 |  | Post-Harvest Losses of Fish in the Tropics (NRI) | ||
 |  | Physical losses | ||
|  | (introduction...) | ||
| | Examples of complete physical loss | ||
| | Handling and processing losses | ||
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Physical losses of fish after harvest can be regarded in two distinct ways. Firstly, there is what might be termed complete physical loss. Quantities of fish may spoil completely, becoming completely inedible. The by-catch from shrimp trawling is thrown overboard. Related to these losses are the under-utilization of resources, when small fish are converted into fish meal instead of being used for human food. Also there are many less popular fish which are seldom used for human consumption.
The second type of physical loss, which can be regarded as a loss of material, is a result of poor handling and processing of both fresh and cured fish.
Glue catches
For many fish species, glut catches occur, and then the distribution and marketing system may be unable to cope with the quantities of fish which are landed. In other places the processing facilities such as drying racks may be inadequate. Then much of the fish has to be left to rot.
Losses during distribution
If fish is handled carelessly, it may be crushed or broken during handling and can become unsaleable. Sometimes there may be transport problems, for example if floods block roads or bridges are damaged, and it is simply not possible to get fish to market before it rots.
Shrimp trawler by-catch
When shrimp is being trawled, other fish is caught incidentally; this is known as shrimp by-catch. The proportion of by-catch is very high, often 95% or more of the total material taken on board. The by-catch is very mixed, with many species of fish, some large, some small, and other species such as crabs. Usually the by-catch is thrown away at sea. Some of it, particularly the larger fish, could be landed and sold, but even those fish are of much lower value than the shrimp. Chilled or frozen storage facilities on board the trawlers are limited, so they have to be kept for the shrimp. Also, sorting the by-catch would require a lot of the time of the trawler's crew. It is currently estimated that between 5 and 20 million tonnes per year of shrimp by-catch are wasted in this way.
Under-utilization of small pelagic fish used for fish meal
Fish meal is widely used, particularly in the developed countries of the world as animal feed. Fish meal is made from small pelagic fish such as sardine, mackerel and anchovy, and it is estimated that some 20 million tonnes of this fish are converted into, fish meal. Much of this quantity could be used directly as human food. Producing fish meal makes an indirect contribution to human food since it is used to feed animals which will then be eaten by human beings. Unfortunately this fish has disadvantages; it is fragile, easily spoilt and damaged so it is difficult to distribute and market in good condition It is acceptable in canned form, but in many countries canning is expensive.
Meso-pelagic fish
It was mentioned earlier (see Introduction p. I) that stocks of the more popular and accessible fish are becoming fully exploited. One way of relieving the shortage of fish could be the exploitation of the less accessible meso-pelagic f is h. However , practicable catching have to be developed . Also consumers are not yet familiar with this fish , so , market acceptance would also , have to develop.
Wet fish
Much of the fish caught by small-scale fishermen in developing countries is sold soon after landing, either for human consumption or for processing. However wet fish is highly perishable, and is subject to both bacterial and autolytic spoilage as soon as the fish dies. Often it is in poor condition when it is landed, especially if it has been physically damaged by poor handling techniques or carelessness.
Bacterial spoilage Bacteria are present on the skin and gills and in the gut of live fish, although the flesh itself is normally sterile. Bacterial growth and invasion of the flesh are prevented by the body is natural defence system during life, but after death the defence system breaks down and the bacteria multiply and invade the flesh. Bacterial spoilage is characterized by softening of the muscle tissue and the production of slime and offensive odours Damaged fish are particularly susceptible to´' bacteria! spoilage e. Bacterial spoilage is slow at 0°C, in ice. It is increasingly rapid as the temperature is increased, and at tropical temperatures it is very rapid.
Autolytic spoilage Autolytic spoilage is caused by the continuation of the normal enzymatic digestive processes after the fish has died, and is in fact self-digestion of the fish flesh. The flesh of the fish softens, particularly in the gut area where the digestive processes are active. This process is also accelerated by high ambient temperatures.
Bacterial and autolytic spoilage generally occur simultaneously, and for most practical purposes can be considered together as a single cause of loss.
Poor Landing techniques Physical damage to fish can begin as soon as it is caught, and while it is being brought aboard. Small pelagic fish such as mackerel, anchovy and sardine is fairly delicate, and can easily be bruised and broken when being pulled out of the nets, and damaged by being trodden upon. Mishandling can occur after landing and the fish is more prone to physical damage if bacterial and autolytic spoilage have already weakened the flesh. Environmental filth introduced by washing the fish with contaminated or polluted water, or by the fish coming into contact with dirt on land, will accelerate spoilage generally, and encourage contamination by pathogenic bacteria.
Delays in distribution and marketing Some fish is landed at or near the place where it is to be sold, for example close by a major city. Then the fish can be sold quickly after landing and spoilage should not be too serious However much fish is landed in distant, often remote places. This is especially the case for artisanal, village, fishing. Then the fish may spoil while it is being taken to market, and there is much loss of quality. If there are unexpected delays, spoilage can be especially serious, as explained above.
Although physical losses in fresh fish post-harvest are generally assumed to be significant, it is extremely difficult to carry out quantitative studies, and few have been made. James (1977) suggested a figure of 10% as being typical, based largely upon a qualitative assessment, and this implies that losses are about 2 million tonnes per year (see Table 1).
Cured fish
Fish is cured by the traditional processing methods of salting, drying or smoking, which result in a product which is less perishable than wet fish. Depending on how much it is dried, it will keep for several weeks or even months. Cured fish constitutes an important part of the diet of many low income consumers in the developing world, and traditional methods are well suited to local circumstances, as they are cheap and require only simple equipment or facilities. The simplest method only involves laying the fish on the sand to dry in the sun. Alternatively the fish can be smoked over a fire to accelerate drying and to deposit anti-microbial compounds on to the fish. For either process, fish can first be salted, to facilitate drying. However, unless processing is very careful, physical losses can occur from microbial spoilage, insect infestation and fragmentation.
Microbial spoilage The type and extent of microbial spoilage depends almost entirely on moisture content or more precisely, the water activity of the product. (The- term 'water activity' refers to the water which is available for bacterial growth. In practice, this means the total water content less the water which is bound to the food tissues, salt, etc.) Normally for fresh fish, spoilage flora will be active in fish awaiting processing, and during the early stages of processing. When the water activity has fallen, during the later stages of processing, most bacteria will become inactive. Other halophilic bacteria (which can grow in salty conditions! and cause a red discolouration of the flesh) and xerophilic moulds will then become the chief causes of microbial spoilage. Microbial growth ceases at low water activity, equivalent to 60%, relative humidity, and it is necessary to, dry the product to a fairly low moisture content to achieve this.
Unfortunately the ambient humidity in many tropical countries does not fall below 60% making it impossible to sun drv the fish sufficiently fully, and microbilal spoilage cannot be completely arrested. However Poulter et al. (1982) showed that there would be a lag, dependent upon the water activity of the product, before spoilage occurs, allowing a limited degree of storage and distribution.
Insect infestation During the early stages of curing, fish is susceptible to infestation from the larvae, or maggots of various species of blow fly (Diptera). More fully or completely cured fish is attacked by a wider range of pests, of which the most important are Dermestes and Necrobia beetles.
Blowflies are a particular problem fish is dried. They are attracted to wet fish, and lay eggs on it. Larvae (maggots) hatch from the eggs and eat the flesh the fish; the fish can be completely hollowed out. However, the maggots can only attack relatively moist material, and excessive loss can be prevented in good drying conditions Small fish will be less vulnerable to blowflies as it dries more quickly Larger fish, and especially fatty fish' take much longer to dry and then blowfly infestation can be particularly serious. Salt often reduces blowfly attack the fish, although salt-resistant blowflies exist in parts of the world for example, Indonesia.
Beetles, particularly Dermestes and Necrobia are major causes of infestation in unsalted dried fish If undisturbed, they can consume the flesh and soft tissue until only the bones and some hard tissues remain. Loss of cured fish due to beetle infestation may not be serious for short periods, but if the fish is stored for any length of time, it can become increasingly serious.
Fragmentation The tendency for dried fish to fragment is increased by damage to the texture of the fish caused by spoilage before processing; by subsequent mould and bacterial growth; and by insect infestation. If the fish is in good condition before drying, it will be reasonably strong On the other hand, if it has deteriorated significantly before drying, the product will be much more brittle and friable. However, fragmentatiton is most serious in hot-smoked fish The high temperatures used in this form of processing damage the connective tissue anti structural proteins of the fish muscle. This means that the fish damages easily and breaks in small fragments, which may be completelv lost.
Fragmentation greatly reduces the value of cured fish, as the pieces sell for much less than intact fish. Generally only the small pieces are lost completely.
Post-harvest losses of cured fish were estimated by the [us National Academy of Sciences in 1978 to be 25% or 3 million tonnes of the total annual production (see Table 1).
In 1988 Ames and his colleagues carried out a review of postharvest losses of cured fish in which they summarized all the published data which could be traced, together with much unpublished material. Material losses of the main types of cure fish were covered, and whether the data were obtained by measurements or they were estimates was specified. This summary is reproduced here as Table 2.
Physical loss of cured fish in the
tropics
It can be seen that although the values vary considerably, the National Academy of Science's figure of 25% falls well ins´' the average range of the data shown.
It should he noted that the measurement or estimation of loss is not straightforward. Even when losses have been measured, the significance of the quantities is uncertain. For example, if material losses have been caused by insect infestation, not all the fish in one particular batch will have been attacked to the same extent, so some fish with little visible damage would he able to be sold at a reasonable price, while severely damaged fish would have to be discarded, and a percentage loss of whole fish would not be equivalent to the same percentage loss in weight of fish in a particular batch.
None of the measurements was made over any prolonged period There could be very large differences at various times of year. For example, in the dry season fish can be dried quickly and losses could he much less than in the wet season, when drying may be slow or impossible, and when mouldiness can be more serious. The losses may vary greatly in different parts of the country. They can change a lot from one y ear to the next, due to climatic variations, levels of insect pests, etc. No measurements of losses have been carried out ever wide areas or over a significant period of time.
Many estimates of losses are, of necessity, purely visual, so the observer's assessment is subjective rather than objective, however good his intentions. Probably all the estimates were based on observations in a small number of places on a very few occasions; this cannot produce a valid overall figure, and the rigorous observer will naturally qualify his comments accordingly. Unfortunately, qualifying phrases such as 'about', 'of the order of' or 'may be as much as' are likely to be omitted w hen the report or publication is referred to by others, and what was a guess comes to appear as a statement of fact.
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