Bull sharks, a large predatory shark, they are stocky, fierce, adult individuals are 2 to 4 meters long, sometimes even other sharks inevitably fall prey to them. Even such a powerful character faces many dangers in infancy, such as the gray shark, which is larger than the bull shark, and its juvenile shark has unfortunately become a regular on the menu of some large sharks. Fortunately, cow shark mothers have unique insights and solutions to the problem of parenting. It is true that, like other sharks, bull sharks are not tasked with protecting their offspring after the baby shark is born, but it is possible to choose a less dangerous place for the baby sharks to use as their nursery.
The rivers of northern Australia are one of these ideal places, where there are many water systems, resources that can fully cope with the growth of young sharks, and predators that can threaten them are generally no other than Australian saltwater crocodiles. Unique mediation capabilities allow the bull shark to enter fresh water and survive for several years, the mother of the bull shark drags her heavy body thousands of miles to the river where she was born, some of them choose to give birth at the mouth of the river, and the most responsible mother shark continues to swim upstream, providing the best security for the future of the baby in the safest freshwater waters. Next, it is all up to the baby sharks themselves, and today, Zoology will talk about how the calf sharks started their own survival path in the North Australian River.
Adult bull sharks that live in the ocean
Calf sharks in the river
In order to study the ecological information of juvenile bull sharks, scholars Suchbourne et al. collected samples from the river between the Fitzroy River in Western Australia and the Robinson River in the Northern Territory, and the specific collection location is shown in the green dot in the figure.
Zoologists collect bull sharks in the Bei'ao River
In the above area, scholars have collected a total of 111 juvenile bull sharks (57 females and 54 males), their body length is 69 to 138 cm, the following is the body length distribution map, you can see that most of the individuals are under 100 cm, this is indeed a veritable nursery.
The body length of the freshwater calf shark
As mentioned earlier in Zoological Records, bull sharks can survive in fresh water for several years, to be precise, calves generally leave fresh water at the age of four to live in estuarine areas, so how old are they from birth to each year older when they leave fresh water? According to the picture below, they are about 86 cm at the age of one, 101 cm at the age of two, 117 cm in the third year, and 134 cm at the age of four, at which point the baby shark can go to the next habitat, the estuary area.
Growth of small bull sharks in freshwater
So, between the ages of 0 and 4, how do calves solve the most important thing – the food problem , and what exactly do they feed on? By examining the stomach contents of these sharks, we can get a glimpse of their recipes: there is no doubt that the bony fish is the main food for the young sharks, especially the New Sea Catfish (up to 60 cm in size), the Bei'ao Sea Bream (up to 48 cm, generally less than 20 cm) and the fine-scaled YellowTail (up to 65 cm) with the highest frequency.
In the stomach volume examination, the most surprising thing was that the juvenile sharks had sporadic sawfish, wild boars and Australian freshwater crocodiles in their stomachs.
The small tooth sawfish, a giant cartilaginous fish with a body length of up to 7 meters, is generally less than 3 meters in freshwater rivers, and like the bull shark, it also uses fresh water as a childcare place, and the northern part of Western Australia contained in the expedition area is one of the most important calves in the world. Surveys have been conducted in the Fitzroy River in Western Australia showing that 39 sawfish 96 to 158 cm long, 23 with predator bite marks, of which 21 Australian freshwater crocodiles attacked and 3 bull sharks were responsible. Picture 1 below shows a 183.5 cm sawfish under investigation with bull shark bite marks on its body, and Zoology has calculated that the attacker should be less than 3 years old. Figure 2 below shows the wreckage of a sawfish in the belly of a calf shark.
Sawfish have bull shark bite marks on their bodies
Sawfish remnants in the stomachs of juvenile sharks
The average adult boar in the North Australian region is 87 kg, the female is 71 kg, I believe that the news of the flooding of Australian wild boars has been heard, the following is the wild boar hair in the shark's belly.
Wild boar hair was found in the stomach of the calf shark
Freshwater crocodile bones have also been found in the stomach of juvenile sharks, freshwater crocodiles with a maximum body length of more than 3 meters and a weight of nearly 100 kilograms, although the size is far less than that of its relatives saltwater crocodiles, but it prefers upstream habitats to be a great threat to juvenile sawfish.
Freshwater crocodile remains found in the stomach of a calf shark
Australian freshwater crocodiles prey on sawfish
But we can't help but question that the wild boar and freshwater crocodile in the stomach do not mean natural predation, but may also be scavenging. If you want to talk about predation, how do calf sharks do it? Marshall, a local Aboriginal elder in Derby, Western Australia, gave us part of the answer by providing some observations of shark attacks:
When the boar entered the water to drink, several calf sharks bit the boar's leg and dragged it into the deep water, drowned and quickly began to eat. Marshall also gave a vivid example of crocodiles - the target was a large freshwater crocodile, a calf shark quickly bit off one of the crocodile's front legs, and many other bull sharks appeared and then devoured the remaining limbs of the crocodile, after which they began to attack the trunk.
Marshall is certainly not a professional shark researcher, but scholars such as Saubern have great respect for her, and in their thesis acknowledgment, they especially thank Marshall for their help with their projects and review of documents, and also mention her records in the main text as evidence. Therefore, Zoology believes that Elder Marshall can be used as a reliable source, and the general direction of the events she provides should not be a big problem, and whether she will be careless in the details as a non-professional scholar needs to be further illustrated by video materials.
The early growth of the calf shark is introduced here, and the zoology will introduce more ecological information about adult bull sharks in the future. Here I would like to say that no matter what age the bull shark, no matter what kind of shark, should not bear too much of the notoriety of man-eaters, even if they occasionally attack terrestrial animals, but that is mostly based on the compensation strategy after the lack of aquatic resources; as for them attacking humans in the water, the reason is that the low visibility of the water is one, the inspection bite is the second, and the result is that the vast majority of people have not been bitten off any limbs - unloading the prey is their best skill. Compared with the tragic situation of prey, it is too different, so when we think about it, do we still think that sharks regard humans as prey? So we really shouldn't be too hostile to these magnificent top predators, and harmony between man and nature is the best way to start the road to the future.