Otter is a mammal, so right there it has a better brain and the upper hand against a scaled fish . Hahahah go mammals.
bull sharks like shallow water and the shark can breath underwater, the otter can't all it would take is a quick bite from below (like they attack seals) and there would be all kinds of dead otter.
But when sharks attacks seals, like at Shark Alley in South Africa. The sharks attack from below in deep water. This line of attack would not be possible in a river system. Also, the turning ability and maneuverability of an otter would out class any shark that didn't have a surprise attack. The obstacles in a river system such as the amazon, logs, trees, branches and sandbanks would definitely give the otter the upper hand I feel.
On top of all of this, the otters are not solitary creatures and rely quite heavily on group participation for protection and detection of predators. If the otters were that easy for bull sharks to catch would they not come to get them more frequently instead of a few isolated incidences perhaps?
completly agree with you. The otters are much more agile than a shark. And we should not forget that the otters know their area very well. The shark can´t do his surprise attack in the shallow waters of the amazon. On the other hand Otters are high intelligent and social animals while bullsharks are dumb as bread. The alpha female of any otter clan will never accept such a predator in her clans territory. Chances are high that as soon the bullshark is spotted a pack of otters will hang on it, rip open the gills and destroy the fins.
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