Teachers' notes - Professor Helene Marsh, environmental scientist

By-catch threats to marine wildlife

And around the Queensland coast?

That’s a very different situation. On the Cape York coast there is no evidence of serious depletion in the short term – that is, since the mid-’80s when we started developing aerial survey techniques. But on the urban coast of Queensland, loosely defined as Port Douglas south, there is evidence from a number of sources of quite serious depletion. On the anecdotal evidence, in the historical records the numbers of dugongs are so large it’s almost unbelievable. Secondly, our aerial survey evidence indicated a decline between the mid-’80s and the mid-’90s, although recently we’ve done work which suggests that animals have now moved back into the northern part of that area, probably from the more remote regions further north.

We have just analysed a 40-year data set of dugong by-catch in shark nets set for bather protection. (Such a long data set in biology is quite rare, and we got hold of this one only after a lot of debate, the Department of Primary Industries being fairly reluctant to release it.) There are shark nets set at a number of beaches on the Queensland coast with a view to making the beaches safer to bathers, not by providing a wall to stop sharks from coming to the beach but rather as a fishing device to deplete the local population of sharks. Unfortunately, there is a serious by-catch of marine wildlife such as dugongs and dolphins which get tangled in the nets and drown, and our analysis indicates a significant decline in the dugong catch per unit effort – of the order of 8.7 per cent a year, for 40 years.

The program also has a serious by-catch of small sharks. They are certainly not fatal to humans, but I guess shark attacks are such an emotive issue that it would now be very difficult for the government to stop the program. Advocates have said, ‘Well, there has been no fatal attack at a meshed beach since it was introduced.’ Other data suggests that the catch per unit effort of major man-eating sharks such as tiger sharks off Townsville, for example, has not changed over the 40-year period, and that the tiger sharks are coming into the region to breed. Because they are not resident in the region, the shark nets may be having no effect at all. But shark meshing on the Queensland and New South Wales coasts is a response to the sort of hysteria and emotive media response we saw last week when two people were killed by white pointer sharks off South Australia.

Whether the number of dugongs caught in shark nets is a reliable index of the dugong population, by the way, is debatable. There is no evidence that the dugongs could learn to avoid the nets, but the human use of the beaches over that 40-year period may have meant that dugongs are less likely to use those areas. We have no data on that. But we do have a data set which indicates a significant depletion in catches over a very long period, and when I combine that with the anecdotal information and the aerial survey data and triangulate, adding in the belief of the indigenous people who use that coast that there has been a serious decline, I think the evidence suggests quite a significant depletion.

‘Think about losing 1000 square kilometres of habitat’

Is the problem that people want to bathe at the beaches where the seagrass grows?

In some areas. The causes of the dugong decline are very complicated and difficult to disaggregate, and probably vary in different regions. In the Cairns region, for example, we have the account by Colin Bertram of his visit to that area. He was a biologist who worked on dugongs in the 1960s, and he says that when he went to the Yarrabah community in 1965, those indigenous people claimed to be catching 200 dugongs a year. But now when we’ve done aerial surveys in that region we have seen dugongs in such low numbers that we can’t make a reliable population estimate. In some regions we certainly find evidence of dugongs being caught in gill nets set by commercial fishermen; in other areas there is evidence of significant habitat loss.

Unfortunately we don’t have a 40-year data set for habitat, but we do know the effects of some extreme weather events. For example, in 1992 there were two floods and a cyclone in very quick succession in the Maryborough region. The floods themselves were not remarkable, although one was the fifth highest flood this century, but the fact that there were two of them, three weeks apart, was very unusual. And 1000 square kilometres of seagrass in Hervey Bay was destroyed.

By silt formation?

It was deepwater seagrass – that is, seagrass growing near the limits of its light tolerance in greater than 15 metres – and with a prolonged flood plume it just had to die. A seagrass survey had been done only a few months before, and then it was repeated. Well, dead dugongs were found all along the New South Wales coast, and some live ones; about 100 dugongs were recovered in the few months after that event, and more were found that year on the New South Wales coast than in all other years put together. So something very strange was going on.

Think about losing 1000 square kilometres of habitat. It has come back, but the concern is that we have extreme weather events coupled with bad land-use practices. For example, the sediment going into the Great Barrier Reef lagoon per year has increased by four or five times since European settlement. We have dugongs in both intertidal and subtidal areas. The intertidal seagrass is probably okay, because it’s getting light when the tide goes out, but the likelihood is that the depth margin of the subtidal seagrass has shrunk. We have no idea, over those sorts of time scales, what habitat loss there has been. But it’s hard to believe, given the Hervey Bay story and similar events, that it hasn’t been quite serious.