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Growth models, age frequency and size frequency distributions of sea urchin populations (Heliocidaris erythrogramma) in native macroalgal beds and sea urchin barrens
Growth models were constructed for two sea urchin populations over a two-year period using a tag-recapture study in the Mercury Passage, Tasmania. Sea urchins were tagged using tetracycline and calcein and growth models constructed using measurements taken from sea urchin jaws.
Underwater visual census surveys were conducted at 15 sites in eastern Tasmania to quantify the abundance of the sea urchin Heliocidaris erythrogramma and several putative predators.
Sea urchin tagged using tetracycline display visible marks in jaw elements and test plates that can be used to determine growth and subsequently an individuals age. Natural growth lines in the same structures can be used to determine age if natural lines are deposited at a consistent rates independent of age. Sea urchins were tagged using tetracycline and age determined from both fitted growth models and the number of bands deposited. Rates of natural line deposition were also quantified.
The spatial extent of the long spined sea urchin, Centrostephanus rodgersii, was estimated by divers using underwater visual census methods to survey rocky reef habitats at 13 regions along Tasmania's east coast (between Eddystone Point and Recherche Bay). Within each region 3 subsites were surveyed, and within each subsite 4 belt transects were surveyed. Divers recorded depth, percent substrata type, percent C. rodgersii barrens habitat; abundance of urchins (C. rodgersii and H. erythrogramma), rock lobster (J. edwardsii) and abalone (H. rubra). Divers also recorded algal cover (estimated), C. rodgersii barrens and substratum type using set categories (see below).
A pilot management program to maximise Tasmania's Sea Urchin (Heliocidaris erythrogramma) resource: Meredith Point, 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 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 Meredith Point.
Management options to minimise formation of urchin barrens: measuring changes in marked incipient barren patches
A photographic and in situ diver survey of marked incipient Centrostephanus rodgersii barren patches, in eastern Tasmanian, was used to assess any changes in the grazed area and shape at experimental (lobster translocation/ research reserve protection sites; and abalone diver urchin cull sites) versus control sites.
Biogenic marine habitats are increasingly threatened by a multitude of human impacts, and temperate coasts in particular are exposed to progressively more intense and frequent anthropogenic stressors. In this study, the single and multiple effects of the urban stressors of nutrification and sedimentation on kelp bed communities were examined within Australia’s largest urbanised embayment (Port Phillip Bay, Victoria). Within this system, grazing by sea urchins (Heliocidaris erythrogramma) plays an important role in structuring reef communities by overgrazing kelp beds and maintaining an alternative and stable urchin barrens state. It is therefore important to explore the effects of urban stressors on kelp bed dynamics related to urchin abundance, and test the relative strengths of bottom-up and / or physical drivers (e.g. elevated nutrients and sediment) versus top-down (e.g. urchin grazing) forces on kelp bed community structure. The interactions of these drivers were assessed to determine whether their combination has synergistic, antagonistic, or additive effects on kelp beds. It was found that kelp responds positively to nutrient enhancement, but when combined with enhanced abundance of grazing sea urchins, the local positive effect of nutrient enhancement is overwhelmed by the negative effect of increased herbivory. Turf-forming algae behaved very differently, showing no detectable response to nutrification, yet showing a positive response to urchins, apparently mediated by overgrazing of canopy-forming algae that limit turf development. No direct effects of enhanced sediment load (at twice the ambient load) were found on intact kelp beds. Collectively, the results demonstrate that the ‘top-down’ control of urchin grazing locally overwhelms the positive ‘bottom-up’ effect of nutrient enhancement, and that intact kelp beds demonstrate resilience to direct impacts of urban stressors.
Sea urchins have the capacity to destructively overgraze kelp beds and cause a wholesale shift to an alternative and stable ‘urchin barren’ state. However, their destructive grazing behaviour can be highly labile and contingent on behavioural shifts at the individual and local population level. Changes in supply of allochthonous food sources, i.e. availability of drift-kelp, is often suggested as a proximate trigger of change in sea urchin grazing behaviour, yet field tests of this hypothesis are rare. Here we conduct a suite of in situ behavioural surveys and manipulative experiments within kelp beds and on urchin barrens to examine foraging movements and evidence for a behavioural switch to an overgrazing mode by the Australian sea urchin Heliocidaris erythrogramma (Echinometridae). Tracking of urchins using time-lapse photography revealed urchin foraging to broadly conform to a random-walk-model within both kelp beds and on barren grounds, while at the individual level there was a tendency towards local ‘homing’ to proximate crevices. However, consistent with locally observed ‘mobile feeding fronts’ that can develop at the barrens-kelp interface, urchins were experimentally inducible to show directional movement toward newly available kelp. Furthermore, field assays revealed urchin grazing rates to be high on both simulated drift-kelp and attached kelp thalli on barren grounds, however drift-kelp but not attached kelp was consumed at high rates within kelp beds. Time-lapse tracking of urchin foraging before/ after the controlled addition of drift-kelp on barrens revealed a reduction in foraging movement across the reef surface when drift-kelp was captured. Collectively results indicate that the availability of drift-kelp is a pivotal trigger in determining urchin feeding modes, which is demonstrably passive and cryptic in the presence of a ready supply of drift-kelp. Recovery of kelp beds therefore appears possible if a sustained influx of drift-kelp was to inundate urchin barrens, particularly on reefs where local urchin densities and where grazing pressure is close to the threshold enabling kelp bed recovery.
Management options to minimise formation of urchin barrens: assessing dynamics of incipient urchin barrens patches in eastern Tasmania using timed swims
Timed GPS-tracked swims using SCUBA (45 and 30 minutes), were used to assess the changes in frequency and size of Centrostephanus rodgerii incipient barrens in eastern Tasmania. The number of C. rodgerii and Heliocidaris erythrogramma in each incipient barren were also assessed.
Mortality rates of the sea urchin Heliocidaris erythrogramma where estimated under experimental conditions to determine the importance of natural and predation mortality in structuring sea urchin populations. Data was obtained from tethering, tagging and caging experiments at four sites within Mercury Passage and the Derwent Estuary in eastern Tasmania.