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N. Sivasothi,
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Fri 29 May 2009

300 entangled horseshoe crabs rescued at Mandai Besar mangrove

Category : marine

Carcinoscorpius rotundicauda is one of four species of horseshoe crabs in the world. It is found in the inshore waters of the Indo-West Pacific and associated with the mangrove-mudflat ecosystem. In Singapore waters, from eyewitness accounts alone, their numbers have depleted significantly since the 80's. This is not unexpected given the massive quantity of habitat loss and increased pollution over the past three decades along the Johor Straits.

They still are found in the mangrove-mudflats of north-western Singapore, off the Johor Straits. However, nearshore fishing is unregulated, and Mandai mangrove and mudflats are not provided with any special protection by the law. It's fate appears to be undecided as yet. Meanwhile something else threatens its ancient denizen.

Telsons poking out of the net give an indication of the number of trapped animals in this section of a long net

Frequently over the past decades, monofilament gill nets have been left behind over several tidal cycles in several mangrove patches in north-west Singapore. These poorly frequented areas without popular access are commonly only visited only by recreational fishermen and researchers. Fishermen normally harvest fish caught in a gill net after a single tidal cycle as the dead fish would rot very quickly. However, many nets I have encountered are unattended for days and are regarded as "ghost nets". Entangled fish will die once exposed but arthropod mangrove residents, can survive trapped in the net for several tidal cycles. Eventually it is the heat of the sun during the exposed low tide or starvation that kills them.

skitched-20090529-183140.jpgIn several instances, I have found entire lengths of "ghost nets" with the still-trapped carcasses of animals in an advanced state of decay. In other instances, I have been fortunate enough to detect them early enough to release the living indiviudals. A few of such instances are featured in Habitatnews over the years, e.g. in March 2005 and July 2008. Members of the Nature Society Singapore Horseshoe crab research and rescue team scour Kranji mangroves on a quarterly basis to release trapped individuals as well [link].

Given the reduced population size of these Xiphosurans ("sword tail"), the the impact of indiscriminate capture by gill nets has been heightened and it tragic to see the senseless slaughter of these animals, mirroring the larger-scale problem in the world's oceans.

Two nights ago (27 May 2009) I chanced upon a gill net and the sound of horseshoe crabs struggling to be free at about 11pm. My labmates from Ecolab and myself had been in the area for about four hours and were leaving as the tide rose. Regretfully, we left behind the trapped horseshoe crabs.

I reassured myself that the incoming tide would keep them relatively safe, even if they could not bury themselves.

The next morning I returned alone to release the animals, thinking it would be a one or two hour job as was usually the case. Instead I discovered I had badly miscalculated the numbers trapped in the darkness of the previous night. And I had come directly without help or breakfast! But it was getting hot so I decided I'd best just work as fast as I could.

Mandai Besar mangrove, 27 May 2009

In the end it took five hours (8.45am - 1.45pm) to remove some 300 mangrove xiphosurans from a 100 metre gill net. They were in relatively good condition, being able to struggle reasonably vigorously. There was no mortality except for predator attack (possibly heron) on a two mature gravid females. Otherwise the other females were not full of eggs.

I cut the net into several sections and dragged them into different lengths of a stream through some thigh-deep mud. I turned the thick bundles over every couple of hours (or when I thought the carapace looked alittle too dry) to ensure that all the individuals were kept wet. Thankfully there were intermitted clouds that reduced that stress on the animals. I cut out an individual at a time, removing the entangled net with the help of a pair of scissors.

I tried to keep the cuts to a minimum to reduce littering the mud with short lines. By first removing the main tense filaments restricting the animal, the scissors was then used like a pair of forceps to pull loose any line entangled amongst its segmented legs. This often required cuts at either ends to remove knots before pulling a line free with the scissor tip - it helped that I was using a narrow-point scissors with a slightly dulled blade tip.

I had to be careful not to cut and injure a grasping leg or a telson - the latter looks like a twig so I kept saying to myself, "there are no twigs"! So the release was a very deliberate exercise and it was important to be vigilant while working quickly. Sand grains and the filaments were wearing into my skin and I was thankful the water was brackish and not salty.


As each animal was cleared of all filament, it was released away from the net to prevent re-entanglement - horseshoe crabs can move away quite fast in precisely the wrong direction sometimes! I bundled the net fragments away from the stream for collection later. Thunder crabs and forceps crabs were set aside as they require a much finer pair of scissors and would take too much time. Those I would bring back to the Systematics & Ecology lab at NUS for release and return later.

It was hot and I ran out of water but that spurred me to work faster before these creatures dried up. By creating sections, I turned a large job into a series of smaller jobs to encourage myself with intermediate success. By 10.30am (1 hour 45 minutes) I estimated that I had released over a hundred horseshoe crabs. By then I was sitting in the mud and pretty much covered with it as bending over and even squatting was too tiring.

As the tide rose, I went back to turn over individuals that had returned to a belly up position. I was encouraged when I saw some crawl off but did not examine them. As they were submerged, I cleared the cut nets and stuffed them into large plastic bags I found amongst the trash on the forest floor. Rain threatened but did not come, unfortunately. Eventually, the last individual was released about five hours after I had first started.

I dreamt of them later and woke up in the middle of the night hoping they were all okay.

Until the next ghost net appears.

Photos on Flickr

Posted at 10:50AM UTC by N. Sivasothi | permalink | email | Raffles Museum news