Evaluate migration in marine species

Every year, migration of animals becomes very apparent to us, when thousands of birds form big groups in autumn. They then leave our realms to move southwards into warmer habitats. Migration is the active movement of a group or population of animals of one species moving from one habitat to another. It can be short moves within a single habitat or long travels such as in the bird migration and naturally occurs on a seasonal basis. Migration is very common in many species. The most important reasons for migratory animals to change their habitats are feeding behaviour, avoiding food shortages or disadvantageous environmental factors in different seasonal times and breeding behaviour. However, in most cases the reasons for migration are still poorly understood.
Moving around is very common in many species. Not only terrestrial animals migrate, but it is widespread among marine organisms as well. There are several completely different types of movements going on in the oceans that have different reasons. These can be distinguished into three different main types.

During the nighttime one will find a great many animals near the surface and the shallow waters beneath, which rather are associated with the deep sea. This is due to a great and unexpected movement of organisms, that takes place in the open ocean daily. In the evening, mainly zooplanktonic animals and small fish start to move up the water column, leaving their original habitats. The way from the deep zones to the shallow water zones is approximately 50’000 times their body length long. (Prager et al., 2000)
Light decreases with increasing depth and after about 200 metres the ocean is completely dark. Hence, no photosynthesis can take place and there only are few nutrients. The only food available comes from above falling down into the Mesopelagic and lower zones. Therefore, one suggested main reason by Prager (2000) for the diunal vertical migration is, that organisms move upwards to feed in the nutrients-rich surface waters. The animals feed at night when it is dark to avoid predation. When it gets light again, they go back in the deep to hide, where they are poorly noticeable by possible predators during the day. Furthermore, Ellen J. Prager suggests, that due to Zooplankton and Fish being poikilotherm, the metabolisms of the animals slow down in the cold deep sea waters. Thus they do not need much energy and thereby no food during the daytime. (Prager et al., 2000)

Another migration in the marine environment occurs in the intertidal zones, when animals migrate up and down the shore with the tides. Most of the intertidal organisms are vulnerable to changing environmental conditions, especially heat and light. When the water is gone and they are exposed to the sun and the air, the animals have to cope with extremes in heat and cold which can lead to desiccation or freezing. Some species have structural or physiological adaptations to these effects like light coloring, different cell thermostabilities and special body structures. Most mobile species, however, go with the water. When the tide is going out, they move downwards in lowshore areas and when the tide is rising they migrate with the tide to the highshore. Thus, it actually is a migration within one habitat of the organisms as it occurs in the intertidal zone. Nevertheless, the movement can take place over a relatively long distance.
Additionally, the creatures can prevent predation by birds or terrestrial animals, as they would be easy prey if they were exposed. Furthermore, they will avoid foodshortages during the low-tide times.

However, the most prominent migrations are the long-distance travels occurring in the oceans. According to Levinton (2001), many bigger marine species such as fish, marine mammals and turtles have a reproduction cycle that leads to long-distance migration between spawning, mating or nursing grounds and feeding grounds. In about 5% of fish species, movement is not only in the oceans, but cover both marine and freshwater habitats. These are called diadromous. Thus, they either live in salt or freshwater and migrate to the opposite to spawn. Salmons (Salmo salar), for instance, are born in freshwater and therefore are anadromous. During their different larval stages, they live up to 3 years in this area. When they are old enough, they form groups and move seawards. As adults, they live several years in the sea, before they start to migrate again. In 90% the salmons return to the stream where they were born to spawn.
Catadromous fish live the other way around. They generally spwan in oceanic environments and return to live in freshwater rivers, before they start to migrate again. An example is the common eel (Anguilla anguilla).
It is common in this migration-behavior, that the different age-classes live in fairly different habitats. Furthermore, often drifting is part of the migration cycle, when larvae are not actively moving, yet but been moved in a planktonic way by currents, rivers, floods and so on.
As a long migration costs a big amount of time and energy, there must be good reasons for these species to approach these efforts. Levinton suggests, that the individuals take advantage of the optimal habitats respectively for feeding and spawning. Therefore there is a higher reproduction rate and reproduction success. Furthermore there might be different needs for food in the different age classes. Besides, if adults and juveniles live in diverse habitats, there will not be food competition among individuals of the same species.
Another migratory species are sea turtles. In the green turtles (Chelodnia mydas) females cover distances of up to thousands of kilometres to reach a very small number of certain beaches to lay their eggs. Any individual consistently returns to the same beach. Further on, this only occurs during a very specific time of the year. When the hatchlings have eclosed, they themselves start the long journey to the actual feeding grounds of the turtles.
A third example of long-distance-travellers are cetaceans, big whales in particular. Most of the Baleen whales (Mysticeti) feed in the nutrients-rich arctic and antarctic waters, where they will find great amounts of Krill – their primary food resource. Nevertheless the best areas to give birth to their young are in warm equatorial waters. Hence, the whales have a annual migration between these two regions. They mate in the polar feeding grounds and afterwards start moving to the south where they give birth. During their journey the animals fast. on the contrary, in Sperm Whales (Physeter macrocephalus) only males migrate. The females stay in warm waters during the whole year, whereas the males travel from the cold at the end of the summer to meet with the others and form big harems. Thus, the cetaceans migrate from feeding grounds to form nurseries and grow their young. (Carwardine, 2003)

The ability to move over distances is of great importance for many marine species. Even though the migration is connected with great efforts and time, the animals take big advantages which outweigh the costs. There are several different reasons that cause the behaviour. However, three main points are obvious, which are interlinked. Naturally, the animals live in habitats, which are rich of nutrients and offer good food resources. Still, these areas can be dangerous places for prey animals like in the shallow surface waters or low-tide zones. Thus the creatures migrate to avoid predation and yet get the best food, as the places where they hide are generally low on nutrients. A food-predation-avoiding connection is obvious and one main factor for migration.
A second interconnection is between feeding grounds and breeding grounds. It appears that the areas, where the animals prefer to feed for different reasons like richness of nutrients, are not good for spawning or nursing. Therefore the animals migrate between those two areas. Further on migration to mating grounds gives solitary animals such as whales the possibility to easily find a mating partner, which otherwise would be difficult in the giant area of the ocean.
Hence, the ability to migrate is of great value for the animals. It permits the organisms to get good food on one hand and make it easier to survive due to predation-avoiding and breeding in favourable areas on the other.
However, the exact reasons and particularly mechanisms of the marine migrations are yet to be fully understood and give a great field of questions and debate among scientists, whether it is in the movement of the smallest organisms on a daily basis or the annual travels of the giant whales through whole seas.

Campbell, N A and Reece, J B. 2003. Biologie. Spektrum Akademischer Verlag GmbH Heidelberg, Berlin.

Carwardine, M. 2003.Wale und Delfine in europäischen Gewässern. Delius Klasing Verlag, Bielefeld.

Prager, E J and Earle, S A. 2000. The Oceans. McGraw-Hill, New York.

Levinton, J S. 2001. Marine Biologiy. Oxford University Press, Inc, New York.

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