A size-dependent migration strategy in Atlantic salmon smolts: Small smolts favour nocturnal migration

Migration theory states that migration behavioural strategies should be optimised to maximise fitness. Many studies have shown that in downstream migrating Atlantic salmon Salmo salar L. smolts, mortality from predation is high and negatively size dependent. The most common predators are birds and piscivorous fish that are mainly daylight feeders. Given the high mortality during this stage we should expect to observe smolts to follow predator avoidance strategies that may be affected by body size. We tested the hypothesis that small smolts have a higher tendency to exhibit predator avoidance strategies (i.e. nocturnal versus diurnal migration) than larger smolts. The number and size of out-migrating/downstream-migrating wild Atlantic salmon smolts was recorded as they passed through a glass-sided channel during April-May, 1996–1999. In all years, the mean size of nocturnal migrating smolts was significantly lower than the mean size of diurnal migrating smolts. Analysis of the size of smolts, during early and late stages of the migration period showed size-dependent nocturnal migration behaviour up to the end of April. After this, no such size dependent migration pattern was observed. However, small smolts (<100 mm) were absent during this period. We suggest that nocturnal migration is an adaptive behaviour that small Atlantic salmon smolts have to avoid predation by large daylight feeding visual piscivorous predators (e.g. pike Esox Lucius L. and fish eating birds).     

Predation on hatchery-reared and wild smolts of Atlantic salmon, Salmo salar L., in the estuary of River Orkla, Norway

Predation on wild and hatchery-reared Atlantic salmon smolts was studied in the estuary of River Orkla. Cod and saithe congregating in the estuary were the most serious predators on smolts. There was no difference between the mortality rates of wild and hatchery-reared smolts. Predation by cod was estimated at 20%. No evidence was found to indicate selective predation on the smallest wild and hatchery-reared smolts.     

Predation on salmon smolts, Salmo salar L., in the estuary of the River Surna, Norway

The post-smolt stage of salmon, Salmo salar L., is subject to heavy mortality in Norway. Each year 35 000 hatchery-reared smolts are stocked in the River Surna. Smolt stocking at sea and in the fjord resulted in recovery rates of adult salmon which were up to three times better than those resulting from river stockings. This was supposed to be due to by-passing some predators. In 1984 and 1985 predators and their predation on smolts were studied in the R. Surna estuary. Cod, Gadus morhua , were found to prey heavily on smolts, and a total mortality up to 24.8% was found in a small restricted area. Cod are thought to assemble in the Surna estuary in the spring, foraging on the smolt run.     

The interrelationships between stress and osmoregulation in a euryhaline fish, Oreochromis mossambicus

Three groups of Oreochromis mossambicus, progeny of a single couple, acclimated to freshwater, seawater and iso‐osmotic water, respectively, were confined for 4 h following the same experimental procedures. Confinement stress evoked a ‘simple’ stress response that produced a significant rise in cortisol and glucose and slight osmotic disturbances for each tested salinity. It was speculated that the mechanisms securing osmotic homeostasis in this extremely euryhaline species favour it from a wide osmotic stress‐evoked disturbance.     

Life histories of Atlantic salmon altered by winter temperature and summer rearing in fresh- or sea-water

Increased growth during winter increased the incidence of age 1+ Salmo salar smolts in spring. High condition factor in spring and good growth in summer was associated with a high incidence of sexually mature males in autumn. In two experiments, groups (n=160–237 per group) of individually identified parr, either ungraded (lower and upper modal groups: LMG, UMG) or size-graded (LMG only), were reared at either 10, 6 or 3 °C overwinter (Nov to May). At 10 °C, up to 51% of parr originally in the LMG became smolts in spring at age 1+. In contrast, at either 6 or 3 °C (control), < 6% of LMG parr became smolts. The probability of being recruited into the UMG overwinter was positively related to initial body size, and was increased by size-grading. Smolt recruitment was two-fold higher among females compared to males; a proportion of males by age 0+ had already opted to mature at age 1+ rather than smolt at age 1+. In contrast, smolting at age 1+ was not inhibited in males previously mature at age 0+. During summer (May to Nov), all experimental groups were reared at ambient temperature, each subdivided between fresh water (max 21 °C) or seawater (max 15 °C). Good growth in seawater of winter recruits into the UMG confirmed they had completed smolting. Mortality in seawater among parr was 41–83%, and among smolts was 10–22%. Specific growth rate during summer was inversely related to winter rearing temperature. The incidence of sexual maturity in autumn at age 1+ among male parr was positively related to winter rearing temperature, fork length and condition factor in May, but there was large variation among individuals with respect to body size and maturity. Summer rearing in seawater reduced growth and the incidence of maturation. Parr and post-smolt maturity was 84–99% and 100% in fresh water respectively, 21–58% and 0% in seawater.     

The osmorespiratory compromise in sculpins: impaired gas exchange is associated with freshwater tolerance

We acclimated two species of sculpin, the freshwater prickly sculpin (Cottus asper) and the closely related marine Pacific staghorn sculpin (Leptocottus armatus) to freshwater ( approximately 0 g/L), brackish water (15 g/L), and seawater (30 g/L) for at least 4 wk and examined the relationships between respiration, ion regulation, gill morphology, and freshwater tolerance. The prickly sculpin successfully acclimated to all three salinities and did not experience appreciable changes in plasma osmolality, [Cl-], or mortality. Gill Na+/K+-ATPase activity was lowest in prickly sculpins acclimated to freshwater, their native salinity, and increased during acclimation to seawater. Furthermore, prickly sculpins acclimated to freshwater had a 30% higher P(crit) than fish acclimated to brackish water or seawater; P(crit) is the environmental P(O2) below which an animal can no longer maintain a routine (.-)M(O2), and an increase in P(crit) represents a compromise of respiratory gas exchange. The higher P(crit) observed in prickly sculpins acclimated to freshwater is likely a consequence of their having small, relatively thick gills that increase in thickness (by approximately 1 microm) during freshwater exposure. In contrast, the marine Pacific staghorn sculpin successfully acclimated to brackish water and seawater, but high mortality (25%) was observed after 3 wk of exposure to freshwater. Pacific staghorn sculpins exposed to freshwater suffered significant, 15%-20%, reductions in plasma osmolality and [Cl-], and these losses in plasma ions resulted in a 1.4-fold increase in gill Na+/K+-ATPase activity. Pacific staghorn sculpins have large, thin gills that are not modified in response to salinity acclimation, and as a result, these animals show no respiratory compromise during freshwater acclimation, as evidenced by the lack of change in P(crit), but show significant ion regulatory disturbance. Overall, this study suggests that gill thickening and the resulting respiratory compromise are necessary for freshwater tolerance in sculpins.     

Changes in carbonic anhydrase activity in coho salmon smolts resulting from physical training and transfer into seawater

Carbonic anhydrase (CA) activity was measured in blood and in gill tissue of coho salmon smolts during chronic exercise and subsequent transfer into seawater. The mean level of CA activity was higher in blood than gill tissue in both freshwater and seawater. CA activity in gill tissue increased significantly after the smolts had adapted to seawater. CA activity in blood decreased significantly in the group of fish given the highest exercise level after they had adapted to seawater. There were no significant differences in CA activity in gill tissue between control and exercise groups in either freshwater or seawater.     

No evidence of nonlinear effects of predator density,refuge availability,or body size of prey on prey mortality rates

Predator density, refuge availability, and body size of prey can all affect the mortality rate of prey. We assume that more predators will lead to an increase in prey mortality rate, but behavioral interactions between predators and prey, and availability of refuge, may lead to nonlinear effects of increased number of predators on prey mortality rates. We tested for nonlinear effects in prey mortality rates in a mesocosm experiment with different size classes of western mosquitofish (Gambusia affinis) as the prey, different numbers of green sunfish (Lepomis cyanellus) as the predators, and different levels of refuge. Predator number and size class of prey, but not refuge availability, had significant effects on the mortality rate of prey. Change in mortality rate of prey was linear and equal across the range of predator numbers. Each new predator increased the mortality rate by about 10% overall, and mortality rates were higher for smaller size classes. Predator–prey interactions at the individual level may not scale up to create nonlinearity in prey mortality rates with increasing predator density at the population level.     

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

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

Intestinal water transport in juvenile Atlantic salmon (Salmo salar L.) during smolting and following transfer to seawater.

1. The rate of mucosal to serosal water movement was measured in vitro in non-everted midgut segments in Atlantic salmon (Salmo salar L.) during parr-smolt transformation (February-July) and following transfer of smolts to seawater in May. 2. The rate increased significantly during smolting from 5.61 microliters/cm2/hr in the parr (February) to 11.03 microliters/cm2/hr in smolts in May. 3. Measured at intervals over a period of 20 days in seawater, the rate of water transport was not significantly different from that found in the freshwater-adapted smolts (11.20 microliters/cm2/hr). 4. Intestinal water transport is sodium-linked and inhibited by ouabain but is not stimulated by cortisol.     

Effect of glutathione depletion, hyperthermia, and a 100-mT static magnetic field on an hsp70/luc reporter system

Heat shock proteins, in particular hsp70, are induced under conditions of cellular stress. It has been reported that environmental stimuli such as hyperthermia, oxidative stress, and exposure to magnetic fields increase levels of hsp70. It has also been reported that hyperthermia in combination with magnetic field exposure results in a synergistic increase in hsp70 production. We tested the hypothesis that oxidative stress induced by glutathione (GSH) depletion in combination with static magnetic field (SMF) exposure will produce a similar synergistic increase in hsp70 production. We exposed cells to heat, SMF, and diethylmaleate (DEM), which depletes GSH levels alone and in combination with each other, and measured hsp70 production using an hsp70/luciferase reporter and mRNA levels using PCR. We found that treatment with DEM significantly reduced the rate of luciferase bioluminescence production, particularly in the presence of heat. There was no significant effect of a 100-mT SMF exposure either alone or in combination with heat, DEM, or both on bioluminescence, however there was a significant interaction between SMF and DEM on hsp70 mRNA levels. Therefore, under our exposure conditions, GSH depletion reduced hsp70 levels but a synergistic effect of combining this stress with other external stimuli was only observed at the level of mRNA.     

Changes in growth hormone and prolactin messenger ribonucleic acid levels during seawater adaptation of amago salmon (Oncorhynchus rhodurus).

To examine the changes in secretion of growth hormone (GH) and prolactin (PRL) with reference to their osmoregulatory roles, changes in pituitary mRNA levels and plasma concentrations of these hormones were examined during seawater adaptation in silvery juveniles (smolts) and precociously mature males (dark parr) of amago salmon (Oncorhynchus rhodurus). Transfer to seawater increased plasma sodium levels in both smolts and dark parr. Smolts adjusted their plasma sodium to the level associated with seawater-adaptation (165 mEq/liter) within 3 days, whereas no adjustment was seen in dark parr; the latter failed to survive in seawater for more than 3 days. In smolts, plasma GH levels increased significantly 1 day after transfer, whereas there was no significant change in dark parr. An increase in GH mRNA levels was observed in smolts in association with increased plasma GH, whereas there was no change in dark parr. In contrast, a reduction in plasma PRL levels was consistently observed in both smolts and dark parr after transfer to seawater. However, there was no significant change in PRL mRNA levels in either smolts or dark parr. These results suggest that both gene expression and release of GH are activated by seawater transfer only in smolts with adequate seawater adaptability, whereas PRL gene expression is decreased after seawater transfer regardless of seawater adaptability.     

The Effect of Carlin Tags on Survival and Growth of Anadromous Arctic Charr, Salvelinus Alpinus

Fish tagging methods are important tools in the research and management of salmonids; the Carlin tag is widely used, especially in anadromous salmonids. However, this tagging method may interfere with fish behaviour and increase post-smolt mortality. To estimate Carlin-tag-inflicted mortality in Arctic charr, we used jet injection of alcian blue (colour tags) as a control. Recapture rates were higher for the colour-tagged fish than for the Carlin-tagged fish for both one- and two-year-old smolts. For one-year-old smolts in 1992, the recapture rate was 18.9% for colour-tagged fish and 10.0% for Carlin-tagged ones. For two-year-old smolts in the same year, 34.0% of colour- and 28.6% of Carlin-tagged fish were recaptured; in 1993, recapture rates were 17.2% and 6.3% for colour- and Carlin-tagged fish, respectively. Colour-tagged one-year-old Arctic charr also grew faster than their Carlin-tagged counterparts. No differences in growth rates were found between tagging methods in two-year-olds. These results imply that for hatchery-reared Arctic charr, survival rates using Carlin tagging may be half to one third that of colour tagging. Growth was also higher among colour-tagged fish, indicating that Carlin tags alter the behaviour of fish in the sea.     

Implications of Russian wheat aphid, <Emphasis Type="Italic">Diuraphis noxia</Emphasis>, falling rates for biological control in resistant and susceptible winter wheat

The restriction of aphid reestablishment onto plants by epigeal predators represents a critical component of integrated pest management. To further realize the potential that these predators might have in control programs, it is necessary to quantify such behavior as aphid falling rate to reveal the number of aphids that are available as potential prey. This study calculated the falling rate of the Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Sternorrhyncha: Aphididae), and tested whether this aphid more likely fell from wheat plants that differed between flat leaf architecture versus those with furled leaves. Specifically, the hypothesis was tested that a resistant wheat line (flat leaves) will have a higher aphid falling rate than a susceptible closely related line (furled leaves). The experiment was performed at Fort Collins and Akron, Colorado, USA, from May through July, 2008. Aphids were sampled from infested wheat rows to estimate aphid density, and sticky traps were used to capture falling aphids and to measure falling rate. Falling rates ranged from 0.7 to 69.5% in Fort Collins and from 1.4 to 59.5% in Akron. The falling rate of D. noxia was more influenced by plant growth stage than aphid densities, with the highest falling rate occurring after wheat senescence. Wheat plants with flat leaf architecture did not significantly increase aphid falling rate. Diuraphis noxia falls at a higher rate at lower aphid densities, which is when epigeal predators could have their greatest biological control impact.     

Changes in tissue free amino acid contents, branchial Na+/K+ -ATPase activity and bimodal breathing pattern in the freshwater climbing perch, Anabas testudineus (Bloch), during seawater acclimation

This study aimed to examine effects of short- or long-term acclimation to brackish water or seawater on the climbing perch, Anabas testudineus, which is an aquatic air-breathing teleost living typically in freshwater. A. testudineus exhibits hypoosmotic and hypoinoic osmoregulation; the plasma osmolality, [Na+] and [Cl-] of fish acclimated to seawater were consistently lower than those of the external medium. However, during short-term (1 day) exposure to brackish water (15 per thousand) or seawater (30 per thousand), these three parameters increased significantly. There were also significant increases in tissue ammonia and urea contents, contents of certain free amino acids (FAAs) in the muscle, and rates of ammonia and urea excretion in the experimental fish. The accumulated FAAs might have a transient role in cell volume regulation. In addition, these results indicate that increases in protein degradation and amino acid catabolism had occurred, possibly providing energy for the osmoregulatory acclimation of the gills in fish exposed to salinity stress. Indeed, there was a significant increase in the branchial Na+/K+ -ATPase activity in fish exposed to seawater for a prolonged period (7 days), and the plasma osmolality, [Na+] and [Cl-] and the tissue FAA contents of these fish returned to control levels. More importantly, there was a significant increase in the dependence on water-breathing in fish acclimated to seawater for 7 days. This suggests for the first time that A. testudineus could alter its bimodal breathing pattern to facilitate the functioning of branchial Na+/K+ -ATPase for osmoregulatory purposes.     

Ultrastructural features of chloride cells in the gill epithelium of the Atlantic salmon, Salmo salar, and their modifications during smoltification

To elucidate the ultrastructural modifications of the gill epithelium during smoltification, gills of the Atlantic salmon (Salmo salar) were examined by electron microscopy at three stages of this process, which were defined as follows: "parrs" were freshwater fish that had not yet started their transformation; "freshwater smolts" were freshwater fish that were ready to enter seawater; and "seawater smolts" were smolts that had been transferred from fresh water and maintained for 4 days in seawater (35%). In the gill epithelium of parrs, there were two types of chloride cells. The large chloride cells contained deeply stained mitochondria and numerous apical, irregular, dense, membrane-bound bodies that formed 77% of the chloride cell population and were distinguished easily from small chloride cells that have distinctly paler mitochondria and no dense bodies in their apical cytoplasm. In freshwater smolts, the large chloride cells formed 95% of the chloride-cell population. In contrast to the small chloride cells that were not modified, they almost doubled in size. Their tubular system developed extensively to form a tight network with regular meshes significantly smaller than those observed in parr chloride cells. Forty percent of the large chloride cells were associated with a new type of cell, the accessory cell, to which they were bound by shallow apical junctions. Half of these accessory cells were not seen to be in contact with the external medium. In seawater smolts, 80% of the large chloride cells were associated with accessory cells. Most accessory cells reached the external medium and sent numerous cytoplasmic interdigitations within the apical portion of the adjacent chloride cells. As a result, a section through the apical portion of the chloride cells and their associated accessory cells revealed a mosaic of interlocked cell processes bound together by an extended, shallow apical junction. It was concluded that the Atlantic salmon develops in fresh water most of the ultrastructural modifications of the gill epithelium which in most euryhaline fish are triggered by exposure to seawater. The effective transfer into seawater would act only as a final stimulus to achieve some adequacy between the freshwater smolt and its new environment.     

Effects of starvation and - or -alanine administration on the free - and -alanine levels in the muscle and hepatopancreas of the crayfish, Procambarus clarkii

Changes of - and -alanine and other free amino acids were examined in muscle and hepatopancreas of the crayfish Procambarus clarkii before and after a 1-month starvation in freshwater, 50% seawater, and seawater. Total free amino acids and - and -alanine in both tissues decreased during starvation. The largest decrease was found in the animals starved in freshwater. In 50% seawater and full-strength seawater, the ratio of -alanine to total alanine increased in both tissues, although the level was reduced a little. A large dose of -alanine administered into the tail muscle of freshwater crayfish was converted to -alanine and -alanine returned to the control level within a week. A large amount of -alanine was transported from muscle to hepatopancreas. -Alanine injected into freshwater crayfish was converted to -alanine, but was not transported to the hepatopancreas. In the case of -alanine injection into the muscle of crayfish acclimated to 50% seawater, large amounts of - and -alanine were maintained in the muscle during the experimental period of 4 days. These data indicate that -alanine is metabolized actively in crayfish tissues and accumulated as an important osmolyte during osmotic stress.     

Combined effect of microbial and chemical control agents on subterranean termites

Termite mortality was measured when fungi were combined with bacteria or a chemical termiticide to determine whether a synergistic effect occurred. The fungus Beauveria bassiana was combined with the non-repellant chemical termiticide imidacloprid. Of the three B. bassiana strains tested one, B. bassiana ATCC 90519, was sufficiently pathogenic on its own that the advantage of a supplementary chemical treatment was marginal. The mortality caused by another fungal strain, B. bassiana ATCC 26037, was improved in combination with imidacloprid at both of the tested chemical concentrations over the first 14 days. The remaining fungal strain, B. bassiana ATCC 90518, demonstrated an overall mortality rate in combination with imidacloprid of 82.5%, versus a rate of 65.0% for the fungus alone. The fungus Isaria fumosorosea (Ifr) was combined with the bacterium Bacillus thuringiensis (Bt). On day 5, Ifr, Bt, and the combined treatment at a 10⁶ spores or cells/ml dosage caused 8.8%, 22.5%, and 15.0% mortality, respectively. The Bt and combined mortality rates are not significantly different. Control mortality on day 5 was 5.0%. On day 13 the combined 10⁶ treatment mortality rate was 91.3%, which was significantly higher than all other treatments: control at 17.5%, Ifr at 36.3% and Bt at 35.0%. When Ifr and Bt were applied at a 10⁹ spores or cells/ml dosage, Ifr alone caused a mortality rate of 97.5% as early as day 5. The combination with Bt could not significantly increase the effectiveness of this dosage. These data demonstrate the potential for synergistic effects of fungal and chemical treatment methods, thereby broadening the use of microbial control agents and reducing the quantity of chemical agents necessary to effect control.     

Hydropower-related mortality and behaviour of Atlantic salmon smolts in the River Sieg,a German tributary to the Rhine

We studied downstream migration of 256 radio-tagged Atlantic salmon smolts passing a low-head power station where technical facilities have been improved to allow safe migration via several bypass routes. Extra mortality was 7 and 17% (two years) in the power station reservoir, and a minimum of 10 and 13% at the power station compared to in a control stretch. The majority of the smolts followed the main flow at the power station, towards the turbines. Sloped bar racks with 10 mm bar spacing hindered smolts from entering the turbines, hence there was no turbine mortality. Smolts used surface openings in the racks, which directed them to a bypass route outside the turbines. The extra mortality in the reservoir and at the power station was related to physical injuries in bypass routes and to predation. The mortality risk in the reservoir and at the power station decreased with increasing migration speed. Migration speeds increased with water discharge. Migration was slower when the smolts passed the power station than on other stretches. This study shows that hydropower regulation caused elevated mortality and delays for downstream migrating fish, even with improved technical facilities to reduce mortality.     

High temperature acclimation alters the emersion behavior in the crab Neohelice granulata

An increase in environmental temperature can deleteriously affect organisms. This study investigated whether the semiterrestrial estuarine crab Neohelice granulata uses emersion behavior as a resource to avoid thermal stress and survive higher aquatic temperatures. We also examined whether this behavior is modulated by exposure to high temperature; whether, during the period of emersion, the animal loses heat from the carapace to the medium; and whether this behavior is altered by the temperature at which the animal has been acclimated. The lethal temperature for 50% of the population (LT₅₀) was determined through 96-h mortality curves in animals acclimated at 20 °C and 30 °C. The behavioral profile of N. granulata during thermal stress was based on monitoring crab movement in aerial, intermediary, and aquatic zones. Acclimation at a higher temperature and the possibility of emersion increased the thermotolerance of the crabs and the synergistic effect of acclimation temperature. The possibility of leaving the hot water further increased the resistance of these animals to thermal stress. We observed that when the crab was subjected to thermal stress conditions, it spent more time in the aerial environment, unlike under control conditions. Under the experimental conditions, it made small incursions into the aquatic environment and stayed in the aerial environment for a longer time in order to cool its body temperature. The animals acclimated at 20 °C and placed into water at 35 °C remained in the aerial zone. The animals acclimated and maintained at 30 °C (control) that were placed in water at 35 °C with the possibility of emerging into hot air transited more frequently between the aquatic and aerial zones than did the animals that were put in water at 35 °C with the possibility of emerging into a cooler air environment. We conclude that emergence behavior allows N. granulata to survive high temperatures and that this behavior is influenced by acclimation temperature.