Regarding food studies of fish, food is simply referred to as any ingested material that can be digested, assimilated and utilized for energy production. Each group of living has its range of materials, which can be effectively utilized as food.
For most organisms, food is the major source of energy for locomotion in search for food, escape from predators and migration for migratory species.
Food is also the major source of energy for reproduction for species perpetuation, growth for elaboration of all organelles, respiration for production of energy and excretion for removal of wastes
Basic food nutrients are carbohydrates (glucose), protein (amino acids) and fats, (fatty acids and glycerol).
These nutrients are required in various proportions and in combination with vitamins and minerals in the right proportions. In other words, they form a balanced diet which is essential for optimal growth of fishes.
A complete study on the food and feeding habits of a fish species would not only investigate the dietary composition, their quantity and selectivity but would also involve an examination of the functional morphology and physiology of the alimentary system as well as the fauna and flora of the environment, particularly, those that are food for fish to reveal their interrelationship with each other and the fish species
Methods of Food Studies of Fish
1. Collection of Fish
The best collection method is the use of gears that produced unbiased samples. This makes the use of a combination of gears most appropriate.
Collection is best around the feeding period of fish, firstly to remove the bias posed by differential digestion of materials and further, to avoid catching fish with mainly empty stomachs. Any method of collection used should be consistent at all times.
Sampling for fish should include all sizes, ages and sexes of the particular species. The date, time of collection, location of capture and method of collection should also be recorded.
2. Preservation of Fish
The preservation of sample must begin immediately after capture and must be consistent. Thus, 4-10% formalin or deep freezing can be used to maintain some degree of good condition and moistness of food in the stomach.
This prevents decay and breakdown of food organisms. If samples are to be deep-frozen, they should be transported in ice chest to the laboratory (if facilities are not available for freezing immediately after capture). Preservation in formalin can commence immediately after capture in the field
3. Preparation of Fish
This involves dissection of the fish and then the stomach or/and in some cases the intestine after measuring the length and weight of the fish.
The sample should be mopped dry with filter paper or clean towel (after thawing for frozen samples) before weighing. Irrespective of the part chosen (stomach or intestine), what is expected to be analyzed is last meal of the fish.
4. Examination of Stomach/Intestine
The condition of the stomach should be determined visually before dissection and categorized as follows:
0/4 – empty stomach
¼ – one quarter full stomach 2/4 – half full stomach
¾ – three quarter full stomach 4/4 – full stomach
This will shed light on the commencement of feeding and feeding period of the fish. The condition of the stomach also provides information on whether the fishes are feeding properly or not. The content of stomach are emptied into a petri dish and examined.
A few drops of water may be added into the dish to allow for proper separation of the items so that they can be identified. Depending on the method of analysis to be employed, the number, weight and /or volume of items are taken.
Objectives of Food Studies
The major objectives of food studies in fisheries include the following:
– Food studies reveal the status of the foraging fish species e.g. predator (piscivorous and non- piscivorous), plankton feeder, herbivore, omnivore and detritivore. These are essential when assessing food and feeding inter-relationship to provide ideas of the niches of the various species within the ecosystem.
– Food studies can be used to determine the rate of growth of fish since growth represents the food converted.
– The types and magnitude of food available can give information on seasonal life history changes in fishes.
– Information from food studies can be used during species selection in fish culture. This is particularly useful in polyculture because proper selection of fishes with different feeding habits will prevent or significantly reduce competition during culture.
– Data on the food items and their selectivity by fish are important in the selection of suitable food organisms for the culture of live foods.
Live foods are known to be useful to fish especially during the early stages of development (fry/fingerlings) when the alimentary canal is not fully equipped to digest artificial or formulated feeds.
– Food studies provide information on possible pollution when such species which constitute fish food are indicators of pollution e.g. the euglenoids, Euglena and Phacus; the blue green alga, Cladophora and diatoms, Synedra, Tabellaria, and Gomphonema.
All these are known to thrive in polluted water. In addition, macroinvertebrates which thrive in polluted water (hence indicating pollution) but form the food of many fishes includes, the annelid worm, Tubifex, chironomid and chaoborid insect larvae, Chironomus and Chaoborus.
Information on the biochemical composition and the energy levels of the ingested food and its absorption in the alimentary canal provides base-line data useful in artificial feed formulation for fishes during their culture.
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Such information can save the farmer a lot of money during feed formulation. Generally, the cost of producing adequate feed for predators is higher because they require a lot of protein in their while the feed of herbivores is cheaper since they require less protein.
Analysis of Food/ Stomach Contents
Food of fishes could be analyzed using the following methods:
- Numerical method
- Frequency of occurrence method
- Volumetric method
- Gravimetric method
- Point method
- Dominance method
1. Numerical Method
This method involves counting the number of each food item present in the stomach of a fish and summing up this number to obtain the grand total number of all food items found in its stomach.
The number of each food item is then expressed as a percentage of the grand total number of all food items. It is usually expressed as:
Percentage number of a food item = Total number of the particular food item x 100
Total number of all food items
This method expresses the numerical importance of different food items. It gives the relative importance of each food items.
A major demerit with this method is that, often, it over- emphasizes the importance of small-sized organisms since such organisms occurring in large numbers may not necessarily constitute the most important food items.
For example, a hundred copepods may not necessarily give a predator enough satisfaction as a single prey fish or shrimp. Another demerit of this method is that, often, it may be difficult and cumbersome to count the number of small-sized organisms that occur in large numbers.
A good example is counting the number of cells of phytoplankton, particularly, colonial and filamentous algae which form a significant part of the food of plankton feeder like tilapias.
It also does not give the frequency with which the food items are consumed. There may also be the problem of counting broken organisms or parts of organisms.
2. Frequency of Occurrence Method
This involves counting the number of times a particular food item occurs in the stomach and expressing this as a percentage of the total number of stomachs with food (empty stomach excluded). This is usually expressed as:
Percentage occurrence of a food item =
Total number of stomachs with the particular food item x 100 over Total number of stomach with food
This method presents the food spectrum of the species. Hence, the importance of the food items relative to the population of the species could probably be guessed. This method is, however, biased due to accumulation of digestion-resistant materials.
This makes the apparent frequency with which they occur seemingly more than the actual frequency. Besides, it does not show the number of the different food items and does not give the bulk of the food items.
3. Volumetric Method
This method involves measurement of the quantity of food. In this case, the volumes of the different food items are measured by water displacement, using graduated cylinder or convenient apparatus.
The volume of each food item is then expressed as a percentage of the total volumes of all the food items in the stomach as follows:
Percentage volume of a food item = Volume of the particular food item x 100
Total volume of all food items
This method shows the bulk of the different food items. Thus, it removes the wrong impression created by less important but numerically abundant food items.
In this method, however, large voluminous food items tend to overshadow small-sized food items and their importance may be over-emphasized. Furthermore, materials resistant to digestion (e.g. chitin, cellulose) may accumulate from series of feeding and may influence the results. Errors could also be accrued from differential rates of digestion of food items.
4. Gravimetric Method
This method is similar to volumetric method, but, the weights of food items are measured instead of volume. It is expressed as:
Percentage weight of a food item = Weight of the particular food item x 100
Total weight of all food items
This method has the same merits and demerits as those for the volumetric method.
5. Point Method
This method involves scoring points to different food items depending on their numbers and sizes; one large organisms being equivalent to many small organisms.
All the points accumulated by each food item are summed-up and expressed as a percentage of the total number of points accumulated by all the food items as follows:
Percentage point of a food item = Number of points of the particular food item x 100
Total number of points of all food items
This method has been described as an approximate volumetric method. Others described it as numerical-volumetric hybrid applied subjectively because it is a compromise between numerical and volumetric methods.
It is useful for on-the-spot analysis of food items especially in the field. It is a fast and easy method. This method is, however, subjective thereby introducing bias. Different workers may score different point to a particular food items; even an individual may be biased in allotting points.
Due to its subjective nature, it is impossible to statistically analyze data from this method. Furthermore, it is impossible to compare data from different workers using this method even for the same species and from the same habitat.
6. Dominance Method
This method involves counting the number of stomachs in which each food item occurs as the dominant food. This is then expressed as a percentage of the number of fish with food in their stomach as follows:
Percentage dominance of a food item =
Number of stomachs that the particular food item is dominant x 100
Total number of stomachs with food
The merits and demerits of this method are similar to those of frequency of occurrence method.
In summary, no matter the method applied, usually when large numbers of samples are examined, the results using different methods come out similar.
However, it is advisable to apply a combination of at least two, but preferably three methods for accuracy, putting emphasis on number, occurrence and bulk of food.
Accordingly, food studies have been categorized into that which discerns dietary composition, competition, feeding rhythm and periodicity and that which attempt to estimate the total amount of food consumed by a population.
The latter is simply described as quantitative aspects of feeding. Most studies have emphasized more of qualitative rather than quantitative.
Hence, the more common has always been what the fish has eaten and approximately in what proportion rather than show how much each species has eaten.
Quantitative studies in food consumed are time consuming, and this is likely responsible for fewer quantitative studies on food and feeding habits of fishes.
In fisheries, the objectives of fish studies are manifold, and include to know the status of foraging species, the growth-rate of the species, and the types and magnitude of food available etc.
Food and feeding habits of food could be analyzed using numerical method, frequency of occurrence method, volumetric method, point method and dominance method.
A combination of at least three methods taking cognizance of number, occurrence and bulk of food give the best results.
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