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background
The domestication of Atlantic salmon for commercial aquaculture has resulted in farmed salmon showing much higher growth rates than wild salmon under cultured conditions.
In contrast, the difference in growth between farmed salmon and wild salmon is much smaller compared to wild salmon. The mechanism behind this contrast between environments remains largely unknown.
Farmed salmon may have adapted to high-energy pellets developed specifically for aquaculture, causing growth differences to swell when fed on this diet.
We studied the growth and survival of 15 farmed, wild and F1 hybrid salmon families that were fed three contrast feeds under hatchery conditions, commercial salmon pellet feed, commercial carp pellet feed and mixed natural diet, including preserved invertebrates commonly found in Norwegian rivers.
outcome
For all groups, although all diets provided the same number of calories, up to 68% and 83% reduction in overall growth relative to the salmon diet were observed in carp and natural diet treatments, respectively, with farmed salmon outperforming crossbred and wild salmon in all treatments.
The relative growth differences between wild and farmed fish were highest in carp diets, moderate in salmon diets and lowest in natural eclipses.
conclusion
No signs of genetically-based adaptation to the form or nutrient content of commercial salmon feed were found in farmed salmon, so we conclude that diet alone is not the main reason for the huge difference in hatchery and wild salmon growth, at least in the absence of other environmental stressors.
Furthermore, we conclude that genetically increased appetite may be the main reason why farmed salmon exhibit higher growth rates than wild salmon when temporarily fed under hatchery conditions.
background
Aquaculture is now the fastest growing food sector in the world, providing more than half of the world's fish protein. One of the most economically important aquaculture species is Atlantic salmon, an anadromous salmon endemic to the northern hemisphere's Atlantic West and East Coast rivers.
Atlantic salmon farming originated in Norway in the late 1960s, and in recent years the industry has grown globally, including commercial efforts in many countries within and outside the species' natural range. Global production of Atlantic salmon currently exceeds 30,000 tonnes, more than half of which is produced only in Norway.
Selective breeding programs began shortly after the first commercial farming in Norway began, and current salmon strains have undergone up to twelve or more generations of directed selection to acquire commercially important traits.
The initial breeding goal of salmon farming was to increase growth rates, followed by delayed sexual maturation, and soon expanded to include disease resistance, flesh color and body composition.
It is estimated that the genetic gain in salmon growth rate is 10-15% per generation, therefore, under hatching conditions, selection increases the growth rate of farmed salmon several times compared to wild salmon.
In intensive aquaculture, feed is continuously provided in the form of high-energy pellets formulated to provide fish with the specific nutritional needs of all species while maximizing feed utilization.
In commercial salmon aquaculture, feed is one of the highest operating costs, which can be as high as 60% of production costs. As awareness of Atlantic salmon's nutritional needs grows, commercial diets continue to refine to more closely meet energy and nutrient needs while striving to utilize more cost-effective ingredients.
Salmon is carnivorous and requires a diet high in protein and containing essential fatty acids. Traditionally, these nutrients are obtained by adding large amounts of fishmeal and fish oil to the salmon diet.
However, given sustainable intensification, marine sources of protein and lipids in salmon diets are slowly declining in favor of plant alternatives. As a result, the commercial salmon diet deviates not only from the wild diet in terms of form, but also in terms of energy content and nutrient content.
The natural diet of wild fish can vary greatly in terms of prey type and form, calorie density, and nutrient content, and in freshwater habitats, juvenile salmon typically feed on drifting and benthic invertebrates, the availability of both will depend on specific habitat characteristics such as flow rate and substrate.
Domestication involves adapting to captivity, which is very different from the natural environment experienced by wild conspecifics. These differences can lead to phenotypic and behavioral differences between domesticated and wild individuals, and domestication-mediated genetic changes may occur within a generation.
These changes are the result of direct and indirect responses to artificial selection and relaxed natural selection, and low mortality associated with the home environment can lead to phenotypes persisting without persisting in the wild.
Farmed salmon have been fed with pelleted feed since the beginning of commercial salmon farming, while in the wild fish are opportunistic feeders and actively seek out feed, often changing their diets to obtain the essential nutrients they need to grow.
Thus, adaptation to commercial salmon pellets may partly explain why such large differences in growth between farmed and wild salmon have been observed under cultured conditions, while much smaller differences have been observed under natural conditions.
We hypothesize that if farmed salmon are genetically adapted to the nutrient content or form of pelleted salmon feed, they will not be able to maintain a large relative growth difference compared to wild salmon when fed a commercial pelleted feed with unfamiliar nutritional content, or when fed a diet similar to a natural diet.
The Mowi strain is the oldest domesticated salmon strain in Norway. The Mowi strain was first established in the 1960s by salmon populations in rivers along Norway's west coast, with major contributions coming from the Borstad and Oroi rivers.
Among other traits, this strain was mainly selected to increase the growth rate and underwent more than ten generations of selective breeding.
As a result, the offspring of Mowi farmed salmon show significantly higher growth rates under standard hatchery conditions compared to the offspring of wild salmon. In the wild, however, this cultured strain grows only slightly faster than wild conspecifics.
discuss
When cultured together in hatcheries and under commercial farming conditions, farmed salmon grow significantly faster than wild salmon, but in the wild there is little or little difference in growth between these groups.
However, the mechanisms behind this contrast between environments remain more or less completely unknown, and unraveling these mechanisms is important for our understanding of the genetic changes that occur as a result of domestication in farmed salmon and the long-term evolutionary consequences of breeding escapees and wild conspecifics of salmon hybridization.
grow
The carp diet contains about 4.5 times more carbohydrates than salmon feed, one-third lower protein and half the lipid content of salmon feed.
The ability of fish to utilize carbohydrates varies by species and carbohydrate complexity, and salmon is not as effective as some other fish in this regard.
Commercial salmon diets typically contain low levels of carbohydrates because salmon do not require high levels of carbohydrates in their diets, unlike warm-water species such as carp, and while adding small amounts of carbohydrates can promote the utilization of other nutrients, most of the energy needs of farmed salmon come from high dietary levels of lipids and protein.
Therefore, the lower growth relative to the control diet observed in carp treatment is most likely due to a mismatch in dietary levels of specific nutrients resulting in all fish being unable to make full use of or digest food efficiently.
Previous studies have shown that high levels of dietary carbohydrates negatively affect feed utilization and growth in several species of fish, including Atlantic salmon, European sea bass, and Wuchang seabream.
Domestic selection for growth affected a variety of feeding-related traits, including appetite and FCE. Thodesen et al. found that, under controlled conditions, farmed salmon consume more food than wild conspecifics, use food more efficiently, and attribute this to genetic changes in domesticated fish growing through direct selection.
Similarly, Handeland et al. found that under controlled conditions, farmed salmon smolt had significantly higher growth and overall FCE compared to wild salmon.
Adaptations to the nutritional content of commercial diets were indirectly tested by comparing the growth of farmed and wild salmon when nutritionally compared with commercial pelleted feed and a diet composed of natural prey.
Salmon are generally found to grow lower than household conditions under natural conditions, and growth is closely related to water temperature, and growth is also associated with the metabolic costs associated with actively hunting for prey, defending territory, avoiding predators, and abundant food and energy in river systems.
Since this study was conducted in hatcheries without predation, where there were no restrictions on food and fish did not have to actively seek prey, the overall increase in natural dietary treatments, i.e. an 83% reduction in growth, was less likely to be attributed to any of the above.
While efforts are made to ensure that a natural diet contains similar calorie content to other diets, fish may not be able to obtain and utilize the right nutrient balance to maximize growth. Or simply, fish may not be able to consume enough of this water-rich food to match the calorie content of the two formulas, which limits their growth.
As mentioned above, farmed, hybrid, and wild salmon exhibited similar growth response norms between control and natural diet treatments, i.e., all groups showed an 83-84% growth reduction relative to their respective growth in the control treatment.
If farmed salmon are adapted to a commercial diet, or if wild salmon simply do not eat pellets like farmed salmon, one would expect the relative growth difference between farmed and wild salmon to be significantly lower when fed a natural diet compared to pelleted feed.
It has been suggested that appetite may be the main driver of the observed differences in growth between farmed and wild salmon when fed on temporary diets.
Due to differences in environmental experience relative to wild salmon, farmed salmon that escape into the wild may not initially be accustomed to actively seeking and selecting prey. Release experiments have shown that farmed salmon previously farmed with pellets are less likely to actively feed in their natural environment than wild conspecifics and are more likely to ingest prey with lower nutritional value.
In general, after a period of domestication, farmed fish exhibit similar feeding behavior to their wild conspecifics, although this usually depends on the stage of life.
In this study, the natural diet consists of dead organisms, so a natural diet may be too readily available for fish, and using live food where fish have to chase their prey on their own may elicit different responses between salmon populations.
Although the absolute growth differences observed in this study between the farmed, hybrid, and wild salmon experimental groups were lower than previously observed under hatching conditions, it is clear that multigenerational selection resulted in farmed salmon growing more than their wild conspecifics, although this effect was not pronounced in the wild.
survive
Studies have shown that fish raised in captivity and fed only commercial feed have low survival rates in the wild once they are released or escape, as they cannot effectively switch from pelleted feed to natural feed initially.
The Comparative Survival in the Field study found that farmed fish had lower freshwater survival rates compared to wild conspecifics, and hybrids generally showed moderate survival. Skala et al. observed a significant reduction in survival of the offspring of farmed fish grown as eggs in natural river systems relative to their hybridized and wild conspecifics.
Similarly, Fleming et al. found that the offspring of farmed fish had lower early survival rates in the wild than wild conspecifics, although there was no difference in survival rates at later stages.
Among other things, lower survival rates in farmed salmon may be the result of inefficient feeding behaviors and differences in behavior, such as increased aggression or reduced predator awareness, which may also expose fish from farmed backgrounds to more predation than wild conspecifics.
Farmed fish may also have adapted to the form and nutrient content of commercial salmon feed and therefore lost their ability to forage in the wild, resulting in their low survival rates in nature. If farmed salmon lose their ability to digest natural feed, they are expected to show the lowest survival rate in natural handling.
However, farmed salmon show the highest average survival rate in natural handling. Therefore, there is no evidence that farmed fish have adapted to the form and nutrient content of commercial salmon feed, and when fed entirely to mimic the natural diet in the wild, it affects the survival of their offspring.
In fact, as mentioned above, studies have shown that the dietary composition of wild-farmed salmon tends to overlap with that of wild salmon.
Egg size is significant and positively correlated with survival rates, indicating that larger egg size is conducive to survival under these conditions. Studies have shown that egg size has a positive effect on salmon survival, which may explain why the average egg of wild fish in this study was larger than that of their conspecifics.
conclusion
This study provides insights into the potential genetic changes that occur as a result of domestication in salmon, as well as the underlying mechanisms of genetic and ecological interactions between farmed escapees and wild salmon after crossbreeding in the wild.
In addition to the sustainable development of aquaculture, understanding the impact of differences in farming, hybridization and growth of wild fish is important for the conservation and management of wild fish.
Overall, these results suggest that increased appetite under temporary feeding conditions compared to wild salmon is the main reason for the increased growth rate of farmed salmon.
Therefore, we encourage further research in wild or semi-natural conditions to clarify why farmed salmon do not grow widely in their natural environment.
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