surveys and smolt numbers determined parr densities by springtime trapping in the river mouth. During the smolt trapping period captured parr and smolts were aged from scales. Water temperature was measured daily. Mean length decreased from 137 mm (TL) to 129 mm among 2‐year‐old smolts, and from 150 mm to 139 mm among 3‐year‐olds. Median date of the smolt migration was 10 days earlier, from early June to late May during the study period, linked to the rise in air temperature in May at the nearby Kemi‐Tornio airport. However, the median day temperature and the mean daily water temperatures during the second (Q₂) and third (Q₃) migration quartiles did not change. This implied that migration began when a suitable water temperature was reached, independent of the date.     

Influence of sea temperature and initial marine feeding on survival of Atlantic salmon Salmo salar post-smolts from the Rivers Orkla and Hals, Norway

The abundance of returning adult Atlantic salmon Salmo salar, in the River Orkla in mid‐norway (1 sea‐winter, SW, fish) and River Hals in north Norway (1–3 SW fish), was tested against the early marine feeding and the seawater temperature experienced by their corresponding year classes of post‐smolts immediately after entry into the Trondheimsfjord (Orkla smolts, 22 years of data) and Altafjord (Hals smolts, 17 years of data). In both river–fjord systems, there was a significant positive correlation between the abundance of returning S. salar and the mean seawater temperature at the time of smolts descending to the sea. The number of 1SW fish reported caught in River Orkla was positively correlated to the proportion of fish larvae in the post‐smolt stomachs in Trondheimsfjord. The abundance of returning S.salar was, however, neither correlated to forage ratio (R_F) nor other prey groups in post‐smolt stomachs in the two fjord systems. In the Altafjord, the post‐smolts fed mainly on pelagic fish larva (70–98%) and had a stable R_F (0·009–0·023) over the 6 years analysed. In the Trondheimsfjord, however, there was a higher variation in R_F (0·003–0·036), and pelagic fish larvae were dominant prey in only two (50 and 91%) of the 8 years analysed. These 2 years also showed the highest return rates of S. salar in River Orkla. These results demonstrate that the thermal conditions experienced by post‐smolts during their early sea migration may be crucial for the subsequent return rate of adults after 1–3 years at sea. Pelagic marine fish larvae seem to be the preferred initial prey for S. salar post‐smolts. As the annual variation in abundance of fish larvae is related to seawater temperature, it is proposed that seawater temperature at sea entry and the subsequent abundance of returning adult S. salar may be indirectly linked through variation in annual availability of pelagic fish larvae or other suitable food items in the early post‐smolt phase.     

Assessment of a retrofitted downstream fish bypass system for wild Atlantic salmon (<Emphasis Type="Italic">Salmo salar</Emphasis>) smolts and kelts at a hydroelectric facility on the Exploits River,Newfoundland, Canada

In 2002 and 2003, the Bishops Falls hydroelectric generating facility on the Exploits River, insular Newfoundland, Canada, underwent extensive refurbishing including replacement of turbines and installation of a ‘retrofitted’ bypass and fish handling system. The effectiveness of this new bypass system has been assessed during the annual downstream run of wild Atlantic salmon smolt and kelt in 2003 and 2004. In 2003, 195 smolt were radio tagged and released between June 9 and July 2, in the forebay of the hydro plant (19 releases) and one upstream (in-river) release. Fish guidance efficiency (FGE) of the system overall was 63% (123 of 195 fish) with 36 fish passing through the turbines, and six known mortalities. In 2004, between June 9 and July 2, a total of 358 smolt and 103 kelt were released in the forebay in 45 and 13 releases (n = 8 per release), respectively. The FGE of the system for smolt was 71.7% (257 of 358 fish) and for kelt was 92.3% (95 of 103 fish). In 2004, 96 tagged smolt passed through the turbines and 43 (44.8%) were detected at a downstream station confirming they had survived turbine passage, suggesting an overall survival of smolt passage of the Bishops Falls hydro facility in the order of 85%. A total of seven kelts (6.8%) passed through the turbines and were not detected 1.5 km downstream suggesting they did not survive turbine passage. Smolt spent on average 39.8 h in the forebay before exiting in 2003 and forebay residency averaged 26 h in 2004. In both years, most smolt selected their passage route, actively or passively, within the first 10 h with secondary peaks at 25–30 h and 50–55 h, corresponding to evening passage in the second and third night, after release. Few smolt were bypassed or entrained into turbines during daylight hours. In both years turbine passed smolt spent more time in the forebay suggesting the longer fish reside in the forebay the greater the likelihood of turbine entrainment. Kelt were either bypassed or turbine entrained relatively quickly, within 2 h of release, and virtually all kelts were bypassed/turbine entrained during the hours of 18:00 and 01:00. These data on fish behaviour and residency in the forebay will assist further refinement of operations of the bypass facility to optimize survival.