Monday, August 4, 2008

Recovery of Chek Jawa thus far

5am, one small team of us left Changi jetty, volunteering selflessly to do a health check at Chek Jawa. Though all of my regular gang were busy to make it, I am very grateful that Ginny, Pamela, Tze Hwee and Han Sheng could come and help put and also experience Chek Jawa for their first time.

It has been one year since I've arrived at Chek Jawa so early. But we get to enjoy the spectacular sunrise over the horizon of Pulau Tekong.

This stretch of Chek Jawa to the west of the coral rubble, near the House No. 1 jetty has very good growths and large numbers of adult carpet anemones (Stichodactyla haddoni).

It is postulated that this part of Chek Jawa seemed to have minimal freshwater impacts due to headlands that diverted the river effect from Johor River. Therefore, numerous carpet anemones are still thriving here and that this location also acts as a seeding site for recovery to the other parts of Chek Jawa.

The way they are found on the seagrass area of this part of Chek Jawa can come in random, clusters,

and interestingly, linear fashion, where five of them forms a straight line. I guess this is just purely coincidental.

But it is also possible that the carpet anemones here could be migrated from the other parts of Chek Jawa that were badly affected by the Jan 2007 mass death.

This is because they can uproot themselves in stressful situations and drift with the currents to a less stressful area.

However, I couldn't find the colourful variety of sponges at the coral rubble that I've seen before during December 07. Only many of these long branched purple sponges, as also observed by Ria earlier on, can be found.

According to our regular trips observation from northern shores, these long branched purple sponges are very common because they are quite tough and can be found even in inconducive conditions. What happen to the other "more sensitive" sponges? Was the tide not low enough to reveal the other sponges?

Ginny and I were doing checks at the coral rubble, one of which is to find the adult knobbly sea stars (Protoreaster nodosus). I think the tide wasn't super low enough (though 0.0m) to find the adults. But Ginny found this juvenile knobbly sea star that was enough to make me excited.

This means new blood of the knobbly stars for Chek Jawa! Can't wait for them to grow up.

Other sea stars spotted included a couple of the cake sea stars (Anthenea aspera).

This is the other cake sea star found at the coral rubble area.

As usual, there are many biscuit sea stars (Goniodiscaster scaber) that we commonly find at the northern shores nowadays.

I've also found a patch of zebra coral (Oulastrea sp.) growing in the interior of an abandon tyre.

Also spotted are zoanthids and flowery soft corals that I couldn't take a photo of due to the murky waters from the returning tides.

Next, we checked the sand bar and the seagrass lagoon area.

Several groundscape photographs were taken for monitoring over time and also to compare with the state of Chek Jawa before the mass death.

Tze Hwee, alone, was posted to search for the Button Shells (Umbonium vestiarum) at a far far northern sandbar. She couldn't find them though, even at the usual spots where I once recorded GPS points.

While everyone was hard at work, Pamela and Han Sheng were searching for the mussel beds within the seagrass lagoon.

To my surprise, we could not find any mussel bed today! This was in line with the missing mussel beds from Changi and also later on I realized those at the Ubin jetty were gone too. What was left behind were the mounds of mud that I believed were once trapped by the byssus threads of the once-existent mussels. Could their absence now be due to a seasonal reason or that the predators of these Asian mussels (Musculista senhousia) have completely wiped them out?

A study was done by Kushner and Hovel (2006) to the response of native predators (gastropod Pteropurpura festiva) to an invasive marine bivalve, the Asian mussel Musculista senhousia (Mollusca: Bivalvia: Mytilidae). The results of the study suggested that in overall, "intertidal or subtidal areas with high seagrass cover and high predator abundance may be resistant to invasion by Asian mussels, but the likelihood that a population of M. senhousia will become established will depend on the relative densities of predator and prey, M. senhousia settlement and growth rates, and the spatial and temporal scale being considered."

The study by Kushner and Hovel (2006) also stated that "native predators may be able to confer invasion resistance to local communities, but are unlikely to be able to control large, established M. senhousia populations"

Similar to the native gastropod Pteropurpura festiva from Kushner and Hovel (2006), many of the drills (Thais sp.) were once found on top of the mussel beds.

And the eggs of the drills were also present on the surface of the mussel beds.

It is possible that the predation of the drills on the mussels could have slowly but surely led to the elimination of the mussels from the mussel bed. Though Kushner and Hovel (2006) stated that it is difficult to control large populations, the seemingly shrinking of the mussel beds observed in May could be a factor that aided the predation of the Asian mussels which led to control and thus elimination.

Nevertheless, the real reason behind such an elimination is difficult to account for. A better knowledge of the relative densities of predator and prey, M. senhousia settlement and growth rates, and their spatial and temporal scale will be needed to give a scientific explanation.

There are many sand dollars towards the northern sandbar that are teeming on the sand.

Some of the tubes from the tubeworms were also photographed. This one is the bigger one.

And there is another type of tubeworms that are much smaller.

There were about less than ten carpet anemones starting to be found growing at the sandbar. Before the mass death, the sandbar was filled with these carpet anemones like landmines.

Hopefully with time, more of these carpet anemones will return to the sand bars.

The seagrass lagoon has yet another surprise for us, another juvenile knobbly sea star! Ria has also found two of them in July as well. This is heartening.

Underside of the juvenile knobbly sea star.

And for the finale....

Common sea stars (Archaster typicus) are still around!

However, they seem to be only found at a specific spot that I always return to search for them through GPS. I hope there are more elsewhere that we have no chance to stumble across because Chek Jawa is too large to properly do a search.
All too soon, the tide still had to return and we left the shore promptly.

My friends couldn't resist climbing the Jejawi viewing tower and it was a good opportunity to take a group photo of them. Thank you all for your help today!

A more light hearted post with other sightings during the trip:

Kushner, R.B. and Hovel, K.A. (2006) Effects of native predators and eelgrass habitat structure on the introduced Asian mussel Musculista senhousia (Benson in Cantor) in southern California. Journal of Experimental Marine Biology and Ecology 332(2): 166-177.

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