A pilot management program to maximise Tasmania's Sea Urchin (Heliocidaris erythrogramma) resource: Hope Island, urchin density manipulations
The main aim of this research program was to determine the potential for reducing the density of urchins to encourage the return of seaweeds and an improvement in urchin roe quality and quantity from remaining urchins. Tasmanian Sea Urchin Developments used two widely-separated sub-tidal experimenta...
| Summary: | The main aim of this research program was to determine the potential for reducing the density of urchins to encourage the return of seaweeds and an improvement in urchin roe quality and quantity from remaining urchins. Tasmanian Sea Urchin Developments used two widely-separated sub-tidal experimental lease areas. One of these areas was at Meredith Point, on the east coast, and the other at Hope Island, on the south coast. Both sites had been subject to some overgrazing by urchins. At Meredith Point, the study area was divided into plots containing urchins at three densities: artificially enhanced, continually harvested and control (undisturbed). At Hope Island, controlled clearings of urchins and limpets from barrens areas were conducted. Recovery of vegetation was monitored as well as urchin roe quality and quantity. The data represented by this record was collected at Hope Island, and includes results from an inital survey collected at the site before the main study commenced. Initial transect survey (further details can be found in Section 3 of preliminary report) Knowledge of the vegetation, substrate and urchin density distributions at the this site was required before the pilot study was to proceed. The site were divided into equidistant sections. These have been marked with numbered boards at the site to enable re-location. Transects were run perpendicular to the shore at each of these marks. The transect line consisted of a lOOm length of 7mm rope marked every meter with lead 'net' weights and canvas 'flags' every 5 meters with the distance inscribed. Using a meter length of PVC pipe, urchins were counted for five meter lengths of the transect line within a distance of a meter from the line. Two divers were used for each transect. The divers did alternate 5 meter sections contiguously. Diver 1 (principal investigator) used a 0.25m2 quadrat, laid at each 10m mark (always starting from '0') to record vegetation cover. The quadrat was divided by seven lines running horizontally and vertically giving 49 intersection points. These and one of the corners gave 50. Substrate was divided into Reef, Boulders (0.5 1.0m diam.), Rocks (0.1-0.5m diam.), Rubble (0.02-0.lm diam.), Sand and Shells. Algae were divided into species as best as possible underwater. Algal cover and substrate were given a reading based on the number of points intersected. Total cover for algae can come to a number greater than 50 due to successive layers of algae. Recordings were made only for the extent of hard substrate as urchins are not generally not found on sand. Main survey (further details can be found in section 4 of main report) Initial clearings at Hope Island resulted in little recovery of macroalgal vegetation. This indicated that intended clearings of urchins at this site to improve roe quality would be unlikely to succeed. It was postulated that limpets may be an additional herbivore controlling vegetation so manipulations were conducted of urchins and limpets to determine the principle herbivore. Macrocystis spores and plants were also introduced to each of the manipulated squares. To determine the cause of the barrens on the island, four treatments were proposed. These were: 1. No urchins (U) 2. No urchins, abalone and limpets (UAL) 3. No abalone or limpets (AL) 4. Control (C) These were done in quadruplicate for 16 squares divided off by approx. 50 cm high netting. The squares measure 10m x 7.5m. There are two blocks of four squares each on each side of the island with randomly allocated treatments. Monitoring was conducted using 10 1 x 1 m quadrats within each of the squares for counts of urchin, abalone and limpets. Quadrats were regularly arranged in the square: one at each of the corners, one at each of the mid points of the sides and two in the middle. Ten 0.5 x 0.5 m quadrats (50 point intercept) are used for the algae placed within the 1 x Im quadrat used for urchin numbers. In order to provide a source of spores, as spore availability may affect re-vegetation, Macrocystis plants were introduced to each of the squares. Ten plants were placed in each. Juvenile plants (0.2-1 .0 m high) were obtained from Dodges Ferry, kept moist under hessian sacks and within 24 hours of harvest, replanted at Hope Island in June 1994. Individual plants were attached to bricks using rubber bands over the haptera at the base of the plant. Urchin divers maintain that best roe recovery comes from urchins harvested from beds of Macrocystis. Monitoring was conducted approximately once every four months after the pre - harvest survey. Minimal change was observed in re-vegetation for the first spring-summer period over which the experiment was conducted (1994) so it was run until spring-summer of the second year (1995). Clearing sessions were conducted at the beginning of the experiment (June 1994) and again in the autumn of 1995. |
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