Shiprock 12 Hour Dive 2017

Repeating a Milestone in Marine Research after 50 years

In November 1966 URG conducted a 12 hour survey of Shiprock in Port Hacking. The documentation of this site by URG and subsequent submissions by URG contributed to its gazettal as an Aquatic Reserve in 19821. In the words of Clarrie Lawler, past URG President:

“during the early months of 1965 the author and other members of the Underwater Research Group of NSW began diving in this area and were astonished at the profusion of marine fauna that we found in this seemingly ordinary estuarine situation”.

Over 50 years later, and the 12 hour feat has never been repeated. These days, people generally only dive the site on the run of the high tide. We feel it’s high time that we returned for another 12 hour survey.

In 1966, divers commenced at 6 am with a team of three entering the water to lay the guide lines and take initial measurements. Thereafter divers entered the water on the hour and recorded:

At the surface:

  • Water temperature
  • Water sample for subsequent analysis

At the bottom of the reef (13 m):

  • Horizontal visibility, using a 30 cm Secchi disc
  • Direction of current

At the deepest area (Pinnacles, 20 m):

  • Water temperature
  • Water sample for subsequent analysis
  • Direction of current
  • Observe sessile life – sea pens and Carijoa which emerge / contract on a cycle

See below for actual account of the survey.

Plans for 2017

We plan to return in November, to repeat these measures plus conduct biodiversity surveys. Sunday 12th November looks best in terms of tides. It would allow us to add a Reef Life Survey on the low tide (10 am), high tide (4:30 pm) and possibly other times (dawn, high current time). More will follow in future months – but for now, if you’re interested, pencil it in your diaries!

September 1966 Bulletin

SHIPROCK: 12 HOUR SURVEY (Planning Stage)
C. J. Lawler

To help in our studies we propose to spend 12 hours at Shiprock, diving at 2 or 3 hourly intervals to take water samples, temperatures, visibility distances, record pertinent phases of certain marine animals and observe details of current flow.

Factors which were taken into account when selecting a suitable day were:-

a. A Week end day
b. A major spring tide occurring near noon.
c. A reasonably long period of daylight.

These conditions have been met in selecting either Saturday 5th or Sunday 6th of November. A high tide of 5′ occurs at 1 p.m. on Saturday or 2.12 p.m. on Sunday, lows being around 6.30 and 7.30 a.m. and 8 and 9 p.m. The period of sunlight is quite satisfactory, sun rising at 4.50 and setting at 6.30, air temperatures at this time of year should be quite pleasant (mean max. 74°) and water temperatures high enough at 66° to be comfortable but not so high as to risk danger from sharks during early morning and late afternoon dives.

To make this project a success as many divers as possible will be needed to be able to allocate not more than 2 or possibly 3 dives per person. At least 3 divers will be needed at every dive. The author after completing his allocated dives intends to remain on duty during the day to co-ordinate divers, collect tabulations, filled sampling bottles etc. Probably some form of refreshments will be organised on shore for those waiting between dives and during breakfast and lunch periods.

Would members interested in taking part in this survey contact the author as early as possible so that a roster can be drawn up and dives allocated?

November 1966 Bulletin

At last the long awaited 6th November arrived, the sun was shining and we were away on our 12 hour survey dive. There had been some worry during the previous week when gale force southerly winds lashed the coast and light rain began to fall about midweek. Air temperatures were about 8 to 10 degrees below normal and our hopes for the weekend were not great. But the rain eased after Thursday, although the southerly winds and resulting rough seas persisted. The sun shone strongly during Saturday and we were all set to go at first light in Sunday morning.

The first dive was scheduled for 6 a.m., the 3 divers of the first team arriving at Shiprock at 5.30 a.m. The first team would, as well as carry out the first observations and sampling, lay the guide lines for subsequent divers and position the U.R.G. boat above the survey area as both a warning and precautionary measure. These tasks were completed in a little over 35 minutes in very dark water with a maximum visibility of only 10 feet. The recording and sampling operations to be carried out during this and all succeeding dives were as follows:-

  1. Record surface water temperature.
  2. Take sample of surface water, descend guide line to the base of the reef where:
  3. Horizontal visibility measured with a 30 cm (12″) dia. white disc (secchi disc).
  4. Record direction of bottom current. Follow second guide line to the deepest (60′) area near the pinnacle where:
  5. Record bottom water temperature.
  6. Take sample of bottom water.
  7. Record direction of bottom current,
  8. Observe marked sea pens (Cavernularia obesa) at pinnacle.
  9. Observe state of soft coral (Telesto smithi) in whole area – return to surface.

Back on shore the results were entered onto a prepared sheet together with divers names and dive duration times. These survey dives, which averaged about 15 minutes duration, were repeated hourly until 6 p.m. Low water of 2’0″ occurred during the 8 a.m. dive and high water of 5′ 10″ occurred between the 2 p.m. and 3 p.m. dives. The weather deteriorated a little during the early morning becoming cloudy with some slight drizzle about 8 a.m., but the skies cleared and the weather remained warm for the rest of the day reaching a maximum temperature of 66° at 2 p.m. The last dive of the day was held in rapidly dimming conditions, the time for sunset being 6.28 p.m.

Some of the results of the survey are shown in the graphs on the following pages. It was disappointing that water visibility was far below our average (taken over the past several months at high tide) of 24 feet. The average during the survey was only 14 1/2 feet. This very poor visibility also ruined our chances of obtaining accurate observations of Telesto smithi.

Water temperatures were normal for the time of year, varying between a minimum of 63.0°F on the bottom at 6 a.m. just before low tide to a maximum of 66.0°F on the surface just after high tide. No sharp thermocline was noticed during any dive. Greatest variation of surface and bottom temperatures was found to occur about low tide and the least variation around high tide. There appeared to be a slight overall warming of the water during the course of the day and the beginning of a cooling towards evening, but this could be associated with the tidal phase.

Figures on the salinity which are being determined by officers of the C.S.I.R.O. Fisheries Laboratories at Cronulla will not be available for a week or so, but figures for previous samples which averaged 35.25 surface and 35.32 bottom indicate a salinity of slightly less than that of oceanic water off Sydney which is about 35.5 parts per thousand.
The current at station 1 in 45 feet of water was found to flow in a south-easterly direction during most of the day, in fact it flowed continuously in this direction from 6 a.m. to 1 p.m. reaching its greatest velocity between 10 a.m. and 1 p.m., just before high tide.

shiprock1

During the high tide slack interval the velocity died and the direction swung to S.E., then east at 3 p.m. coming back to south at 4 p.m. At 5 p.m. it was seen to reverse direction to north for a short period before settling back to S.W. again at 6 p.m. At this time the current on the surface was moving steadily in the opposite direction, i.e. north-east.
At station 2 the current was most variable and at no time as strong as at station 1. at 6 p.m. there was very little current set, at 7 a.m. it was flowing south, at 8 a.m. N.W. Between 9 a.m. and 1 p.m. it was flowing either due west or W.S.W. At 2p.m., almost high tide, there was no discernable movement but at 3 p.m. there was a set to east swinging down through S.E. and south and finishing at S.W. at 6 p.m. The current here is probably affected to some degree by the large mass of the nearby rock pinnacle. This pinnacle is roughly triangular in plan, about 20′ a side and 15′ high.

Three sea pens (C. obesa) were tagged during the 6 a.m. dive and two more at the 8 a.m. dive. These 5 sea pens were observed during most of the day although 1 disappeared during the afternoon. Many additional pens were seen during the 10 a.m. to 3 p.m. period. at 4 p.m. only 1 marked sea pen was left visible above the sand. From this behaviour it seems that C. obesa sea pens respond to increases in light intensity and possibly spend the hours of darkness retracted beneath the sand.

Incomplete and irregular reports on the incidence of Telesto polyps being ‘out’ make it very difficult to draw any conclusions. There did however seem to be very little of it in the expanded stage during the first dives. At about high tide the author was able to observe fairly extensive patches of Telesto expanded and participants of the very late dives reported a fairly high incidence of expanded polyps.

In view of the success of this survey it might be well to repeat it in autumn, perhaps picking a ‘high-low-high’ tide cycle. Slight changes in procedure may be necessary. We may need a third station further south into Port Hacking for additional current data. Some method of measuring current flow rates would be valuable and surface flow direction should be included. Sea pen tags should be more numerous and more easily seen, possibly by using some floating marker.

shiprock2

A small area of Telesto should be marked out and all observations made on the one area. A guide line from the last station should lead back to the shore rather than have divers retrace their  route using precious air.

Lastly I would like to thank all divers and members who took part in this survey and who performed their jobs in a very workmanlike manner in spite of swift currents and filthy water.

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