As plants and animals grow from fertilised eggs to maturity their individual biomass increases greatly. However, mortality of the early life stages is usually heavy and population biomass does not increase as steeply, large numbers of low biomass early stages resulting in small numbers of large and mature individuals. The organic matter added to the biomass of a population in a unit of time is referred to as production and we can make comparisons and contrasts between habitats of different trophic status by looking at their production. Values of primary (plant) and secondary (animal) production for a range of aquatic habitats are given in Tables 6.1 and 6.2.

Table 6.1. Some estimates of primary production in aquatic habitats (all given in g C m^-2 y^-1). F = freshwater plants; M = marine plants; E = estuarine/salt marsh plants. From Wotton R.S. (ed.) (1994) The Biology of Particles in Aquatic Systems, after authors.

Table 6.2. Some estimates of secondary production of animals from different habitats (all given in g C m^-2 y^-1, except ^* = per season). F = freshwater animals; M = marine animals; E = estuarine/salt marsh animals. From Wotton R.S. (ed.) (1994) The Biology of Particles in Aquatic Systems, after authors.

As expected, primary production in any habitat is higher than secondary production (showing well in a comparison of phytoplankton and zooplankton data). One reason for this is that plant food is not consumed with 100% efficiency and part of the energy assimilated by herbivores is used in respiration.

Data within Table 6.1 also confirm that oligotrophic waters have a substantially lower production than do eutrophic waters. Where waters are fertilised by nutrients at turnover, from upwellings, or other inputs, blooms of algae frequently occur. These contribute high primary production but not at the levels of kelp beds and coastal marshes, where the biomass of plants is very high and the plants grow very rapidly. Although tropical oceans are oligotrophic, high levels of primary production are found over reefs and at coastal margins in the tropics. In these habitats, there is high light intensity throughout the year, warm temperatures, and good availability of nutrients. These factors combine to provide the best conditions for plant growth.

Very high secondary production is often found in estuaries, coastal marshes, and at lake outlets (Table 6.2). Suspension-feeding collectors abound in these habitats and their high biomass usually contributes to high secondary production. Estuaries and coastal marshes have high primary productivity that supports the biomass of collectors with good quality food (both DOM and POM) and there is also a good particle delivery system, with twice daily tides bringing food. There are also inputs from rivers to estuaries and runoff from the surrounding land to both estuaries and marshes. Streams draining from the surface waters of stratified lakes in summer also contain good quality DOM and POM and the flowing water keeps up the supply of food for suspension feeders, at least for a short distance downstream from the lake. Whether in lake outlets, estuaries or marshes, the advantage of the suspension-feeding collector strategy is that space can be taken up by the animals alone. No substratum is needed for primary production to support the biomass of the suspension feeders, as their food is carried to them and does not grow in situ. In addition, estuaries, marshes, and outlets draining the epilimnion of lakes have warm water during the summer and this provides equable conditions for growth.

Some organisms are large and long-lived and thus have high population biomass. As a result, their annual production, although sometimes substantial, can only be a fraction of this biomass. In contrast, organisms with short life cycles, completing many generations in one year, have annual production in excess of mean population biomass. The ratio of annual production to mean biomass thus gives an indication of how different populations use available resources and each species has evolved its own characteristic life cycle and mode of life.

The production of organisms is important for looking at the quantities of organic matter cycling and available at various trophic levels. Where organisms are slow-growing, and of high biomass, their main contribution to trophic structure is in their processing of organic matter, or in their secretions. Where organisms are fast growing they frequently have a high mortality and their contribution to annual production is often by their bodies, in addition to their activities while alive.

Calculating production of aquatic organisms is not only useful as a means of understanding trophic structure and dynamics; it also has applied value. Some production that normally cycles within a water body is harvested by humans. Of special interest in this respect are aquaculture and fishing, both for sport and commercially.

Sport fishing is one of the most popular leisure activities in the World. Most anglers use rods and lines to catch fish in streams, rivers, lakes and coastal waters, although small nets are also used. Some anglers fish for the enjoyment of the chase, some for specimen fish, and others for fish to eat. If overexploitation occurs it is possible to restock waters with fish and these fisheries are usually monitored, and managed at the local, regional, or national level. In some areas of the world there is a conflict between sport and commercial fisheries, especially for prized fish such as salmon. Migratory salmon, netted in the sea or entering rivers are killed and this obviously reduces both the run of fish upstream and the recruitment of young after breeding. Damming rivers also affects migratory fish and "fish ladders" [6.1.] allowing fish to bypass dams are a familiar feature of many impoundments on large rivers.

In some areas of the world, sport fishing is accompanied by eutrophication of lakes and other still waters. Anglers fishing these waters for bottom-feeding fish frequently add large quantities of ground bait to tempt fish to a baited line. This results in an input of organic matter far in excess of that which naturally enters such waters and they rapidly become turbid and highly eutrophic. The disturbance of bottom sediments by fish adds to this eutrophication as buried nutrients are passed once more into the water column.

Most sport fishing, whether freshwater or marine, is on a small scale and has only local effects on fish stocks. Commercial fisheries have created much more significant problems.

Yield curves [6.2.] allow us to assess when stocks are being damaged. In the rising, linear sector of the curve catches increase with effort put into fishing. This cannot be sustained and the rising sector of the curve begins to plateau, with much more effort required to return only slightly higher catches, the asymptote being at the maximum sustainable yield (MSY). Beyond this point, more and more effort is required and yet yields decrease so that overfishing is now taking place and stocks are being depleted. It is no longer a cropping of resources and there is a danger that fish stocks become so depleted that the fishery is no longer viable. Fish stocks then need careful management to allow recovery.

Commercial fishing is largely marine (88% of the total World fishery [6.3.]) and was previously organised on a local scale in developed countries. It continues to be maintained on this scale in much of the Third World, where fishing is less commercialised and more a means of providing animal protein for local populations. Like other developing industries, commercial fishing became organised on a large scale with a smaller number of centres for landing and processing fish, and a distribution network to carry the fish and fish products to retail outlets. Even fifty years ago, many developed countries had small fishing ports dotted around their coastline with boats catching coastal fish and returning them to a local market. There have also been deep-water fisheries for centuries and the increasing ease of transport has made fishing a worldwide industry with the distance between catcher and consumer often being thousands of miles. This coincided with the development of technology so that sonar, satellite imaging and many other devices are used to locate fish that are caught and then processed by ocean-going factories, as well as being returned as refrigerated wet fish to market.

There is debate between the fishing industry and conservation biologists as to whether stocks of fish are being overexploited. However, there is little doubt that some previously vigorous fisheries have undergone periods of collapse. Among the best known of these is the anchoveta (Engraulis ringens) fishery off the coast of Peru. The west coast of South America has a major upwelling of deep ocean water rich in nutrients and this fertilises the surface waters so that production is markedly increased in all trophic levels, including production of the planktivorous anchoveta. Unfortunately, El Niño events occluded the upwellings in the early 1970s and the supply of energy to the trophic systems was greatly diminished. With the continued extensive cropping of the fish stocks this resulted inevitably in overexploitation a temporary collapse of the fishery. This had important consequences for Peru, as anchoveta were processed mainly for use as an animal feed [6.4.] and the export of fish meal made a significant contribution to the national economy.

There is evidence that many commercial fishing stocks are being overexploited, or are close to being so. As a result, regulation of fishing is now commonplace and this involves several measures: (i) use of quotas and control of the number of fishing vessels - with catches by fishing fleets limited by law; (ii) regulation of net mesh size and minimum size of fish of each species landed - preventing the capture of small fish which can then develop to breeding age and thus maintain stocks; and (iii) prevention of fishing near spawning grounds and at certain times of year - to encourage recruitment of stocks. It is no surprise that these regulations cause conflict in the fishing industry where large capital sums are invested in machinery and where the industry is very important to local and national economies. Moral arguments about the capture and drowning of dolphins in long trailing nets and the suffering of fish have also been used against the fishing industry.

Whaling provides another example of overexploitation of stocks [6.5.]. Being air-breathers, whales are caught at the surface using harpoons and different species are recognised by the type of blow (exhalation) they produce. When whales were abundant, the best returns were on the largest whales and these were thus the most hunted by whalers, being processed ashore or on board factory ships sailing with the whaling fleets. With the need to return to whaling stations on land removed by the commissioning of factory ships, the way was open for decimation of stocks and the catch of smaller individuals of each species. As a result, the population density of some whale species has become so low they have approached extinction, and whaling for these species is banned. A comparison of whale reproductive biology with that of fish shows why whale stocks are especially threatened. Whereas fish produce huge numbers of offspring per individual in an annual breeding season, whales produce usually one offspring, perhaps every second year. In addition, whale calves are dependent on their mothers for milk, whereas larvae of commercial fish stocks are independent of their parents.

Like local fishing, there has always been local whaling and this does not have a deleterious effect upon populations. There are still some large whale stocks that provide a sustainable yield, but popular opinion in many countries has acted as a brake on the whaling industry.

Aquaculture, the farming of aquatic organisms, has ancient origins. It is probable that fish were caged and fed in streams flowing past early human settlements, perhaps utilising waste products that were carried into the water. Such practices are still employed in some parts of the World today.

Fish that cope well with changing conditions have been farmed for centuries. For example, carp are tolerant of warm temperatures and lowered oxygen tension and are reared in ponds that may be natural, or artificially dug and allowed to fill with water [6.6.]. The ponds are stocked with fish caught from nearby lakes with the addition of organic matter, e.g., of cut leaves, by-products of agriculture, even silkworm pupae (once the precious silk has been unwound). Fish are speared or netted as required and provide a source of animal protein for the local community.

Ancient aquaculture was not just of freshwater organisms. Shellfish have been collected for thousands of years by those living near the sea and the production of bivalves is increased by growing them on ropes or poles. They are then carried clear of the turbulent, mineral-rich bedload and are able to feed by filtering good quality food from the upper part of the photic zone. Unlike those on poles, molluscs on ropes attached to buoys are also little affected by tides, so their feeding is continuous. Once seeded with spat (the immature early stages) the ropes and poles become covered with growing molluscs that are then harvested easily. Just as with aquaculture of fish in ponds, this ancient practice continues today in many parts of the World. [6.7.]

Modern aquaculture has become more commercial with large-scale production and often processing, of seaweeds [6.8.], shellfish [6.9.] and bony fish. In addition to the rearing of commercially prized fish such as salmon, there is also a demand for many by-products of processed organisms for the food and pharmaceutical industries. If organisms are kept in cages or tanks (as are salmon [in coastal inlets] and trout [in ponds]), rather than being exposed to natural conditions, it follows that they require feeding and that disease organisms must be kept at bay. Excreta and the presence of excess food cause local eutrophication and chemicals added to control disease are also potentially damaging to natural communities [6.10.]. However, given the commercial demand for animal protein and major problems facing terrestrial agriculture, it is likely that aquaculture will continue to expand, and develop. Cropping of algae gives the most efficient form of aquaculture, as these are primary producers and harvest solar energy, with energy loss only to their own respiration. Production of herbivores is less efficient, and of carnivores very inefficient in terms of energy conversion. Of course, in aquaculture it is not essential that the whole trophic structure is in situ, as foods are added to the water by humans. However, recent experience with terrestrial agriculture should make us cautious as to the nature of added fish foods.

Thus far, aquaculture has been of organisms that are familiar parts of the human diet. Inevitably, we will see the development of manufactured foods from bacteria, protists and invertebrates processed into forms that can be marketed using the power of effective advertising. One way of doing this is to use colonies of algae that do not produce toxins, engineered genetically to utilise conditions optimally and thus to bloom. Cropping excess production from blooms may result in "Sea Salad" or another more imaginative brand name appearing on supermarket shelves.

Whatever its future in food marketing, it is important that we recognise the significance of aquaculture to the developing World [6.11.].

6.1. http://www.worldofstock.com/slides/NFI1807.jpg
6.2. http://www.fao.org/docrep/009/a0212e/A0212E06.htm
6.3. http://www.fao.org/fishery/en
6.4. http://www.caretas.com.pe/1464/mdf/90-1.jpg
6.5. http://archive.greenpeace.org/comms/cbio/case.html
6.6. http://www7.taosnet.com/platinum/data/whatis/history.html
6.7. http://www.fao.org/docrep/T8598E/t8598e05.htm
6.8. http://www.seaweed.ie/aquaculture/index.php
6.9. http://media.nola.com/business_impact/photo/aquaculture-oyster-farm-mainejpg-e24265c48d151b6a.jpg
6.10. http://www.salmonfarmmonitor.org/documents/questions.html
6.11. http://www.fishfarming.com/