Investigation of stomach content has traditionally been an important field of activity in fisheries biology, but it is one in which there are great difficulties in correlating the results with the research made in the other fields. Investigations of the food of the fish cannot be considered in isolation but have to be discussed in relation to the whole marine environment, of which the fish constitute single elements. Therefore, a brief survey of the most important processes in aquatic ecology must be made, with particular reference to feeding.
Living organisms interact with each other and with their non-living (abiotic) environment in many ways; no organism exists independently of its environment. It is the study of these interrelationships which is called ecology. It is possible to study the ecology of one species in relation to its environment or a whole group of species and their interactions with both each other and their physical surroundings. Thus, ecology is concerned not only with the biological disciplines but also the physical and chemical sciences.
Any area of nature in which materials are being exchanged between living organisms and their abiotic environment forms an ecological system or ecosystem. This concept is useful as it stresses the interdependence of the components involved. Although it is hardly possible to demarcate any area in nature that is not influenced by neighbouring areas, one may, nevertheless, consider a pond, a lake or even part of a forest as an ecosystem.
To understand the dynamics of such a self-sufficient ecosystem as a functional unit, its component parts must be looked at in some detail first.
The following steps in the operation of the ecosystem (Figs. 6.1 and 6.2) can be recognized from a functional point of view:
The main producers are the chlorophyll-possessing green plants; synthesizing bacteria usually play only a very minor role.
The consumers comprise all the other living components present. They include the herbivores (plant-eaters), which feed directly on the producers, and the carnivores (predators), which feed on the herbivores or other carnivores. They also include parasites, scavengers, (carrion-eaters) and saprophytes. Although it is convenient to list the decomposers, consisting of bacteria and fungi, as a separate entity because of their specific role and their indispensability they are of course also consumers.
The inorganic nutrients comprise a large number of elements present in the form of dissolved salts. The most important are nitrogen and phosphorus followed by potassium, calcium, sulphur and magnesium. Some elements are needed in extremely minute amounts and are therefore referred to as micro-nutrients.
The production of organic substances (food) by photosynthesis is a process involving transformation of light energy into potential chemical energy. The transfer of this food energy from the producers through a series of consumers is called a food chain, each organism through which it is passed being a link in the chain.
For sake of simplicity three different food chains may be recognized:
In reality food may be passed through parts of all three chains before it is finally decomposed into inorganic nutrients by the bacteria and fungi found at the end of every food chain. In other words, the species population within a community or ecosystem form many food chains which interconnect, anastomose or cross each other in a complex pattern, which is usually referred to as the food web.
Organisms which belong to the same link of the food chain as counted from the producer level are said to belong to the same trophic level. Thus the plants constitute the first trophic level, the herbivores the second, and the carnivores feeding on herbivores the third trophic level. Secondary carnivores feeding on third level carnivores belong to the fourth trophic level and so forth. However, there is a very definite limit to the number of possible links in a food chain, and consequently also to the number of trophic levels in any ecosystem. The reason for this is that only about 10 percent of the available energy is assimilated in passing from one trophic level to the next. At the top of the food chain there are usually only one or two major predators. The number of species in each trophic layer increases with approach to the first layer, giving rise to what is called a pyramid of numbers. For the major predators introduction of small amounts of pollutants into the first trophic layer can have fatal consequences because it is eventually concentrated in them.
The laws of thermodynamics state that energy cannot be created or destroyed, but also that it cannot be transformed from one type to another without partial dispersion into heat energy. This means that the transformation of light energy into potential chemical energy in the form of organic compounds in the plants cannot be 100 percent efficient. Only a very small portion of the light energy absorbed by green plants that is transformed into food energy (gross production) because most of it is dispersed as heat. Furthermore, some of the synthesized gross production is used by the plants in their own respiratory processes, leaving a still smaller amount of potential energy (the net production) available for transfer to the next trophic level.
The loss of energy is generally referred to as the respiratory loss because the organisms utilize the food energy by oxidizing it. Because of the respiratory losses the food chains cannot be very long and the number of trophic levels in natural communities is therefore seldom more than four or five and often only three. It also means that the total amount of food available decreases with increasing trophic level. For this reason, the largest animals are found feeding on either plants or other animals which are in a low trophic level as, for example, whales on krill and elephants on plants.
Among animals the gross production corresponds to the food assimilated, which means food ingested and absorbed by the intestine. The net production is here equal to food assimilated minus respiration.
While most of the energy lost within an ecosystem is due to the respiratory processes, there are other losses which affect the individual organisms. Some of the potential food is not ingested, but is either decomposed directly or is stored or is exported out of the system or community. Another source of loss is that not all of the food ingested is actually assimilated; some passes through the alimentary canal and is lost as faeces.
As stated earlier, as the organisms die they are attacked by the decomposers, which derive their energy from them by reducing their organic contents to inorganic nutrients. As also indicated earlier, these nutrients can then be used by the producers anew with the result that the materials involved are continuously circulating in the system. However, the energy flow is strictly passed along a “one-way street”. To keep an ecosystem going light energy must be continually supplied.
As we have seen, true production of organic matter takes place only in the chlorophyll-possessing plants and certain synthetic bacteria, and this has been referred to as the primary production. However, copepods and euphausids, for example, are often referred to as meat producers or “key industry” animals because they convert plant material into protein that can be assimilated by the animals which eat them but which themselves could not exist on plant material. In reality, of course, they only assimilate and store energy derived from the primary producers. To avoid confusion it would be better to call them secondary producers, a term which of course fits animals at higher trophic levels just as well because they too - although indirectly - utilize the primary production of the plants.
From a practical point of view it is often desirable to find out how big is the secondary production of certain animals in a given area, say a fishing bank, or even more important, whether a known production can be increased. Production estimates must be based on such factors as standing crop (biomass), rate of removal of materials and rate of growth, including growth of young born or hatched during the census period. The turnover rate is also of interest when short-lived species are involved as is practically always the case within ecosystems in the sea. The biological production must be expressed per time unit. A large standing crop is by no means synonymous with a large production rate. To take an easily visualized example, a pasture grazed by cows may have a very small standing crop of grass because the production is being eaten as it is being produced, but it may nevertheless have a higher production rate than a neighbouring ungrazed pasture with a very large standing crop.
Quantitative relations between the various trophic levels can be calculated provided the production rates are known for each level concerned. Relationships of this nature within trophic levels are also of considerable interest. Expressed as percentage ratios the results of such calculations are often referred to as ecological efficiencies because they are concerned with the efficiency of energy transfer at different points along the food chain. Thus they are important to our understanding of the dynamics in ecosystems. Moreover, most of the efficiency ratios are meaningful with regard to single species populations as well as to whole trophic levels.
Unfortunately much confusion exists in the terminology used by various authors, and it is not always clear to which efficiency ratio an author has really wished to refer. Odum (1959) has made an attempt to define the various ratios, based on his energy flow diagram, and this is certainly a good method of illustrating the complexities involved (Fig. 6.3).
Among fishery biologists the most common way of describing the efficiency is by the conversion factor, i.e. the ratio of the weight of the food consumed by the fish and the growth in weight. Some authors express this conversion factor as the nutritional coefficient. From Fig. 6.3 it is seen that the conversion factor is the reciprocal value of what Odum calls the ecological growth efficiency. The value of the conversion factor is traditionally defined as 10, but in fact it can be both much higher and much lower, e.g. in trout farms in northern Europe the conversion factor is about 5.
However, it is important to stress that none of the ecological efficiencies are constant for any species population or for a whole trophic level. They are dependent on a number of abiotic factors such as temperature and salinity, as well as biotic factors such as type, abundance and distribution of available food, and the age of the consumers; for example, larvae and young ones of all species investigated have much lower conversion factors than older animals.
Almost all the work that has been done upon the food intake of fishes has been qualitative, rather than quantitative. That is, workers have described the occurrence of food found in the digestive tract, usually in the stomach only. This tells what the fish has eaten and approximately in what proportions but it does not describe how much of each food species is eaten. The reaon for the lack of quantitative work is that it is very time-consuming.
As soon as possible after the fish has been caught all the digestive tract is dissected out, and if the investigation cannot be finished at once, possibly because of lack of laboratory facilities, the material must be preserved in 4 percent buffered formalin.
Later in the laboratory the different sections of the digestive tract are opened and the individual groups of organisms are sorted out foridentification. This is most easily done with a binocular-dissecting microscope. To illustrate the methods in use and to describe their advantages and disadvantages a series of results will be considered based on the examination of the stomach contents of two Lates niloticus (Table 1).
The occurrence of each food item is recorded and expressed as the percentage occurrence of all food organisms. The method is very quick and easy but it underestimates the importance of the larger species. In the example, 40 percent of all the food occurrences were Odonata nymphs. The stomach contents of these two L. niloticus could have consisted almost entirely of nymphs, each with a very small Tilapia galilaea but the results give no indication whether this might be a true interpretation of the data.
The numbers of each food item are recorded and the results expressed as a percentage of the total number of food items present. This method overestimates the importance of small food organisms, in this case the Odonata nymphs. It is not satisfactory with plant food, except with microscopic examination of algae. In the example given the plant material could be a small piece of grass or a large piece.
The method is similar to the frequency of occurrence method except that the results are expressed in terms of the number of fish in which the food item occurred expressed as a percentage of the total number of fish. The method gives an indication of what are the main food species but does not indicate their relative importance as sources of energy.
The entire volume of the stomach contents is measured. The contents are then sorted into different types of food and the volume of each determined. Results are usually reported as percentage of the total volume.
This method is more time-consuming than any of the previous methods but it describes accurately the relative importance of each food species and the total volume (∑) indicate the quantity of each item being eaten.
This method is identical to the previous method except that the volume of each food item is first expressed as a percentage volume of the total stomach contents from which it was removed and then an average taken for all the fish sampled. The advantage of this method over the previous one is that the final results will be more representative of the feeding habits of a group of fish if one or two individuals in that group have been feeding very heavily on one particular food item. The disadvantage is that the information on the actual volume of the stomach contents is lost.
Total weights of all food items are determined. This method is very similar to the volumetric method, over which it has little advantage unless the sample is dried so that only the dry weight is determined. Results are usually presented as percentages of the total. If wet-weights are determined the food items must be placed on a blotter for a moment to remove excess moisture. This problem of removing excess moisture makes the method somewhat subjective and therefore dry-weight is better, although much more time-consuming.
Points are given to each item. The number of points depends upon both whether the organism is very common in the stomach contents (highest number of points) or rare (lowest number) and upon its size (more points for large than small size). The method may also be modified to take stomach fullness into account. It is rapid, easy and requires no special apparatus; with experience the method can be very accurate. In the example shown (Table 6.1) the points allocated give a result similar to both the volumetric and gravimetric methods.
Further detail about these methods is given by Rounsefell and Everhart (1953).
All these methods describe, with a greater or lesser degree of precision, the amount of food which the two L. niloticus, which were used as an example, had in their stomachs; the best being the volumetric and gravimetric methods. The sum of the actual volumes and of the weights also indicates the degree of fullness of the stomach, which enables feeding cycles to be described. The points system is also good if used objectively and has the added advantages of ease and quickness. It can also be adapted to take into account fullness of the stomach.
However, none of these methods describes the quantity of each food item being eaten. In the example used, T. galilaea may take a long time to digest and be identifiable long after Odonata nymphs cease to be. During the time it takes to digest one T. galilaea 20–30 Odonata nymphs might have been eaten and digested but at any one time only 4–6 might be recognizable in the stomach. On the other hand, some food items might have been regurgitate upon capture. While the qualitative methods allow the construction of food chains they do not describe the energy flow through those food chains nor allow the importance of say, predation by one commercially-valuable species on another equally valuable species, to be determined. If answers to these problems were known it might in some instances be considered advisable to reduce the abundance of a predator species to allow an increase in abundance of its prey, especially if the prey were at a lower trophic (more efficient) level. These questions can be answered by qualitative studies only.
Quantitative feeding studies take considerable time and are therefore the work of the specialist. Only a brief outline of the methods used will be given. Firstly the unit in which energy flow is to be measured is chosen, calories and nitrogen being the two commonest. Secondly, feeding experiments are conducted in aquaria with the different food materials which the species under study eats in order to determine how efficient it is in converting the food item into fish flesh. For calories, this means determining the calorific value of the selected food, feeding the fish a known weight of this food, collecting its faeces and determining the calorific value of these. In order to translate field observations on the weights and calorific values of food found in stomachs digestion rates must be established by feeding fish and then killing them at known intervals. This may need to be done for more than one temperature. From this information is built up a picture of the daily food intake of fish of various sizes of the same species, how much of the energy from this food it utilizes to maintain itself and to grow. One such study by Daan (in press) shows that large cod in the southern North Sea probably have a big influence on the size of year-classes by eating small O-group cod and that they also eat large quantities of herring, and soles. Boddeke (1971) has also shown that recent large year-classes of cod have reduced the abundance of brown shrimp (Crangon crangon) off the Dutch coast. Daan has suggested that the total production of commercial species from the southern North Sea might be increased by reducing the abundance of cod by heavy fishing upon the stock. Solution of such predator-prey relationships might alter the manner in which some fish assessments were made.
Allee, W.C. et al., 1949 Principles of animal ecology. Philadelphia, W.B. Saunders
Boddeke R., 1971, The influence of the strong 1969 and 1970 year classes of cod on the stock of brown shrimp along the Netherlands coast in 1970 and 1971. ICES C.M. Shellfish and Benthos Committee. Paper K32:3 p. (mimeo)
Clarke, G.L., 1954 Elements of ecology. New York, Wiley and Sons, 534 p.
Daan N., A quantitative analysis of the food intake of North Sea cod. Gadus morhua, L. Neth.J.Sea Res., (in press)
Hardy, A.C., 1959 The open sea; its natural history. Pt. 2. Fish and fisheries with chapters on whales, turtles and animals of the sea floor. London, Collins, 322 p.
Laevastu, T., (comp.), 1965 Manual of methods in fisheries biology. FAO Man.Fish.Sci., (1):pag.var.
Marshall, N.B., 1966 The life of fishes. Cleveland, Ohio, World Publishing Co., 402 p.
Nikolskii, G.V., 1963 The ecology of fishes. London, Academic Press, 352 p.
Odum, E.P., 1959 Fundamentals of ecology. Philadelphia, W.B. Saunders, 546 p.
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|Table 6.1||Example of results obtained using different methods of estimation of stomach contents for two Lates niloticus|
L. niloticus 1.
|1 Tilapia galilaea 20 cm long, weight 80 g, volume 100 ml 6 Odonata (dragon fly) nymphs each 1 cm long, 0.5 g, 1 ml Plant material, plus weight 5 g, volume 7 ml|
L. niloticus 2.
|1 T. galilaea 10 cm, 10 g, 15 ml 4 Odonata nymphs each 1 cm, 0.5 g, 1 ml|
|Food||Method||Fish||Ln 1||Ln 2||%||Total of which % expressed|
all food occurrences
all food organisms
total food volume
total weight of food
Fig.6.1 Principal steps and components in a self-sufficient ecosystem (from Clarke, 1954)
Fig. 6.2 Simplified example of a marine ecosystem. Energy flow shown by open arrows
|Fig. 6.4||Various types of ecological efficiencies. Symbols are as follows (see diagram): L = light (total), LA = absorbed light, PG = total photosynthesis (gross production), P = production of biomass, I = energy intake, R = respiration, A = assimilation, NA = ingested but not assimilated, NU = not used by trophic level shown, t = trophic level, t-1 = preceding trophic level (from Odum, 1959)|