According to the definition most widely accepted by ichthyologists, scientists who study fish zoology, migratory fish are those that regularly change their habitats, undertaking “vertical” migrations upstream and downstream in rivers and entering and leaving lakes and streams, or that make long journeys in the oceans. Migrations take place at different times; some species migrate once a year and others undertake their journeys more frequently.
Fish usually make such movements in search of feeding grounds or to procreate. They are sensitive to subtle changes in the water conditions in their environments such as altered acidity and oxygen concentrations, or changes in water temperature that tell them when it is time to make their journeys.
Migratory fish are classified into five categories according to the origins and destinations of their migratory journeys. There some species, for example, that spend most of their life in the open seas but enter freshwater rivers when they wish to reproduce, and there are other species that leave their freshwater habitats and seek the oceans or other bodies of saltwater in order to reproduce.
The seasonal upstream migration of fish to spawn in Brazilian rivers is a well-known phenomenon and is referred to locally as the Piracema (pronounced Pirassema). In the course of their journey upstream and later back down the fish have to face many natural obstacles like rocks, waterfalls, and predators, all of which make the migration a dangerous venture. The word Piracema derives from the indigenous Tupi language and means Fish (Pira) Depart (Sema) – Fish Departure.
In spite of being so well-known, there have been very few studies of migratory fish species in the Brazilian Amazon.
Biologist Rosalvo Duarte Rosa believes that one of the reasons for the lack of studies on migratory fish species is the very high cost of conducting them. “How can you manage to accompany the movements of a fish that may travel up to 5 thousand kilometres? It is extremely difficult and there are not many organisations that have the funds to needed to support such a project”, he explains.
The best technique for tracking migratory fish is called ‘Tagging’ and, as the name suggests, it consists of marking individual fish in some way , usually with a tag or a clip or some other very small device, and by that means keeping track of the animal’s movements.
However it is a costly and time consuming method and is liable to various setbacks such as the devices becoming detached or deteriorating over time or being torn off by predators or fishermen. Nevertheless it is still the best method available for this kind of study.
Biologist Reginaldo Carvalhos dos Santos declared that studying the more important migratory species will lead to a greater knowledge of the Amazon region which, in turn, will be very useful in the elaboration of regional conservation projects.
“Some species, like the huge Piraiba catfish for example, travel a lot but they always seek out undisturbed river environments with vegetation along the water’ edge and good quality water. Accompanying the Paraiba’s movements means getting to know areas of the Amazon that are still intact and discovering environments as yet unknown to man or at least where his presence has not yet interfered”, he explained.
Reginaldo added that a large number of other fish species (somewhere between 350 and 450) are associated to the the Piraiba catfish’s (Brachyplatystoma filamentosum) habitats in the Amazon and Teles Pires rivers, as well as big mammals, and arthropods.
“Thus, guaranteeing the integrity of the environment and the health of the ecosystems for the Piraiba also helps to foster the conservation of a series of other life forms”, said the biologist.
The coordinator of the Juruena River Migratory Fish Species Expedition, Ayslaner Gallo, said that it is important to get to know the rivers and environments used by migratory fish in order to be able to contribute effectively to their conservation.
“All the information we can get hold of is important to support our thinking as to how these species can be conserved and perpetuated. We need to get to know the places they visit and whether they leave one river basin to enter another making use of the Madeira, Tocantins and Xingu River basins, for example”, said the coordinator.