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154. Schooling Snappers




Subject:

A school of snappers is facing a very light current in Mauritius. By watching carefully, one can distinguish two different species; the Bluestripe Snapper (Lutjanus kasmira) with a dark band between its eye and snout, and the Bengal Snapper (Lutjanus bengalensis) who is completely white from its lowest horizontal stripe down.


A group of fish that stay together for social reasons are shoaling. If the group is swimming in the same direction and in a coordinated manner, they are schooling. Generally, all fish in a particular school are of the same size, shape, and colour. Therefore, it is often true that different species that look alike regarding colour, size and markings often congregate in big schools, just like in this underwater video clip.


When a school of fish stops to forage on the reef, the school takes on a random form and becomes a shoal. Shoals are more vulnerable to predator attacks, and so is the odd individual in the shoal or school. When an individual is different in colour, shape, or size, it will stand out like a sore thumb and becomes an easy prey. The shape a shoal or school takes depends on the fish species and their behavior. Schools that are traveling can form long thin lines, ovals, or strange non-defined shapes. Fast-moving schools usually form a wedge shape, while shoals that are feeding have no particular shape.


Schools provide a certain number of securities to the fish. By sticking together, fish mimic the pressure wave of one big animal instead of many smaller ones. The predator is deceived, and the fish are left alone. Another explanation for why fish school is what is known as "safety in numbers". The bigger the school, the lesser the chance each individual has of becoming a predator's dinner. Besides protection from predators, there is the increased success when foraging or a greater chance of finding a mate, and maybe even increased hydrodynamic efficiency!


The mechanisms of schooling, like moving like one and changing direction simultaneously, are not fully understood. There is certainly a visual factor involved. Many schooling fish have distinctive traits. The stripes and spots of these snappers are most likely visual aids or reference marks. Some say forming schools is probably a learned ability. This theory is based on the observation that juveniles practice in pairs in order to develop the skill of schooling. More recent studies show that schooling is in a species' genes. The behaviour is controlled by a genomic region that is also involved with the development of the lateral line, the sensory structure that fish likely use to keep the right distance from each other.


Technique:

Big objects like in this case, schooling fish, are always difficult to illuminate completely with standard video lights. The use of an orange filter as an alternative is to be considered in certain circumstances, like filming when there is enough daylight present. On deeper dives, most orange filters are less efficient. There are, however, orange filters that are specially made for deeper and thus darker water.


In post-production, the yellow of the snappers and the blue of the ocean were a bit saturated to make the image more vibrant.


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