420 Dive Log Australasia OCTOBER 2026

Bull sharks can swim in freshwater rivers

drink just a few litres of sea water. It has dramatic effects on our blood and every aspect of living from kidney function to heart function and more. Fish have ways of excreting excess salt. Most Marine fish die quickly if placed in a fresh water aquarium. The Bull shark, Carcharias leucas , has an extra ordinary ability to adapt to life in brackish and even fresh water as well as in the open ocean. The salt level that is lethal to humans is not a problem for them. Research in Fiji has shown that mother Bull sharks swim upstream to inland rivers to give birth. The pups spend their first few months in fresh water. One possible explanation is that this enables them to avoid marine predators who cannot venture into fresh water rivers. A perfect survival mechanism to escape danger as they grow. In the sea, Environmental Pressure comes in the form of being able to find enough food, being able to avoid predators, surviving competition from members of the same species, finding a mate and a secure place to live in free from danger. Darwin and Wallace would observe that a Parrotfish sets up a territory and fights to maintain it. Intruders of the same species are driven off. When an Alpha male succeeds in establishing a territory, he can attract an Alpha female and they can produce offspring. The territory has to be big enough to provide enough

food and shelter for mum and dad Parrotfish and their offspring. Rival Parrotfish fight for the right to own their own territory or try to find a vacant reef that will be able to provide for them and their offspring ( in a way that is similar to magpies ) . SURVIVAL OF THE FITTEST Every day of your life, you are under subliminal attack from some sort of pathogen. Your immune system deals with the threat and you do not even realise that you have survived a potentially near death attack from Meningococcal disease, for example. Microbiologists tell us that the meningococcal bacteria are hundreds of times This Bull shark most probably died from the massive wound on her back

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Dive Log Australasia #420 October ’26

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