Response of herbivorous fish to crown-of-thorns-starfish, Acanthaster planci, outbreaks in the central Great Barrier Reef

Three adjacent pairs of reefs, located in the central region of the Great Barrier Reef were chosen for this study. One reef of each pair had been impacted by a crown-of-thorns starfish (COTS) outbreak in the previous 5 to 7 years. The reefs chosen were: the mid-shelf reefs, Grub Reef (impacted) and Centipede Reef (non-impacted) and 2 pairs of outer-shelf reefs, Yankee Reef (impacted) and Bowl Reef (non-impacted), and Dip Reef (impacted) and Coil Reef (non-impacted). Surveys were carried out on the windward reef slope of each reef at depths between 3 m and 7 m.Substratum cover and the density and size of herbivorous fish were surveyed on three occasions (June 1991, January 1992 and March 1993). On each occasion 6 sites were selected at each reef and 3 replicate, 45 x 6 m transects surveyed at each site. Seven species of roving herbivorous fish were selected for analysis of density and size: Acanthurus nigrofuscus and Zebrasoma scopas (Acanthuridae), Scarus frenatus, Chlorurus sordidus, and Scarus niger (Scaridae), and Siganus corallinus and Siganus vulpinus (Siganidae). An observer swam slowly along each transect (10 m/min), estimating size classes and counting all fish occurring 3 m on either side of the transect as it was being laid out.On completion of the fish surveys, the observer swam back along the transect identifying the substratum cover at a point at each 1 m interval. This resulted in 46 point intercepts (0 to 45 m). To distinguish between intensively grazed surfaces and damselfish territories, turf algal cover was split into 2 arbitrary categories: =1 mm.Feeding rates of two species of herbivorous fish (Acanthurus nigrofuscus and Scarus frenatus) were determined by counting the number of bites taken during a 5 min observation period for each individual. Seven replicate observations were made at each of 8 sites within each reef between 1000 hrs and 1600 hrs. Fork length of Acanthurus nigrofuscus was estimated in 1 cm size classes. Observations of Scarus frenatus were made on initial phase females only.The diet of Acanthurus nigrofuscus was analysed for 53 specimens collected from impacted reefs and 41 from non-impacted reefs. Fish were collected with spear guns and monofilament nets (15 mm mesh size) at all reefs on 2 separate occasions, April and October 1992. The stomach was dissected out and preserved in 70% seawater formalin. In the laboratory, the gut was split open and the contents washed into a petri dish, mixed with tap water and broken up with a dissecting needle. A 'clump' of this material was placed on a microscope slide, and further teased apart under low power. At 100x magnification, 4 areas of each slide were examined using an ocular micrometer with 25 intersection points. Any alga falling across one or more of these points of intersection was identified and counted. This procedure was repeated 3 times for each stomach sample, resulting in a total of 300 points of intersection.Acanthurus nigrofuscus were also collected with spear guns and monofilament nets (15 mm mesh size) at all reefs on 3 separate occasions, April and October 1992 and April 1993 for age and growth studies. Fish were measured (fork length (cm)) and weighed (g). The sex of individuals was determined according to appearance of gonads, and selected gonad samples preserved for histological examination. The head of each specimen was tagged and frozen for later removal of otoliths.The length and width of each otolith were measured using a dissecting microscope and an ocular micrometer. All age determinations were based on transverse sections of the sagittae. All sections were examined under a dissecting microscope and counts were made in the same region, which showed identifiable and readable growth rings. Transverse sections were photographed under a compound microscope (40x magnification) using colour 200 ASA film.In May 1992, 300 individuals were tagged using tetracycline on an isolated coral bommie at Bowl Reef to validate age estimates of Acanthurus nigrofuscus. Fish were collected and initially placed in a submerged holding tank, after which they were transferred a few at a time to the boat, measured (FL), injected with the required dosage of tetracycline, and released. All specimens were tagged externally with fine T-bar anchor tags inserted in the dorsal musculature between the 2nd and 3rd dorsal spines. In April 1992, 1 specimen was recaptured and a further 4 specimens were recaptured in November 1993. Sagittae were examined under a compound microscope with an ultraviolet light source to identify the fluorescent band resulting from the treatment with tetracycline.
This research was undertaken to examine the response of roving herbivorous reef fish to increased food following outbreaks of the crown-of-thorns starfish, Acanthaster planci (COTS). Comparisons were made between impacted and non-impacted reefs for the following:1. substrate cover and feeding ecology of roving herbivores 2. density and biomass of selected species of herbivorous fish3. age, growth rates, mortality and maturity indices of an 'indicator' species, Acanthurus nigrofuscus

本研究选取大堡礁中部区域的三组相邻珊瑚礁对作为研究对象。每组礁对中，有一个礁体在过去5至7年间曾遭受长棘海星（crown-of-thorns starfish, COTS）暴发灾害影响。本次选取的礁体包括：陆架中部礁群——受影响的格鲁布礁（Grub Reef）与未受影响的蜈蚣礁（Centipede Reef）；以及两组陆架外缘礁群——受影响的扬基礁（Yankee Reef）与碗礁（Bowl Reef）、受影响的迪普礁（Dip Reef）与科伊尔礁（Coil Reef）。
所有调查均开展于各礁体的迎风礁坡，水深范围为3米至7米。研究分别于1991年6月、1992年1月及1993年3月开展三次调查，内容涵盖底质覆盖度以及植食性鱼类的密度与体型参数。每次调查中，每个礁体选取6个样点，每个样点设置3条重复的45×6米样带。
本次分析共选取7种漫游型植食性鱼类：刺尾鱼科（Acanthuridae）的黑刺尾鱼（Acanthurus nigrofuscus）与栉齿刺尾鱼（Zebrasoma scopas）；鹦嘴鱼科（Scaridae）的绿唇鹦嘴鱼（Scarus frenatus）、驼背鹦嘴鱼（Chlorurus sordidus）与黑鹦嘴鱼（Scarus niger）；以及蓝子鱼科（Siganidae）的爪哇蓝子鱼（Siganus corallinus）与狐蓝子鱼（Siganus vulpinus）。
观察者以10米/分钟的速度沿样带缓慢游动，估算鱼类的体型等级并计数样带两侧3米范围内的所有鱼类。完成鱼类调查后，观察者沿样带返回，在每1米间隔处记录对应点位的底质覆盖类型，共计获得46个有效点位采样（覆盖0至45米区间）。为区分重度摄食表面与雀鲷领地，本次研究将表层藻垫覆盖度划分为两类：=1 mm。
两种植食性鱼类（黑刺尾鱼与绿唇鹦嘴鱼）的摄食率通过记录单条个体在5分钟观测期内的咬食次数进行测定。于每个礁体的8个样点处开展7次重复观测，观测时段为10:00至16:00。黑刺尾鱼的叉长（Fork Length, FL）以1cm为等级进行估算。本次研究仅对初始种群的雌性绿唇鹦嘴鱼开展摄食观测。
本研究共采集受影响礁区的53尾、未受影响礁区的41尾黑刺尾鱼样本，用于食性分析。于1992年4月与10月两次在所有礁体使用鱼枪及网目尺寸为15mm的单丝网开展样本采集。解剖取出胃囊后，将其置于70%海水福尔马林溶液中保存。实验室阶段，剪开肠道并将内容物冲洗至培养皿中，加入自来水混合后使用解剖针捣碎。取该混合物的一小块置于载玻片上，在低倍镜下进一步分散样品。在100倍放大倍率下，使用带有25个交叉点的目镜测微尺对每个载玻片的4个区域进行观察，凡是横跨一个或多个交叉点的藻类均进行鉴定与计数。每个胃样本重复该操作3次，共计获得300个交叉点采样数据。
另于1992年4月、10月及1993年4月三次在所有礁体采集黑刺尾鱼样本，用于年龄与生长相关研究。测量记录样本的叉长（cm）与体重（g），根据性腺外观确定个体性别，并选取性腺样本保存用于组织学检查。将每个标本的头部标记并冷冻，用于后续取出耳石。使用解剖镜与目镜测微尺测量每枚耳石的长度与宽度。所有年龄鉴定均基于矢耳石的横切面。所有切片在解剖镜下观察，在同一区域计数可识别且可读的生长环。使用彩色200ASA胶片在复合显微镜（40倍放大倍率）下拍摄横切面照片。
1992年5月，在碗礁的一处孤立珊瑚丘上使用四环素对300尾黑刺尾鱼个体进行标记，以验证其年龄估算结果。采集鱼类后先将其置于水下暂养箱，随后分批转移至船上，测量叉长并注射所需剂量的四环素后放归。所有标本均通过在第2与第3背棘之间的背部肌肉植入细T形锚定标签进行外部标记。1992年4月重捕1尾标记个体，1993年11月重捕4尾标记个体。使用带有紫外光源的复合显微镜观察矢耳石，以识别四环素处理产生的荧光标记带。
本研究旨在探究漫游型植食性礁区鱼类对长棘海星（Acanthaster planci, COTS）暴发后食物资源增加的响应。对比受影响与未受影响礁区的以下指标：1. 漫游型植食性鱼类的底质覆盖与摄食生态；2. 选定植食性鱼类物种的密度与生物量；3. 指示物种黑刺尾鱼的年龄、生长速率、死亡率与成熟度指数。