Physiological And Environmental Factors Involved In Smoltification In Salmon Aquaculture

Salmon Facts and Types

Question:

Discuss about the Different Types Of Salmon Aquaculture.

Salmon is a ray finned fish which to the family of Salmonidae. There are other fishes that belong to the same family, like whitefish, grayling, char, trout. Salmon is natively found in the tributaries of Pacific Ocean and North Atlantic Ocean. The species that are found in the Atlantic Ocean belong to the genus Salmo and the species that are found in the Pacific Ocean belong to the genus Oncorhynchus. Salmon was introduced in North America and also in South America, within non-native environments of Great Lakes and Patagonia respectively (Aas et al., 2010). The fish is intensively farmed in different parts of the world. An adult Atlantic salmon weighs around 10 pounds, while a king Salmon weighs around 23 pounds. However, fishes weighing around 50 to 80 pounds are also common. There are different types of Salmon like Cham Salmon, Coho salmon, Sockeye salmon, and pink salmon. Salmon fish are anadromous, which means that the fishes lay their eggs in freshwater and then return to the ocean. The fishes again return to the freshwater to reproduce. Different species of Salmon exhibit the anadromous character, while other species of Salmon exhibit the freshwater characteristics (Miramichi Salmon Association, 2018). This study is based on the discussion on the physiological and the environmental factors involved in smoltification in salmon; and how the manipulation of these factors revolutionized the salmon aquaculture.

In order to understand osmoregulation (active regulation), it is necessary to discuss about the osmosis (passive regulation). Cells contain a lot of different solutes (polysaccharides, proteins, ions) and water, which creates a specific concentration inside the cell membrane. The membrane is Semi permeable which means that, it allows only water to pass through it and does not allow the solutes. The law of osmosis can be seen when a cell is kept inside solution exhibiting a different concentration. Due to osmotic pressure water moves into the solution from a high concentration to a region of low concentration via a semipermeable membrane. The movement of water takes place to balance the concentration (Bbc.co.uk, 2018).

Maintaining a homeostatic balance is a big issue for the fishes in both the marine water and fresh water, because the metabolic processes inside the body of a fish occur specifically in certain chemical and physical environment. To keep up with the constantly with the internal environment, a continuous adaptation with respect to the concentration of oxygen, carbon dioxide, glucose, calcium ion, potassium ion, sodium ion, pH and temperature. The main problem of the fishes is the osmoregulation. Active regulation of the osmotic pressure in order to maintain the concentration of the salts and the fluid balance is called osmoregulation (Whittamore, 2012).

Osmoregulation in Salmon

Firstly the freshwater fishes, the salt concentration inside the body is higher in comparison to the surrounding water, thus water moves in to the body because of osmosis. If there is no regulation at this point, then the fishes would swell up. In order to compensate this, kidneys produce a large amount of urine and this signifies loss of salt from the body. Thus to compensate for this loss, specialized cells in gills (called chloride cells) absorbs ion from water and is directly transported to the blood stream (Whittamore, 2012).

The marine fishes face a different situation, the concentration of salt in the blood is much lower in comparison to the sea water. Does fishes constantly lose water and build up salt concentration inside the body. To compensate for the loss of water from the body fishes drink a lot of water. Due to the small size of the kidney and the inability to excrete a large amount of urine, marine fishes excrete salt from the gills. The gills have specialized cells called to chloride cells which function in just the opposite way the gills in freshwater fishes function (Edwards & Marshall, 2012).

Smoltification is a process in which the behavioral, morphological and physiological changes that a young salmon fish undergoes during migration from freshwater to a saltwater region. Considering the life cycle of a salmon fish, it starts its life in the freshwater and it gets prepared to enter in to the oceanic water where the concentration of salt is high. Three important changes take place at this stage of the life cycle. Firstly, the fish starts to drink a lot of water. Secondly, the urine production reduces to great extent by the kidneys. Thirdly, the molecular pumps or the chloride cells in the gills function reversely. Which means that, the chloride cells pumps sodium out instead of absorbing it. When the fishes reenter in to the freshwater, the body of the salmon fishes starts to acclimatize automatically. The fishes stay within the estuarial zone in order to get acclimatized (Lerner, Sheridan & McCormick, 2012).

The process of smoltification also occurs in the other species like sharks. In the estuaries, where the sea water meets the freshwater changes in concentration occur gradually. Sharks are known to move further in to the freshwater zone and the process of osmoregulation also occurs differently in comparison to the salmon. Sharks convert ammonia to urea and are capable of retaining it within the blood stream which is slightly more concentrated than the sea water.  Through this way, the loss of water through the osmosis is prevented and the excess salt is excreted via the rectal gland. These processes are regulated by hormones and controlled by brain. Hormones like thyroid and cortisol are the effective regulators of osmotic pressure which influences the direction and the rate of the ions that are pumped through the chloride cells (Cramp, Hansen & Franklin, 2015).

Smoltification Process

Contaminant exposure- It has recently been found that the change in water chemistry has a detrimental effect on the early marine survival and the process of smoltification. This can be attributed to the non-point source of industrial pollutants, exposure to trace heavy metal, mineral deposit drainage. In addition the trace heavy metal from the drainage of the mineral deposits, intensive forest areas, ranges and agricultural areas. This results into the movement of low level concentration of herbicides in to the rearing of the juvenile salmon (Solomon et al., 2013).

Water temperature- In order to increase the growth and reduce the time required for the production of the smolts, elevated temperature is used. However, care must be taken because artificial temperatures potentially affect the process of smolting as well as the growth (National Oceanic and Atmospheric Administration, 2018).

Photoperiod- research has shown that there is a positive correlation with the behavioral and the physiological aspects with respect to the endocrine system. Because there is direct chemical link with the physiological and the environmental changes in fish. There is a strong link of the photoperiod with the seasonal cycle of growth in the juvenile salmon and the smolting process (National Oceanic and Atmospheric Administration, 2018).

Presently a major problem faced by salmon industry is that they are unable to produce and maintain a continuity in the product sizes in accordance to the market demand which are existing throughout the year. Such problems exist because fishes had introduced into the sea for a very short period of time during the early summer and the spring. Due to this the production is not even according to the size of the market and also results in an unavailability What the Fish products. At certain times of the year, the transfer of smolts into the ocean water has implications on the costing and the production on the smolts. Similar to spawning and maturation the smoltification timing is affected by the patterns of the season and day length changes. Extension cord compression of the annual rate of change of photoperiod leads to both the delay and advancement in the completion of smoltification. Photoperiods that change seasonally can be replaced by the combinations of constant short and long days. Bringing modifications in the combinations of short and long days as well as seasonal changes can effectively increase the production of smolts during several months of the year (Zydlewski, Stich & McCormick, 2014).

Osmoregulation and Smoltification in Other Fish Species

The major complications that arise with the manipulation of the photoperiod is the timing of the process. Similar to the process of reproduction which is controlled seasonally, the process of smolting is dependent on the endogenous clocks or mechanisms. Under the ambient conditions the endogenous clocks can be identified or determined through the seasonally changing day length. Under the modified light conditions, the different components of this system that include hypo osmoregulatory ability, body silvering, condition factor, factors that control growth will collectively affect the smoltification process. The components might get desynchronized and the probable effects are reflected in the poor rates of survival when transferred to see water. Manipulation of the photoperiod result in affecting the commercial viability of the fishes and affect the maturation of the pre and the post smolts. Thus, any alteration in the photoperiod result in the reduction of the number of smolts, reduced rates of survival when the smolts are transferred to the sea water affecting the early maturation and also cause vital problems for the production management of the grow out farms (Imsland, Handeland & Stefansson, 2014).

Environmental factors- contaminant exposure is one of the prime environmental factor which is affecting the smoltification. The gill ATPase enzyme present in the salmon fishes is highly sensible to the levels of the trace heavy metals dissolved in water. The exposure to the copper during the transformation of parr-smolt can potentially inactivate the gill-ATPase enzyme. The biological damage is not apparent unless the fish moves to the sea water. Just during this movement, the severe mortalities begin to occur. Another negative consequence of such a phenomenon is that the migratory also gets suppressed due to the inactivity of the gill ATPase. Cadmium levels of 4 microgram per liter in the freshwater also result in mortality when the coho smolts are transferred to the thirty percent sea water. Chromium or nickel although does not affect the migratory behavior, but exposure to mercury severely affects the migratory behavior. Other implication of mercury exposure can be related to the malformed development of the embryos when the exposure is only of 2.5 microgram per liter. The herbicide concentration also increase into the waterbodies due to the surface run off from the agricultural fields, and the accumulation of herbicide in to the body of the coho salmon smolts resulted in hampered migratory behavior (Thorstad et al., 2012).

Water temperature and the rearing temperature have been found to be directly impacting the gill ATPase activity and the hypoosmoregulatory activity during the process of smolting. It has been seen in certain species that increase in temperature accelerates the onset of smolting and on the other hand also delays the time period of desmoltification. Coho salmon at the 6 degree Celsius shows rise in the gill ATPase activity, and a precocious development is found at 2o degree Celsius. However, the process of desmoltification enhances due to the increase in temperature. Not all species of salmon show positive responses to the temperature rise. Steelhead trout are found to be potentially affected by the elevated level of temperature rise, while temperature above 13 degree Celsius inhibits smolting. The Atlantic salmon are not similar to the steel head trout. When the temperature rises around 10 degree Celsius, the activity of the downstream activity of salmon rises (Björnsson, Stefansson & McCormick, 2011).

Environmental Factors That Affect Smoltification

Photoperiod plays a significant role in the migratory behavior and the development of the smolt characteristics. The rate of change of the photoperiod played an important part in the modification of the periods of smolting. It has been found that presmolts kept in 7 hour dark period and 17 hour light significantly increases the gill ATPase activity and the plasma thyroxine. However, prolonged periods of exposure to the day light inhibit smolting and growth.

Physiological process- According to the normal life history of Salmon, it migrates as juveniles from fresh water into the sea. Atlantic salmon undergoes transformational changes to get adapted to the oceanic life. The parr-smolt transformation involves both the physiological and the morphological changes in order to get acclimatized to the high saline conditions in sea water. The development of smolt is controlled by both the environmental and developmental information. Due to the exposure of increased day length, components of neuroendocrine axis become sensitive. Plasma levels of growth hormone increases due to the activation half right brain pituitary axis. This results in increased levels plasma levels of growth hormone. It has also been found that the levels of Plasma cortisol increases, and this hormones control the biochemical and cellular changes inside the gill. This increases the gill sodium potassium ATPase activity and results in increased salt tolerance. Triiodothyronine, thyroxine, thyroid hormones increase during the smolt development. These changes are actively thought to alter morphological and behavioral characteristics of the fish (McCormick et al., 2013).

Photoperiod is an important determinant in initiating sexual maturation in the salmon fishes. More than the specific day length, the direction of photoperiod plays a major role in orchestrating sexual maturity and reproduction. Prolonged exposure to long day lengths increased the attainment of sexual maturity in this fishes. Aquaculture of the salmon fishes gives a better control over the adverse environmental conditions. Aquaculture provides a better opportunity to alter the photoperiod easily and helps in the production cycle. While at the same time gives a better control over certain environmental variables like water temperature. Water temperature on the other hand is one of the important environmental parameter which directly influences the physiology of the fishes. The internal temperature of the fish is directly dependent on the external ambient temperature. Physiological development of eggs and larvae, egg hatching, time of spawning, growth rate, metabolic rate all her directly related to the natural environment and water temperature. It has been found that aquaculture of salmon at temperatures ranging 2 to 16 degree Celsius effectively increases the growth rate (Thyholdt, 2014).

The Role of Temperature and Photoperiod in Smoltification

Conclusion

Thus, from the above discussion it can be concluded that the life cycle of salmon fish is directly influenced by the environmental factors like water temperature, photoperiod, herbicides and traces of heavy metal. Salmon fishes can survive both in freshwater and sea water which makes it a complex fish for aquaculture. In order to aquaculture salmon fishes, several parameters like water temperature, photoperiod and also the environmental conditions must be kept at check. Only after keeping all the parameters at optimal conditions can favor the growth of salmon fishes rapidly.

References

Aas, Ø., Klemetsen, A., Einum, S., & Skurdal, J. (Eds.). (2010). Atlantic salmon ecology. John Wiley & Sons.

Bbc.co.uk. (2018). BBC - GCSE Bitesize: Osmosis in cells. Bbc.co.uk. Retrieved 15 February 2018, from https://www.bbc.co.uk/schools/gcsebitesize/science/add_gateway_pre_2011/greenworld/waterrev2.shtml

Björnsson, B. T., Stefansson, S. O., & McCormick, S. D. (2011). Environmental endocrinology of salmon smoltification. General and comparative endocrinology, 170(2), 290-298. DOI https://doi.org/10.1016/j.ygcen.2010.07.003

Cramp, R. L., Hansen, M. J., & Franklin, C. E. (2015). Osmoregulation by juvenile brown-banded bamboo sharks, Chiloscyllium punctatum, in hypo-and hyper-saline waters. Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology, 185, 107-114. DOI https://doi.org/10.1016/j.cbpa.2015.04.001

Edwards, S. L., & Marshall, W. S. (2012). Principles and patterns of osmoregulation and euryhalinity in fishes. In Fish Physiology (Vol. 32, pp. 1-44). Academic Press. DOI https://doi.org/10.1016/B978-0-12-396951-4.00001-3

Imsland, A. K., Handeland, S. O., & Stefansson, S. O. (2014). Photoperiod and temperature effects on growth and maturation of pre-and post-smolt Atlantic salmon. Aquaculture international, 22(4), 1331-1345. DOI https://doi.org/10.1007/s10499-014-9750-1

Lerner, D. T., Sheridan, M. A., & McCormick, S. D. (2012). Estrogenic compounds decrease growth hormone receptor abundance and alter osmoregulation in Atlantic salmon. General and comparative endocrinology, 179(2), 196-204. DOI https://doi.org/10.1016/j.ygcen.2012.08.001

McCormick, S. D., Sheehan, T. F., Björnsson, B. T., Lipsky, C., Kocik, J. F., Regish, A. M., & O'Dea, M. F. (2013). Physiological and endocrine changes in Atlantic salmon smolts during hatchery rearing, downstream migration, and ocean entry. Canadian journal of fisheries and aquatic sciences, 70(1), 105-118. DOI https://doi.org/10.1139/cjfas-2012-0151

Miramichi Salmon Association. (2018). Life Cycle of the Atlantic Salmon - Miramichi Salmon Association. Miramichi Salmon Association. Retrieved 15 February 2018, from https://miramichisalmon.ca/education/atlantic-salmon/

National Oceanic and Atmospheric Administration. (2018). Spo.nmfs.noaa.gov. Retrieved 15 February 2018, from https://spo.nmfs.noaa.gov/mfr426/mfr4261.pdf

Solomon, K. R., Dalhoff, K., Volz, D., & Van Der Kraak, G. (2013). Effects of herbicides on fish. In Fish physiology (Vol. 33, pp. 369-409). Academic Press. DOI https://doi.org/10.1016/B978-0-12-398254-4.00007-8

Thorstad, E. B., Whoriskey, F., Uglem, I., Moore, A., Rikardsen, A. H., & Finstad, B. (2012). A critical life stage of the Atlantic salmon Salmo salar: behaviour and survival during the smolt and initial post?smolt migration. Journal of Fish Biology, 81(2), 500-542. DOI: 10.1111/j.1095-8649.2012.03370.x

Thyholdt, S. B. (2014). The importance of temperature in farmed salmon growth: Regional growth functions for Norwegian farmed salmon. Aquaculture Economics & Management, 18(2), 189-204. DOI https://doi.org/10.1080/13657305.2014.903310

Whittamore, J. M. (2012). Osmoregulation and epithelial water transport: lessons from the intestine of marine teleost fish. Journal of Comparative Physiology B, 182(1), 1-39. DOI https://doi.org/10.1007/s00360-011-0601-3

Zydlewski, G. B., Stich, D. S., & McCormick, S. D. (2014). Photoperiod control of downstream movements of Atlantic salmon Salmo salar smolts. Journal of fish biology, 85(4), 1023-1041. DOI: 10.1111/jfb.12509