Predatory black swans...

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Neil Fitzgerald
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Predatory black swans...

Postby Neil Fitzgerald » Tue Jul 10, 2018 8:17 pm

The local newspaper (Waitomo News) has a front pager about irate Kawhia residents planning "a massive cull" of black swans, because "whoever is supposed to be, isn't looking after it [the swan population] properly". No question there are a lot of black swans and Canada geese on Kawhia Harbour (the geese are also mentioned as a problem), but I hadn't realised they are such a problem for flounder. Yes, "a few years ago, Mr Taylor says he opened a dead swan and found close to 80 small flounder inside", and since "Kawhia and Aotea are the flounder nurseries of the North Island", well, people around these parts are wearing angry faces.

Has anyone ever encountered evidence of swans eating fish? If so, perhaps Mr Williams will need to update the NZ Birdsonline page, as it currently says "Black swans are entirely herbivorous".
Jim_j
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Re: Predatory black swans...

Postby Jim_j » Wed Jul 11, 2018 11:49 am

I guess if there is a problem with the flounder fishery we can automically rule out:-
- Human overfishing
- Anything to do with land use from farming or forestry causing sediment run-off
- Anything to do with human or dairy waste runoff
Must be those swans - case closed!

cheers
Jim
Ian McLean
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Re: Predatory black swans...

Postby Ian McLean » Wed Jul 11, 2018 2:00 pm

Neil, can someone write a a letter to the editor of the Waitomo News ASAP, advising that the assumption that Black Swans eat flounder is completely incorrect. I have heard of similar rubbish talk about fish eating swans in the Tauranga Harbour previously & it the people spreading such nonsense have the intention of "the more you say it, the more people will believe it". I would be keen on writing a letter of rebuttle myself, but I would prefer a Waikato local did this, rather than a letter coming from an Aucklander !
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David Riddell
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Re: Predatory black swans...

Postby David Riddell » Wed Jul 11, 2018 4:27 pm

Perhaps we shouldn't be quite so quick to dismiss this. True, all the reference books say that black swans are "almost entirely herbivorous" or some variant on that - the phrase occurs in the black swan entry in the Handbook of Australian New Zealand and Antarctic Birds, for example. But it then goes on to mention "small flounder" among other food records. The reference for this is Wilson, R. A. (1957): The Royal Spoonbill. Notornis, 7(4), 107-109. The relevant passage reads:
We saw the Spoonbills fly to the nests to feed the young, but they were too high to observe any feeding particulars. Mr Ken Nolan, the caretaker there, however, says he has seen the Spoonbills carrying fish in their bills to the nests. This fish, it would seem, could only have been small flounder. No doubt in the mud where the Spoonbill feeds with a swinging motion of its bill there must be numbers of small flounders, and these could no doubt be caught by the Spoonbill. This theory is confirmed by the fact that on a Bluff estuary the Black Swans that are shot often have young flounders in their crops.

Wilson doesn't provide any evidence for this claim, but if it can be taken at face value then it would seem black swans feeding on small flounder is something that has been observed before.
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Re: Predatory black swans...

Postby Ian McLean » Wed Jul 11, 2018 7:22 pm

Thanks for the information David, it would be great to know for certainty if Black Swans do indeed prey on flounder ? On first glance this is hard to imagine, swans are slow swimmers, whilst they can upend they cannot dive & their bill is for grazing & does not have the hook of a fish eating bird such as a shag or merganser. Secondly, despite the fact that thousands of Black Swans are shot each year, no one seems to have ever recorded flounder in their stomach contents & despite the prevalence of mobile phone cameras no one appears to have photographed it. You would reckon that if someone found "about 80 flounder" in the swans stomach, they would have a least counted exactly how many there were, photographed it & maybe even recorded the date ! Wilson's comments are interesting but are based on second hand information given to him by duck hunters & I regret to advise that there are a few duck hunters out there who seem to struggle to know the different between a Black Shag & a Black Swan ! If anyone knows for sure that Black Swans eat flounder or any other fish, please advise as I am keen for the facts. Cheers, Ian
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Re: Predatory black swans...

Postby Walpole » Thu Jul 12, 2018 2:07 am

My question is how do people then make this big of a mistake and blame the black swans? Could it be malicious for some reason?
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David Riddell
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Re: Predatory black swans...

Postby David Riddell » Thu Jul 12, 2018 9:36 am

I really don't know if we can be sure this is a mistake. It could be that someone has opened up a swan and misidentified some crop contents (leaves?) as young flounder. Or maybe as Ian says they've misidentified a black shag as a black swan. Maybe it's a malicious lie by someone who hates swans for some reason. Or maybe the report's accurate and black swans really do eat small flounder. It sounds unlikely, but it's not that long ago that possums were also thought to be almost entirely herbivorous, and reports of kea killing sheep were dismissed as stories put out by farmers looking to blame something else for stock deaths caused by their own mismanagement.

I do agree that Wilson's comment in his Notornis paper is inadequate in many respects. He doesn't make it clear whether this is something he's observed himself, or if not, how reliable the source of the information was. He calls it a "fact" that black swans on a Bluff estuary "often" have young flounder in their crops, but how well confirmed is that fact, and how many swan crops were actually examined? There's a real lack of clarity around this, and yes, it would be great to see some actual photos. But when there's a reference in the scientific literature to black swans eating small flounder, however vague, we shouldn't rush to judgement.

I see even Fish and Game are sceptical of this, one commentator querying whether the "flounder" might be parasitic flukes.
https://fishandgame.org.nz/news/both-ba ... -aug-2016/
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philbattley
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Re: Predatory black swans...

Postby philbattley » Thu Jul 12, 2018 9:52 am

I heard the same story about swans at Kawhia some years ago, but in absence of anything to disprove the story, I'm not about to state that it is wrong. I have a student studying godwit foraging at the Manawatu Estuary, and guess what one of the food items were they were feeding on last spring? Baby flounder. I can't see that a black swan dabbling along a seagrass bed would not be able to encounter baby flounder.
Cheers, Phil
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Neil Fitzgerald
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Re: Predatory black swans...

Postby Neil Fitzgerald » Thu Jul 12, 2018 12:26 pm

I think some of the claims made by locals are misinformed or meant to antagonise, but I'm not ready to dismiss it all as rubbish. I am genuinely interested in whether this sort of thing has been reported before, and thanks to the feedback above it appears it has. Not good proof, but a faint smell of smoke. Maybe it will be worth pointing out how interesting this observation of flounder in swans is, if they can provide some evidence...
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Re: Predatory black swans...

Postby Jim_j » Thu Jul 12, 2018 12:46 pm

Are there not two things here:-
- Do Black Swans eat baby flounder
- If the answer is Yes does it have any material affect on the flounder population

The first is an interesting question which would be good to know.

The second... I struggle to believe that no matter how many swans are jammed into these harbours that the affect on the flounder population would be that significant to warrant a swan cull for THAT reason.
I imagine there are a multitude of creatures that feed on baby flounder.
As I mentioned I suggest that the people claiming reduced flounder numbers first look at their own activities (overfishing) and then the local environmental conditions (silt run-off from human land use in paricular) - these I suspect are a much more likely cause of reduced flounder numbers.

cheers
Jim

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