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.     

Physiological preparedness and performance of Atlantic salmon Salmo salar smolts in relation to behavioural salinity preferences and thresholds

This study investigated the relationships between behavioural responses of Atlantic salmon Salmo salar smolts to saltwater (SW) exposure and physiological characteristics of smolts in laboratory experiments. It concurrently described the behaviour of acoustically tagged smolts with respect to SW and tidal cycles during estuary migration. Salmo salar smolts increased their use of SW relative to fresh water (FW) from April to June in laboratory experiments. Mean preference for SW never exceeded 50% of time in any group. Preference for SW increased throughout the course of smolt development. Maximum continuous time spent in SW was positively related to gill Na⁺, K⁺‐ATPase (NKA) activity and osmoregulatory performance in full‐strength SW (measured as change in gill NKA activity and plasma osmolality). Smolts decreased depth upon reaching areas of the Penobscot Estuary where SW was present, and all fish became more surface oriented during passage from head of tide to the ocean. Acoustically tagged, migrating smolts with low gill NKA activity moved faster in FW reaches of the estuary than those with higher gill NKA activity. There was no difference in movement rate through SW reaches of the estuary based on gill NKA activity. Migrating fish moved with tidal flow during the passage of the lower estuary based on the observed patterns in both vertical and horizontal movements. The results indicate that smolts select low‐salinity water during estuary migration and use tidal currents to minimize energetic investment in seaward migration. Seasonal changes in osmoregulatory ability highlight the importance of the timing of stocking and estuary arrival.     

Rhythmic aspects of estuarine migration of hatchery-reared Atlantic salmon (Salmo salar) smolts

Seasonal, diel and tidal rhythmic activity of hatchery-reared Atlantic salmon ( Salmo salar ) smolts migrating through a large estuary was studied by ultrasonic tracking of 46 individuals during two seasons. Prior to 10 May each year most smolts were inactive and remained near shore in shallow water. After 10 May nearly all smolts moved away from the release point into swift water and made rapid seaward progress. Initiation of migration each year occurred when river and hatchery water temperatures rose above 9°C. Migration in the estuary was largely passive drift, and as a result there were distinct tidal rhythms of ground ('swimming') speed and net seaward travel. There were no diel rhythms in ground speed or net seaward travel; smolts drifted seaward on the tides during both day and night. Smolts may be slightly deeper during day than night.     

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.     

Ultrasonic tracking of the movements of atlantic salmon smolts (Salmo salar L) in the estuaries of two Scottish rivers

Ultrasonic telemetry was used to follow the movements of two groups of smolts in the estuaries of two small Scottish rivers. Hatchery reared smolts released into a typical wedge flow, partially mixed estuary had movements which were dominated by the influence of tide on the direction of water flow. The net movements of wild native smolts in a two layer flow estuary, in which freshwater flow dominated, was downstream but were intermittent consisting of short steps and numerous long pauses. The hatchery reared smolts escaped from the estuary within a tidal cycle, moving out on an ebb tide. The wild smolts remained in the estuary for periods up to 108 h, none escaping within one tidal cycle.     

Riverine, estuarine and marine migratory behaviour and physiology of wild and hatchery-reared coho salmon Oncorhynchus kisutch (Walbaum) smolts descending the Campbell River, BC, Canada

Eighty coho salmon Oncorhynchus kisutch smolts (40 wild and 40 hatchery-reared) were surgically implanted with acoustic transmitters and released into the Quinsam River over 2 days. Differences in physiology, travel time and migratory behaviour were examined between wild and hatchery-reared fish. In addition, tagged and control fish of both wild and hatchery-reared stock were raised for 3 months following surgery to compare survival and tag retention. Detection ranges of the acoustic receivers were tested in the river, estuary and ocean in a variety of flow conditions and tide levels. Receivers were placed in the river, estuary and up to 50 km north and south from the river mouth in the marine environment. Wild smolts were significantly smaller by mass, fork length and condition factor than hatchery-reared smolts and exhibited significantly higher levels of sodium, potassium and chloride in their blood plasma than hatchery-reared smolts. The gill Na⁺K⁺-ATPase activity was also significantly higher in the wild coho smolts at the time of release. Ninety-eight per cent of wild and 80% of hatchery-reared fish survived to the estuary, 8 km downstream of the release site. No difference was found in migration speed, timing or survival between smolts released during daylight and those released after dark. Wild smolts, however, spent less time in the river and estuary, and as a result entered the ocean earlier than hatchery-reared smolts. Average marine swimming speeds for wild smolts were double those of their hatchery-reared counterparts. While hatchery smolts dispersed in both a northward and southward direction upon entering the marine environment, the majority of wild smolts travelled north from the Campbell River estuary. The wild coho salmon smolts were more physiologically fit and ready to enter sea water than the hatchery-reared smolts, and as a result had higher early survival rates and swimming speeds.     

Movement of adult Atlantic salmon in the Usk estuary, Wales

A total of 56 salmon was tagged in the Usk estuary using combined acoustic and radio tags. Those fish migrating within the estuary oscillated with the tide over c . 10 km, being towards the seaward end at low water and moving upstream on the flood tide. Fish migrating through the estuary moved upstream on the flood tide and stemmed displacement downstream during the ebb. These findings, together with information on the hydrodynamics of the estuary, indicate that the fish utilize tidal currents to migrate passively in their preferred direction.     

The seaward movement of Atlantic salmon smolts in the Usk estuary, Wales, as inferred from power station catches

The greatest numbers of Atlantic salmon smolts were caught at Uskmouth power station during the day on the flood tide and the least on an ebbing night tide. Catch rates during the day on an ebb tide and at night on a flood tide represented an intermediate state. The results, together with tracking data, suggest that salmon smolts prefer to migrate seaward through the lower Usk estuary during the night Oncc an ebbing tide.     

Migration of Atlantic Salmon, Salmo salar, Smolt through the Estuary Area of River Ellidaar in Iceland

Synopsis We tagged both wild and hatchery Atlantic salmon, Salmo salar, smolts from River Ellidaar (64 ° 08′ N, 21 ° 50′ W) with ultrasonic tags. We caught the wild smolts in a smolt trap and selected the largest individuals from the run. We implanted the transmitters in the abdominal cavity of the fish and then released them in River Ellidaar close to the estuary. We used four ultrasonic receivers; one in the river’s estuary, one outside the estuary and two further away on both sides of an island in the estuary zone. The receivers recorded all transmitters within a 600 m radius. The hatchery smolts were larger than the wild smolts. Some of the smolts were lost on the way through the estuary. Only 4 out of 9 wild smolts and 14 of 17 hatchery smolts were recorded all the way through. The tags and the tagging likely affected the survival of the smolts especially the smaller fish. There were no differences in the smolt migration between the 2 years of study and no differences in the migration behavior between the wild and the hatchery smolts. After being released the smolts stayed on average for 10 h in the river then migrated into the estuary were they stayed for 54 h on average. Then they migrated straight through the area to the sea at approximately 0.2 fish lengths per second. We recorded large differences in the migration.     

Time and size at seaward migration influence the sea survival of Salmo salar

Whether time of seaward migration of young Atlantic salmon Salmo salar influences their subsequent survival and growth was investigated in the River Imsa, south‐western Norway. Salmo salar were tagged when moving downstream through a trap near the outlet between 1976 and 2010 and recaptured on their adult return. Most descended as smolts in April and May, but some descended during the other months of the year. Annual variation in timing of the smolt migration was significantly correlated with variation in water temperature during spring. Mean total body length of the descending S. salar varied with month of seaward migration. The sea survival of S. salar emigrating from the River Imsa between January and May was 2·8 times higher than for those descending between June and December. The sea survival of the various cohorts decreased with increasing river temperature in April to May, prior to the smolt migration, and decreasing day number when the smolts moved to sea. The size of smolts descending the river between April and May did not affect the survival at sea as much as it affected the survival of migrants descending in any other month of the year. The majority of the downstream migrating S. salar were 2 years old, but proportionally, more 1 year olds moved downstream in the autumn than in the rest of the year. Mean duration between downstream migration of the young and the return migration of the grilse was shortest (12·7 months) for those descending in July and August and longest for those descending in October (21 months). Mean monthly specific growth rate was highest for those migrating downstream between May and July and lowest for those emigrating in September. Based on the present results, it was hypothesized that S. salar emigrating between April and August migrated directly out into the ocean, while those that emigrated between October and March stayed in the estuary until the subsequent spring.     

Behaviour and diet of sea trout post-smolts in a Norwegian fjord system

In order to study the behaviour of sea trout Salmo trutta L., in the early post-smolt phase, reared smolts were tagged with acoustic transmitters and released in the estuary of the River Aurland (Western Norway) and observed for 31–137·5 h. The study was conducted in May-June 1991 and 1995 on a total of seven tagged smolts. The fish moved back and forth along the littoral zone close to the surface and migrated net distances of 100–8000 m. In 1991 the tagged fish were usually observed in schools averaging 24–55 fish, while the tendency to school was less pronounced in 1995. Post-smolts of wild and reared origin were caught by 25 m long gillnets in the littoral zone. In 1991 the mean distance to shore at capture was 4·3 m and the mean depth 0·6 m, and the corresponding numbers in 1995 were 8·0 and 0·8 m. The distribution of the post-smolts is explained by a general preference for shallow water caused by osmoregulatory problems in high-salinity deep water, together with a preference for the sheltered littoral zone and a strong predation pressure in the open sea from fast swimming pelagic fish predators and gulls Larus sp. Schooling seemed to restrict predation by cod Gadus morhua L., in the littoral zone in 1991, while no cod were observed in this habitat in 1995. In both years the stomach contents of the netted fish consisted mainly of terrestrial insects. Reared fish chose prey items similar to those taken by wild fish, but consumed less food.     

Associations of testate rhizopods (Protozoa) in the plankton of a Malaysian estuary and two nearby ponds

Plankton samples were collected from the Chukai-Kemaman estuary in Malaysia in order to determine the distribution of testate rhizopods in relation to salinity. Two nearby freshwater ponds were sampled for comparison.
In the ponds, 35 species of testate rhizopods were found, 13 of which were not found in the estuary. From the estuary 53 species were found, of which 31 were not found in the ponds. Some species appeared to have been lifted into the plankton by the rising tide. At a generic level, the Arcella species were very similar in both ponds and estuary (Sorensen Index 80), the Difflugia species were moderately similar (Sorensen Index 47), while Nebela and Quadrullela were present in the estuarine samples, but absent from the pond samples.
The salinity gradient ranged from sea water down to a conductivity of 27 μS. All the estuarine stations were tidal. The first freshwater testate rhizopods were found in a region where the conductivity was between 3000 and 4000 μS at high tide. Where the conductivity was below 1000 μS at high tide the number of species rapidly increased, and reached a maximum of 25 in the samples from the lowest conductivities. The flowering tree, Hibiscus tiliaceus , appears to be an indicator of the region where the first freshwater testate rhizopods might be encountered upstream from the sea.     

Migration of Atlantic salmon smolts and post-smolts in the Rivière Saint-Jean, QC north shore from riverine to marine ecosystems

The characteristics of the Atlantic salmon (Salmo salar) smolt run of the Rivière Saint-Jean, Quebec, Canada, in 2009 and 2010 were determined using acoustic telemetry. Tagged smolts were tracked from freshwater release sites for 17 km, then through the estuary and for their entry into the Gulf of St. Lawrence. The smolt migration began in both years at water temperatures of 10 °C, but lasted twice as long in the cooler year. The smolts in 2009 crossed the river to ocean boundary faster than the smolts of 2010, despite being similar in size. Smolt speed over ground increased from the river to the estuary to the marine environment. Smolts migrated both during the day and the night, but most movements began near or just after sunset, with increasingly nocturnal movements in the ocean. Smolts crossed the estuary during ebbing tides and moved faster during the night than the day. The tide cycle also influenced estuarine smolt travelling rates, but only secondarily to the diel cycle.     

Migratory behaviour and survival rates of wild northern Atlantic salmon Salmo salar post‐smolts: effects of environmental factors

To study smolt behaviour and survival of a northern Atlantic salmon Salmo salar population during river descent, sea entry and fjord migration, 120 wild S. salar were tagged with acoustic tags and registered at four automatic listening station arrays in the mouth of the north Norwegian River Alta and throughout the Alta Fjord. An estimated 75% of the post‐smolts survived from the river mouth, through the estuary and the first 17 km of the fjord. Survival rates in the fjord varied with fork length (L_F), and ranged from 97·0 to 99·5% km^?1. On average, the post‐smolts spent 1·5 days (36 h, range 11–365 h) travelling from the river mouth to the last fjord array, 31 km from the river mouth. The migratory speed was slower (1·8 L_F s^?1) in the first 4 km after sea entry compared with the next 27 km (3·0 L_F s^?1). Post‐smolts entered the fjord more often during the high or ebbing tide (70%). There was no clear diurnal migration pattern within the river and fjord, but most of the post‐smolts entered the fjord at night (66%, 2000–0800 hours), despite the 24 h daylight at this latitude. The tidal cycle, wind‐induced currents and the smolts' own movements seemed to influence migratory speeds and routes in different parts of the fjord. A large variation in migration patterns, both in the river and fjord, might indicate that individuals in stochastic estuarine and marine environments are exposed to highly variable selection regimes, resulting in different responses to environmental factors on both temporal and spatial scales. Post‐smolts in the northern Alta Fjord had similar early marine survival rates to those observed previously in southern fjords; however, fjord residency in the north was shorter.     

Post-smolt migration of Atlantic salmon, Salmo salar L., from the Simojoki river to the Baltic Sea

The migration pattern of Atlantic salmon ( Salmo salar L.) post-smolts in the Baltic Sea was investigated based on tag recoveries of Carlin-tagged wild and hatchery-reared smolts released in the Simojoki river in 1972–2005. Exact date of sea entry was known only for the wild smolts. Tag recoveries of wild salmon in the estuary within 10 km from the river mouth were received on average 3.5 days (±2.0 SD) after release. Time required for emigration from the estuary was dependent on the sea surface temperature (SST) off the river ( R ² = 0.625, P = 0.004), being shorter in years with warmer than colder sea temperatures. Outside the estuary, the wild and hatchery-reared post-smolts migrated southwards along the eastern coast of the northern Gulf of Bothnia, the tag recoveries coinciding with the warm thermal zone in the SST occurring along the coastal area. After arriving in the southern Gulf of Bothnia in late summer the post-smolts mostly migrated near the western coast, reaching the Baltic main basin in late autumn. The relationships between the marine conditions and migration patterns are discussed.     

Morphological and physiological changes of female tropical eels, Anguilla celebesensis and Anguilla marmorata, in relation to downstream migration

Acoustic telemetry and microchemical analysis of otolith strontium-calcium ratios were used to evaluate how exotic brown trout Salmo trutta have responded to Japanese riverine environments of south-western Hokkaido by observing their migratory patterns. The existence of anadromous S. trutta was also verified. Most S. trutta caught in rivers for otolith analysis were freshwater residents (95·6%), whereas those caught in the sea were mainly smolts (91·3%), which had just migrated from rivers during spring. Anadromous S. trutta (n = 6) were captured in rivers and in the sea, confirming the existence of mature pre- and post-spawning fish. According to telemetry results, both mature and immature S. trutta used the river in winter, and their estimated sea-run timings showed individual differences. Through the combination of these two methods, migratory patterns on various spatio-temporal scales were observed. This first documentation of the presence of both male and female anadromous S. trutta in the same region within Japan indicated the risk of further colonization of exotic S. trutta via oceanic migration.     

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.     

Does reduced feeding prior to release improve the marine migration of hatchery brown trout Salmo trutta smolts?

The aim of this study was to test the hypothesis that hatchery brown trout Salmo trutta smolts, with 50% reduced or no feeding over the last 5 months before release, were more likely to migrate to the sea than individuals with standard feeding ratios. The juvenile fish were divided into three groups 176 days before release: (A) with no feeding, (B) with 50% and (C) with 100% feeding. To study their seaward migration, 40 fish from each feeding group were tagged with acoustic transmitters and tracked by automatic listening stations in the River Nidelva, Trondheim, Norway, its estuary and in the nearest marine environment. At the time of release, mean condition factor was significantly lower in group A and the fish from groups A and B had higher levels of Na+, K+‐ATPase. Significantly more fish from group A migrated to the sea, but the rate of downstream progression from release to the estuary did not differ between the three groups. In conclusion, the S. trutta smolts with no access to food in the last 176 day before release were more likely to migrate to the sea. Fish from all three feeding groups, however, appeared to smoltify and had the same rate of downstream progression to the estuary. This indicates that differences in migratory behaviour between individuals from the three feeding groups begin from the time when the fish reach saline waters. It is suggested that feeding in hatcheries has to be greatly reduced (by 50% or more) over several months to have a pronounced effect on the migratory behaviour in S. trutta.     

Tidal effects on the organic carbon mineralization rate under aerobic conditions in sediments of an intertidal estuary

Laboratory experiments and field measurements were conducted to examine the effect of tide on the organic carbon mineralization rate in sediments under aerobic conditions of an intertidal estuary. Core samples of surface sediments were collected from an intertidal estuary of the Kurose River, Hiroshima, Japan. To mimic low and high tide in the intertidal estuary, organic carbon mineralization rates in the samples were measured in the laboratory under both air-exposed and submerged conditions. Mineralization rates under air-exposed conditions were two to five times higher than those under submerged conditions. Field measurements of the rate of CO₂ emission from the sediment surface revealed a rapid increase in the rate as the sea level fell during ebb tide. The estimated amount of daily organic carbon mineralization assuming a constantly submerged condition was 30% less than that estimated when considering the semi-diurnal fluctuation in sea level. These results indicate that tide has a marked impact on the organic carbon mineralization rate in sediments under aerobic conditions on an intertidal estuary, and tidal effects need to be considered when the amount of mineralized organic carbon is estimated.     

Diluted seawater promoted the green tide of Ulva prolifera (Chlorophyta,Ulvales)

Ulva prolifera (Müller) J. Agardh is the main causative species of the 2008 Yellow Sea green tide incident. We investigated the influences of diluted seawater on the vegetative growth and reproductive cell formation of the alga. The thalli that were cultivated under low salinities (10‰ and 20‰) and low pH values (pH around 7.0) showed obvious and steady biomass gain, while those cultivated under high salinities (40‰) and relatively higher pH conditions (pH around 8.0) manifested significant biomass loss. The trend was, however, completely the opposite for reproductive cell formation and there were indications that enrichment, over very wide concentration ranges of both nitrogen and phosphate, could significantly promote vegetative growth. Results also indicated that relatively low salinity and low pH regimes boosted vegetative growth but were unfavorable for reproductive cell formation and vice versa. Based on these results, the possible origin and development mechanisms of the green tide event are discussed. Eutrophication in the Changjiang River estuary and adjacent sea areas, as well as extremely high freshwater inflows before, and during, the flood of 2007 – due to the full operation of large‐scale water facilities in the area – may have both played an important role in the formation and development of the green tide event.