The Migrations of North Esk Sea Trout

The migrations of North Esk sea trout were studied by tagging all stages of sea trout taken in a stationary trap situated on Kinnaber Mill Lade which drains its water from the main river. The majority of recaptures were reported from the Montrose area, and evidence of a great deal of intermixing between the sea trout of the North Esk and neighbouring rivers was found. After their initial migration as smolts a proportion of the tagged sea trout returned to fresh water within the same year. Some tagged fish undertook several journeys back and forth between sea water and fresh water during (his period. Numerous examples of tagged sea trout travelling appreciable distances (>100 km) along the coast were obtained. In addition four tagged fish were recaptured in excess of 500 km from the North Esk, three off the Scandinavian coast and one in the River Barvas, North West Lewis.     

The performance of sea trout,Salmo trutta L., stocks from the Burrishoole system western Ireland, 1970–1994.

Abstract Since 1975, with partial recoveries in 1979 and 1984–1986, the numbers of finnock and adult sea trout, Salmo trutta L., in the Burrishoole system, western Ireland, have declined. The population decreased sharply from 1987, and collapsed in 1989 and 1990. Finnock were virtually absent in 1989 and one-year maidens equally so in 1990. Sea trout smolt recruitments from 1992 to 1994 were the lowest recorded since 1970. The modal fork length of finnock decreased from 28 cm in 1987 to 26 cm in 1990. Up until 1989, the percentage return of smolts, as finnock, ranged from 11.4 to 32.4%. In 1989 a minimum was reached at 1.5%, with values of 10.0, 3.7, 7.4 and 9.9% from 1990 to 1994. Marine survival of smolts, to total first return to fresh water, historically ranged from 19 to 66%. Survival dropped to 1.8% in 1989. Observed changes in the structure of the sea trout population and the collapse in marine survival suggest that the current west of Ireland sea trout problem is based in the marine habitat.     

The commercial exploitation of sea trout, Salmo trutta L.

Abstract. The net and coble catch of sea trout in the River North Esk was sampled between 1977 and 1979. A comparison of the length frequency distribution of these samples with those of sea trout taken in a trap which was assumed to be unselective revealed that the net and coble fishery was very selective. Only isolated fish below 35·0 cm were encountered in the commercial catch samples, yet between 25% and 41% of the upstream migratory sea trout taken in the trap during the netting season were below this length. The sea trout in the commercial catch samples were predominantly maiden, one-sea-winter fish. The dominant age class each year was the 2·1 + group, 3·1 + fish comprising the other major age class. The freshwater age of the maiden sea trout sampled declined as the season progressed. The proportion of previously spawned fish in the samples ranged between years from 12% to 15%. The mean length and mean weight of sea trout sampled from each year's catch showed little variation, ranging from 42·9 cm to 43·3 cm and 971 g to 972 g, respectively. The ratio of males to females in the 1979 sample was 1·00:2·18. The use of morphometric characteristics, including head length, pectoral fin length and adipose fin length was investigated with the aim of distinguishing the sex of fish taken early in the season from external features alone. Although in each month the mean head length as a percentage of fork length of males was greater than that of females, this difference was not sufficient to enable the rapid determination of sex during commercial catch sampling routines. The exploitation rate of sea trout reaching the most upstream major fishing station, Morphie Dyke, was estimated from the recapture of upstream adult sea trout removed from a trap and released in the main river 2 km downstream of Morphie Dyke. The percentage exploitation rate at this fishing station was found to range between 6·4% and 12·2%, and the percentage exploitation rate for the entire net and coble fishery was at a level of approximately 30% during the period of the study. The sea trout statistics of the North Esk net and coble fishery for the period 1925–1979 were examined, and the effect of a change in fishing effort on these figures is described. Recent catch figures, although below those reported during the mid-1960s, are above the level of catches reported prior to 1952.     

Diel migration pattern of Atlantic salmon (Salmo salar) and sea trout (Salmo trutta) smolts: an assessment of environmental cues

The timing of smolt migration is a key phenological trait with profound implications for individual survival during both river descent and the subsequent sea sojourn of anadromous fish. We studied relationships between the time of smolt migration, water temperature and light intensity for Atlantic salmon (Salmo salar) and sea trout (Salmo trutta). During 2006–2012, migrating smolts descending the southern Norway River Storelva were caught in a rotary screw trap located at the river mouth. The date of 50% cumulative smolt descent correlated significantly with the date when the river temperature exceeded 8°C for both Atlantic salmon and sea trout smolts. In 2010, smolts of both species were passive integrated transponder (PIT)‐tagged, and the diel timing of their migration was precisely documented. The degree of night migration decreased in both species as the river temperature rose, and at temperatures above 12–13°C, more smolts migrated during day than during night. A multinomial model was fitted for estimating temperature and species effects on probabilities of migration during night, daytime, dusk and dawn. Atlantic salmon smolts preferred migrating under lower light intensities than sea trout smolts during early, but not late spring when both species migrated during bright daylight. In accordance with the early‐season tendency to migrate at night, Atlantic salmon smolts migrated more during darker hours of the day than sea trout. In both species, smaller smolts migrated under dark conditions than during light conditions. Most of the findings on thermal, light and temporal effects on the observed smolt migration pattern can be explained as adaptations to predation avoidance.     

Comparison of physiological smolt status in descending and nondescending wild brown trout (Salmo trutta) in a Danish stream

Abstract –  Downstream movement of a wild population of brown trout was examined in a small Danish stream in relation to morphological and physiological smolt status from March to May. Downstream movement was monitored in a Wolf-type trap covering all possible passage routes in the stream. Trout caught in the trap were classified as parr, pre-smolt or smolt based on morphological criteria and compared with trout randomly caught by electrofishing upstream of the trap. Representative gill samples from trap-caught and electrofished trout were analysed for gill Na⁺, K⁺-ATPase activity and used as a measure of physiological smolt status. Only a few parr occurred in the trap. Few pre-smolts occurred in the trap evenly in March and early April. In late April, pre-smolt movement peaked. By comparison, the main downstream movement of smolts occurred in distinct peaks through late March and April. The majority of fish caught in the trap were judged as pre-smolts or smolts based on morphological criteria's and they were characterised by relatively high gill Na⁺, K⁺-ATPase activity compared with trout judged as parr. Trout caught by electrofishing upstream the trap, were classified as parr, pre-smolts and smolts early in the season (March). During and after the main smolt-run in April the distribution of the remaining trout in the brook became skewed in favour of pre-smolt and parr. The study suggests that smolting trout initiate downstream movement once having reached a certain physiological smolt condition (judged by increased gill Na⁺, K⁺-ATPase activity).     

Linking habitat characteristics with juvenile density to quantify Salmo salar and Salmo trutta smolt production in the river Sävarån,Sweden

Abstract Habitat mapping along 85 km of river was related to juvenile (15 years of electric fishing) and smolt (3 years of screw‐trapping) abundance data to estimate salmon, Salmo salar L., and sea trout, Salmo trutta L., smolt production in the River Sävarån, northern Sweden. Spawning site selection by radio‐tagged salmon (n = 12) and sea trout (n = 4) was also assessed. Fifty‐one hectares of potential spawning and nursery habitat was found in the main stem river, representing 25% of the total river area. These areas were estimated to yield 1300–7580 salmon and 630–3540 sea trout smolts based on juvenile densities, equating with 3 years of screw‐trap data (2990–5080 salmon and 680–2520 trout smolts, respectively). A hypothetical maximum production of about 19 900 salmon smolts was predicted for the river at a density of 40, 0+ salmon 100 m^?2. Tracking adults during the spawning period identified optimal and potential reproductive areas.     

Competition between Asian pink salmon (Oncorhynchus gorbuscha) and Alaskan sockeye salmon (O. nerka) in the North Pacific Ocean

The importance of interspecific competition as a mechanism regulating population abundance in offshore marine communities is largely unknown. We evaluated offshore competition between Asian pink salmon and Bristol Bay (Alaska) sockeye salmon, which intermingle in the North Pacific Ocean and Bering Sea, using the unique biennial abundance cycle of Asian pink salmon from 1955 to 2000. Sockeye salmon growth during the second and third growing seasons at sea, as determined by scale measurements, declined significantly in odd‐numbered years, corresponding to years when Asian pink salmon are most abundant. Bristol Bay sockeye salmon do not interact with Asian pink salmon during their first summer and fall seasons and no difference in first year scale growth was detected. The interaction with odd‐year pink salmon led to significantly smaller size at age of adult sockeye salmon, especially among younger female salmon. Examination of sockeye salmon smolt to adult survival rates during 1977–97 indicated that smolts entering the ocean during even‐numbered years and interacting with abundant odd‐year pink salmon during the following year experienced 26% (age‐2 smolt) to 45% (age‐1 smolt) lower survival compared with smolts migrating during odd‐numbered years. Adult sockeye salmon returning to Bristol Bay from even‐year smolt migrations were 22% less abundant (reduced by 5.9 million fish per year) compared with returns from odd‐year migrations. The greatest reduction in adult returns occurred among adults spending 2 compared with 3 years at sea. Our new evidence for interspecific competition highlights the need for multispecies, international management of salmon production, including salmon released from hatcheries into the ocean.     

Differences in sea migration between wild and reared Atlantic salmon (Salmo salar L.) in the Baltic Sea

The effect of origin, smolt size and year of release on the sea migration pattern of Atlantic salmon (Salmo salar L.) in the Baltic Sea was examined by tagging experiments conducted in 1991–1993 on wild and reared smolts of the Simojoki river salmon stock. The tag recovery data analysed by log-linear models revealed significant differences in both spatial and temporal sea migrations between the wild and reared salmon; the variation was attributed to the year of release and to the origin of the fish. Grilse accounted for the majority of reared returners (76%) but for a smaller proportion (46%) of the wild fish. The effect of smolt size could be studied only in the smolt groups tagged in 1991. Wild fish were more frequently (71%) caught in the Baltic Main Basin than were reared fish (51%) during their second sea year, and the size variation between wild and reared smolts did not explain the recovery site. No such differences in spatial distribution were found during the third sea year. The tagging place (hatchery/trap) of the reared fish did not affect their later sea migration. The differences in sea migration patterns suggest that the wild salmon are more vulnerable to the intensive salmon fishery in the Baltic Main Basin than are reared fish.     

Survival of reared Atlantic salmon and sea trout in relation to marine conditions of smolt year in the Baltic Sea

The marine survival of hatchery-reared Atlantic salmon (Salmo salar) and sea trout (Salmo trutta) was examined in relation to marine conditions during post-smolt migration and in relation to stock traits. In 1970–2001, Carlin-tagged smolts were released in the Iijoki and Oulujoki rivers, the northern Baltic Sea. When both species were analysed together, the abundance of the three prey fish, herring (Clupea harengus), smelt (Osmerus eperlanus) and vendace (Coregonus albula) correlated positively with the survival of salmonids. In addition, the increase in smolt size appeared to improve the survival rate. Sea surface temperature (SST) may have affected indirectly through the abundance of prey fish during the post-smolt migration of salmon and sea trout. The smelt and vendace showed a statistical effect on survival only when the temperature effects were not included in the models. In sea trout, an increasing smolt length was not significantly correlated with the survival in good herring recruitment years, but in poor years survival increased very rapidly with increasing smolt size. The recapture rates of the salmonids tended to decrease between the years 1970 and 2001. During the same time period, the June SST slightly decreased. The positive correlation between the annual summer SST and recapture rate of salmon may partly explain the decreasing trend in recapture rates. An increase in smolt size did not compensate for the decline in the recapture rate of either species.     

The movement of wild sea trout, Salmo trutta L., smolts through a river estuary

Abstract  The movement of sea trout. Salmo trutta L., smolts through the estuary of the River Avon, southern England was studied using miniature 300 kHz acoustic transmitters and an array of strategically placed acoustic sonar buoys. Sea trout smolts were trapped 3.25 km above the head of tide as they moved downstream and the transmitters were surgically implanted into the peritoneal cavity of the fish. Subsequent movement of the smolts through the estuary was predominantly nocturnal and occurred mainly during an ebb tide or the period of slack water between two high waters. During the hours of darkness the ground speed of the smolts through the estuary was similar to that of passively drifting objects. However, where movement did occur during daylight hours ground speeds of smolts were significantly slower. Vertical salinity profile measurements of the estuary indicated that individual smolts may have experienced salinities ranging from fresh to 100% sea water during their migration. The behaviour of the smolts is discussed in relation to the possible environmental and physiological cues controlling the estuarine migration.     

Early marine survival and movements of escaped Atlantic salmon Salmo salar L. juveniles from a land‐based smolt farm during autumn

The aim of this study was to examine early marine survival and movements of simulated escaped Atlantic salmon Salmo salar L. pre‐smolt and smolt from a commercial smolt farm during autumn. One‐third of the pre‐smolt most likely died in the immediate vicinity of the release location, whereas the corresponding mortality for smolts was lower (8.5%) during the 5‐week study period. The surviving pre‐smolt left the farm area after 2–3 days, predominantly along the shore. In contrast, most of the surviving smolts left the farm area during the first day and 54% seemed to move away from the shore and adopt a more pelagic movement pattern than pre‐smolt. The number of surviving fish recorded in the fjord decreased throughout the study period, possibly due to a combination of fish migrating out of the fjord or undetected mortality. Compared with existing knowledge on migration of released farmed smolts during spring, our results indicate less directional and slower movement rates during autumn. Only two of the tagged fish were detected upstream in the rivers following release. A rapid dispersion of escapees indicates that the potential for recapturing escapees is limited unless recapture efforts are initiated immediately after escape. Hence, there is a need for development of technology that detects and prevents escapees to enter the sea.     

Strontium levels in scales and vertebrae of wild and hatchery-reared Atlantic salmon, Salmo solar L., smolts

Abstract. Analyses of the Sr-contenl in scales of Atlantic salmon, Salmo saiar L ., smolts from two hatcheries revealed significant ( P < 0·001) differences. These differences were reflected in the Sr-conients of their respective diets. Sr-values in scale samples taken from wild salmon smolts from 10 Norwegian rivers showed a considerable variation. Analysis of variance showed that sampling site was the major source of variation. In general the Sr-content in scales cannot be used to distinguish between hatchery-reared and wild salmon smolls, although the present results nevertheless suggest that such a distinction is possible in some localities.
The Sr-contenl of the vertebrae was somewhat lower than that of the scales in both hatchery-reared and wild smolls. The Sr-conient of the scales of sea trout, Salmo trutta L., smolts from one of the sampling sites was significantly ( P < 0·001) higher than that of scales of Atlantic salmon smolts from the same site. The Sr-content of the scales of all the salmon smolts sampled from a lake was higher than the values recorded for sea trout and salmon after their stay in the sea. Use of the Sr-contenl of the scales todistinguish between migratory and resident individuals should therefore be done with care.     

Marine mortality in the river? Atlantic salmon smolts under high predation pressure in the last kilometres of a river monitored for stock assessment

The River Bush (Northern Ireland) is an index river for the estimation of Atlantic salmon, Salmo salar L., stock size, population dynamics and marine survival rates. Marine survival estimates are based on the number of smolts counted at a trap 3.5 km upstream of the river outlet. The survival from release to coastal inshore waters for acoustic‐tagged smolts released at the Bushmills trap varied between 32% and 68%, with both year and brightness during river exit playing a significant role in explaining the variations in survival. This constitutes an important survival bottleneck. Contrary to true marine mortality, this significant loss of smolts in the river and nearshore environments could be reduced by focused management actions. More studies on other rivers, where smolts are enumerated above the head of tide, could further partition smolt and post‐smolt mortality, help differentiate true marine survival and help understand fluctuations in adult returns.     

The effects of ration size on migration by hatchery‐raised Atlantic salmon (Salmo salar) and brown trout (Salmo trutta)

Abstract – The possibility to increase the proportion of migrating hatchery‐reared smolts by reducing their food ration was studied. Lake‐migrating, hatchery‐reared salmon (Salmo salar) and trout (Salmo trutta) smolts were either fed normal rations, based on recommendations from the fish‐farming industry, or reduced (15–20%) rations. They were released into the River Klarälven, western Sweden, and followed as they swam downstream to Lake Vänern, a distance of around 25 km. For both Atlantic salmon and brown trout, smolts fed a reduced ration migrated faster than fish fed a normal ration. Furthermore, a higher proportion of salmon smolts fed reduced rations migrated to the lake than fish fed normal rations in 2007 but not in 2006. This difference between years corresponded to greater treatment differences in size and smolt status in 2007 than in 2006. For trout, the proportion of migrating individuals and smolt development did not differ with ration size. Trout migrants fed a normal ration had a higher standard metabolic rate (SMR) than nonmigrants, whereas there was no difference in SMR between migrating and nonmigrating salmon. These results show that it is possible to use a reduced food ration to increase the migration speed of both Atlantic salmon and brown trout and to increase the proportion of migrating Atlantic salmon.     

Predation by cormorants, Phalacrocorax carbo (L.), on the salmonid populations of an Irish river

Abstract. Counts were made of cormorants, Phalacrocorax carbo (L.), feeding on the River Bush. County Antrim. Northern Ireland during the post-dawn period on three occasions. Two of the counts during May 1986 indicated that up to 264 birds may have been feeding at least once per day throughout the catchment during the salmon, Salmo salar L., smolt run. The number of feeding birds had dropped to an estimated 61 by the time of the third count on 1 July 1986. Stomach samples from shot birds showed that upstream feeding was concentrated on wild smolts and brown trout. Salmo trutta L. However, cormorant predalion downstream from the salmon hatchery at Bushmills was restricted solely to hatchery smolts. Estimates of the total daily predation rates were calculated at 653–1214 wild smolts. 107–231 hatchery smolts and 422–785 brown trout. The possible impact of this level of predation on the salmonid stocks of the river was assessed.     

Seasonal differences in smolt traits and post-smolt survival of wild Atlantic salmon, Salmo salar, migrating from a northern boreal river

Abstract  Seasonal differences in smolt traits and the post-smolt survival of wild Atlantic salmon, Salmo salar L., were investigated by smolt trapping and Carlin-tagging in the River Simojoki, northern Finland between 1991 and 2004. The annual trapping season was divided into two halves based on the median catch date. Smolt length was significantly higher during the first half of the season, while differences in smolt weight were typically small. Smolt age was always significantly higher during the first half of the season because older smolts tended to migrate earlier in the season. Many smolts migrating during the early season and almost all smolts migrating later had started their new growth, indicating that smolts grow in the spring before sea entry. The differences in recaptures between smolts tagged during the first and second halves were insignificant. Although variations in smolt traits and environmental conditions can produce inter-annual variation in post-smolt survival, their seasonal differences seem to be too small to have an effect.     

Reduction of brown trout, Salmo trutta L., salmon, S. salar L., and rainbow trout, Oncorhynchus mykiss (Walbaum), smolt bycatches in eel pound nets

Two types of bycatch reduction devices (BRDs) were tested in the Randers Fjord, Denmark, aimed at deflecting the surface-oriented smolts of brown trout, Salmo trutta L., salmon, Salmo salar L., and rainbow trout, Oncorhynchus mykiss (Walbaum), before they became trapped, while retaining the catches of eel, Anguilla anguilla (L.). The methods tested were: a) placing a floating guard net between the wings; and b) submerging the pot net (i.e. last enclosure before fyke net) 55–100 cm below mean sea level. After each haul the pound net-setups were changed from the control to one of the types of BRD, and vice versa. Both methods significantly reduced the bycatch of brown trout smolts, while catches of legal-sized eels were not affected. Submerging the pot net reduced smolt catches of all species; mean reductions were 91.1% for brown trout, 74.5% for rainbow trout, and 86.1% for salmon.     

Delayed smolt migration of stocked Atlantic salmon parr

Abstract The timing of the smolt run in the Dale River in western Norway was monitored from 2002 to 2007 after annual stocking in late autumn 2000 to 2005 with 5000–10800, 11–16 g, Atlantic salmon, Salmo salar L., parr. The releases yielded an annual smolt production of 1000–2000 individuals, mainly 1+ smolts. Almost 5700 stocked smolts were trapped during smolt migration and 60% of these were genotyped for family identification. The date for 50% descent varied by 14 days from year to year. For the most part, however, the 2+ stocked smolts and the majority of the wild smolts left the river in May, while the 1+ stocked smolts migrated 23–26 days later in June. It was concluded that the strategy of stocking large parr in late autumn may conflict with the natural timing of smolt migration the following spring.     

Survival of migrating sea trout (Salmo trutta) and Atlantic salmon (Salmo salar) smolts negotiating weirs in small Danish rivers

Abstract – The survival of brown trout and Atlantic salmon smolts during passage over small weirs was estimated in two small Danish rivers during the spring of 1998. Parallel groups of smolts were released upstream and downstream of the weirs and recaptured in traps further downstream. The results showed a smolt loss varying from 18 to 71% for trout and 53% for salmon. Furthermore, the surviving smolts from the upstream groups were delayed for up to 9 days compared to downstream groups. The study demonstrated that an increased proportion of total river discharge allocated to fish passage increased the smolt survival. Losses may be because of fish penetrating grids erected at fish farm inlets, predation and delays, which may lead to desmoltification. The low survival may seriously threat both the long-term viability of wild populations of anadromous salmonids and the outcome of the intensive stocking programme in Denmark.     

Seawater tolerance in Atlantic salmon, Salmo salar L., brown trout, Salmo trutta L., and S. salar?×?S. trutta hybrids smolt

High levels of hybridization between Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) have been reported in the Gyrodactylus salaris infected Rivers Vefsna and Driva in Norway. The survival and behaviour during the sea phase of such hybrids is unknown. The reported work documents ionoregulatory status after 24 h seawater challenge tests (24hSW) and gill Na⁺/K⁺-ATPase (NKA) activity of migrating wild smolts of Atlantic salmon, brown trout and hybrids at two sampling dates during the 2006 smolt run in River Driva. Salmon, trout and hybrids contributed to 27, 52 and 21% of the catches, respectively. The large contribution of hybrids suggests both a high hybridization rate and a high survival rate from fry to smolt. Both salmon and hybrids had a well-developed seawater tolerance at the time of downstream migration, revealed by small ionoregulatory effects and no or low mortality rates during the 24hSW tests. The trout were not fully adapted to seawater, and high mortality rates were observed (71 and 92%) during the 24hSW tests. The NKA activity was not significantly different between salmon and hybrids. Most of the hybrids were physiologically capable of direct entry to full strength seawater. The incomplete seawater tolerance in trout compared to salmon corresponds well with differences in life-history patterns between these two species. The life history strategy of the hybrids during the sea phase is not known, and further investigations on the marine behaviour and survival is needed to evaluate the role of hybrids in the risk of spreading G. salaris to nearby river systems.