Resurvey of the longspined sea urchin (Centrostephanus rodgersii) and associated barren reef in Tasmania-猫眼课题宝

对塔斯马尼亚长刺海胆（Centrostephanus rodgersii）和相关荒礁的重新调查

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2018

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S. Ling;J. Keane

2016/2017 年，潜水员和水下拖曳视频对塔斯马尼亚东部海带床的 Centrostephanus 丰度及其影响程度进行了重新调查，并相对于 2001/2002 年建立的基线进行了评估。重新调查涉及横跨塔斯马尼亚东部 13 个地点的 156 个潜水横断面，从 Eddystone Point 到 Recherche Bay 相距约 20 公里（图 1）。从这些样带来看，2001/02 年至 2016/17 年期间，4 至 18 m 深度范围内的珊瑚礁上的 Centrostephanus 丰度从每公顷 1,036 个海胆平均密度增加至 1,818 个海胆。 C. rodgersii 的增加并没有均匀地发生在整个海岸，在塔斯马尼亚南部（布鲁尼生物区）的许多地点，Centrostephanus 仍然很少见，密度低于每公顷约 20 只。相反，从 2001 年 2 月到 2016 年 2017 年，从菲欣纳生物区到塔斯曼岛（地点 1 至 9，图 1），C. rodgersii 的平均密度从每公顷 1,495 只增加到 2,623 只。这意味着该地区的 C. rodgersii 密度在 15 年来增加了 75%，相当于每年人口增长 3.8%。将观测到的 C. rodgersii 在 4 至 18 m 深度范围内的可用岩礁栖息地密度相乘，估计 C. rodgersii 数量在 2001/2002 年至 2016/17 年间从约 670 万只增加到 990 万只（在 15 年期间增加了 48%，或平均每年约 200,000 只海胆）。考虑到每个调查期间海胆的平均个体重量，这相当于估计生物量从约 1,850 吨增加到约 3,000 吨，即每年平均增加约 80 吨。扩大整个可用珊瑚礁宽度（4 至 40 m 深度）的海胆密度，估计 C. rodgersii 的数量在 15 年期间从约 1,100 万增加到超过 1,800 万（在 15 年期间增加了 60%，或平均每年约 460,000 个海胆）；相当于生物量从约 3,000 吨增加到约 5,500 吨，即每年平均增加 170 吨。包括巴斯海峡肯特群珊瑚礁上的大量种群在内，到 2017 年，塔斯马尼亚州水域的 C. rodgersii 种群数量估计已超过 2000 万只。潜水员对塔斯马尼亚东部所有地点 4 至 18 m 深度范围内的海胆放牧情况进行的评估显示，2001/02 年至 2016/17 年期间，海胆荒地覆盖率从 1.6% 增加到 6.3%。仅考虑塔斯曼岛以北至埃迪斯通角 (Eddystone Point) 的塔斯马尼亚东部遗址，C. rodgersii 现在很常见，潜水员对 4 至 18 m 深度范围内的贫瘠之地进行的评估显示，贫瘠土地的珊瑚礁覆盖率从 2.3% 增加到 9.0%。使用拖曳式水下视频对整个珊瑚礁宽度（深度为 4-40m）进行采样，观察到塔斯马尼亚东部 1-9 号地点的贫瘠珊瑚礁百分比从 2001/2002 年到 2016/2017 年从 3.4% 增长到 15.2%，相当于在 15 年期间每年增加约 10.5%。 15 年期间，C. rodgersii 密度的增加和相关贫瘠之地的扩张在 18 至 30 m 之间以巨石为主的珊瑚礁上最为明显。在塔斯马尼亚东部海岸线上，C. rodgersii 和贫瘠之地的最大变异发生在不同地点（即约 20 公里的尺度），而在更精细的公里（子站点）或亚公里（横断面）尺度上，海胆丰度和贫瘠之地覆盖率的变化相对较小。值得注意的是，除了荒地覆盖面积增加之外，在调查期间海藻床结构的变化也很明显，一些藻类物种的覆盖面积增加，而另一些藻类物种的覆盖面积则下降。海带 Ecklonia radiata 是 C. rodgersii 大量吃草的一种藻类，其覆盖面积有所增加。也就是说，虽然贫瘠之地的覆盖范围普遍增加，但剩余的海带床似乎变得更厚，正如大型藻类覆盖范围的总体增加所表明的那样。这一看似不符合直觉的结果很可能是对巨藻巨囊藻广泛衰退的持续反应，巨藻巨藻历史上主导着塔斯马尼亚东部的珊瑚礁，在那里它击败了较小的林下海藻，例如辐射藻。然而，根据测量海底覆盖范围的潜水员的评估，塔斯马尼亚东部的巨型海带减少了 42%，从 2001/02 年出现的 10 个地点中的 7 个消失了。再次证实了 2001/2002 年基线调查的结果，在海胆荒地上观察到鲍鱼和岩龙虾的密度较低。鉴于清除足够的海胆以扭转贫瘠土地变得越来越困难，继续积极主动地管理海胆过度放牧至关重要。观察到的海胆吨位每年增加的规模已经达到了控制的规模，例如通过重建捕食者和扩大扑杀和/或收获的规模似乎是合理的。本次调查期间获得的关于贫瘠之地覆盖范围和过度放牧风险最大的位置的空间信息将有助于进一步采取有针对性的干预措施。另请参阅：http://www.imas.utas.edu.au/urchins

The abundance of Centrostephanus and the extent of its impact on kelp beds in eastern Tasmania was re-surveyed by divers and underwater towed-video in 2016/17 and assessed relative to baselines established in 2001/02. The re-survey involved 156 diver transects spanning 13 eastern Tasmanian sites spaced ~20 km from Eddystone Point to Recherche Bay (Fig. 1). From these transects, the abundance of Centrostephanus on reefs within the 4 to 18 m depth range increased from an average density of 1,036 to 1,818 urchins per hectare between 2001/02 and 2016/17. The increase in C. rodgersii has not occurred evenly across the coast and there are many sites in southern Tasmania (Bruny Bioregion) where Centrostephanus remains rare, occurring at densities less than ~20 individuals per hectare. Conversely, across the Freycinet Bioregion to Tasman Island (sites 1 to 9, Fig. 1), C. rodgersii increased from an average density of 1,495 to 2,623 urchins per hectare between 2001/02 and 2016/17. This represents a 75% increase in C. rodgersii density over 15 years in this region, equating to a population increase of 3.8% per annum. Multiplying observed C. rodgersii densities across available rocky reef habitat within the 4 to 18 m depth range leads to an estimated increase in the C. rodgersii population from ~6.7 million to 9.9 million individuals between 2001/02 and 2016/17 (a 48% increase over the 15 year period or an average of ~200,000 urchins per year). Factoring by the average individual weight of urchins in each survey period, this equates to an estimated biomass increase from ~1,850 to ~3,000 tonnes, or an average increase of ~80 tonnes per year. Scaling urchin densities across the full width of available reef (4 to 40m depth), the population of C. rodgersii is estimated to have grown from ~11 million to more than 18 million over the 15-year period (a 60% increase over the 15 year period or an average of ~460,000 urchins per year); equating to a biomass increase from ~3,000 to ~5,500 tonnes, or an average increase of 170 t per year. Inclusive of the sizeable population on Kent Group reefs in Bass Strait, the population of C. rodgersii in Tasmanian State Waters is estimated to have exceeded 20 million individuals by ~2017. Diver assessment of urchin grazing within the 4 to 18 m depth range for all eastern Tasmanian sites, revealed an increase in urchin barrens cover from 1.6% to 6.3% during the 2001/02 to 2016/17 period. Considering only the eastern Tasmanian sites north of Tasman Island to Eddystone Point where C. rodgersii is now common, diver assessment of barrens within the 4 to 18 m depth range revealed an increase in barrens from 2.3% to 9.0% cover of reef. Using towed underwater video to sample the full width of reefs (from 4-40m depth), the percentage of reef as barrens across sites 1-9 in eastern Tasmania was observed to grow from 3.4% to 15.2% from 2001/02 to 2016/17, equating to a ~10.5% increase per annum over the 15-year period. Increase in the density of C. rodgersii and expansion of associated barrens over the 15-year period was greatest on boulder-dominated reef between 18 to 30 m. Across the eastern Tasmanian coastline, greatest variability in C. rodgersii and barrens occurred from site to site (i.e. at scale of ~20 km), with relatively lower variation in urchin abundance and cover of barrens at finer kilometre (sub-site) or sub-kilometre (transect) scales. Notably, beyond increasing barrens cover, changes to the structure of kelp beds were also apparent between survey periods, with some algal species increasing in cover while others declined. The kelp Ecklonia radiata, which, is an algal type heavily grazed by C. rodgersii, showed increase in cover. That is, while barrens cover generally increased, remaining kelp beds appeared to become thicker as indicated by overall increase in macroalgal cover. This seemingly unintuitive result is likely an ongoing response to widespread decline of giant kelp Macrocystis pyrifera, which historically dominated eastern Tasmania reefs where it outcompeted smaller understorey kelps such as E. radiata. However, giant kelp as assessed by divers measuring coverage of the seafloor was observed to decline by 42% across eastern Tasmania, disappearing from 7 of the 10 sites where it was present in 2001/02. Re-confirming findings of the original 2001/02 baseline survey, low density of both abalone and rock lobster were observed on urchin barrens. Continued proactive management of urchin overgrazing is critical given that removing sufficient urchins to reverse barren grounds becomes increasingly difficult. The observed annual increase in tonnage of urchins has been of a scale that control, such as by rebuilding of predators and upscaling of culling and/ or harvesting would appear plausible. The spatial information on barrens coverage and locations at greatest risk of overgrazing as obtained during this survey will assist further targeted interventions. See also: http://www.imas.utas.edu.au/urchins

Resurvey of the longspined sea urchin (Centrostephanus rodgersii) and associated barren reef in Tasmania

对塔斯马尼亚长刺海胆（Centrostephanus rodgersii）和相关荒礁的重新调查

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