Believe it or not, white sharks are just as physically unique as people – you just need to know which features to look at for comparison. The obviousness of these features covers a broad spectrum, from at-a-glance unmistakeable to minute attention to detail using high quality photographs. In this blog, I’ll take you through the different methods that I use to tell apart the sharks that I work with, as well as introducing you to some of my favourite individuals. All pictures were taken from Slashfin, an ecotourism boat run by the fantastic Marine Dynamics (www.sharkwatchsa.com) – the best white shark cage diving operation in South Africa and supporter of my PhD field work to date.
The size of the shark you are looking at is the first and most basic indicator of who it is you are looking at. Things get trickier when you have a lot of sharks of similar size but you aren’t going to confuse a 2m juvenile with a 5m mature individual! Accurately (within centimetres) estimating the length of a free-swimming shark is actually pretty difficult – experience, water distortion, viewing angle, water clarity and the availability of a reference object against which you can compare the length of your shark are just some of the factors that can effect a size estimate. In a later blog post, I’ll tell you how I am using lasers to accurately measure free-swimming white sharks. In addition to looking at length, girth can also be a useful diagnostic feature in individual ID – white sharks don’t just get longer as they mature, they get disproportionately more bulky with length, and some individuals are visibly more bulky than others.
This is another common enquiry – how do you tell if a shark is a male or female? Carefully is usually a good start 😉 Male and female sharks can be told apart by looking at their pelvic fins. The pelvic fins of male sharks have a modified form, where they effectively curl round to form two sausage shaped structures called claspers. The claspers perform a similar function to the mammalian penis, whereby they are used to transfer sperm to females during mating.
The rule of thumb to tell if a male shark is sexually mature is to see if their claspers extend more than a third beyond the posterior edge of their pelvic fins or to check if they have become calcified (this makes them rigid as opposed to the pliable claspers of immature individuals). For mature individuals, it is relatively easy to see which sex they are, even if they don’t accommodatingly roll or leap right in front of you. For immature individuals however, it can be very difficult to tell unless you manage to get a decent photo of their pelvic fins.
OK, so you have the size and possibly sex of your shark to help you narrow down who they are. One of the next, and sometimes most obvious, things to look at is their scars. White sharks are highly active, curious creatures that often earn themselves a good handful of scars throughout their lifetime. These can be scrapes, scratches or bites from an unknowable number of sources, including interactions with each other. Mature females in particular may have bite marks from mating – male sharks don’t have hands to hold on with so they bite females during copulation, usually in the head/gill/pectoral fin regions. White sharks have impressively tough and thick layers of skin and muscle and bites like these don’t pose a threat to their health. Fresh wounds are pink, healing to white and eventually fading to black. The location and appearance of scars can be a great help in identifying your shark.
“Rosie” is a term given to white pigmentation marks on the skin of white sharks. They are typically, and most noticeably, located on the dorsal and caudal (tail) fins but can occur in a wide variety of locations on a shark’s body. They vary in size and shape and can be an invaluable distinguishing characteristic.
Sharks frequently pick up skin parasites and these critters can be very useful in helping to identify individuals. Parasite load and the position of singular or patches of parasites can help to ID individuals over a short time frame.
White sharks are famous for their counter-shaded colouration – grey on top and white below. The shape of the countershading line is unique to each individual sharks and is often one of the more subtle ways used to tell sharks apart. Some researchers find the countershading pattern on the pelvic fins and in the gill slits especially useful for individual ID.
Dorsal Fin Shape:
The shape of the trailing edge of the dorsal fin of white sharks has been found to act kind of like a white shark fingerprint. The nicks, notches and tabs of the trailing edge can be traced from photographs into free computer software called DARWIN (http://darwin.eckerd.edu/), which stores all traced fins into a database and matches new fin traces to the whole database to tell you if you have seen a shark before or if it is new. Dorsal fins do change in appearance over the course of a few years so it’s important to get up to date photos of the sharks to trace, but DARWIN can save hours of manually sifting through a large photo library and trying to see if two fins look similar. All DARWIN matches should be checked manually (no system is perfect) but this software has proved to be a great tool in the arsenal of researchers wishing to individually ID sharks to help estimate things like population sizes, mortality rates, residency periods, migration patterns and much more.Back to all News