All Shark & No Bite

The slender gummy shark is often seen both exploring and resting along the sandy ocean floor.

The slender gummy shark is often seen both exploring and resting along the sandy ocean floor.

Victorian waters are home to a significant range of shark species, including iconic species such as the Great White (Charcharodon charcharias) and the Mako (Isurus oxyrinchus). However, amongst these majestic creatures, there are also many less-renowned species, like the Gummy (Mustelus antarcticus). Unlike their more famous cousins, these sharks don’t exactly have ‘teeth’, but rather bony plate-like structures forming what look like ‘gums’ - hence their unusual names. Notably, the gummy also plays quite the role in Victoria’s commercial and recreational fisheries.

The gummy shark is from a family known as the Houndsharks (Triakidae), and is usually born measuring approximately 30 to 35 cm in length with some even reaching up to around 1.8 metres and the larger sex generally being the female at maturity. However, this species is definitely not endemic to Victorian waters, as they are also found along the entire south-eastern coastline of Australia, from as far north as New South Wales to the more southern waters of Tasmania. The south-western portion of the Western Australian coast is also known to be a home to them.

Distribution of the gummy shark around the southern coast of Australia.  

Distribution of the gummy shark around the southern coast of Australia.  

Gummy sharks can be easily identified by their slender bodies and characteristically white spotted dorsal (or upper) side, and are docile animals that will happily cruise around the lower sections of the water column, often ‘resting’ on the sea floor due to their incredible ability to stop moving and still breathe. This is accomplished via the small holes behind their eyes known as spiracles, which allow the gummy to pump water through to its gills so that it is able to breathe without the need to move. Their diet usually consists of shellfish and other marine invertebrates such as polychaetes (sea worms), which they are able to crush with their specially designed plate-like gums. They do on occasion, however, eat various small fish and squid.

In Victoria, gummies are frequently seen and caught in both Port Phillip and Western Port Bays. Large females give birth in these enclosed bays that provide ‘nurseries’ to shelter and protect their young from the threats of the open ocean. Mothers tend to enter the bays and give birth in the warmer months from October through to February. In these warmer waters, food is usually plentiful and supports a faster growth rate in pups, allowing them to grow big enough to avoid larger predators as soon as possible. Pups also tend to stay in these shallow nursery areas for a few years before they reach a large enough size to venture out and brave the open waters. Here, they will mature, meet potential mates and, like their parents, return to the bays to breed again.

A juvenile gummy shark caught in Western Port Bay for research purposes. Notice the plate-like gums used for crushing small prey. 

A juvenile gummy shark caught in Western Port Bay for research purposes. Notice the plate-like gums used for crushing small prey. 

Gummy sharks are not just commercially valuable for their flesh (many may know this as ‘flake’ at your local fish ‘n’ chip shop), but are also popular targets in recreational fisheries. Unfortunately though, gummy populations (like those of many sharks) are quite sensitive to fishing pressures. Gummies mature slowly and give birth to relatively few young, as females take approximately five years to mature, with a subsequent gestation period of one year and litters ranging from 15 to 30 pups in size. This is quite a small number if you consider that only a small portion will survive to adulthood as a result of predation and fishing activity. This slow reproductive rate therefore makes this species particularly vulnerable to overfishing.

However, despite this fact, gummy sharks have been commercially fished since the 1920s, providing a lucrative trade in Victorian and Tasmanian markets. From the 1920s until the late 80s and early 90s, gummy shark populations were subjected to intense and often unregulated fishing (at least in the early years), which saw stocks overfished and resulted in severe declines in population numbers. Consequently, fisheries directed their attention to researching the biology, population structure and dynamics of the gummy shark. These studies proved fruitful, many still continuing today, with results leading to changed fishing practices and regulations. As a result of our awareness and ability to implement change, gummy stocks have increased since their decline in the late Twentieth Century, with continually monitored fishing practises now allowing the stock to be sustainably fished in Victoria.

These remarkable creatures are only one of many species of shark in Victorian waters, and although popular culture and - to some extent - instinct makes us frightful of these creatures, we must open our minds and understand the crucial role they play in delicate marine ecosystems. Yes, some sharks are indeed dangerous and aggressive, but we must still pay them the respect they deserve. In doing so, we can learn to live with them, helping to create and maintain a healthy, functioning and productive ocean for all to enjoy.

 

 

 

 

First image courtesy of www.oceanwideimages.com