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Wen 丨 Yujie History Museum
Editor丨Yujie History Museum
Since the 80s of the 20th century, China's large-surface cage culture technology has gradually developed and expanded. This technology takes full advantage of the excellent large-surface fishing environment, combined with efficient cage culture methods, thus changing traditional farming and fishing patterns.
This innovation has produced high-quality aquatic products, brought strong market competitiveness to the aquaculture industry, and promoted the vigorous development of the large surface cage aquaculture industry.
Cage culture adopts a variety of methods, mainly using floating platforms built of pontoons, and the bottom of the water is kept in captivity with fishing nets, forming a natural fishing ground, and high aquaculture density is also the most direct and efficient way to use water resources.
In recent years, in order to pursue high yield and economic benefits, people have blindly expanded the farming density, but have not achieved the expected high yield, but have seriously affected the aquatic environment and the welfare of fish.
Fish benefits include freedom from hunger and disease, freedom from pain and external stimuli, comfort in life, and mental well-being.
Although fish welfare still needs to be improved relative to animal welfare, its importance is gradually gaining recognition with the attention of the public, legislatures and the research sector.
So, let's follow Yujie to see: in the process of raising rainbow trout, the change of reservoir cage density has an impact on the welfare and physiological condition of rainbow trout.
Rainbow trout rearing and fish welfare
Rainbow trout, a salmon family known for having clear rainbow-like bands on one side of its body, is native to the Pacific coast of North America, especially in mountain streams in California, USA, and prefers to inhabit clear and unpolluted cold water, with strict water quality requirements.
Rainbow trout can only grow in water temperatures below 20°C, the most suitable temperature is 16-18°C. Water temperatures below 7°C or above 20°C can lead to decreased appetite and slower growth.
When the dissolved oxygen is lower than 4.3mg/L, there will be a "floating head" phenomenon, less than 3mg/L will be fatal, in addition, rainbow trout requires the pH of the water body to be between 6.5-8, ammonia nitrogen content less than 0.5mg/L.
Rainbow trout are carnivorous fish that feed on zooplankton, benthic animals and aquatic insects in juvenile stages and fish, crustaceans, shellfish and aquatic insects in adults.
In artificial farming, the bait of rainbow trout is rich in animal protein, and the meat of rainbow trout is tender, delicious, high in nutritional value, rich in protein and unsaturated fatty acids, so the market demand is increasing.
Animal welfare mainly covers two core aspects: one is the definition of animal welfare, and the other is how to measure the indicators of welfare, after all, the definition of animal welfare is diverse.
But no matter how you explain it, the basic elements of animal welfare include: physical, environmental, hygienic, behavioral and psychological.
The concept of animal welfare can be applied to a variety of animals, including some fear-prone animals and higher animals, and the welfare of fish has been widely debated in recent years, especially regarding whether fish are sentient.
Some studies have shown that fish lack important brain tissues or functional parts of perceptual perception, making it difficult to determine whether they feel pain or fear.
Other studies have provided anatomical, physiological, and behavioral evidence that fish have pain receptors and therefore may experience pain and fear during their life cycle.
The brain structure of fish is different from that of humans, so they are unable to produce pain or fear similar to that of humans and the corresponding emotional responses.
This complicates determining whether fish respond to pain and fear, and if fish may be affected by pain or discomfort during their life cycle, then their perception of pain or fear becomes important.
To ensure the healthy growth of fish, we need to provide them with good environmental conditions. While good health is an important part of fish welfare, it's not the only aspect of concern.
At present, due to limited scientific research means and technology, it is difficult to systematically and objectively measure other welfare indicators except for diseases and physical health.
Current research assesses fish welfare primarily through a combination of physiological, biochemical and behavioral indicators, rather than relying solely on changes in a single indicator.
For example, physiological stress responses, such as the release of cortisol, are only natural responses to external conditions and do not alone represent whether fish welfare is threatened or whether external conditions pose a threat.
Similarly, a behavioural analysis of fish welfare alone is not sufficient to provide reliable information, so the study of fish welfare requires a combination of physiological and behavioural indicators.
So, how does fish welfare affect changes in water temperature?
Fish welfare is affected by water temperature
Temperature is one of the important environmental factors for fish growth and physiological and biochemical indicators.
Different species have different requirements for optimal growth temperature, and the optimal growth water temperature of Sturgeon is 21.53°C. Below or above this temperature significantly reduces its specific growth rate, daily gain weight, food conversion ratio (FCR), and feed intake rate (FR).
It grows best in the range of 23-32°C, and its absolute and relative weight gain rates increase as the temperature increases, but decreases below 23°C or above 32°C.
Juvenile anchovy show higher feeding rates and intra-gastrointestinal feeding coefficients at water temperatures of 15 °C and 20 °C, with feeding rates up to 100% and 80% at 10 °C.
Grass carp grew significantly better than other temperature treatment groups at 32°C, and the feeding rate and food conversion rate were also higher, but the mortality rate of grass carp was higher at 35.5°C.
Gem bass showed the greatest daily gain weight at a water temperature of about 30°C, which was significantly different from other temperature treatment groups. Turbot showed the highest survival rates at 15°C and 18°C water temperatures, 97.5% and 95%, respectively.
When the water temperature is low, the metabolic level and digestive enzyme activity of the fish body decrease, resulting in a decrease in appetite, thereby affecting the growth, and the protease activity and amylase activity of Sturgeon juveniles are significantly lower than those at 21°C at 14°C.
Higher water temperature will lead to an increase in metabolic rate, produce more CO2 and ammonia nitrogen substances, affect the physiological and biochemical processes of fish, and threaten its normal growth.
Blood indexes will also change accordingly under different temperature treatment, and the number of red blood cells, glutamate aminotransferase (ALT) enzyme activity, total protein (TP), globulin and other indicators of rainbow trout will change with the increase of temperature at different temperatures.
Temperature has an important impact on fish growth, physiological and biochemical indexes and blood indicators. High temperatures can promote the proliferation of harmful microorganisms, adversely affecting the health and immune system of fish.
In addition, what is the effect of farming density on fish growth?
Effect of culture density on fish growth
Farming density has different effects on the growth and production of various fish species, and here is a compilation of some of the results of research on different fish species and their culture density:
Effects of culture density have been reported in fish such as rainbow trout, Nile tilapia, grouper, Arctic char, African catfish, gilthead seabream and lake sturgeon. In general, low-density farming helps to improve growth and survival, but there may be differences between species.
Less research has been done on benthic fish such as turbot, toothfish, Atlantic flounder, caninth, etc., but farming density may also affect their growth and production.
In general, low-density culture contributes to improved growth and survival, as evidenced by fish such as African catfish (Toko et al.), Thai catfish, sturgeon, silver catfish and Philippine tilapia, but not all fish species follow this pattern, such as spotted forktail catfish, Nile tilapia and luteal ray.
In domestic research, low-density aquaculture is not conducive to the growth and development of spot-banded grouper, and too low density may waste the aquaculture water.
When the farming density increases to a certain extent, it will also have a negative impact on the growth of the fish, because the high density will increase the fish's own energy consumption.
Different fish species differ in terms of culture density, and the optimal culture density depends on the fingerling and the specific culture environment.
The adjustment of culture density can affect the production performance of fish by affecting the physiological and biochemical processes of fish, such as feed intake, growth indicators and neuroendocrine, in fish farming, the choice of culture density needs to be adjusted and optimized according to the specific situation.
So in addition to these, what other effects do changes in farming density have on rainbow trout?
Effect of culture density on serum cortisol content
Stock density plays a key role in fish production, with profound implications for fishers' economic performance.
High stocking densities are often seen as an environmental stress that negatively affects fish growth and development. Several studies have pointed out that many indicators of fish growth are closely related to stocking density.
The effect of stocking density on fish growth has been studied in a variety of fish species, including toothfish, tail spotted carp, African catfish, filamentous heterogeneous catfish and Senegalese sole.
High-density stocking affects fish growth indicators through crowding stress and changes in water quality. Some studies have found elevated plasma cortisol levels in fish in overcrowded environments.
High-density farming may lead to deterioration of water quality, which can affect fish growth by reducing feed intake or food conversion rate, and the effect of this stress on feed rate or food conversion rate suggests that the distribution of energy from food between growth capacity and metabolic capacity may not be constant.
In terrestrial vertebrates, stress has been shown to increase the metabolic activity of the body, thereby increasing the metabolic rate, resulting in slower growth.
Stress can also trigger an increase in the fish's energy requirements, which affects the activity of digestive enzymes, so the body composition of the fish may also change.
While stress has been reported to increase fish energy requirements, almost all of the data comes from acute stress.
Acute stress can lead to instantaneous changes in fish metabolic function, including increased plasma cortisol levels, increased blood sugar levels and changes in fish behavior, etc., whether long-term chronic stress can cause changes in fish growth and physiological indicators, more experiments are needed to verify.
When rainbow trout's weight and specific growth rate remain within a suitable growth range, their growth increases significantly, but once they deviate from this range, the growth rate slows down.
Interestingly, the study also found that different feeding densities had little effect on the liver body index of rainbow trout, but the liver index (HSI) was closely related to water temperature.
As the water temperature increases, the HSI of rainbow trout decreases, i.e. as the water temperature increases, the liver body index decreases.
This is because the elevated water temperature causes the metabolic rate of rainbow trout to rise, which reduces fat storage in the liver, so the hepatosomal index is reduced, and the serum cortisol content is also related to the water temperature.
This suggests that water temperature can be considered a stress factor that affects the growth of rainbow trout.
In general, since the 80s of the 20th century, China has developed and expanded the technology of large-surface cage aquaculture, making full use of the rich large-surface fishery environment.
The use of efficient cage culture methods has changed the traditional breeding and fishing methods, brought market competitiveness to the aquaculture industry, and promoted the prosperity of the large surface cage aquaculture industry.
Water temperature is critical for the growth of rainbow trout, the optimal temperature is 16-18 °C, below 7 °C or above 20 °C will lead to decreased appetite and slower growth, and elevated water temperature also reduces liver body index and affects serum cortisol content, indicating that water temperature is a stress factor affecting the growth of rainbow trout.
Density is also an important factor, and low-density farming helps to improve the growth and survival of rainbow trout, but high-density farming can cause stress and affect feeding rates and growth
In addition, we highlight the need to consider the health and well-being of fish in the farming process to ensure sustainable farming practices.
The success of rainbow trout farming requires management taking into account water temperature, culture density and fish welfare in order to achieve the production of high-quality aquatic products and the sustainable development of aquaculture.