The Physiological and Metabolic Adaptations of Salmonids with Respect to Nutrient Requirements and Environmental Influences.

The life of salmonids is greatly dictated by the availability of food and how it is utilised. The nutrition gained from feeding is categorised and then directed down the correct pathways enabling fishes continued growth and reproduction, their sole purpose in life. The environmental influences and changes in lifestyle that create physiological and metabolic adaptations in fish will be discussed in this report along with the basic nutritional requirements of these situations.

A fish needs to be capable of performing chemical processes to survive; these processes are known as their metabolism and measured as a metabolic rate. To calculate this, the carbon dioxide output, the oxygen intake and the excretion of nitrogen together with the calorific value of the excreta needs to be determined (Brown 1957). This happens to be a very complicated process therefore the measurement of oxygen intake alone is commonly used to determine the metabolism. Whilst fish are in a resting state the consumption of oxygen per unit time can be measured, this is known as the basal or minimum metabolic rate. The important factors affecting the metabolism of fish are temperature, oxygen demand and levels of activity. These parameters are closely linked as temperature directly affects the amount of oxygen available in the water and the activity rate of the fish is the largest single factor affecting oxygen consumption.

The metabolic pathway is divided into two distinct routes; the anabolic pathway that uses materials in the formation of new body tissues and the catabolic pathway that assimilates then degrades food into the digestive system. These two pathways are in competition and for growth to occur the anabolic production must be the greater of the two. The supply of materials needed for the basic maintenance of a fish are provided by nutrients, this means that the energy needed for every day activity comes directly from their food. Energy cannot be gained just by consumption of food; firstly, it needs to be absorbed by the digestive system. The energy that can be gained from the food needs releasing by oxidation; this is why the intake and use of oxygen can be a measure of the metabolism.

The main aim of feeding is to provide energy for the metabolic process with the surplus being used for reproduction and growth. The diet of a fish contains various macro-nutrients such as fats, which are available in the form of lipids, proteins (amino acids) and carbohydrates; with micro-nutrients (vitamins and trace elements) being the essential co-factors in the metabolic transfer.

Generally salmonids are predatory so their intake of food consists mainly of animal material therefore carbohydrates are not a substantial part of their diet. When carbohydrates are ingested they have the tendency to become glucose and then stored as glycogen, others are broken down into pyruvate. The completed oxidation process occurs as part of the citric acid cycle, the final progress for most elements of ingested food and in doing so freeing amino acids for growth. Excess carbohydrates will form lipids which can appear as muscle oils, visceral fats or cause fatty degeneration around the liver (Smith 1982). Cheap forms can be added to artificial salmonid diets in quantities of approximately 20% dry weight. Carbohydrates contain around 4.1 Kcal/gram of energy and manipulation of this varies significantly with the ability to digest it efficiently.

The quantities of protein that salmonids require vary with water temperatures and the age of the fish. Smith (1982) suggests that the rapid growth in salmonid fry can only be maximised when a 50% protein content is present in the diet. This compares to a one-year-old fish that only needs a 35% content. His studies on chinook salmon (Oncorhynchus tshawytsha) revealed that 40% of protein at a temperature of 7°c was needed for maximum growth, rising to 50% when the temperature was increased to 15°c. Activity levels increase when oxygen contents rise, which in turn vary with temperature fluctuations. The intake of certain food types must therefore satisfy the metabolism before excess is supplied for growth. The example of the chinook shows that small increases in these variables can lead to huge variations in nutritional demands, which are needed to continue the curve of natural growth. The salmonid can deal efficiently with proteins, for example, there are 4.65 Kcal/gram of energy available with 3.9 Kcal/gram being manipulated, an 85% efficiency rate.

Proteins consist of twenty-one amino acids that are all utilised by the fish. There are ten 'essential' amino acids that determine the quality of the protein produced; these include arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine (Hoar & Randall 1969). Fish struggle to break down the essential amino acids and in trying to do so they waste more energy that can be gained therefore the non-essential amminos will be the first used for energy production.

Escalating water flow is associated with increases in oxygen levels, but requirements for energy will also rise. Although the initial thought would be directed towards energy requirements needed for the swimming against currents, it is commonly forgotten that energy is also required when fish are just trying to hold position in their chosen habitat.

An interesting part of some salmonid lifecycles includes migration, which entails periods of starvation. An experiment conducted on starved brook trout found that energy requirements reduced daily for the first three days, then stabilised for the following ten days. Beamish (1964c) concluded that he had discovered a minimum energy level occurring at the point of stabilisation.

The process of reproduction causes fluctuations in the metabolism for various reasons. Energy is required in the formation of ova and milt, which can be up to 33% of the body weight in females, and 3.7% in males (Hoar & Randall 1969). With these organs being so large, energy gained from food that would have been utilised in other areas of the body system will be lost in the form of calories stored in the ova and milt. This will increase the energy requirements of the fish with extra energy needed during spawning and recovery. Salmonids can be categorised into having indeterminate growth patterns. Although at ever decreasing rates, growth will continue through out the life of the fish, in contrast, fish that have a fixed final size are said to have determinate patterns (Purdom 1993). The difference being salmonids will use surplus energy in creating body mass, which will be turned into sexual organs when necessary. This is compared to the direct metabolism of food immediately into gametes, which is the practise of fish with determinate growth patterns. All of these points will contribute to significant changes in the nutrient requirements and oxygen demands, both affecting the metabolism of the fish.

The role of cortisol production has been found highly important to a salmon when running to its spawning grounds. Due to the high stress levels encountered when travelling such distances, the fish will respond by releasing cortisol, the main interrenal steroid of teleosts. The secondary and tertiary responses involved with energy metabolism can be a direct or indirect result of this hormone (Iwama & Nakanishi 1996). The cortical tissue secretes gluco-corticoids, which have an effect on the level of blood sugars produced via carbohydrate metabolism. There is also some influence on the osmoregulation; this is down to the secretion of mineral-corticoids (Lagler et al. 1977). While Smith (1982) was investigating the cortisol levels in salmon he discovered that fish caught offshore and held in captivity increased their levels 4-5 times over a period of 24 hours. These results were compared to running salmon that increased their levels of cortisol naturally as they migrated upstream. The levels peaked at the spawning grounds and the quantities were found to be the same as his fish being held in captivity. He concluded that the starting level of cortisol in salmon was not important as the final quantity was fixed to reach a standard maximum. Cortisol functions in intermediary metabolism are fundamental, as teleost fish require them as an essential factor of life (Hoar & Randall 1988). It is suggested by Hoar & Randall (1988) and Iwama & Nakanishi (1996) that the immune system and related stress actions may need cortisol as a major catalyst.

At sometime in a salmon's life cycle, they feel the need to turn downstream and head towards the sea. Unlike the mortal spawning migrations of many species of salmon, smolt migration must be successful so the fish are available to return to spawn. Major physiological changes occur, which involve the loss of their parr marks that make way for silvery body coloration's. The parr tend to become restless just before the migration; Smith (1982) suggested that an increased thyroxin level that increases the basal metabolism was accountable for this behaviour. Table 2 shows many of the physiological characteristics of smolts in comparison to their original state as parr.

These changes in physiology are essential in gaining the smolt the correct balance in their body system when having to face sudden increases in salinity. Although smolts and adults are both subjected to these changes in salinity, lower stress levels in smolts could suggest that they adapt more efficiently to the new environment. The age of the fish does not seem to dictate the time of smoltification, but their size seems to be very influential. Smith (1982) realised that coho salmon (Oncorhynchus kisutch) could start running safely as soon as they reach a fork length of 68-70mm. This length can be gained in 1.5 years, but some have been found to run in the same year that they have hatched.

The final point on smoltification involves the possibility that migration is not completed. Any of the changes a parr may have experienced will revert to the conditions suited to freshwater. The metabolic rate that had increased will reduce and the displaying of unsettled behaviour patterns will cease, completing the full return to parr characteristics.

It is clear that fish have many situations to overcome when trying to complete their journey through life. As with most animals the careful selection of correct food types will dictate their existence, reproductive qualities and growth rates. This report has opened my eyes to the complex world of fish nutrition, metabolic rates, and the effects they have on their life styles and a good understanding of previously unknown areas has been the result.