High densities trigger floodplain migrations

Posted on June 26, 2010


Many animals, including species of insects, mammals, fish and birds, undergo migrations from seasonal environments. Surprisingly little is known about the factors that trigger these movements, although unfavourable conditions, limited resources, predation pressures and competition have all been proposed.  The Guineatilapia (Tilapia guineensis) is the commonest fish species on the Gambia River floodplain.  This species  occurs in pools,  normally at densities of  around 11 fish / mbut at times as high as 500 fish / m2, and rapidly colonises new habitats that become available in the rainy season.  The willingness of Guinea tilapia to migrate was assessed by counting the number of fish that traversed a shallow ramp to escape from a 1 m x 1 m outdoor tank over a seven day period.  The proportion of the population that migrated was significantly higher when fish were stocked at a density of 40 / m2 than when they were at low or medium densities (10 or 25 fish / m2), probably because of increased territorial competition at high density.  In other experiments, withholding food from fish stocked at medium density had no significant effect on migration rate, although there was an increase in exploratory behavior (attempts to move onto the ramp).

Reference:  Louca, V., Lindsay, S.W. & Lucas, M.C.  (2009).  Factors triggering floodplain fish emigration: importance of fish density and food availability.  Ecology of Freshwater Fish 18, 60–64.

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Posted in: behaviour, fish