Distribution,prevalence and severity of Parvicapsula minibicornis infections among anadromous salmonids in the Fraser River,British Columbia,Canada

Polymerase chain reaction (PCR) and microscopic examination of stained kidney sections were used to diagnose infections with the myxozoan parasite Parvicapsula minibicornis in maturing Fraser River salmon. In 2 series of collections, the parasite was detected in 109 of 406 migrating sockeye salmon Oncorhynchus nerka belonging to Early Stuart, Early Summer and Summer run-timing groups, mainly upper Fraser River stocks. However, the parasite was detected neither in fish at sea nor once they had migrated several 100 km upstream. Prevalence then increased to 95% or greater at the spawning grounds. Histological examination of kidney was less sensitive than PCR in detecting the parasite in salmon collected from the earliest sites in both collections found positive by PCR. Severity of infection was greatest at the spawning grounds. Development of infection in sockeye, measured by prevalence, severity or by the rate of false-negative histological diagnoses, appeared to be a useful estimate of in-river residence time. Prevalence and severity of infections in sequential samples of Harrison River and Weaver Creek sockeye stocks collected from the Harrison River indicated that more time had elapsed since parasite transmission than would be predicted based on migration distance alone. Pink salmon Oncorhynchus gorbuscha, coho salmon O. kisutch and chinook salmon O. tshawytscha were found to be infected with the parasite. Development of P. minibicornis in pink salmon was most similar to that in sockeye. Pink and coho salmon may be at risk to the pathological consequences of P. minibicornis infection.     

Survival of migrating salmon smolts in large rivers with and without dams

The mortality of salmon smolts during their migration out of freshwater and into the ocean has been difficult to measure. In the Columbia River, which has an extensive network of hydroelectric dams, the decline in abundance of adult salmon returning from the ocean since the late 1970s has been ascribed in large measure to the presence of the dams, although the completion of the hydropower system occurred at the same time as large-scale shifts in ocean climate, as measured by climate indices such as the Pacific Decadal Oscillation. We measured the survival of salmon smolts during their migration to sea using elements of the large-scale acoustic telemetry system, the Pacific Ocean Shelf Tracking (POST) array. Survival measurements using acoustic tags were comparable to those obtained independently using the Passive Integrated Transponder (PIT) tag system, which is operational at Columbia and Snake River dams. Because the technology underlying the POST array works in both freshwater and the ocean, it is therefore possible to extend the measurement of survival to large rivers lacking dams, such as the Fraser, and to also extend the measurement of survival to the lower Columbia River and estuary, where there are no dams. Of particular note, survival during the downstream migration of at least some endangered Columbia and Snake River Chinook and steelhead stocks appears to be as high or higher than that of the same species migrating out of the Fraser River in Canada, which lacks dams. Equally surprising, smolt survival during migration through the hydrosystem, when scaled by either the time or distance migrated, is higher than in the lower Columbia River and estuary where dams are absent. Our results raise important questions regarding the factors that are preventing the recovery of salmon stocks in the Columbia and the future health of stocks in the Fraser River.     

Effects of river temperature and climate warming on stock‐specific survival of adult migrating Fraser River sockeye salmon (Oncorhynchus nerka)

Mean summer water temperatures in the Fraser River (British Columbia, Canada) have increased by ～1.5 °C since the 1950s. In recent years, record high river temperatures during spawning migrations of Fraser River sockeye salmon (Oncorhynchus nerka) have been associated with high mortality events, raising concerns about long‐term viability of the numerous natal stocks faced with climate warming. In this study, the effect of freshwater thermal experience on spawning migration survival was estimated by fitting capture–recapture models to telemetry data collected for 1474 adults (captured in either the ocean or river between 2002 and 2007) from four Fraser River sockeye salmon stock‐aggregates (Chilko, Quesnel, Stellako‐Late Stuart and Adams). Survival of Adams sockeye salmon was the most impacted by warm temperatures encountered in the lower river, followed by that of Stellako‐Late Stuart and Quesnel. In contrast, survival of Chilko fish was insensitive to the encountered river temperature. In all stocks, in‐river survival of ocean‐captured sockeye salmon was higher than that of river‐captured fish and, generally, the difference was more pronounced under warm temperatures. The survival–temperature relationships for ocean‐captured fish were used to predict historic (1961–1990) and future (2010–2099) survival under simulated lower river thermal experiences for the Quesnel, Stellako‐Late Stuart and Adams stocks. A decrease of 9–16% in survival of all these stocks was predicted by the end of the century if the Fraser River continues to warm as expected. However, the decrease in future survival of Adams sockeye salmon would occur only if fish continue to enter the river abnormally early, towards warmer periods of the summer, as they have done since 1995. The survival estimates and predictions presented here are likely optimistic and emphasize the need to consider stock‐specific responses to temperature and climate warming into fisheries management and conservation strategies.     

Foraging, bioenergetic and predation constraints on diel vertical migration: field observations and modelling of reverse migration by young-of-the-year herring Clupea harengus

This study is the first to characterize temporal changes in blood chemistry of individuals from one population of male sockeye salmon Oncorhynchus nerka during the final 6 weeks of sexual maturation and senescence in the freshwater stage of their spawning migration. Fish that died before the start of their historic mean spawning period (c. 5 November) were characterized by a 20-40% decrease in plasma osmolality, chloride and sodium, probably representing a complete loss of osmoregulatory ability. As fish became moribund, they were further characterized by elevated levels of plasma cortisol, lactate and potassium. Regressions between time to death and plasma chloride (8 October: P < 0·001; 15 October: P < 0·001) indicate that plasma chloride was a strong predictor of longevity in O. nerka. That major plasma ion levels started to decline 2-10 days (mean of 6 days) before fish became moribund, and before other stress, metabolic or reproductive hormone variables started to change, suggests that a dysfunctional osmoregulatory system may initiate rapid senescence and influence other physiological changes (i.e. elevated stress and collapsed reproductive hormones) which occur as O. nerka die on spawning grounds.     

Changes in structure of tissues and in plasma cortisol during the spawning migration of pink salmon, Oncorhynchus gorbucha (Walbaum)

Changes in the structure of the gonad, skin, interrenal, liver, kidney, stomach, gill and pituitary gland, as well as blood cortisol and haematocrit values were investigated in adult pink salmon during their migration through the Fraser and Thompson Rivers to the spawning grounds. At the commencement of their freshwater migration the gonads of both males and females were in an advanced state of development, the pituitary contained a large complement of well-granulated gonadotrops, and hypertrophy was evident in the interrenal tissue and in the epidermis of the skin. At this time, no change from the normal sexually immature salmon was evident in the structure of the gill, liver or stomach. Sclerosis of the glomeruli was noted in the kidney. The plasma cortisol level was consistent with concentrations in unstressed salmon.
Migration of the fish through a turbulent section of the Fraser River evoked a marked increase in both blood cortisol concentration and in interrenal nuclear diameters.
On arrival at the spawning grounds, 10–15 days after entry into fresh water, a general but not marked deterioration of the tissues was evident. The results are discussed in relation to the spawning migration of other species of Pacific salmon.     

Timing and route of migration of mature female blue crabs in a tidal estuary

Information on migration patterns is critical to using no-take migratory corridors and marine reserves to protect the spawning stock of commercially exploited species. Both active and passive acoustic tracking methods quantified movement of commercially and ecologically important blue crabs in the White Oak River estuary, NC, USA. We targeted post-mating female crabs migrating down-estuary to oceanic spawning grounds. Crabs travelled approximately 14.1 km mainly in deeper channels and over 12–26 days from mating areas to spawning grounds. No crabs were detected migrating down-estuary in the autumn and only 30% were detected migrating down-estuary in spring. None of the crabs detected near spawning grounds were detected or recaptured back up-estuary, suggesting that they either (i) do not return to the estuary after a one to two week period in the spawning area or (ii) were captured by fishermen. The results from this study demonstrate that (1) acoustic transmitters coupled with passive acoustic receivers provided reliable and valuable data on migration patterns of mature female blue crabs and (2) mature female blue crabs are capable of migrating primarily within deep channels to spawning grounds shortly after insemination.     

Physiological mechanisms of imprinting and homing migration in Pacific salmon Oncorhynchus spp

After several years of feeding at sea, salmonids have an amazing ability to migrate long distances from the open ocean to their natal stream to spawn. Three different research approaches from behavioural to molecular biological studies have been used to elucidate the physiological mechanisms underpinning salmonid imprinting and homing migration. The study was based on four anadromous Pacific salmon Oncorhynchus spp., pink salmon Oncorhynchus gorbuscha, chum salmon Oncorhynchus keta, sockeye salmon Oncorhynchus nerka and masu salmon Oncorhynchus masou, migrating from the North Pacific Ocean to the coast of Hokkaido, Japan, as well as lacustrine O. nerka and O. masou in Lake Toya, Hokkaido, where the lake serves as the model oceanic system. Behavioural studies using biotelemetry techniques showed swimming profiles from the Bering Sea to the coast of Hokkaido in O. keta as well as homing behaviours of lacustrine O. nerka and O. masou in Lake Toya. Endocrinological studies on hormone profiles in the brain-pituitary-gonad axis of O. keta, and lacustrine O. nerka identified the hormonal changes during homing migration. Neurophysiological studies revealed crucial roles of olfactory functions on imprinting and homing during downstream and upstream migration, respectively. These findings are discussed in relation to the physiological mechanisms of imprinting and homing migration in anadromous and lacustrine salmonids.     

Migrating silver eels return from the sea to the river of origin after a false start

The European eel''s singular spawning migration from European waters towards the Sargasso Sea remains elusive, including the early phase of migration at sea. During spawning migration, the movement of freshwater resident eels from river to sea has been thought to be irreversible. We report the first recorded incidents of eels returning to the river of origin after spending up to a year in the marine environment. After migrating to the Baltic Sea, 21% of the silver eels, tagged with acoustic transmitters, returned to the Narva River. Half returned 11–12 months after moving to the sea, with 15 km being the longest upstream movement. The returned eels spent up to 33 days in the river and migrated to the sea again. The fastest specimen migrated to the outlet of the Baltic Sea in 68 days after the second start—roughly 1300 km. The surprising occurrence of returning migrants has implications for sustainable management and protection of this critically endangered species.     

Temporal differentiation and conditions of reproduction of chum salmon <Emphasis Type="Italic">Oncorhynchus keta</Emphasis> (Salmoniformes,Salmonidae) from the Tugur River basin (Khabarovsk Krai)

Chum salmon Oncorhynchus keta from the Tugur River of Khabarovsk krai was studied. It was found that, in the river basin, chum salmon of two ecological groups reproduces: the first spawns at river sections with a pronounced underflow and the second spawns at river sections with a groundwater outlet. In the first anadromous migration, usually three maxima of the number of approaches of migrating fish are observed. The first in time maximum is represented by early (or summer) chum salmon, the second includes migrants of early and late (or autumn) form, and the third is represented by chum salmon of only late form. Forms of chum salmon from the Tugur River basin different in dates of the run and the spawning sites differ not only in the ecology of reproduction but also in biological indices. The later chum salmon is larger; its gonads at entry into the river for spawning are less mature as compared with early chum salmon. The autumn form of chum salmon reproducing at key spawning grounds is distributed more widely than the summer form. It is found along the Asian coast from Chukotka in the north to Japan and Korea in the south. Summer chum salmon occupies only part of the range of the autumn form.     

Spawning migrations of the chub in the River Spree, Germany

Out of 52 chub Leuciscus cephalus tagged with internal radio‐transmitters, 46 migrated to spawning grounds in the River Spree, Germany. All chub started their first spawning migration on 21–22 May in both years of study, 1995 and 1996. They arrived at three different spawning grounds within a day and left after 1 to 6 days to return to their original sites. On 17 and 18 June 1995 and 1996 chub started their second spawning migration with the same pattern as the first. About 60% of all migrating chub moved between 1 and 13 km upstream to the nearest spawning ground. Some exceptions were observed: a few chub did not use the nearest spawning ground, some chub used different spawning grounds at first and second spawning or moved downstream to a spawning ground. Spawning grounds were characterized by 0·4 m s⁻¹ current velocity, shallow depth (0·1–0·8 m) and a stony bottom, where most eggs adhered.     

Physiological and molecular endocrine changes in maturing wild sockeye salmon, <Emphasis Type="Italic">Oncorhynchus nerka</Emphasis>, during ocean and river migration

Maturing adult sockeye salmon Oncorhynchus nerka were intercepted while migrating in the ocean and upstream in freshwater over a combined distance of more than 1,300 km to determine physiological and endocrine changes associated with ionoregulation. Sockeye migrating through seawater and freshwater showed consistent declines in gill Na⁺/K⁺-ATPase (NKA) activity, plasma osmolality and plasma chloride concentration. In contrast, plasma sodium concentration became elevated in seawater as fish approached the river mouth and was then restored after sockeye entered the river. Accompanying the movement from seawater to freshwater was a significant increase in mRNA for the NKA α1a subunit in the gill, with little change in the α1b subunit. Potential endocrine signals stimulating the physiological changes during migration were assessed by measuring plasma cortisol and prolactin (Prl) concentrations and quantifying mRNA extracted from the gill for glucocorticoid receptors 1 and 2 (GR1 and GR2), mineralocorticoid receptor (MR), growth hormone 1 receptor (GH1R), and prolactin receptor (PrlR). Plasma cortisol and prolactin concentrations were high in seawater suggesting a preparatory endocrine signal before freshwater entry. Generally, the mRNA expression for GR1, GR2 and MR declined during migration, most notably after fish entered freshwater. In contrast, PrlR mRNA increased throughout migration, particularly as sockeye approached the spawning grounds. A highly significant association existed between gill PrlR mRNA and gill NKA α1a mRNA. GH1R mRNA also increased significantly, but only after sockeye had migrated beyond tidal influence in the river and then again just before the fish reached the spawning grounds. These findings suggest that cortisol and prolactin stimulate ionoregulation in the gill as sockeye salmon adapt to freshwater.     

Spawning migrations in landlocked Atlantic salmon: time series modelling of river discharge and water temperature effects

River discharge and water temperature are frequently cited as controlling the upstream migration of adult salmonids to their spawning areas. The results of earlier studies on the effect of these environmental factors were examined. The statistical methods employed in some of these studies failed to consider the serial correlation often found in migration time series. To assess the effects of discharge and temperature on the migratory activity of the landlocked Atlantic salmon (ouananiche, Salmo salar ), 12 years of data on spawning migrations in the Mistassini R., Quebec, Canada, were analysed and the results obtained by ordinary least squares regression and time series regression were compared. In six of the 12 years, upstream migratory movement was related negatively to changes in river flow, suggesting that fish favour falling water phases for ascent. Water temperature appeared to have little effect on migratory movement. The mean body size of migrating fish decreased significantly throughout the summer; early migrants were on average 11.4% larger (mean fork length 522mm) than late migrants (469 mm). Larger, 3-lake-year salmon migrated 7.2 days earlier than 2-lake-year salmon. Because the residuals from ordinary regression exhibited strong autocorrelation, time series regression was more appropriate than ordinary regression for the analysis of migration time Series.     

Elevation of plasma cortisol during the spawning migration of landlocked kokanee salmon (Oncorhynchus nerka kennerlyi)

Kokanee salmon (Oncorhynchus nerka kennerlyi ), a landlocked subspecies of sockeye salmon, exhibited hypothalamic-pituitary interrenal (HPI, adrenal homologue) axis activation and an increase in plasma cortisol concentration up to 639 +/- 55.9 ng/ml in association with upstream migration in the upper Colorado River even though they were not exposed to a change in salinity and lengthy migration. Kokanee salmon were collected at various stages of migration and concomitant sexual maturation. The pattern of cortisol elevation in kokanee is similar to that in ocean-run sockeye salmon (O. nerka nerka). The presence of plasma cortisol elevation in an upstream migrating, landlocked Pacific salmon suggests that stressors previously considered to cause the cortisol increase, such as long-distance migration and changes in salinity, may not be primary causes of the HPI axis activation.     

Olfactory navigation during spawning migrations: a review and introduction of the Hierarchical Navigation Hypothesis

Migrations are characterized by periods of movement that typically rely on orientation towards directional cues. Anadromous fish undergo several different forms of oriented movement during their spawning migration and provide some of the most well‐studied examples of migratory behaviour. During the freshwater phase of the migration, fish locate their spawning grounds via olfactory cues. In this review, we synthesize research that explores the role of olfaction during the spawning migration of anadromous fish, most of which focuses on two families: Salmonidae (salmonids) and Petromyzontidae (lampreys). We draw attention to limitations in this research, and highlight potential areas of investigation that will help fill in current knowledge gaps. We also use the information assembled from our review to formulate a new hypothesis for natal homing in salmonids. Our hypothesis posits that migrating adults rely on three types of cues in a hierarchical fashion: imprinted cues (primary), conspecific cues (secondary), and non‐olfactory environmental cues (tertiary). We provide evidence from previous studies that support this hypothesis. We also discuss future directions of research that can test the hypothesis and further our understanding of the spawning migration.     

Ontogenetic Behavior and Migration of Gulf of Mexico Sturgeon, Acipenser oxyrinchus desotoi, with Notes on Body Color and Development

We observed Suwannee River Gulf sturgeon, Acipenser oxyrinchus desotoi, in the laboratory and found free embryos (first interval after hatching) hid under rocks and did not migrate. Thus, wild embryos should be at the spawning area. Larvae (first interval feeding exogenously) initiated a slow downstream migration, and some juveniles (interval with adult features) continued to migrate slowly for at least 5 months, e.g., a 1-step long larva-juvenile migration. No other population of sturgeon yet studied has this migration style. A conceptual model using this result suggests wild year-0 sturgeon have a variable downstream migration style with short-duration (short distance) migrants and long-duration (long distance) migrants. This migration style should widely disperse wild fish. The model is supported by field studies that found year-0 juveniles are widely dispersed in fresh water to river km 10. Thus, laboratory and field data agree that the entire freshwater reach of river downstream of spawning is nursery habitat. Foraging position of larvae and early juveniles was mostly on the bottom, but fish also spent hours holding position in the water column, an unusual feeding location for sturgeons. The holding position of fish above the bottom suggests benthic forage in the river is scarce and fish have evolved drift feeding. The unusual migration and foraging styles may be adaptations to rear in a river at the southern limit of the species range with poor rearing habitat (low abundance of benthic forage and high summer water temperatures). Suwannee River Gulf sturgeon and Hudson River Atlantic sturgeon, A. o. oxyrinchus, are similar for initiation of migration, early habitat preference, and diel migration. The two subspecies differ greatly for migration and foraging styles, which is likely related to major differences in the quality of rearing habitat. The differences between Atlantic sturgeon populations show the need for geographical studies to represent the behavior of an entire species.     

Changes in plasma thyroid hormone levels in pink salmon, Oncorhynchus gorbuscha, during their spawning migration in the Fraser River (Canada)

Plasma thyroid hormone levels were measured in pink salmon, Oncorhynchus gorbuscha , during their spawning migration in the Fraser River, British Columbia. The plasma levels of both l-thyroxine (T₄) and triiodo-l-thyronine (T₃) were significantly higher in males than in females. In both sexes the hormone levels were maintained, or increased somewhat, during the early stages of migration, but fell thereafter. In females the plasma T₄ and T₃ levels of salmon collected on the spawning grounds were at or below detectable levels of the assays. The changes in thyroid hormone levels are correlated with changes in plasma insulin, gonadotropin, gonadal steroid hormones, cortisol and vitellogenin levels measured in the same specimens.