Learn more about the physiology of rainbow trouts
1. The integumentary system
Skin and Appendages
The skin of the Trout has two major roles, waterproofing the fish and protecting it using the armour plating of the scales. This function of keeping the water out, and fish tissue-fluids in, is performed by the epidermis. This is a very delicate clear covering which is draped over the scales, and possesses tiny glands, the goblet cells. These increase in number when the fish is stressed, and help to secrete the mucus, a protective coating of thick, infection-resistant fluid. Usually it is clear but sometimes skin parasite infections cause the epidermis to secrete a thicker, more viscid mucus, which gives it a bluish tinge.
Beneath the epidermis lie the scales - ovoid plates of bony material which are formed in small pockets, or scale beds. Scales develop on salmonids at the fry stage, and once a fish has its full quota of scales, it does not develop more as it increases in size. Consequently, the scales must grow in step with the fish. Scales grow by accumulation of material round their edges, laid down in the form of concentric rings. When a fish is growing rapidly these rings are spaced far apart so that in the summer (or in the sea) the distance between rings is much greater than in winter. At spawning time, the salmonids do not feed, and in order to obtain enough calcium for eggs or sperm they withdraw calcium from the outermost scale rings. This results in permanent scarring of the scale at that place and by examination of scales, you can assess the age, number of spawning's, and even the fish's size at the end of each year of life. Occasionally a scale may be damaged, and a new scale grows in the scale pocket. This scale cannot recapture the previous life history of the fish and that area of the new scale is therefore blank.
Over the scales are the pigment cells -. melanophores (black cells), iridophores (silver cells), and xanthophores (yellow and red). The black cells are under both nervous and hormonal (ie chemical) control. When fish are on a dark background, they emphasize their black pigment cells, and on a light background, the silver cells are more obvious. When they are depressed due to disease, fish frequently become darker in colour.
The strength of the skin is in the dermis - the layer below the scales. This is a very fibrous layer with considerable tensile strength.
2. The muscular system
Muscles and Bones
Most of the fish muscle, the muscle which we eat, is known as 'white muscle.' This is a very powerful type of muscle, used principally for escape, or pursuit of prey. Red muscle, which is like the muscle of higher animals, is only found along the lateral line and in some specialized sites such as the base of the fins, the eye and gills.
The main swimming muscles of salmonids are arranged in a series of blocks of white muscle or myotomes. This gives them considerable driving force on the tail. The myotomes are attached to the spine, the central bone, which is very flexible. The fins are moved by small independent red muscles. In addition to the usual fins, the salmonid fish are all characterized by a small appendage on the back, just in front of the tail, which is known as the adipose fin because of its fatty internal structure.
The skeleton of the young fish, as with other animals, is formed of cartilage, which becomes calcified later and can have an important influence on certain diseases.
3. The respiratory system
Fish breathe by means of gills, a system of four sets of very fine flattened capillaries or tubes, on either side of the throat, through which the blood flows, and over which water is continually passed. In passing through the gills, the blood gives up its carbon dioxide to the water, and obtains oxygen from the water, through the gill wall.
The respiratory surfaces of the gills, the secondary lamellae, must be very delicate so that the oxygen and carbon dioxide can be readily exchanged. They also contain mucus-producing cells and cells which excrete any excess salt from the blood as it passes through them, and they also excrete ammonia. Such a delicate structure on the outside of the body is highly vulnerable to injury via the water. The gills are protected on the outside by a bony shield called the operculum, and on the inside of the throat, they have a set of comb-like structures called gill rakers, which help to guide the food down the gullet rather than over the gills.
4. The circulatory system
Heart and Blood vessels.
The circulatory system is the blood-transport system of the fish. The pump in the system is the heart, a muscular organ occupying the area at the base of the throat. It is a two-chambered pump (lacking the auxiliary pump for taking blood to the lungs, which is a feature of man and the higher animals). The blood passes from the triangular, very muscular ventricle, which provides the main pressure, into the white, elastic-walled bulbus arteriosus. This is an elastic pressure-balance, converting the pumping of the heart into a steady surge of blood to the gills, from where it passes to the rest of the body to deliver oxygen to the tissues.
Once it has passed through the gills its pressure is much reduced and its passage through the tissues is relatively slow. In the fine circulatory network of the tissues, the capillaries, the oxygen of the blood is replaced by carbon dioxide and waste products.
The blood then returns via the vena cava or great vein, passing through the kidney on its way back to the heart. As the blood passes through the capillaries, some fluid, known as lymph, is lost to the tissues. This is the watery fluid which runs from a fresh fillet of fish. The lymph is returned to the circulation by a separate set of vessels, the lymphatics, which return it to the bloodstream just before the heart.
5. The digestive system
Mouth, Stomach, Intestine and Associated glands
This system is a relatively simple tube in the salmonids. It starts at the mouth, where the teeth are designed for capture not chewing. When ingested the food is quickly passed down the gullet or oesophagus to the stomach, a U-shaped organ which can expand greatly to take large meals. It is in the stomach that the food is really chewed, ie it is broken down by the action of acid and digestive enzymes as well as the crushing contractions of the muscles in the wall of the stomach. At the posterior end of the stomach where it joins the small intestine, there is a group of blind-ending sacs, the pyloric caeca. These usually number 30 to 80 in Salmonids, and they lie conspicuously across the stomach when the fish is opened. They are covered with a considerable amount of white fatty tissue unless the fish has been starved.
From the stomach, food passes through a one-way valve, the pylorus, to the intestine, where the disintegrated food is acted upon by further enzymes. These break down the food to its constituent sugars, fats, and amino acids (from proteins), which then pass into the bloodstream of the intestinal wall for transport to the liver. The remaining food - roughage, snail shells, etc, travels on to the large intestine and is voided as faeces.
Associated with the digestive tract are two very important glands. One, the liver, is a large organ situated just in front of the stomach. It is a pinky-brown colour, soft, and easily ruptured. The liver is the main factory of the body, to which food molecules are taken in the blood from the intestine, for manufacture into the proteins, carbohydrates and fats of the fish's body. Inserted in the top of the liver is a small greenish sac - the gall bladder. When incised, this usually releases a greenish fluid called bile, which under normal conditions passes to the intestine through the bile duct, and aids with food breakdown.
Because of its importance in food metabolism, disease of the liver is very significant. The most common liver abnormalities are excessive infiltration by unsuitable dietary fats and parasites. Parasites are also frequently found in the gall bladder.
The other important associated digestive gland is the pancreas. This is a very diffuse structure which cannot be seen with the naked eye as it is scattered throughout the fat surrounding the pyloric caeca. The pancreas has two functions, the production of pancreatic enzymes, which pass via the pancreatic duct to the intestine, and the production of insulin, which controls sugar and protein metabolism and prevents fish from becoming diabetic. The pancreas is very significant in viral diseases, because it is a favourite site of multiplication for two of the most important salmonid viruses.
Many species of fish including salmonids possess a swim bladder, which is a hydrostatic organ used to trim buoyancy at the appropriate depth. The swim bladder may also have a function as a hollow organ for receiving deep, low-frequency sounds. In the salmonids, it has a connection with the back of the throat so that the fish can quickly squeeze out air and drop to the bottom. Any blockage of this duct, or damage to the swim bladder wall, can result in considerable swimming problems for the fish.
6. The excretory system
Kidney and Bladder
The kidney is the main filter of the body. It filters blood through a sieve-like apparatus called the glomerulus and passes it through tubes to paired ducts, the ureters, which carry it to the bladder. In salmonids this is a small thin-walled structure above the anus. The duct from the bladder drains via the urogenital opening, which is also the exit for eggs.
The kidney of the Trout is a long black structure in the top of the abdomen, extending from the back of the head to the vent. The vena cava runs through the centre of the kidney and on its outer surface may be seen the narrow white ureters, wending towards the bladder. In higher animals the kidney is purely a selective filter, but in the fishes, it also contains the haemopoietic tissue, especially at the front end of the kidney. This is the tissue that makes the oxygen-carrying red blood cells, and defensive white blood cells, and also stores them until they are needed. The other site where this takes place is the spleen, a large black organ attached to the wall of the intestine. This haemopoietic tissue of kidney and spleen is very important in disease as it is affected by a number of serious bacterial and viral agents. It also contains a network of traps, the fixed macrophage cells, which catch any microbes passing through the blood stream and usually succeed in destroying them.
7. The reproductive system
Ovaries and Testes
The gonads of Trout comprise paired ovaries in the female and testes in the male. In the immature or resting state they lie in the anterior of the abdomen, above and on either side of the stomach. At sexual maturity, under the control of the hormones from the pituitary gland, they develop to extend the full length of the abdomen.
The ovary consists of germinal cells, some of which grow to the size of a pea to form the orange-coloured ova or eggs. Others stay small as the cells for subsequent spawning.
At spawning, the skin of both male and female becomes thicker and shinier, and the urogenital opening swells up. Eggs are released into the abdomen as the supporting capsule ruptures and are pushed on a tide of fluid to the urogenital opening by contractions of the female's abdominal muscles and by small sweeping hair-like cilia inserted in certain parts of the lower abdominal wall. Semen, known as 'milt‘, in fish, is excreted from the testes by bodily contraction and passes into the water as a cloud of living, wriggling sperm cells. In the wild, this occurs in the redd (nest) prepared by the female, but in the hatchery, this process can be done artificially.
8. The nervous system
Brain, Spinal cord and Nerves
The nervous system of salmonid fish reflects their behaviour. Salmon home on their sense of smell, hunt with their eyes, and are creatures almost entirely of reflex. Consequently, they have a well-developed olfactory area at the front of the brain, which connects directly with the nostrils. These are paired sacs on the snout with a continual flow of water around them, which is completely clear in normal fish, but becomes cloudy in certain disease conditions.
The eye is also one of the sites with extremely delicate blood vessels and is therefore very vulnerable to rupture of capillaries, eg by gas bubbles in certain circumstances. The ears of salmonid fish do not have an outlet and although they may detect some vibrations, their main function is balance. They are located within the skull, just behind the eyes, and when they are damaged by disease the fish is unable to balance properly.
9. The endocrine system
Pituitary, Adrenal, and Other hormone-producing glands
The endocrine glands are small groups of cells which have a significance for the body way beyond their size. They secrete chemicals into the bloodstream, hormones, which act on distant sites such as the gonads, skin or blood vessels. The most important endocrine gland, the pituitary, which has been called the 'conductor of the endocrine orchestra,' is in a very secure site below the brain.
The adrenal, or inter-renal as it is often called in fish, is a gland producing several important hormones including the fear hormone, adrenaline. It is located within the haemopoietic tissue at the anterior end of the kidney. The thyroid gland, producing growth hormone, is elusive in the salmonids, often scattered randomly around the tissues of the throat area. Salmonids and other fish also have two endocrine structures with as yet unknown functions.
These are the Corpuscles of Stannius, small white spots, placed laterally in the mid-kidney tissue (usually three or four can be seen on the surface of the kidney) and the pseudobranch, a red, vestigial gill-like structure on the inside face of each operculum. The pseudobranch has been associated with an endocrine hormone function but recent evidence suggests it is more likely to be associated with control of oxygen and carbon dioxide levels in the blood.
There are three other endocrine glands of fishes, which do not have equivalents in higher animals. These are the ultimobranchial glands concerned with calcium metabolism, the urophysis, which is a swelling near the end of the spinal cord, and the pineal, at the top of the head, which is thought to be light-sensitive and associated with pigment cell control. The ovary and testis also have an endocrine function, producing sex hormones.
Growth and survival of Troutlodge production fish evaluated after transfer to seawater.
Troutlodge Seawater Adaptation
Size at Entry (November Strain)
Date: April 29, 2019
Author(s): Robert Iwamoto, Cortney Jensen, Kyle Martin
The growth and survival of Troutlodge production fish were evaluated after transfer to seawater. The fish were triplodized progeny of the November strain and were transferred at four different weights (100g, 150g, 250g, and 300g) to circular rearing tanks at NOAA’s Manchester Research Station. Transfers began in September 2018 and ended in January 2019. Fish transferred at 150g and larger had significantly higher survival (87% to 99+%) than the 100g group (38%). The majority of the mortalities occurred within 7 days post-transfer and were primarily of fish at the lower end of the size range at transfer. Although the project was not designed to evaluate optimal fish growth, the results indicated that the November strain has the capacity for good growth and feed conversion in seawater.
Dates: July 28, 2018 - April 18, 2019
Genetic Group: November 2017 YC triploids
Location: Manchester Research Station (Northwest Fisheries Science Center; NOAA Fisheries Service)
Although Troutlodge supplies a limited number of eggs destined for seawater production, the potential for rainbow trout production in seawater is increasing. For the industry to grow, however, baseline data to formulate best practices for the industry are necessary. Among the most critical bottlenecks for successfully producing rainbow trout in seawater is the transition from the freshwater to seawater environment. Key criteria to evaluate successful adaptation to seawater include both survival and growth. Fish that are poorly adapted after the transfer (maladaptation) may continue to survive but demonstrate poor growth, low condition factor, the reappearance of parr marks compared with successfully transitioned peers.
Rainbow trout undergo the process of smoltification in which they become prepared for the transition from fresh to seawater. The process has been well studied, resulting in a thorough understanding of factors affecting readiness for seawater entry. Photoperiod, physiological changes characterizing smoltification, genetic background, the salinity of receiving water, and fish size all influence the success of seawater adaptation.
Timing seawater entry to coincide with smoltification optimizes opportunity for successful growth and survival in seawater. Smoltification and osmoregulatory capacity are closely linked to fish size among other things. The specific objective of this project was to evaluate the survival and early growth of fish transferred to the marine environment at different mean weights to determine the effects of seawater entry size on subsequent adaptation and whether fish size might override the normal controlling effects of photoperiod.
Materials & Methods
1. Experimental Design
Of the four genetic strains maintained by Troutlodge, the February and November spawning strains are most likely to have originated from anadromous steelhead and thus, show good potential for seawater production. To avoid sexual maturation prior to harvest and prevent potential genetic mixing of escaped commercial stocks with wild counterparts, sterile triploid fish are recommended for seawater production. For these reasons, triploid progeny of the November 2017 Troutlodge strain were used in this study.
To observe how size at seawater entry affects early growth and survival, a group of November 2017 YC triploid fish was split into four cohorts. Each cohort was introduced to seawater at a different mean weight-- 100, 150, 250, and 300g average weights, respectively. Each cohort consisted of approximately 500 fish and occupied a single 12-foot diameter circular tank at NOAA’s Manchester Research Station. Pumped seawater from Puget Sound was supplied to each tank. Fish were exposed to ambient light conditions.
Figure 1 shows a timeline of activities associated with the project. Cohort 1 was transferred in September 2018, followed by Cohort 2 in October 2018, Cohort 3 in December 2018, and finally Cohort 4 in January 2019. Although photoperiod can play a significant role in promoting smoltification, no photoperiod manipulation was used in this project. Of particular note, experimental fish were initially exposed to a declining photoperiod at the time of transfer. Successful smoltification is normally associated with spring-like, increasing photoperiods. Fish in this test were therefore exposed to the least optimal photoperiod treatment.
Figure 1: Timeline of activities showing date of vaccination, transfer dates for each of the four cohorts, and sample dates
2. Rearing Conditions
Freshwater Rearing: Fish were initially reared and transferred from Troutlodge’s ELM II freshwater hatchery in eastern Washington to its Tacoma freshwater site at roughly 50g, where they were held until seawater transfer.
- Vaccinations: All fish were vaccinated by injection for furunculosis and vibriosis a minimum of 45 days prior to the first cohort being transferred to saltwater as per manufacturer’s recommendation.
- Acclimation: No acclimation was performed. Fish were transferred directly from freshwater into seawater.
- Feed Rate: Fish were hand-fed at levels ranging from 1.5 to 2.0% body weight per day. Feeding was done once per day excluding weekends.
- Feed: Skretting 6.5mm floating pellet: 12% fat, 46% protein
- Water Temperature: Ambient seawater temperature ranged from a high of 13.4C to a low of 7.1C, averaging 10.5C.
- Salinity: Ambient salinity was a fairly constant 28ppt
3. Data Collected
- Daily mortalities including weights and lengths of individuals were recorded.
- Daily feed ration and feeding behavior were noted.
- Monthly or twice-monthly weight samples of 50 fish per cohort were collected. At the final sample, all surviving fish were hand-counted to determine final inventory and overall survival.
Results and Discussion
Cohort 1 resided in seawater for 217 days, Cohort 2 182 days, Cohort 3 128 days, and Cohort 4 98 days. Successful adaptation to seawater is defined by both survivals as well as growth. Fish that demonstrate poor adaptation may die soon after transfer or linger and exhibit poor growth and condition (maladaptation). The experimental groups showed little evidence of maladaptation (Table 1)—fish either survived and grew or died almost immediately post-transfer. The incidence of maladapted fish averaged slightly more than 1% for all four cohorts at the final inventory.
Table 1. Incidence of maladaptation, disease, and physical deformities recorded at final data collection.
The incidence of sick and deformed individuals was also relatively low overall. Deformities generally involved the caudal peduncle. Diseased fish generally showed visual symptoms of furunculosis. Unlike maladaptation, there were significant differences in survival among the four cohorts (Figures 2 and 3). Cohort 1 experienced very high mortalities (52.5% of the transferees) within 7 days after transfer indicating very poor adaptation to seawater. Although the target size at transfer was a minimum of 100g, individual mortality data indicated that the majority of the mortalities were between 50-75g.
The remaining three cohorts exhibited high to very high survivals by the end of the project, ranging from 87.0% (Cohort 2) to 93.4% (Cohort 3) to a high of 99+% (Cohort 4). In almost every case, mortalities were at or below the average weight of their respective cohort transfer size. As the data indicates, survival in seawater was directly correlated with size at transfer.
Figure 2: Cumulative mortality % for each cohort.
Figure 3. Mortality for each cohort during the first 30 days and to the end of the project.
Although the purpose of this project was primarily to determine the relationship between seawater entry weight and initial survival, growth was also of interest as an indicator of adaptation since maladapted fish do not thrive after seawater exposure. Of particular interest were the effects of a declining photoperiod and declining seawater temperature profile on the capacity of the November Troutlodge strain to grow in seawater.
Due to the limits of the experimental design (hand-feeding once per day, 5 days of the week), we did not attempt to define the maximum growth profiles of the different cohorts. Rather, the primary goal was to determine whether each group continued to grow after transfer.
Each cohort was sampled by randomly collecting 50
individual fish per cohort, monthly or twice monthly, and determining weights and lengths. At the final data collection, each tank was inventoried by hand. Growth profiles for each cohort from transfer to project termination are shown in Figure 4.
Figure 4: Seawater growth curve for each cohort and temperature profile.
Table 2: Average daily gain (ADG), thermal growth coefficient (TGC) and feed conversion ratio (FCR) for each cohort from seawater entry to harvest.
Each of the four cohorts showed significant weight gains during the project duration despite the declining photoperiod and rearing temperature. Cohort 1 had the highest average daily weight gain and thermal growth coefficient, and the lowest feed conversion ratio (Table 2). Two factors may have contributed to that difference:
- Cohort 1 at the time of transfer may have been comprised of the fastest-growing segment of the population in freshwater because of the grading process used to obtain the 100g transfer weight
- The density in Cohort 1’s tank was the lowest of the four cohorts (Table 3) because of the high initial mortalities for that group.
Feed conversion (amount of feed fed/amount of weight gain) ranged from 1.16 for Cohort 1 to 1.54, 1.83, and 1.81 for Cohorts 2-4, respectively from time of transfer to project termination (Table 2).
||Final Density (kg/m³)
Table 3. Final tank densities for each cohort.
Summary and Conclusions
This project demonstrated that Troutlodge’s November strain definitely has the capacity to survive and grow in seawater and that seawater adaptation is correlated with fish size. Furthermore, size at entry ameliorated the negative effects of seasonal declining photoperiod and temperature. Although the precise relationship between size at entry and optimal growth and survival have yet to be determined, the results of this project indicated that November strain triploid progeny will survive if transferred at 100g or greater and that larger sizes would probably lead to higher survival.
This experiment did not conclusively determine the lower size limit for successful seawater transfer for the November Troutlodge strain although there was evidence that fish less than 75g would not adapt well to seawater when transferred in the fall months.
NOAA’s Manchester Research Station provides an ideal platform for smoltification research. We recommend that further testing and evaluation of the seawater adaptability for the other Troutlodge genetic strains, particularly the February and May strains, be conducted at the station whenever feasible. We also encourage family evaluations of fresh- vs seawater rearing to determine the extent of genotype-environment interactions and the need, if any, to specifically select for seawater rearing.
We further recommend that growth and survival trials be conducted with industry partners to evaluate performance under production conditions.
Photographs at final data collection
Manchester Research Station Circular Tanks
All-females and Triploids
Sexual Maturity – All-females and Triploids
Trout farmers seek maximum efficiency to convert fish feed into saleable meat. Sexual maturation prior to harvesting can reduce the production efficiency and quality of the product. During sexual maturation, the nutrients in the feed are diverted away from the production of muscle (meat) towards the development of reproductive organs. Physiological changes during sexual maturation will also affect the quality of the meat by softening the flesh.
Most strains of rainbow trout spawn naturally on a 2-year cycle. However, the faster-growing males in a population can develop reproductive organs precociously and be ready for spawning in the first year. In culture conditions, precocious males can become a significant portion of the stock, with the resultant loss in production volume and value. Female trout rarely mature precociously. For these reasons, Troutlodge has specialized in the production of all-female rainbow trout eggs as our standard product. All-female eggs are preferred by trout farmers around the world for a variety of benefits, such as:
Excellent overall fish quality at harvest.
Firmer and tastier flesh (meat).
Healthier fish due to the absence of males.
Better feed conversion.
No unmarketable, precocious males.
Availability of roe from all-females as an additional marketable product.
Production of Diploids and Triploids
Troutlodge offers both diploid (2N) and triploid (3N) trout eggs. The difference between diploid and triploid trout is the number of chromosome sets contained in the nucleus of each cell.
Within each cell of an organism is a nucleus, which houses the DNA packaged in units called chromosomes. During normal reproduction, offspring receive one set of chromosomes from the mother and one from the father resulting in a diploid organism with 2 complete sets of chromosomes. This is the normal state of being and results in fully viable offspring that can reproduce.
In contrast, triploid organisms have 3 complete sets of chromosomes, 2 from the mother and 1 from the father. Prior to fertilization, an egg contains 2 sets of chromosomes, one of which is called a polar body. Under normal conditions, the polar body is expelled shortly after fertilization, leaving only 2 chromosome sets and creating a diploid organism. However, if a fertilized egg is exposed to either increased atmospheric pressure, or increased temperature, the polar body is not expelled and is retained in the nucleus. Thus, each cell of the organism has 3 chromosome sets and is referred to as triploid. Due to the extra chromosome set, triploid animals cannot produce functional gonads and are reproductively sterile.
For trout production above 3 kg, Troutlodge recommends the use of sterile, triploid (3N) ova. Some trout farmers believe that 3N ova are better than diploids (2N) even for smaller harvest weights, since gonad development is avoided resulting in a better SGR. However, this is not always realized in terms of better meat yield and meat quality for <3Kg harvest weight. Since triploid fish will not mature, they enable a more efficient and productive use of hatchery facilities by avoiding the removal of maturing fish.
Triploid eggs are the perfect solution for producers needing a sterile trout for:
- Growth past the normal period of reproduction.
- Production of ‘trophy-size’ fish for recreational fishing.
- Eliminating any reproduction of fish released into the aquatic environment.
There are several scientific studies of relevance regarding triploidy in rainbow trout. Here you can find some research with Troutlodge genetics regarding this topic in these links:
Photoperiod Control for Managing Maturity
In Trout, onset of puberty is recognized by clear secondary sexual characteristics prior to spawning. Currently, there is much research in progress on puberty control, e.g. by photoperiod treatments, as many factors are still not completely understood.
Alterations of environmental cues, such as the photoperiod, induce changes of activity of the brain-pituitary-gonad axis that in turn regulates the timing of sexual maturation, spawning, sex steroids and thyroid hormone profiles. Photoperiod is considered as the most important environmental cue determining the timing of puberty and reproduction in fish living in temperate waters.
Photoperiod manipulation is used by fish farmers and researchers, to control puberty in farmed fish to prevent or delay unwanted gonadal maturation. Photoperiod can easily be controlled even in outdoor enclosures and under cage culture conditions.
Continuous 24h light photoperiodic manipulation inhibits or modulates the activation of the endocrine cascade from the brain-pituitary-gonad axis, which initiates puberty. It can also affect growth performance directly, by preventing reallocation of energy resources towards gonadal development. Under this light regime, endogenous rhythms controlling reproduction may enter a delayed free-running rhythm or block the onset of gametogenesis altogether (e.g. if applied during autumn after the exposure to natural photoperiod conditions). In this last case, it is highly effective in inhibiting the appearance of precocious males.
Photoperiod manipulation is a realistic therapy on a commercial scale for Trout reared in sea cages or tanks.
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