Reef Monitoring Survey 8-30 October 2012


Dates: 8th – 30th October 2012
Vessel: MV Floreat
Survey leader: Kerryn Johns

Summary

Twenty-two reefs in the Cooktown-Lizard Island sector were surveyed for live coral cover and crown-of-thorns starfish (COTS) using manta tow. Intensive scuba surveys of benthic organisms, reef fishes and agents of coral mortality (scuba searches) were also completed at permanent survey sites on eight of these reefs. These surveys found high densities of COTS on the great majority of inshore and mid-shelf reefs, though they have not yet reduced the coral cover on most reefs. Preliminary results of the manta tow surveys and SCUBA searches are presented in this report.

Image 1: There has been dramatic increase in crown-of-thorns starfish activity on reefs in this sector. Populations of these corallivorous starfish are already above levels seen during the last series of outbreaks in the mid-nineties.

COTS populations were recorded at outbreak levels on 16 of the 22 reefs that were surveyed using manta tow. COTS were observed on all inshore and midshelf reefs that were surveyed, and all but one had populations at outbreak levels. Five reefs were classified as Active Outbreaks (AO) while 11 were classified as Incipient Outbreaks (IO). No COTS were observed on any of the five outer-shelf reefs that were surveyed using manta tow. Dead coral cover was higher than normal on several reefs that were affected by outbreak levels of COTS. There were also large numbers of long-dead coral skeletons that were overgrown by well-established turf algae communities, particularly at reefs with active outbreaks, indicating that COTS had been active for a long period of time.

Image 2: Loose stands of macro-algae, commonly composed of the brown alga Hydroclathrus clathratus and the red algae Hypnea sp. were common on back reef areas of inshore reefs, often entangling hard corals or collecting in shallow depressions on the sea bed.

COTS were also recorded, often in high numbers, during scuba search surveys on six of the eight intensive survey reefs including one outer-shelf reef. Records of scars from COTS feeding activity at reefs with localized outbreaks (>7 COTS across three survey sites) were very high compared with previous surveys. COTS recorded during scuba searches spanned all class sizes, with starfish 15-24cm diameter being the most common. This suggests that current populations on reefs in this sector are the result of multiple recruitment events. Further, in combination with estimates of the size of starfish from manta tow surveys; this also suggests that a large number of recruits from the past two to three years are just beginning to mature.

Hard coral cover was moderate (10-30%) at the majority of survey reefs. High coral cover (30-50%) was recorded at 4 of 22 reefs while low coral cover (5-10%) was recorded from only one reef. Coral cover was the same as in the previous survey for the majority (15) of locations. Coral cover had declined by more than 5% since the last survey at five reefs, however some of these had not been surveyed for more than two years, making it difficult to compare patterns of growth and decline. Coral cover increased since the last survey at only two locations, both of which were previously classified as recovering from COTS outbreaks.

 

Image 3: Some of the exposed outer-shelf front reef areas were denuded with virtually no coral cover. This photo was taken on the front of Carter Reef that is a preservation zone.

Signs of white syndrome disease were rare or absent during scuba searches. This was a decrease from low levels recorded at some locations in 2010. Numbers of Drupella spp. (coral feeding snails) were variable, but generally low relative to historical records.

A summary of the results is presented in Tables 1 and 2. The monitoring program design and sampling methods and an explanation of the COTS outbreak terminology can be found on the AIMS reef monitoring website: http://www.aims.gov.au/docs/research/monitoring/reef/reef-monitoring.html

Reef-wide COTS surveys

There has been a rapid buildup of COTS activity on reefs in this sector (Figure 1). Active outbreaks (>1.0 COTS per two minute tow, meaning that COTS feeding activity is almost certain to reduce coral cover) were recorded at Two Isles Reef, Forrester Reef, Startle East Reef, Irene Reef and South Direction Island Reef. These included both inshore and mid-shelf reefs. The highest density of COTS was observed at Two Isles Reef (mean of 2.7 COTS per tow). Eleven reefs in 2012 were classified as ‘Incipient Outbreaks' (between 0.22 – 1.0 COTS per tow). Incipient outbreak densities mean that COTS activity is likely to reduce coral cover substantially in the future. All outer-shelf reefs and one mid-shelf reef (Reef 15-047) were classified as ‘No Outbreak'. Startle-East was the only reef of those surveyed in 2012 that had a recorded COTS outbreak (Incipient) at the previous survey. The status of the majority of the reefs at the previous survey was ‘No Outbreak', though some reefs (Eyrie, Irene, Ribbon #6 and Three Isles) had not been surveyed within the previous five years. Three reefs (Helsdon, Lizard Island and MacGillivray) were still classified as ‘Recovering' from a previous outbreak when last surveyed in 2010.

Image 4: Extensive stands of bottlebrush Acropora spp. hard corals on inner and mid-shelf reefs provide perfect nursery grounds for juvenile crown-of-thorns starfish where they can feed and hide while they are still small and vulnerable to predators.

Low but gradually increasing numbers of COTS have been recorded on reefs in the Cooktown-Lizard Is sector since 2006.  Observations at the start of the previous wave of outbreaks (1994 on) suggested that numbers build up on reefs in this region over a number of years until populations reach densities that produce very large numbers of larvae that colonize and reach outbreak densities on reefs immediately to the south.  This now appears to be occurring with numbers of COTS observed already greater than those recorded during the nineties. Furthermore, the large proportion of relatively small starfish recorded suggests that the populations on these reefs will continue to increase in the next few years, which will result in a substantial loss of coral from present levels, and the likely initiation of a wave of outbreaks that will pass southwards through the central GBR over the next decade.

Figure 1. Average crown-of-thorns starfish densities (starfish recorded per 2 minute manta tow) on survey reefs in the Cooktown – Lizard Island sector for Inner (I), Mid (M) and Outer (O) shelf reefs. The threshold for Incipient Outbreak status is a mean 0.22 starfish per tow on a reef. * No reefs were surveyed in 2012.

Image 5: Despite the feeding activity of crown-of-thorns starfish, coral cover was vibrant in many places, particularly on inner and mid-shelf reefs.

Reef-wide coral cover

Moderate coral cover (10-30%) was recorded at 17 of the 22 reefs that were surveyed. Coral cover was high (30-50%) at Linnet Reef, Two Isles, Three Isles and MacGillivray Reef, though all these reefs had outbreaking COTS populations (Table 1). Carter Reef was the only reef with low hard coral cover of 5-10%, a decline from 20-30% cover in 2011. The extended period between surveys make it difficult to identify a cause for this decline in coral cover, though there have been a number of storms in intervening years. In addition to Carter Reef, hard coral cover declined by more than 5% since the previous survey (where survey interval was less than 5 years) at Reef 15-034, Forrester Reef, Martin Reef and Two Isles. COTS feeding activity is the likely cause of declines in each case, except for Reef 15-034, an outer-shelf reef where no COTS were observed during manta tow. Conversely, hard coral increased by more than 5% at Helsdon Reef and MacGillivray Reef, both of which were recovering from previous COTS outbreaks. For the remaining 15 reefs, hard coral cover was unchanged from the previous survey, though survey intervals vary from 2 - 10 years and it is not known how coral cover may have varied over this time. The population boom of COTS in this region is relatively recent and there is likely to be a lag before coral cover declines markedly; this should be evident in the next round of surveys in two years time.

Table 1. Summary of manta tow survey results for the Cooktown-Lizard Island sector.

 

Reef

Shelf Position

Tows

COTS

COTS per tow

Median % Live Coral

Cover

Median % Dead Coral

Cover

Median % Soft Coral

Cover

Reef Status

MARTIN

Inner

67

34

0.51

20 to 30

0 to 5

0 to 5

IO

LINNET

Inner

35

34

0.97

30 to 40

0 to 5

0 to 5

IO

TWO ISLES

Inner

25

69

2.76

30 to 40

0 to 5

0 to 5

AO

THREE ISLES

Inner

26

12

0.46

40 to 50

0 to 5

0 to 5

IO

BOULDER

Inner

73

50

0.68

20 to 30

0 to 5

0 to 5

IO

MACGILLIVRAY

Mid

19

5

0.26

30 to 40

0 to 5

0 to 5

IO

LIZARD IS

Mid

86

64

0.74

10 to 20

0 to 5

0 to 5

IO

EYRIE

Mid

67

57

0.85

20 to 30

0 to 5

0 to 5

IO

HELSDON

Mid

48

46

0.96

20 to 30

0 to 5

0 to 5

IO

NORTH DIRECTION IS

Mid

21

15

0.71

20 to 30

0 to 5

0 to 5

IO

SOUTH DIRECTION IS (A)

Mid

45

55

1.22

10 to 20

0 to 5

0 to 5

AO

FORRESTER

Mid

73

152

2.08

10 to 20

0 to 5

0 to 5

AO

MARX

Mid

32

15

0.47

20 to 30

0 to 5

0 to 5

IO

STARTLE (EAST)

Mid

40

49

1.23

10 to 20

0 to 5

0 to 5

AO

15-047

Mid

48

5

0.1

10 to 20

0 to 5

0 to 5

NO

15-077

Mid

41

15

0.37

10 to 20

0 to 5

0 to 5

IO

IRENE

Mid

55

62

1.13

10 to 20

0 to 5

0 to 5

AO

CARTER

Outer

81

0

0

5 to 10

0 to 0

0 to 5

NO

YONGE

Outer

81

0

0

10 to 20

0 to 0

0 to 5

NO

NO NAME

Outer

54

0

0

10 to 20

0 to 5

0 to 5

NO

RIBBON NO.6

Outer

61

0

0

10 to 20

0 to 0

0 to 5

NO

15-034

Outer

32

0

0

10 to 20

0 to 5

0 to 5

NO

Overall GBR average for 2011

All

 

 

0.02

10 to 20

0 to 5

0 to 5

-

Long-term average value

Inner

 

 

0.14

20 to 30

0 to 5

0 to 5

-

Long-term average value

Mid

 

 

0.14

10 to 20

0 to 5

0 to 5

-

Long-term average value

Outer

 

 

0

10 to 20

0 to 5

0 to 5

-

Average last survey – 2011

Inner

 

 

0.02

30 to 40

0 to 5

0 to 5

-

Average last survey – 2011

Mid

 

 

0.1

10 to 20

0 to 5

0 to 5

-

Average last survey – 2011

Outer

 

 

0

10 to 20

0 to 5

0 to 5

-

 

Image 6: Green zones are an effective tool for preserving numbers of top predators. Here a blacktip reef shark (Carcharhinus melanopterus) forages for food in Watson's Bay, Lizard Island.

Intensive SCUBA surveys of agents of coral mortality

Localised active outbreaks of COTS as defined for scuba search surveys (>7 COTS across three survey sites) were recorded at Martin Reef, Linnet Reef, Lizard Island and North Direction Island. The highest COTS count was 48 starfish at Martin Reef (Table 2). COTS numbers at each location are likely to be an under-estimate, as COTS feeding scars were often observed without finding an associated starfish. All class sizes were present and there was no doubt that numbers of small and cryptic individuals remained hidden, particularly amongst stands of bottlebrush Acropora spp. COTS were also observed at MacGillivray Reef but at less than outbreak levels, and a single COTS was recorded on the survey sites of Carter Reef, an outer-shelf reef and Preservation Zone. COTS were present in much lower numbers at each of these locations in the previous survey in 2011 (with the exception of Carter Reef) and much lower numbers of feeding scars were also recorded.

 

Incidence of White Syndrome coral disease seen in scuba searches (Table 2) was generally low. In most cases there was a decrease from the low levels observed in 2011. Skeletal Eroding Band disease (SEB) was observed at or below the long term average at most locations. Temporal patterns in incidence of SEB were inconsistent among locations. It is uncertain what densities of infected colonies lead to significant coral mortality. Low numbers of corallivorous snails, Drupella spp., were found on every survey reef. 527 snails per hectare were recorded for North Direction Island, but this density is still reasonably low compared with some historical records. There was no clear spatial pattern in Drupella spp. numbers among reefs in this year's survey, nor was there a trend compared with previous surveys.

 

Table 2. Summary of SCUBA search survey results for the Cooktown-Lizard Island sector.

Reef

Shelf Position

 COTS (<5cm)

 COTS (>5cm)

COTS (>15cm)

COTS (>25cm)

WS

BBD

BrB

SEB

Drupella

MARTIN

I

1

9

23

15

3

0

3

22

1

LINNET

I

3

12

12

6

0

0

1

13

18

MACGILLIVRAY

M

0

0

1

1

0

0

0

0

24

LIZARD IS

M

0

1

10

6

14

0

1

1

9

NORTH DIRECTION IS

M

0

1

2

6

8

1

12

13

79

CARTER

O

0

0

1

0

8

0

2

1

18

YONGE

O

0

0

0

0

5

0

2

2

4

NO NAME

O

0

0

0

0

10

0

4

1

11

Overall GBR Average for – 2011

All

0

0.021

0.11

0.34

7

0.064

3.6

2.7

11

Long term average

I

0.2

1.4

2.6

3

8.3

0.23

1.9

10

24

Long term average

M

0.063

0.25

2.2

1.8

4.6

0.092

1.4

6.6

47

Long term average

O

0

0.043

0.022

0

26

0.21

2

1.9

15

Average for last survey – 2011

I

0

0.5

2

1.5

31

0

3.5

5.5

14

Average for last survey – 2011

M

0

0

0.33

1.7

8

0

1.3

16

18

Average for last survey - 2011

O

0

0

0

0

23

0

4.3

2

29

WS = White Syndrome, BBD = Black Band Disease, BrB = Brown Band Disease, SEB = Skeletal Eroding Band Disease.

 

 

References

English, S., Wilkinson, C. and Baker, V. (1997) Survey Manual for Tropical Marine Resources (2ndEdition). Australian Institute of Marine Science. Townsville.

This project is partially supported by the Marine and Tropical Sciences Research Facility

For further information contact

Dr. Hugh Sweatman , AIMS
Telephone: +61 7 4753 4470
Fax: +61 7 4753 4288
Email: h.sweatman@aims.gov.au