Swans and Eel grass

In recent discussion of the numbers of Black Swans

... seen on the Mallacoota Inlet it was suggested that one factor leading to reduced numbers of Swans, compared to the very large flocks seen in the past, was the loss of eel grass for them to feed on.   This seemed like an interesting suggestion so I have researched the topic using the resources I have available.

Cutting to the chase, the answer is that it is a possibility, but the consequent question is what caused the eel grass to go away?  I have included some results from overseas articles about Brent Geese, which also feed on eel grass in the Northern Hemisphere.

What is eel grass?

According to Wikipedia it is either Zostera muelleri (salt water) or Vallisneria sp (a mainly tropical, fresh water genus).   I think the latter is disqualified as we aren’t in the tropics and the Inlet isn’t fresh water!

Looking at the Atlas of Living Australia (ALA) there are records for Zostera muelleri in the Inlet, but not enough records to make any statements about change in abundance.

I think this is Eel grass:

That collection was on the edge of the Betka River, where Swans were grazing.  I assume these blades were bits that the swans dropped.

What do Black Swans eat?

The short answer is vegetation.  Commentary in the Handbook of Australian, New Zealand and Antarctic Birds (HANZAB) suggest that while invertebrates are found in the gut contents of Black Swans this is most likely by-catch rather than targeted feeding.

The details given in HANZAB are from examination of contents the oesophagus or gizzards of birds from fresh water sites.  It seems that a fair proportion of the food is pondweed (Potamegeton sp. or Azolla sp.) which is not found in the Inlet.  The Cyperaceae (reeds) are also important.

Possibly more relevant is research in New Zealand[1] and Western Australia[2].  Both of these refer to the Black Swans feeding on Zostera. 

The focus of the NZ work is on whether the grazing by Swans is responsible for declines in Zostera abundance – the answer seems to be probably not, at least not on its own.

The WA work also includes 2 other sea grass species Ruppia meagcarpa and Halophila ovalis  The former seems to have also been recorded in the Inlet (and the Betka River where a flock of Swans have been resident for some time).  It appears that in WA, other things being equal, the Swans and the sea grasses keep a balance between grazing pressure and survival of the sea grass.  It is noted that the presence of algal blooms may upset this balance so that the grass cannot recover from the grazing.  I’m not aware of any algal blooms affecting the eel grass in the Inlet.

Unlike some species of ducks Swans do not dive, but feed by upending

This means that they can only feed where the water is shallow enough for them to reach the food resource. This limits them to sites where the water depth is 1m (+ the height of the food).  That being the case it is interesting how far out in Bottom Lake is that shallow.

The red circles are feeding Swans.  My guess is at least 200m off shore.

Comments from other species

The mention of Eel grass jogged my memory back some 60 years when concerns were expressed that warm water from a proposed nuclear power plant may adversely impact the eel grass at Bradwell in the UK, where it was an important Winter food source for an endangered subspecies of Brent Goose.  Reading two articles about Brents suggests some additional reasons for the decline of  eel grass. 

An article from the WWT[3] (Peter Scott’s organisation) from 1969 discusses this in detail.  Interestingly they comment:

 “The latter [Zostera sp.] was the preferred plant when, in the 1930s, it was attacked by a mysterious disease associated with, if not caused by, the mycetazoan Labyrinthula. Over the whole of its Atlantic and Pacific range the beds of Zostera were wiped out. “ 

They go on to describe how the plant has slowly and (to 1969 at least) incompletely recover. Reference is also made to the control of hunting as a factor in survival of the subspecies of Brent Goose that overwinters in Essex, especially in its Arctic breeding grounds.

Another article[4] discusses a world wide view of the relationship between Brents and eel grass.  As well as the fungal infection mentioned above, increased turbidity or eutrophication are suggested as reasons for reduced Zostera meadows.  Since the area around the Inlet is National Park these reasons seem unlikely to apply here.

What has happened to the Swans and Sea Grass in the Inlet?

Following publication of my article in the Mallacoota Mouth summarising this post I have been contacted by a long term resident of the area who can remember - more than 10 years ago - seeing the Goodwin Sands black with Swans, and large flocks flying overhead.  He considered the estimates of 3500 and 4000 quite believable.

In my informant's opinion they used to feed on sea grass which was abundant in the Inlet,  However when the Inlet was artificially opened the position of the outlet was such that it allowed the sand, which naturally washes up the beach to flood into the Inlet and cover the sea grass, killing it.  In his opinion the number of Swans has not recovered form this event.

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[1] Dos Santos, V. (2011). Impact of black swan grazing and anthropogenic contaminants on New Zealand seagrass meadows (Thesis, Doctor of Philosophy (PhD)). University of Waikato, Hamilton,

[2] https://phys.org/news/2014-10-black-swans-seagrass-delicate.html

[3] Brent Geese, mudflats and Man M. A. OGILVIE and G.V.T MATTHEWS (1969)

[4] Barbara Ganter; Seagrass (Zostera spp.) as food for brent geese (Branta bernicla): an overview