Determination of body shape variation in Irish hatchery‐reared and wild Atlantic salmon

Discriminant function analysis was used to distinguish morphologically between samples of parr, smolts and adult Atlantic salmon Salmo salar from several hatchery and river systems in Ireland. The effect of habitat shift was investigated in Atlantic salmon parr. Parr grown from the eyed‐egg stage with a non‐sibling group in a hatchery environment, came to resemble the mean body shape of their host hatchery Atlantic salmon stock more closely than that of a full sibling group grown at their natal hatchery. Wild Atlantic salmon smolts differed in shape from hatchery‐reared smolts. This difference was less pronounced, but still statistically significant when wild adults were compared with hatchery‐reared adults captured in the coastal drift‐net fishery after a year spent at sea. Rearing conditions had a significant impact on the production and growth of fish body shape. This in turn may have affected adaptability and survivorship of ranched Atlantic salmon in the marine environment.     

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.     

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.     

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.     

Stocking hatchery‐reared brown trout in different densities into a wild population – a comparison of growth and movement

In spring 2001 and 2002 a small stream was stocked with tagged hatchery‐reared yearling brown trout ( Salmo trutta ), in order to study their influence on the resident brown trout population. The stream was separated into six sections: two sections without stocking, two sections where stocking doubled the trout population and two sections where the fish population was quadrupled. The working hypothesis was that due to food limitation (competition) growth of the wild fish will be negatively influenced by stocking, and wild fish will be displaced by the (possibly more aggressive) hatchery fish. Surprisingly, growth rate of wild and stocked fish of the same age was similar and independent of stocking density. Two main reasons may be responsible for this finding: only a low percentage of the stocked fish remained in the stream, and food was not limited during summer. Only 12–19% of the stocked fish were recaptured after six months, in contrats to 40–70% of one‐year old and up to 100% of older wild trout. The wild fish were not displaced by hatchery‐reared fish: During summer the wild fish remained more or less stationary, whereas most of the stocked trout had left their release site. The results indicate that in a natural stream stocking of hatchery reared brown trout does not influence negatively growth and movement of the wild fish independent of stocking density.     

Migration of hatchery‐reared Atlantic salmon and wild anadromous brown trout post‐smolts in a Norwegian fjord system

Hatchery‐reared Atlantic salmon Salmo salar ( n  = 25) and wild anadromous brown trout (sea trout) Salmo trutta ( n  = 15) smolts were tagged with coded acoustic transmitters and released at the mouth of the River Eira on the west coast of Norway. Data logging receivers recorded the fish during their outward migration at 9, 32, 48 and 77 km from the release site. Seventeen Atlantic salmon (68%) and eight sea trout (53%) were recorded after release. Mean migratory speeds between different receiver sites ranged from 0·49 to 1·82 body lengths (total length) per second (bl s⁻¹) for Atlantic salmon and 0·11–2·60 bl s⁻¹ for sea trout. Atlantic salmon were recorded 9, 48 and 77 km from the river mouth on average 28, 65 and 83 h after release, respectively. Sea trout were recorded 9 km from the release site 438 h after release. Only four (23%) sea trout were detected in the outer part of the fjord system, while the rest of the fish seemed to stay in the inner fjord system. The Atlantic salmon stayed for a longer time in the inner part than in the outer parts of the fjord system, but distinct from sea trout, migrated through the whole fjord system into the ocean.     

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.     

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.     

The timing, sex and age composition of the wild and reared Atlantic salmon ascending the Simojoki River, northern Finland

In the Simojoki River in the northern Gulf of Bothnia, reared salmon stocked as smolts produced considerable numbers of ascending one‐sea‐winter (1 SW) males, whereas the proportion of male 1 SW salmon was low among spawning migrants of wild or reared parr origin. The sex ratio among ascending wild fish and reared salmon stocked as parr was similar, with females predominating, while reared salmon stocked as smolts were mainly males. The multi‐sea‐winter (MSW) salmon entered the river annually within a fairly short time period from the beginning of the migration season, independent of their sex or origin. 1 SW males migrated into the river significantly later in the season than MSW males. The results indicate that the delayed opening of the fishing season in the Gulf of Bothnia is effective in reducing the harvest of MSW salmon at sea. However, as the timing of the ascent may vary by several weeks from year to year, the effect of this regulation bound to certain calendar days may also vary considerably from year to year.     

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.     

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.     

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.     

Trophically transmitted parasites in wild Atlantic salmon post‐smolts from Norwegian fjords

The community structure of trophically transmitted intestinal helminths of Atlantic salmon Salmo salar post‐smolts was highly variable among four fjords in Norway. There were no severely pathogenic parasite species. Post‐smolts from the southernmost Trondheimsfjord had a higher diversity of freshwater parasite species compared to the three northern fjords (Tanafjord, Altafjord and Malangen). In contrast, the highest diversity and proportion of marine species was found in the three northern fjords. Post‐smolts were generally more infected with marine parasites in the outer rather than inner parts of all of the fjords. The prevalence of the acanthocephalan Echinorynchus gadi (range: 13–42%) and marine trematodes (range: 14–47%) was higher in post‐smolts in outer zones of the northern fjords than in fish from Trondheimsfjord (0 and 6%, respectively). The within‐fjord variability and north‐south geographical gradient in parasite infection patterns reflected differences in marine feeding of the post‐smolts on potential intermediate hosts such as amphipods ( E. gadi ) and fish larvae (trematodes), which were higher in the northern fjords (range: 27–28 and 67–85%, respectively) than in Trondheimsfjord (5 and 19%, respectively). High intensities of marine parasites suggest that some post‐smolts from northern fjords may have a prolonged fjord‐feeding compared to those from Trondheimsfjord. Parasites of both freshwater and marine origin appear to be suitable as bio‐indicators of feeding and migratory pattern of Atlantic salmon post‐smolts and preadults during their seaward migration.     

Long-term study of the ecology of wild Atlantic salmon smolts in a small Norwegian river

Backcalculated lengths at the end of the first growth season in wild Atlantic salmon Salmo salar differed significantly between parr smolting at age 1, 2 and 3 years over a period of 11 years (i.e. 1983–1993). Mean body lengths of the respective age groups at the end of the first growth period were 11·1, 6·2 and 4·7 cm, respectively. The mean percentage distribution of fish smolting at age 1, 2 and 3 was 14, 78 and 7%, and the mean smolt age was 1·95 years. Mean lengths at smolting of age groups 1, 2 and 3 were 13·6, 15·8 and 17·5 cm, respectively. Females outnumbered males among the downstream migrating smolts with a mean sex ratio (females/ males) estimated at 1·61, with a significant female surplus in 7 of the 11 years sampled. Of the smolts sampled, 14% exhibited enlarged gonads indicative of parr maturation, and all were males (37% of the parr males sampled). Mean annual smolt density from 1975 to 1996 was 13·4 individuals 100 m⁻² ranging between 0·3–31 smolts 100 m⁻². Mean densities (100 m⁻²) of the smolts aged 1, 2 and 3 years were 1·5, 9·3 and 0·9 fish, respectively. Mean annual biomass for the 22-year period (1975–1996) was estimated at 437 g 100 m⁻², with a range of variation from 136 to 683 g 100 m⁻². Smolt age 2 made up 81% of the mean annual biomass (355 g 100 m⁻²) and smolt age 1 and 3, 8% (35 g 100 m⁻²) and 11% (47 g 100 m⁻²), respectively.     

Temporal stability of sea louse Lepeophtheirus salmonis Krøyer populations on Atlantic salmon Salmo salar L. of wild, farm and hybrid parentage

Atlantic salmon salmo salar smolts of wild, hybrid and farmed parentage were individually tagged then reared in a sea cage for 8 months. The fish were sampled three times during this period. On all occasions, farmed Atlantic salmon displayed the highest abundance and density of sea lice Lepeophtheirus salmonis , whilst no significant differences were observed between hybrid and wild Atlantic salmon. Percentage variation between the lowest and highest infected groups was as high as 175 and 144% for L. salmonis abundance and density respectively (sample 2). The temporal stability of interindividual sea lice infection levels was investigated pair‐wise between samples using correlation (sample 1 v . 2, 1 v . 3 and 2 v . 3). When calculated using sea louse abundance, correlations ranged from r ² = 0·11, P  < 0·01 to r ² = 0·39, P  < 0·001, but, when the effects of fish size were controlled for by converting abundance to density, all correlations were <  r ² = 0·1. Therefore, these data indicate that a fish's relative infection level in one sample was a weak predictor of its relative infection level in another sample. This suggests that identification of individual Atlantic salmon that display reduced susceptibility to sea lice, may be problematic.     

Inter- and intra-population morphological differences between wild and farmed Atlantic salmon juveniles

Whether population-specific morphological differences were detectable in small (26–52 mm) Atlantic salmon Salmo salar parr reared under similar conditions was tested. Discrimination based on morphological characters was total (100%) between the fish of farmed origin (from AquaGen) and four wild fish populations. Between the four wild populations the corresponding discrimination was 59·8–86·3%. The inter-population variation in morphological characters was larger than the intra-population variation. The fish originating from the local populations at Driva and Innfjord were narrower in body form, whereas fish from the AquaGen and Innfjord populations had smaller and less pointed heads with smaller eyes. The Driva population fish had the smallest mouth while the longest pectoral fin was found in the Bjoreio population, the river that also has the largest fall gradient. Population-specific morphological characters were thus detectable among Atlantic salmon parr relatively rapidly after yolk absorption.     

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.     

Association between environmental factors, smolt size and the survival of wild and reared Atlantic salmon from the Simojoki River in the Baltic Sea

The survival of Atlantic salmon Salmo salar in the Baltic Sea was examined in relation to smolt traits (length and origin) and annual environmental factors [sea surface temperature (SST) and seasonal North Atlantic Oscillation (NAO) index], and prey fish abundance (herring Clupea harengus and sprat Sprattus sprattus) in the main basin and the southern Gulf of Bothnia. The study was based on recapture data for Carlin‐tagged hatchery‐reared and wild smolts from the Simojoki, a river flowing into the northern Gulf of Bothnia. The survival of the wild and reared groups was analysed using an ANOVA model and a stepwise regression model, with the arcsin‐transformed proportion of recaptured fish as the response variable. The results demonstrated a combined influence of smolt traits and environmental factors on survival. For the reared Atlantic salmon released in 1986–1998 (28 groups), the increasing annual mean SST in July in the southern Gulf of Bothnia and increasing mean smolt size improved survival. If the SST in July was excluded from the model, the NAO index in May to July also had a positive effect on survival (P < 0·10). The log₁₀‐transformed abundance of 0+ year herring in the southern Gulf of Bothnia entered the model (P < 0·15) if the SST and NAO index were excluded. For the wild Atlantic salmon released in 1972–1993 (21 groups), only the increasing SST in July showed a significant association with improved survival (P = 0·004). Prey fish abundance in the main basin of the Baltic Sea had no influence on the survival of reared or wild smolt groups. The interaction between smolt size and the SST in July was not significant. The origin was a better, but not a significant, predictor of marine survival compared to the smolt size or the SST in July. The mean recapture rate of the wild groups was twice that of the reared groups in the whole data. The results suggest that cold summers in the Gulf of Bothnia reduce the survival of young Atlantic salmon in both wild and reared groups. The larger smolt size of the reared groups compared with the wild groups to some extent compensated for their lower ability to live in the wild.     

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.

     

Divergence in smolt production from the stocking of 1-summer-old and 1-year-old Atlantic salmon parr in a northern Baltic river

One‐year‐old and 1‐summer‐old salmon parr have been stocked in the Simojoki, a northern Baltic river, to protect the natural salmon stock from extinction. This enhancement practice was studied during a 5‐year stocking period, when an average of 14.3 1‐summer‐old parr and 5.4 1‐year‐old parr was needed to produce one sea running smolt. The mean yield from stocking was 70 smolts per 1000 stocked 1‐summer‐old parr and 186 smolts per 1000 stocked 1‐year‐old parr. The mean cost of smolts produced by stocking 1‐year‐old and 1‐summer‐old parr was 1.2 and 2.6 times the cost of producing one 50 g smolt in the hatchery, respectively. There was a negative relationship between density of wild parr and length of smolts stocked as 1‐summer‐old parr in the river, but no relationship between wild parr density and length of smolts stocked as 1‐year‐old parr. This indicated that 1‐summer‐old parr were more susceptible than the older parr to competition with wild parr. Considering production costs and smolt yield, stocking of 1‐year‐old parr appeared to be more profitable than that of 1‐summer‐old parr in the enhancement of the endangered salmon stock.