The survival of semi‐wild, wild and hatchery‐reared Atlantic salmon smolts of the Simojoki River in the Baltic Sea

The recapture rate and survival of hatchery‐reared Atlantic salmon Salmo salar stocked as 1 year‐old parr (semi‐wild) with that of hatchery‐reared Atlantic salmon stocked as 2 year‐old smolts and wild smolts of Atlantic salmon in the northern Baltic Sea were compared. This was done through tagging experiments carried out in 1986–1988 and 1992. The recapture rate of the semi‐wild groups varied from 1·0 to 13·1%, being similar in 3 tagging years and lower in 1 year than that of the wild groups (1·7–17·0%). The recapture rate of the semi‐wild groups was similar (in 2 years) or higher (in 2 years) than that of the hatchery‐reared groups stocked as smolts (1·3–6·3%). The survival of semi‐wild smolts during the sea migration was as high as that of wild Atlantic salmon of an equal size and two to three times higher than hatchery‐reared Atlantic salmon stocked as smolts. The survival rate was positively associated with smolt size. The suitability of hatchery‐reared parr and smolts in the management of reduced Atlantic salmon stocks is compared.     

Effect of density on competition between wild and hatchery‐reared Atlantic salmon for shelter in winter

The effect of varying the density of hatchery‐reared Atlantic salmon Salmo salar on the ability of single wild fish to occupy a shelter is assessed. Although there was strong density‐dependence on sheltering overall, the ability of wild Atlantic salmon parr to occupy a shelter was not affected by the presence of hatchery‐reared fish even when outnumbered by four to one. These findings illustrate a competitive asymmetry for shelter in favour of the wild fish at the densities tested.     

Environmental enrichment and prior experience of live prey improve foraging behaviour in hatchery‐reared Atlantic salmon

Atlantic salmon salmo salar L. parr were reared for 3 months under standard hatchery conditions or in a structurally enriched tank (containing plants, rocks and novel objects). Half of each of these fish had prior exposure to live prey in the form of live bloodworm while the other half were fed hatchery‐pellets. After 12 days all fish were tested on a novel live prey item (brine shrimp). A significant interaction between the two factors (prior exposure to live prey and rearing condition) revealed that foraging performance was only enhanced in fish that had been reared in a complex environment and exposed to live prey. It appears that the ability to generalize from one live prey type to another is only enhanced in fish that had been reared in an enriched environment. The findings support the assertion that the provision of enriched environments in combination with exposure to live prey prior to release may significantly improve the post‐release survival rates of hatchery‐reared fishes. As both the environmental enrichment and the prior foraging experience procedures were comparatively simple, the provision of such pre‐release experiences are likely to prove cost effective to hatcheries.     

Life‐history traits of the common pandora Pagellus erythrinus L., interpreted using information from aquaculture experiments

A study was conducted in in the Dale River as a part of a stock enhancement programme. The aim was to compare growth and the incidence of precocious maturation between offspring from precocious and large maturing males, and to study genotype‐environment interactions. River and hatchery performance was compared for 5 × 2 maternal half‐sib family groups, which were stocked as 0+ juveniles or maintained in the hatchery throughout. To identify the offspring, the broodfish were characterized by DNA‐fingerprinting (eight microsatellite markers). Smolt size of 1+ hatchery‐reared smolt and fish caught in a smolt trap, and the size and incidence of precocious maturation among 1+ hatchery‐reared parr and 1+ and 2+ parr caught in the river are compared between the families.     

Effect of hatchery environment on cranial morphology and developmental stability of Atlantic salmon (Salmo salar L.) from North‐West Russia

The study addresses the effect of hatchery rearing on morphological variation and developmental stability of Atlantic salmon parr from North‐West Russia. Totally, we collected nine samples. Four wild samples were collected from each of the rivers Kola, Umba, Keret’ and Shuia. Five samples of hatchery‐reared parr were the first‐generation progeny of wild adults from these rivers reared separately at the four hatcheries (one hatchery was represented by two samples). Ten meristic and 48 morphometric cranial characters were analysed. We studied the morphological divergence between wild and hatchery fishes of the same river of origin. To analyze developmental stability we used fluctuating asymmetry (random deviations from perfect bilateral symmetry). It was found that hatchery‐reared parr significantly differ from wild parr in both meristic characters and the shape of cranial bones. Different hatcheries caused similar effect on morphological variation in all populations. Fluctuating asymmetry in morphometric characters was significantly higher in hatchery fish than in wild from the Shuia River, indicating a higher level of developmental instability. However, wild parr from the Keret’ River had significantly higher fluctuating asymmetry than cultivated parr of the same origin, possible due to a high infection pressure of the parasite Gyrodactylus salaris Malmberg which has led to significant decline of the wild salmon population in this river, or from genetic changes caused by cultivation. The obtained results indicate a notable effect of hatchery environment on Atlantic salmon’s phenotype.     

Differential lifetime success and performance of native and non‐native Atlantic salmon examined under communal natural conditions

The lifetime success and performance characteristics of communally reared offspring of wild native Burrishoole (native), ranched native (ranched) and non‐native (non‐native) Atlantic salmon Salmo salar from the adjacent Owenmore River were compared. Non‐native 0+ year parr showed a substantial downstream migration, which was not shown by native and ranched parr. This appears to have been an active migration rather than competitive displacement and may reflect an adaptation to environmental or physiographic conditions within the Owenmore River catchment where the main nursery habitat is downstream of the spawning area. There were no differences between native and ranched in smolt output or adult return. Both of these measures, however, were significantly lower for the non‐native group. A greater proportion of the non‐native Atlantic salmon was taken in the coastal drift nets compared to the return to the Burrishoole system, probably as a result of the greater size of the non‐native fish. The overall lifetime success of the non‐native group, from fertilized egg to returning adult, was some 35% of native and ranched. The ranched group showed a significantly greater male parr maturity, a greater proportion of 1+ year smolts, and differences in sex ratio and timing of freshwater entry of returning adults compared to native, which may have fitness implications under specific conditions.     

Genetic implications of hatchery rearing in Atlantic salmon: effects of rearing environment on genetic composition

In an experiment to investigate genetic consequences of hatchery rearing in salmon, allozyme variation at five polymorphic loci was examined in Atlantic salmon of known initial genetic composition, which were reared throughout freshwater life in the hatchery or stocked into the wild as swim-up fry. The genetic composition of the juveniles in the hatchery remained homogeneous from fertilization up to stocking, and from stocking to 2+ in the wild, however, those remaining at the hatchery developed genetic differences among smolting and nonsmolting 1+ parr. These differences were attributed to conditions leading to early smolting at 1+ among the hatchery fish, with 1+ smolts diverging from the gene pool from which they were derived, whereas those stocked into the wild did not smolt until a year later and retained the original genetic composition. The results are discussed in relation to hatchery rearing of salmon and implications for the use of reared fish in stocking and enhancement programmes.     

Physiological and behavioural differences of hatchery and wild‐reared steelhead Oncorhynchus mykiss smolts of the same genetic origin

In many parts of the world release of hatchery‐reared smolts has long been used to mitigate for the deleterious effects of habitat loss and overfishing on salmonid populations. Of increasing concern is whether this may cause harm by spreading non‐native stocks and potentially releasing incompetent smolts. The objective of this study was to determine if smolt physiology and behavior of juveniles produced from a recently founded native broodstock differ from their wild (naturally‐reared) counterparts. In the fall of 2002 and 2003 juvenile wild steelhead were captured, PIT tagged, and returned ( n  = 1360 in 2002 and n  = 2708 in 2003) to Abernathy Creek. In winter of 2003 and 2004 hatchery‐reared fish were PIT tagged and later released ( n  = 1100 in 2003 and n  = 1400 in 2004) into Abernathy Creek. Gill biopsies were collected from wild and hatchery fish throughout the rearing and out‐migration season. The timing and speed of outmigration was assessed using two stationary PIT tag antennas (92–97% efficient). Hatchery migrants in 2003 were larger, had significantly lower gill Na⁺, K⁺‐ATPase activities, and migrated slower than wild fish. Results from the 2004 migratory season will also be presented. This study shows that hatchery rearing can result in smolts which are physiologically and behaviourally different from genetically similar wild fish. Whether these differences are critical enough to affect the rate of adult returns will be determined in future years.     

Predator‐induced hyperventilation in wild and hatchery Atlantic salmon fry

Following exposure to a predator stimulus (a brown trout Salmo trutta ), the opercular rate of Atlantic salmon Salmo salar fry increased by 35·3 ± 11·0%(mean ± 95% CI). The time taken for opercular rate to decline to baseline levels depended upon the occurrence of any associated locomotory activity. Opercular rates of fish that dashed when exposed remained elevated for 38·2 ± 20·6 min, whereas those of individuals that did not move ('freezers') recovered within 7·2 ± 2·9 min. The duration that opercular rate remained elevated was positively correlated with the magnitude of the elevation, which was higher in 'dashers' than freezers. The maximum opercular rate in 'freezers' was similar between wild fry and hatchery‐reared fry (from wild parents). There was a significant delay, however, in hatchery compared with wild fry in the time until peak ventilatory response and onset in the decline phase. This difference in opercular response suggests that hatchery fish were slower to realize fully the potential danger from the predator. Any delay in response could be directly attributed to the effect of hatchery‐rearing environment, rather than domestication or hatchery selection effects.     

Bitterling as models for studies of sperm competition

The plight of the world fish stocks is all too well documented. As part of an ongoing attempt to bolster fish stocks for both commercial and conservation purposes, many fish are reared in captivity and released into the wild. It is well known that hatchery‐reared fish have low post‐release survival compared with wild fish of similar age. Part of the reason for this high mortality is that hatchery fish show deficits in virtually all aspects of their behaviour, including prey selection and predator avoidance. Much behaviour requires repeated experience so that it may become fine‐tuned to prevailing circumstances via learning during development. It has been suggested that inappropriate behaviour is encouraged when fish are reared in the unnatural surroundings of the hatchery. However, hatchery fish can be taught to recognise live, novel prey items and predators and the rate of learning is increased in the presence of a more knowledgeable conspecifics. Here we present data showing how social learning protocols can be used to dramatically increase foraging success in juvenile Atlantic salmon. We also outline related aspects of our ongoing research and discuss some of the possibilities for altering hatchery practices to maximize post‐release survival.     

Density‐dependent growth in hatchery‐reared brown trout released into a natural stream

Hatchery‐reared brown trout Salmo trutta stocked in a natural stream in addition to resident wild brown trout grew more slowly than those stocked with an experimentally reduced density of brown wild trout. In both cases, hatchery‐reared brown trout grew more slowly than resident wild fish in control sections. Mortality and movements did not differ among the three categories of fish. The results showed that growth of stocked hatchery‐reared brown trout parr was density‐dependent, most likely as a consequence of increased competition. Thus, supplementary release of hatchery‐reared fish did not necessarily increase biomass.     

Survival and migration patterns of naturally and hatchery-reared Atlantic salmon (Salmo salar) smolts in a Lake Ontario tributary using acoustic telemetry

1. Atlantic salmon (Salmo salar) smolts are often stocked into rivers to supplement natural reproduction, yet hatchery-reared fish have lower survival compared to wild conspecifics. However, few studies have assessed riverine migratory performance and survival differences in hatchery and wild smolts, or more specifically naturally reared smolts (hatchery fish released earlier as parr), particularly in rivers with weirs which may further reduce survival.
2. Using acoustic telemetry, including a subset of fish with novel transmitters that identify predation events, we assessed survival and migration patterns of hatchery- (2017: n = 32; 2018: n = 30) and naturally reared Atlantic salmon smolts (2017: n = 8; 2018: n = 30) in a Lake Ontario tributary with two weirs to better understand their ecology and assess the influence of environmental parameters on migration.
3. Naturally reared smolts were 13.9 times more likely to survive than hatchery-reared smolts and mark–recapture models indicated that weirs did not reduce survival for either group. Survival per km was lowest at the release site, indicating pre-migration mortality, and specifically high stocking-related mortality of hatchery-reared smolts. Speed and times of day fish migrated (i.e. migratory performance) did not vary by rearing group, suggesting that the high mortality of hatchery-reared smolts may be due to other factors related to hatchery and stocking operations. Overall mean (± SD) migration speed for smolts was 0.70 ± 0.39 km/hr and movements occurred significantly more frequently at night (18:00–06:00).
4. Smolts were detected in Lake Ontario after they left the river; however, the array in Lake Ontario was not conducive to providing much detail regarding movement patterns. There was no predation of the two predation tags detected in Lake Ontario, indicating that movements were made by smolts and not predators.
5. With ongoing restoration efforts of Atlantic salmon in Lake Ontario, it was important to understand the smolt migration patterns and success of the stocked fish. Our findings of similar migratory performance yet different relative survival of hatchery- and naturally reared smolts help inform management with regards to stocking strategies that could improve Atlantic salmon reintroduction success.

     

Feeding of wild and hatchery reared Atlantic salmon (<Emphasis Type="Italic">Salmo salar</Emphasis> L.) smolts during downstream migration

In general, hatchery salmonid smolts experience higher mortality during migration than wild smolts, which is suggested to be due to domestication effects and that hatchery fish lack experience of the natural environment. However, possible differences in feeding during smolt migration between hatchery and wild smolts have rarely been addressed. We compared the number of feeding smolts and stomach fullness among wild Atlantic salmon smolts, hatchery-reared smolts released as 1-year-old parr, and hatchery-reared smolts released as 2-year-old smolts during their descent to sea in River Tornionjoki. In addition, estimations of prey selection among the smolt groups were conducted. A high proportion of wild smolts and smolts stocked as parr actively fed during the smolt migration. A lower proportion of smolts stocked as smolts was feeding and their stomach fullness were much reduced in comparison with the two other groups. The study also indicated that the feeding of migrating smolts is selective rather than opportunistic. In conclusion, this study suggests that stocked 2-year-old smolts may enter sea with an inferior foraging behaviour and it is a possibility that this may contribute to the observed low post-smolt survival in the Baltic Sea.     

Atlantic salmon straying from the River Imsa

Mean estimated straying rate for Atlantic salmon Salmo salar L. leaving the River Imsa as smolts during 1976–1999 was 15% for hatchery fish and 6% for wild conspecifics. Hatchery Atlantic salmon selected for production traits during four or more generations strayed >50%. The straying rate was higher for Atlantic salmon staying 2 rather than 1 year at sea before attaining maturity. For spawning, 96% of the strays entered streams within 420 km from the River Imsa, and c . 80% entered streams within 60 km of the mouth of the River Imsa, whether the fish were wild or hatchery released. Within the 60 km zone, the number of strays caught in a river increased with the Atlantic salmon catch in that river, but there was no significant relationship between straying rate and water discharge or distance from the river to the River Imsa. The observed straying rate of hatchery Atlantic salmon decreased with increasing number of fish entering the River Imsa. Sexual maturation as parr did not influence the tendency to stray. The results suggest that the establishment of temporary zones, free of fish farms, outside important Atlantic salmon rivers by the fisheries authorities in Norway should be large, whole fjords, to be effective.     

Differences in the pattern of antipredator behaviour between hatchery‐reared and wild European sea bass juveniles

Shoals of hatchery‐reared and wild sea bass juveniles Dicentrarchus labrax were tested for differences in their antipredator responses towards a potential live predator, the eel Anguilla anguilla . Eight experimental shoals ( i.e . replicates), each composed of 15 individuals from the same stock of juveniles ( i.e . wild or hatchery), were video recorded for 5 min before and after predator exposure. A set of behavioural variables were measured during the pre‐stimulus and stimulus phases of each test and compared between the two groups of replicates. Results showed that in both hatchery‐reared and wild juveniles predator exposure elicited a significant increase in the mean level of shoal cohesiveness and mean shoal distance from the predator, and a significant decrease in the mean shoal distance from the bottom. Shoals of wild juveniles, however, aggregated more quickly and reached higher shoal cohesiveness within the first 20 s of the stimulus period than shoals of hatchery‐reared fish. During this period, the wild fish also reached the highest peak in shoal cohesiveness, which then decreased gradually towards the levels observed before predator exposure. Another component of the antipredator response, the predator inspection behaviour, was fully developed in both wild and hatchery fish. Wild fish, however, tended to inspect the predator at a closer distance than hatchery fish.     

Size and age of maturity of Atlantic salmon correlate with the North Atlantic Oscillation Index (NAOI)

Sea‐age at maturity of Atlantic salmon Salmo salar decreased with increasing values of the seasonal NAOI from February to April. Body mass increment from smolts to adults of one‐sea‐winter Atlantic salmon increased with increasing NAOI in May at the time when the juveniles moved to sea.     

Heart rate responses to predation risk in Salmo trutta are affected by the rearing environment

Both wild‐ and hatchery‐reared brown trout Salmo trutta , 18 months of age and of the same genetic origin, responded with increased heart rates (tachycardia) to a simulated predator attack on 2 consecutive days. Brown trout reared in the hatchery showed a more rapidly induced tachycardia compared with wild‐reared fish at day 1, but not day 2. During an undisturbed period several hours after attacks, hatchery‐reared brown trout maintained higher heart rates compared to wild‐reared fish on both days. Behavioural responses to the attack were very low for all fish, although hatchery‐reared fish tended to be more active than wild fish after the attack day 2. The observed differences may have had a genetic background caused by different selection regimes in the hatchery‐ and wild‐rearing environments, or could have been due to different phenotypic responses in the two environments.     

Interactions between endangered wild and hatchery salmonids: can the pitfalls of artificial propagation be avoided in small coastal streams?

Hatchery and wild juvenile populations of steelhead Oncorhynchus mykiss and coho salmon Oncorhynchus kisutch , in a small coastal watershed in central California, were sampled throughout the year in a stream and at a hatchery. Both species grew faster in captivity than in the wild. Hatchery fish of both species had elevated gill Na⁺, K⁺‐ATPase activity, and thus were ready to enter sea water when planted during the wild fish migration. Downstream migrant trapping and stream surveys indicated that hatchery smolts went to sea soon after planting, consequently avoiding the effects of competition and predation that commonly occur when hatchery‐bred juveniles are released. Adult steelhead were also sampled throughout the watershed. The return of hatchery steelhead was highly synchronized with that of wild steelhead, indicating that hatchery propagation had no adverse effects on the timing of the run. A disproportionate number of hatchery steelhead returned to the tributary where the hatchery was located, despite being planted throughout the watershed. Hatchery steelhead did not differ in mean age or size from wild steelhead. Observations of spawning indicated that hatchery and wild steelhead interbreed. Competition for mates or spawning substratum was rarely observed between hatchery and wild steelhead. Many of the problems commonly associated with artificial propagation can be avoided in small coastal watersheds when wild broodstock are used and fish are released as smolts.     

Gene expression patterns in Atlantic salmon Salmo salar: gene expression during osmoregulation in intestine tissue

Ireland has the world's largest stocks of wild Atlantic salmon. A better understanding of gene expression will benefit conservation of wild stock as well as salmon aquaculture. We describe the PRTLI project designed to advance the fundamental understanding of the genome of Atlantic Salmon Salmo salar . The major objective is to create the first comprehensive database of gene expression and functional information using cDNA libraries and Microarray technology. One key area of interest to salmon biology is osmoregulation, which is critical to the ability of salmon to adapt in seawater. Tissues implicated in this process are the gills, intestine and skin. To initiate studies, SSH (suppression subtractive hybridization) libraries were constructed from intestine RNA extracted from smolts sampled in January and May. A number of potentially interesting clones have been identified, among those a heat shock protein, hsp90 in the reverse library. Others SSH libraries from various tissues (pituitary, hypothalamus, brain, gill, intestine, head kidney and spleen) have also been constructed and will be used to construct a 5000 clone microarray slide. This slide will then be used to elucidate gene expression profiles in various tissues. Further sample collection has been carried out to answer questions regarding biologicaldifferences between one‐ year and two‐year old parr and wild and hatchery smolt.     

The importance of stocking age in the enhancement of River Kymijoki salmon (Salmo salar)

Hatchery‐reared Atlantic salmon parr and smolts are regularly released into the River Kymijoki, Southern Finland, to enhance the local river and coastal fisheries. In 1988–1994, a series of micro‐ and Carlin‐tagging experiments were carried out to examine the influence of stocking age on the stocking results. Five age groups were compared: 1‐year and 2‐year‐old smolts, and 1‐summer, 1‐year and 2‐summer‐old parr. The aim was to determine the most profitable way of producing salmon for fishing in the River Kymijoki. Stocking age had a strong influence on the stocking results, measured as the proportion of adult recoveries. The most favourable results were obtained using 2‐year‐old smolts (survival index 100), followed by 1‐year‐old smolts (52), 2‐summer‐old parr (51), 1‐year‐old parr (37) and 1‐summer‐old parr (24). Available data on rearing costs suggest that 2‐year‐old smolts were also the most economically profitable choice, followed by 1‐year‐old parr, 1‐year‐old smolts and 1‐summer‐old parr. The most expensive way of producing salmon was by stocking 2‐summer‐old parr.