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Breeding obvious - protecting migratory fish


 
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Professor Bill Gladstone, Head of Environmental Sciences at the University of Technology Sydney, is a marine biologist interested in the breeding aggregations of fish and their reproductive behaviour.     
Breeding aggregations refer to those breeding places to which fish migrate each year in order to reproduce. This is not common fish behaviour since many species breed in the habitats where they live. The migration seems to be important in helping to increase their survival, including their offspring. For example, the chosen spawning site may have better protection from predators, a different food supply, different water currents. The same sites are returned to with each season’s migration and often may be quite distant from the home range.
While it is thought that fish have some magnetic sense to assist with migration over large distances in much the same way as sharks do, other mechanisms are regarded as important, for example, some fish use chemical cues to navigate to selected sites.

 
Professor Gladstone has worked on trigger fish living in the Great Barrier Reef as well as wrasse in the Red Sea. He is currently interested in damsel fish which live on reefs in New South Wales.    
One species, the one-spot puller, normally eats the small plankton that inhabit the first few metres of water, near the surface.
When it comes time to breed, they leave the surface area and gather in the water at the bottom of the reef before migrating to some select spawning sites some kilometres away.

When not spawning, one spot pullers spend most of the day feeding on plankton in large schools near the water’s surface

 
This happens during the lunar cycles of a full moon and a new moon, the breeding process taking place a number of times during a four month period. The males arrive first, setting up their territories with combative behaviour. When the females arrive between one and two weeks later, the males display their colours to attract them. The female lays the eggs in the selected male’s territory, favouring the better parts of rocks that will protect them. The male then covers the eggs with sperm and the sticky eggs attach firmly to the rock surface. The female leaves the territory and the male stays for five days, tending to the eggs without feeding. He removes any debris from them, chases other fish away and removes dead eggs which have changed colour as a signal. Only when the eggs have hatched, which always happens at night, will the male return to feeding.
Male one spot pullers (Chromis hypsilepis) gathering in the spawning ground prior to the arrival of the females

 
The newly hatched eggs cannot swim but float to the surface thanks to an oil drop in their gut. Upon reaching the surface they are carried away by the currents so it may well be that tidal currents also play a role in the timing of the migration, and the strength of the current may be influenced by the phase of the moon.

A  male one spot puller defending his territory, in which the females will lay their eggs

 
It is important to be aware of the migratory behaviours and needs of reef fish and such unusual behaviour should be taken into account when planning marine reserves and other protected areas to preserve marine biodiversity. That is, it is not just protecting particular fish species that is required, but equally, the particular breeding sites for migratory fish must also be preserved, thereby protecting their reproductive behaviours.

That should be breeding obvious.

Images from Bill Gladstone
Text: V.B. November 2010



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