Horseshoe crabs: Results from the

Population and Distribution Survey

J Vanitha

Nature Society (Singapore)

An accidental discovery by some people who were

out bird watching

Horseshoe crab in Singapore

C. rotundicauda

Horseshoe crab in Singapore

T. gigas

NSS interest in HSC

exploratory visit to the north western

mangroves saw a constant supply of mangrove

species, some entrapped in fishing nets

Horseshoe crab Rescue and Research

Program (HSCRR)

Objective 1: scientific data

● to establish baseline data on the prevailing

population of Mangrove horseshoe crab

Carcinoscorpius rotundicauda

● to serve as a long term monitoring program

that shows trends across months and years

Objective 2: Rescue trapped hsc

● to release horseshoe crabs trapped in

abandoned fishing nets

Objective 3: public awareness

● developed into an avenue for outreach with

rich educational value

Ｍonthly surveys

● 300 - 800 mangrove horseshoe crabs were

collected and measured each time

● number of horseshoe recorded possibly

affected by

→ number of volunteers

→ number of experienced volunteers

Start of the Island-wide Population

and Distribution survey

● What is the density of horseshoe crabs in SG?

● Where else are horseshoe crabs found?

● Are they found in large numbers?

● Are they found along the same coastlines?

● Can they coexist?

Objectives of the HSC PD Survey

● To find out about presence of horseshoe crab from

fishermen

● To raise awareness about the existence of such

species

● To educate public about the harm of abandoned

fishing nets

FISHERMEN SURVEY

Objectives of the HSC PD Survey

● To find out where else horseshoe crabs exist

● Are they found in as large numbers elsewhere?

● Are there coastal and mangrove species in the

same place?

DISTRIBUTION SURVEY

Objectives of the HSC PD Survey

● To establish estimated population size of mangrove

horseshoe crabs in one site on mainland Singapore

DENSITY SURVEY

HSC PD Survey

The first survey was done in 2009(Lesley Cartwright-Taylor, Yap Von Bing, Hsu Chia Chi, Lou Sieu Tee (2011) Distribution and abundance of horseshoe

crabs Tachypleus gigas and Carcinoscorpius rotundicauda around the main island of Singapore. Aquatic Biology 13:

127-136)

The second survey was done in 2012

The third survey was done in 2015

2015 Survey

date: was 1 day after new moon

time: ebbing spring tide

sites: 9 sites were surveyed on the same day

(Fishermen surveys, distribution surveys and

density surveys)

Density survey

Distribution survey

2015 survey sites

Fishermen survey 2015

Have you seen horseshoe crabs?

Fishermen survey 2015

When did you last see them?

→ < 3 mths, < 6 mths, < 9 mths, more than a yr

When do you see them most often?

→ day / night / high tide / low tide

Which months of the year?

→ Jan, Feb, Mar, Apr, May, Jun, July

Fishermen survey 2015

Can it be eaten?

→ 15 / 27

Would you eat?

→ 11 / 15 said “no”

horseshoe crab

police?

Concerns

Am I

fishing at a

no-fishing

zone?

Are there 2

species?

Distribution Survey (numbers)

T. gigas C. rotundicauda

# of sites found 2* 4**

AdultsFemale 11

Male 20

Amplexus 7

Juveniles 1 21

Dead/moultsFemale 1 1

Male 3

* Pasir ris & Changi

** Changi, Kranji, Pandan, Seletar

● living and dead

● mangrove and

coastal

Distribution Survey (size)

Distribution

● marked difference in numbers found○ sandy habitats did not have mangrove species

○ muddy substrates did not have many coastal species

● Very few coastal species found, hard to

conclude if they coexist

● Salinity difference could be a factor (Mishra 2009)

Density survey methodology

● Visual searches was most plausible

● Method to be replicable for comparison

Density Survey (Mar 2015)

1.5m by 1.5m quadrats

random coordinates were generated

40m

50m

35 quadrats in total

Limitations of March survey

● Weather uncooperative

● 2 weeks prior to the survey

date: algal bloom

40m

63 quadrats in total

Density Survey (May 2015)Density Survey (May 2015)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

2007 2008 Mar-09 Mar-15 May-15

mean

den

sit

y/m

2

Mean densities across years

25 quadrats

(2m X 2m)

35 quadrats

(1.5m X 1.5m)

63 quadrats

(1.5m X 1.5m)

2011

2012

2012

2014

2015

2014

A look back at monthly surveys

A look back at monthly surveys

Discussion

● possible that quadrat survey might not be

effective for horseshoe crabs in Mandai

● larger quadrat sizes

photo from

ERDG

Discussion

● Are we collecting the same numbers of

horseshore crab with a greater number of

volunteers?

In comparison

Singapore

Mar 2015

May 2015

(2009)

Hong Kong

(Shin et al

2009)

Philippines

(Almendral &

Schoppe

2005)

Orissa

(Chatterji et al

1987)

Number of sites

surveyed

1 17 1 -

C. rotundicauda 0.216/m2

0.109/m2

(0.6/m2)

0.023/m2 - -

T. gigas - - - 0.16/m2

T. tridentatus - 0.31/m2 0.0147/m2 -

Moving forward

● Density survey is very important to identify

threats to trends.

● Appropriate survey method needs to be

developed

● “Sites of Special Scientific Interest”

Moving forward

● Too little is known about T. gigas

● Exploratory trips found few T. gigas (most of

them dead)

● Where are they? Where do they go? Where

do they spawn?

Moving forward

So few mangroves left in Singapore

We need an excuse to preserve what’s left!

Acknowledgements

Dr Hsu Chia Chi,

A/P Yap Von Bing,

Dr Lesley Cartwright-Taylor,

Kerry Pereira

To all site leaders for the survey

To all volunteers to participated

any questions before i

disappear into the mud