Establishment of the Problem
There are quite a large number of bird species that are feed on fish in one degree or another. Such birds are called fish-eating or piscivores. These primarily include species belonging to the order of copepods (cormorants, pelicans, loons), many gulls, ciconiiformes (herons, bitterns, etc.), and mergansers etc. There are fish-eating birds in the order of Carnivore such as osprey, eagles, and vulture and even among owls – fish owl of the Far East, for example. In general, the number of bird species generally considered fish-eating is about 80-90 species. Because most species of fish-eating birds are diurnal or active during daylight hours, direct observation is the usual means of confirming bird presence and damage (Brittingham 2016). Considering a quite big amount of existing species of this kind, there emerges the question on how harmful they are for fish population. This essay aims at identifying the interest groups that might be influenced by bird predation and determining whether this type of interaction between species is important.
Sometimes it is said that the "defenders" of fish-eating birds put the nature protection above fisheries interests. Thus, in the sense of more special issue of the preservation or destruction of ichthyophagous birds evolves into a broader subject questioning the protection of nature being opposite to the interests of the economy. However, the protection of fish is also the protection of nature, and ichthyologists are well aware that the struggle for the increase in commercial stock of fish is often managed by limiting catches and even running throuh the protection of livestock from overhunting.
Determination of the Problem
Many ornithologists who study fish-eating birds are convinced that their value is so clear that no one can solve this problem by considering it as such that has no economic value. However, a single opinion is not enough, because one must not forget that there are people with the opposite thoughts on the problem. To clarify the truth, one has to find an answer that would have the power of persuasion of evidence for all.
As mentioned earlier, fish are the basis of the diet of many gulls, guillemots, and Ciconiiformes. In places of congestion of mass fish ichthyophagous birds form large colonies. The most famous colony has been known to desert islands off the coast of Peru, and Bougainville cormorant (Phalacrocorax bougainvillei) for ages being even once called "the most useful bird in the world." These birds have become famous as a source of guano, a valuable fertilizer which played an important role in the development of agriculture in Western Europe. Huge amounts of bird droppings fall into the sea, in the cold oxygen-rich water of Peru Current. Phosphorus and nitrate content increases by hundred times rapidly developing microscopic algae and invertebrates which in turn, serve as the main food for innumerable shoals of anchovies. The best conditions for the supply of fish and the largest number of them are exactly where the fish-eating birds 'fertilize' the sea.
On average, each cormorant consumes up to 300-500 grams of fish a day, and about 60% of this organic matter in the form of excreta returnned to the water. The latter deposits on the bottom of the sea and increases the concentration of nutrients in the lower layers. It promotes the development of ground vegetation, and dying plankton causes the formation of the detritus which serves as the food for the benthic animals. Moreover, the increase in benthos biomass contributes to bottom fish growth. The positive effect of water birds excreta fertilizers may be more significant than damage brought by birds grazing certain parts of fish stocks.
One of the main arguments of those seeking to prove that the harm caused by fish-eating birds is very great is the total count of a number of fish withdrawn from the pond by piscivores. This calculation gives inaccurate results. However, not only this fact is important. The figures cannot be considered as variables showing how the catches are reduced by the presence of fish-eating birds on the pond as eating size is not the size of damage.
One can say that the number of fish-eating birds should include far fewer species than it does so far, and that their quantity is much lower than expected. Diets clarified more or less accurately, and they are greatly reduced in many cases. Fish diet dissected birds on objects with clear cases of weed eating fish useless to a human and sometimes harmful to fisheries. Additionally, the studies found that the contents of the bird stomach are not sufficient to judge its importance for the economy. Furthermore, food facility biology is needed to conclude about what stage of their annual cycle the species are at, how a bird is caught and under which circumstances.