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1.
Abstract  Following a sea trout stock collapse in 1989 and 1990, wild sea trout kelts were successfully reconditioned in brackish water using semi-moist pellets and a commercial broodstock diet. The on-growing of wild sea trout smolts was not so successful. Natural mortality was 12.9%, and 36.1% of the fish failed to adapt to the artificial diets: 82% of these were smolts. Instantaneous growth was highest from June to August (0.74%), with an overall rate for April to September of 0.51%. At the end of the first year. 36 females and 26 males matured; 34 females, with a mean fork length of 33.2 cm, were stripped yielding 22860 ova at a mean rate of 682 ova per female or 1586 ova kg-1 body weight. The survival of green ova to eyed ova was 90–98%, and survival from green ova to hatched alevin was 95%.  相似文献   

2.
The sea trout population of the River North Esk was sampled by means of a stationary trap situated on Kinnaber Mill Lade which drains its water from the main river. Commencing in 1971 the number of trout taken in the trap was noted daily, and from 1976 all stages of trout caught in the trap were tagged. The peak of the sea trout smolt migration was usually in May or June, and over the period of study exhibited a tendency to occur later each season. Two-year-old smolts comprised the largest age class in the smolt samples with 3-year-old fish being the other substantial age class. Samples of smolts taken for sex determination were found to be predominantly female. In 1979 and 1980 the sea trout smolt production of the North Esk was estimated from a mark-recapture experiment to be 51,969 and 78,208 respectively. A proportion of finnock (post-smolts) returned to fresh water in the year of their smolt migration and were taken moving upstream into the trap from July onwards, with a peak of movement occurring in the autumn. The main run of adult sea trout into the trap was found to take place at any time between July and October. The adult sea trout sampled were predominantly maiden spawners of ages 2.1+ and 3.1+ years. Both finnock and adult sea trout samples exhibited a similar freshwater age composition to that of the smolts.  相似文献   

3.
Partial migration is a common phenomenon in many fish species. Trout (Salmo trutta) is a partially migratory species where some part of the population migrate to the marine environment, while another remains in freshwater. In the years 2008 and 2009, a total of 159 wild sea trout smolts were tagged with acoustic and PIT‐tags in the river Villestrup (Denmark) to study the initial postsmolt marine behaviour within a fjord system. We found that the strategies of the sea migrants vary: some stay in the fjord, while others migrate to the sea, suggesting that partial migration occurs even in the marine environments. Overall, a total of 53% of the tagged smolts migrated from the fjord to the sea, and 47% stayed (or potentially died) in the fjord. The ratios of fjord‐resident versus sea‐migrating postsmolts were consistent at the study times, and no differences between the early and late migration periods of the smolts were observed. The individual's size or body condition at the time of tagging did not affect survival or the migratory decisions in the fjord. High overall initial survival (74%) was found 30 days after the fjord entry. We suggest that within a continuum of migration to sea, there is a migratory decision point when sea trout postsmolts encounter a fjord system. At this point, postsmolts will assess the possibility of migration versus the alternative of fjord residency.  相似文献   

4.
An examination of marine growth/marine survival relationships in Atlantic salmon, Salmo salar L., was carried out, based on scale growth measurements in relation to two indices of marine survival in wild fish from the River Bush, Northern Ireland. The survival of cohorts to the Irish/Northern Irish coast (prefishery) and to fresh water was statistically unrelated to variation in growth from smolt migration to the end of the first winter at sea ( P  > 0.1; – P  > 0.7). Marine growth of 1+ smolts decreased significantly during the period of the study ( P  < 0.05), but growth of 2+ smolts did not change ( P  > 0.05). The variability in marine growth was much less than the variation in natural survival at sea, suggesting that factors instead of or in addition to, growth influence natural survival in the marine environment. Survival to fresh water was not related to survival to the coast ( P  > 0.4), although it was inversely correlated with exploitation rate ( P  < 0.01). These results are discussed in relation to the use of freshwater returns to assess marine survival and the potential for the variation in natural marine mortality to influence total life-cycle variation.  相似文献   

5.
This study tests the basic hypothesis that the removal of charr, Salvelinus alpinus (L.), would cause an increase in both the growth and density of a sympatric trout population, Salmo trutta L. The charr population was characterised by slow‐growing individuals, with a high proportion of mature fish, that is typical for so‐called overpopulated populations. A total of 31,000 charr was removed from the lake in the period 1990–1992, and the density of younger trout (1+, 2+), but not older trout (3+, 4+), increased. The growth of older trout (3+, 4+) increased, but the evidence for similar growth increases of younger trout (1+, 2+) was limited. From 1989 to 1990, the proportion of trout increased from 30 to only 40% of the total catch, but from 1991 to 1994, it was significantly higher (60–80%) than that of charr. Total trout biomass increased to a maximum in 1992 and then decreased so that the biomass of 1994 was nearly similar to that of 1989, that is before the start of the charr removal. Back‐calculated lengths of trout from otoliths showed that 2+ and 3+ trout caught in the pelagic were growing consistently faster over previous years than those caught in the littoral, while this was not the case for the 4+ fish. Therefore, the hypothesis was partially supported; the growth rate of trout increased (age groups 1+ to 4+), while the density of juvenile trout (1+, 2+), but not the older trout (3+, 4+), increased after the removal of charr.  相似文献   

6.
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.  相似文献   

7.
The Irish sea trout, Salmo trutta L., broodstock programme was instigated in response to the collapse of sea trout stocks in the west coast of Ireland in 1989 and 1990. Wild sea trout kelts and post-smolts were successfully reconditioned and used as broodstock to produce eyed ova for distribution to affected fisheries. From 1991 to 1999, a total of 8.2 million green ova from four separate stocks were produced. The number of females stripped increased from 34 in 1991 to a peak of 1435 in 1997. Green ova survival to the eyed ova stage ranged from 65 to 96%. The eyed ova produced were distributed to 23 affected fisheries along the west coast. The average cost per eyed ova decreased from €2.02 at the start of the programme to €0.04 at the end. The growth rates recorded for broodstock held in captivity were comparable with those recorded for wild sea trout. Relationships between fish size, egg size and number of eggs produced were examined for each stock and for each stock in each year. Significant positive relationships were found between fish size and egg number and fish size and egg size, with the exception of the first year in which fish were stripped, when a negative correlation between fish size and egg size was found.  相似文献   

8.
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.  相似文献   

9.
Sea trout (Salmo trutta L.) stocks on the West Coast of Scotland and Ireland have decreased due to reduced survival during the marine phase of their life cycle. Lice‐infested sea trout returning to rivers could indicate that parasite burdens are contributing to the decline. Salmon farms represent a potential source of substantial quantities of sea lice, leading to a conjectured link between parasites on salmon fish farms and infestations on wild sea trout. To investigate the potential infective pressure on sea trout in Loch Shieldaig, offshore and sub‐littoral plankton samples were collected and analysed for sea lice nauplius and copepodid stages. During the plankton survey, numbers of sea lice on a fish farm in the loch reached a maximum in November 2001. Soon after, numbers of sea lice larvae peaked in open‐water samples and then in sub‐littoral samples. Nauplii were found adjacent to the farm and occurred less elsewhere. This study reports a concentration of sea lice larvae at the head of Loch Shieldaig and indicates a possible relationship between sea lice numbers on the fish farm and lice larvae densities in the open‐water of the loch and in the sub‐littoral zone.  相似文献   

10.
A dynamic mark-recapture electric fishing study on sea trout, Salmo trutta L., was carried out from July 1989 to January 1990. There was a significant difference in the recapture probability ( P = R / M ) between small (24–39 cm) and large (40–84 cm) sea trout with P- values of 0.31 and 0.62, respectively (χ2 = 16.52, P < 0.001). The vulnerability-size relationship resulted in a slight overestimation of population size (2.5%) using the Petersen method by adding the two size classes together in the computations. Separate computations for the two size classes had no significant effect upon the estimate of the seasonal length distributions but a significant effect on the estimates of total length distribution ( P < 0.001) and the mean weight of the mature stock ( P < 0.001). The present data emphasises that size-selectivity should be taken into account in electric fishing studies on adult sea trout.  相似文献   

11.
Obtaining reliable estimates of marine survival is essential for understanding anadromous salmon population dynamics. Two common approaches to estimating marine survival are (a) dividing abundance of returning adult salmon abundance by abundance of smolts from the same cohort, or (b) tagging a portion of the migrating smolts and estimating the return rate of tagged adults. This study compared these two approaches to estimating marine survival for coho salmon, Oncorhynchus kisutch (Walbaum), across multiple years in three California streams. Abundance‐based survival estimates were higher than tag‐based estimates; average estimates for the two techniques differed from 1.5‐fold to 7.4‐fold across streams. One likely cause for these divergent estimates is migration of juveniles from natal habitat before smolt trapping begins, resulting in an underestimate of smolt abundance and an overestimate of marine survival rate for the abundance‐based method. Estimates of marine survival obtained from abundance estimates and tag returns are not directly comparable.  相似文献   

12.
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.  相似文献   

13.
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.  相似文献   

14.
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.  相似文献   

15.
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16.
Abstract –  Sea trout ( Salmo trutta ) originating from small coastal streams can be found at sea throughout the year, in contrast to conspecifics from larger rivers, which typically spend the autumn and winter in fresh water. Such an extended marine stay has been observed in coastal Skagerrak. We studied the seasonal variation in marine growth of Skagerrak sea trout based on scale increment patterns and body lengths of 563 individuals captured at sea. Growth, measured as increased body length, was rapid during summer while there was no evidence for continued growth during autumn and winter. Growth decreased with increasing age of the fish. Our results suggest that coastal Skagerrak is an important feeding area for sea trout during summer, and that an extended marine stay during autumn and winter may have trade-off benefits other than somatic growth. Alternative benefits might be increased winter survival and decreased migratory costs of juvenile fish.  相似文献   

17.
This study investigated the cumulative impact of weirs on the downstream migration of wild Atlantic salmon (Salmo salar) smolts in the River Foyle, Northern Ireland. In spring of 2013 fish were released in two tributaries of similar length; one tributary (impacted) had seven low‐head weirs along the migration pathway and the other was devoid of such structures (un‐impacted). Salmon smolts fitted with acoustic transmitters were monitored via a passive acoustic telemetry array during downstream migration. In 2014 the study was repeated only in the impacted tributary. Overall freshwater survival rates were high (>94%). There was no significant difference in mortality, movement pattern, delay or travel speeds between rivers or between years at any phase of migration. Escapement of salmon smolts through Lough Foyle (a marine sea lough) to the open ocean was low, approximately 18% in each year. Escapement did not differ between impacted and un‐impacted rivers. This study showed no postpassage effects of weirs on mortality, migration speed or escapement of downstream migrating smolts. This suggests that the elevated mortality at low‐head obstacles described in other studies is not inevitable in all river systems. Migration through rivers with natural riffle‐pool migration may result in similar effects as those from low‐head weirs. Causes of apparent high mortality in the early part of marine migration in this study, are unknown; however similar studies have highlighted the impact of fish predators on smolts.  相似文献   

18.
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.  相似文献   

19.
Sea trout, Salmo trutta L., populations in the Slupia River basin have been affected by mass stocking with smolts and fry. This work is focused on a small‐scale differentiation in sea trout populations from one basin with a strong emphasis on the relationship between wild and hatchery origin fish. A total of 216 sea trout were genotyped at 10 microsatellite loci. Global FST obtained by amova was low at 0.0165. Pairwise FST were significant for all tests except wild and stocked adults. The highest pairwise difference was found between the hatchery sample and Kwacza (FST = 0.038). Analysis of the genetic structure revealed micro‐geographical differentiation with four subpopulations. The quality of the artificial spawning was found not to be adequate with a high risk of adverse effects to the whole population. All future stocking actions in the basin should consider the existing population structures.  相似文献   

20.
Abstract  The prevalence of escaped farmed Atlantic salmon, Salmo salar L., in the River Ewe, western Scotland, was assessed. After the establishment of smolt cages in the catchment and marine cages near the river mouth during 1986–1987, approximately 425 000 parr and smolts, and 122 000 growers have escaped. Between 1987 and 2001, farmed salmon occurred in the rod fishery in 13 of the 15 years, contributing at least 5.8% of the total catch, with a maximum annual frequency of 27.1%. It was estimated that <1% of fish escaping from the marine cages entered the river, but contributed at least 27% of potential anadromous spawners in 1997. Radiotagged, farmed fish in 2001 probably spawned in three subcatchments also used by tagged wild fish. Despite the likelihood of hybridisation there was no change in the median weight or marine age of wild fish, but smolt age decreased significantly ( P  < 0.02). The Ewe has a depleted wild salmon population (≤900 anadromous adults), and further genetic introgression by escapees should be prevented.  相似文献   

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