Posted on Friday, 31 December 2004
I hope you are all enjoying the Festive Season.
December has been a very quiet month for sharks for us so this newsletter is going to be a little brief, maybe a good thing as I am feeling too much in the Holiday mode!
Generally this time of year we begin our off-shore trips to dive with mako and blue sharks, but as last year, the summer winds have keep us off the water and we have only managed one trip. The weather was fantastic, but we got skunked and failed to see a shark. This doesn’t happen very often to us so when it does it serves as a reminder that we are looking in a huge blue desert for sharks as well as makes us appreciate even more when we do see them!
As we were land bound we managed to do a lot of kelp diving to have a look at the small endemic cat shark species we find along our coast. Most people want to see the big glamorous sharks but we also really enjoy seeing the little ones. The species we were seeing were the puff adder shy sharks, striped cat sharks, brown shy sharks, the beautiful leopard cat sharks and once or twice when we were lucky, the spotted-gulley shark. The gully sharks can get as large as 1,8 meters, but the others generally are not longer than 1 meter.
The summer months also attract other migratory shark species to False Bay and these can be caught as by catch by traditional beach seine net fishermen. I have written about this before, but for those who don’t know we measure, tag and then release them after they are caught. This past month 3 bronze whalers were caught as well as a tremendous amount of different ray species. These included Diamond rays, Duck bill rays, Bull rays and the Lesser Sandshark. The fishermen are also not allowed to keep Steenbras which is a collapsed fish species. We are able to measure and tag these. Some of them have been the largest we have measured which is a promising sign. And lastly amongst the by catch there has been a small amount of smooth hound sharks. These have mostly all been too small to tag but we were able to release them back into the water. This species has been particularly hard hit by commercial fishermen and the stocks are decreasing at an alarming rate. Unfortunately there have been no fishing restrictions placed on them and their status is only going to get worse.
Smooth hound sharks are thought to make up a large proportion of the white shark’s diet. The decreasing population of smooth hounds is coincidental with white shark sightings increasing close to shore…interesting…!
A few months ago we were able to catch up with a friend of ours who is doing his PhD study on Wobbegong sharks. I do not know much about wobbys and I was fascinated to hear about Charlie’s study as well as the sharks. Ultimately Charlie’s project will aid in recommending sustainable fishing of wobbegongs. At this time there are no restrictions in place.
Charlie has kindly written a little about wobbegongs as well as a little about his project.
The decreasing population of smooth hounds is coincidental with white shark sightings increasing close to shore…interesting…!
Wobbegongs in peril!
Wobbegongs are cryptic, bottom-dwelling sharks that occur in warm-temperate to tropical waters of the western Pacific. They inhabit rocky and coral reefs or sandy bottoms, from the intertidal area to depths of at least 110 m. Wobbegongs are best distinguished by their flattened head and body, large, nearly terminal mouth situated in front of the eyes. They got their names ‘wobbegongs’ from the dermal lobes in aborigines which can be seen around the mouth and sides of the head. Out of 7 different species of wobbegong sharks worldwide, 6 are found in Australian waters while NSW has two wobbegong species swimming along its coast: the spotted wobbegong and the ornate or banded wobbegong. Some members of the wobbegong family can attain a size of 3 m or more and should be treated with respect as they can move surprisingly quickly and may attack if provoked. Wobbegongs have long, dagger-like teeth for gripping prey which can inflict painful wounds.
Wobbegongs often escape observation by snorkellers and spear fishers due to their excellent camouflage and behaviour of lying motionless on the bottom during the daylight hours. Although further research is necessary to accurately determine biological characteristics such as longevity, fecundity, age and size at maturity, sex structures and possible migrations wobbegongs are believed to feed nocturnally on a variety of bottom-dwelling bony fishes and invertebrates such as octopuses. These sharks are assumed to be ovoviviparity (young are released alive after hatching internally), and have litter sizes of up to 20 or more. The size at birth is about 20 cm.
While wobbegongs are frequently seen by divers and spearos, little is known about their role in the inshore marine ecosystem. It has been recognised that the biology, life history or population sizes of either spotted or banded wobbegong has never been looked at. The aim of our research is to fill the knowledge gap (in biology and ecology) of both species of wobbegong found in New South Wales.
The link between fishers and scientists played a major part in the gathering of data as the latter got most of the catch records and life history data while being onboard fishing vessels. Data obtained from fishing vessels enabled the comparison between each life history characteristics and trade-offs between them. Such data will then be used to build a demographic model to assess wobbegong resilience to current fishing pressure. With the knowledge gathered during this research project, it will then be possible to recommend suitable management regulations in order to protect these species recently listed as ‘Near Vulnerable’ under the IUCN Red List.
For more information, contact Charlie Huveneers (Charlie.firstname.lastname@example.org)
So, if anyone is diving in Australia look out for them, I know I would love to see one!
On “Photo’s of the Month” we have put up some images of the bronze whalers that were caught in the beach nets.
Best wishes to everyone for 2005. I hope that next year I will be able to share with you all more exciting shark encounters!