Great Crested Grebe - diet #3

[Con Boekel <>]

The other day I had an interesting discussion with Steve Wallace about
prey items of Great Crested Grebe and why there was not a permanent
population of Great Crested Grebes in Canberra while, at (relatively)
nearby Pejar Reservoir (Just west of Goulburn) they are seen reliably.

Steve has subsequently posted an image of a GCG with a Weatherloach prey
item.

While reviewing my GCG images I noticed that one of the fish prey items
had eyes considerably larger in proportion to body size than those of
the Oriental Weatherloach. See image.

Unfortunately, the level of detail is insufficient to work out which
species it might be. It may be a young of Redfin or Goldfish, both of
which are known to inhabit the lake but the image fits neither very
well. Other possibilities are Mosquito Fish or Western Carp Gudgeon -
both of which are common elsewhere in the ACT.  But I don't know whether
either of the latter are present in Yerrabi Pond.

I suggest that there is not a year-round supply of accessible small fish
and/or that, at most times of the year, there are not enough prey items
for GCGs.

I have occasionally seen Mosquitofish leap onto land in order to evade
Redfin. It is quite common to see shoals of Mosquitofish leap out of the
water to evade Redfin. Mosquitofish also use dense vegetation as cover.
(Quite a few COGites recently observed Egrets spp and White-faced Herons
successfully hunting Mosquitofish from dense veg at Kelly's.)
Mosquitofish also shoal in extremely shallow water - far too shallow for
GCGs to hunt. In winter, Mosquitofish tend to become inert and to sink
to the bottom.

One reason for a lack of permanent GCGs may be that Redfin Perch are
more efficient predators of small fish in open water and that Redfin and
Mosquitofish interactions make the latter mostly inaccessible to GCGs.

I am not sure about how all this works for Western Carp Gudgeon. From
personal observation, they tend to rest on or near the bottom.

Which brings us to the Yerrabi Pond GCG.

I have yet to see it dive in open water, or to dive into the very
extensive Yerrabi waterplant beds. I am not sure about the name of the
waterplants (they look a bit strap like) but I believe that they do not
provide sufficient cover for Mosquitofish from Redfin. I would be
interested to know whether anyone has seen it feeding in the strap
plants and whether or not it was successful.

The GCG essentially feeds along grassy edges or in and among Typha and
debris. It will poke along grassy edges. It will clamber in, amongst and
over debris. (I note in passing that many of Canberra's lakes have
hardened edges unsuitable for this form of GCG fishing.)

I believe that the Yerrabi Pond GCG may be the first to specialize in
predating the Oriental Weatherloach.

The attached image (captured in the Pelican sculpture cove, Yerrabi
Pond) shows just how far along its learning curve the youthful Yerrabi
Pond bird has gone.

regards

Con

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