Cross shelf distribution and abundance of larval fishes in the central Great Barrier Reef (FIRDC Project 89/28)

In the 90/91 fish breeding season, four cross-shelf cruises (October, November, December and January), each lasting 10 days were undertaken in the central Great Barrier Reef. Each cruise was scheduled between the third and first quarter of the moon, with the exception of December, when Cyclone Joy curtailed operations.

Four light-traps, three at the surface and one at 20m, were anchored immediately downstream of four coral reefs - Keeper Reef, Helix Reef, Faraday Reef and Myrmidon Reef. These reefs, chosen for their similar size, isolation and position on the southern side of the Magnetic passage, span the full width of the reef matrix off Townsville. The anchored light traps were sampled daily, where the sample taken from a trap represented the integrated catch from three periods of fishing during the previous night: 2100-2200, 0000-0100, 0300-0400 hrs EST.

This near reef sampling was complemented by sampling of a cross shelf transect, which consisted of drifting light-traps at five open-water stations in either the GBR Lagoon (Stations CS0-CS4), the Magnetic Passage (CS5-CS9) or the western Coral Sea (CS10-CS14). Depending on the weather, each nightly transect (and hence the entire transect of 160 km) was completed three times in a cruise. At each of the 15 stations, two buoy strings were released 200-300 m apart; each string supported one light-trap at the surface and another at 20m. All traps were recovered after one hour of fishing and their contents were processed as the ship steamed to the next station.

Oceanographic data was also collected from a series of moored current meters, located on both sides of the reef matrix with a third in the Magnetic Passage. This coverage was considered sufficient to model hydrodynamic flows through the matrix and around individual reefs. In the 91/92 breeding season the effort near each of the four reefs was boosted by an extra trap at 20 m. Traps had been upgraded and the depth of the deep drifting traps was increased so that they sampled within 5 m of the bottom, to a maximum of 100 m. Effort was reduced in the Coral Sea due to the extra difficulty of working off the continental shelf at night and the consistently low return from such sampling. Sampling off the shelf was reduced to a single night per cruise but compensated for whenever possible by additional effort at the inshore end of the transect, which was the most productive area.

Sampling was carried out between the third and first quarter moons of September - January. A further 6 sites were established across Palm Passage during this season (PP14-PP18) and sampled in October, November and December. In 92/93, three nights of sampling in the GBR Lagoon (CS0-CS4) was undertaken on each of five cruises during that summer (new moons of October-February). In 91/92 and 92/93, profiles of water temperature, salinity and light-transmission (depending on availability of a transmissometer) were captured with a Seabird (TM) CTD at each of the open water stations. All samples were preserved in 70% alcohol to preserve the otoliths in the heads of these fishes, so that a portion of them could be extracted and measured for age and growth histories by interpreting daily growth rings in their microstructure.

Light trap sampling was used:

(i) to map the distribution of fish larvae along a 160 km cross-shelf transect in the central Great Barrier Reef

(ii) to compare distribution and abundance near and far from reefs

(iii) to measure the temporal variability in abundance of selected stocks of commercial interest at daily, monthly and annual scales

(iv) to determine the influence of water quality and coastal circulation on larval distribution and abundance.

在1990/1991鱼类繁殖季，研究团队于大堡礁（Great Barrier Reef, GBR）中部开展了4次跨陆架巡航，分别于10月、11月、12月和1月实施，每次巡航持续10天。每次巡航均安排在月球上弦至下弦时段，仅12月的巡航因“乔伊飓风（Cyclone Joy）”导致作业中断。

研究团队在基珀礁（Keeper Reef）、螺旋礁（Helix Reef）、法拉第礁（Faraday Reef）及墨米登礁（Myrmidon Reef）4座珊瑚礁的紧邻下游区域锚定了4台光诱捕器（light-trap），其中3台布设于表层，1台布设于20m水层。上述礁体因尺寸相近、相互隔离且均位于磁海峡（Magnetic Passage）南侧而被选中，覆盖了汤斯维尔（Townsville）外礁体复合体的全部宽度。锚定的光诱捕器每日进行采样，单个诱捕器的样本为前一晚三个时段的累计渔获：东部标准时间21:00-22:00、00:00-01:00及03:00-04:00。

近礁采样辅以跨陆架断面采样：该断面在大堡礁潟湖（GBR Lagoon，站位CS0-CS4）、磁海峡（CS5-CS9）或西珊瑚海（CS10-CS14）的5个开阔水域站位布设漂流式光诱捕器。视天气状况，每次巡航期间每晚的断面采样（总长160km的完整断面）需重复开展3次。在全部15个站位中，均布设两组间距200-300m的浮标绳，每组浮标绳分别在表层及20m水层各搭载一台光诱捕器。所有诱捕器在作业1小时后回收，在船舶驶往下一站位的途中对其捕获物进行处理。

研究团队还通过一系列锚定海流计收集海洋学数据，其中两台布设于礁体复合体两侧，第三台布设于磁海峡内。该布设方案被认为足以模拟礁体复合体内部及单个礁体周围的水动力流动。在1991/1992鱼类繁殖季，4座礁体附近的采样强度得到提升：每个站位额外增设一台20m水层的光诱捕器。研究人员对诱捕器进行了升级，同时调整了深层漂流式诱捕器的布设深度，使其在距底5m范围内进行采样，最大作业深度可达100m。由于夜间在陆架外作业难度较高，且该区域采样的渔获量持续偏低，珊瑚海区域的采样强度有所降低。陆架外的采样频次被缩减至每次巡航仅1个夜晚，但研究团队通过在断面近岸端（该区域采样效率最高）增加采样强度，尽可能弥补了采样量的不足。

本研究的采样时段为9月至次年1月的上弦至下弦月期间。1991/1992繁殖季还在棕榈海峡（Palm Passage）增设了6个采样站位（PP14-PP18），并于10月、11月和12月开展采样。在1992/1993繁殖季，该夏季共开展5次巡航，每次巡航均在大堡礁潟湖（CS0-CS4）区域进行3个夜晚的采样，采样时段对应10月至次年2月的新月期。在1991/1992及1992/1993繁殖季，研究团队使用塞伯德（Seabird）品牌的CTD（温盐深仪）在每个开阔水域站位采集水温、盐度及光透射率剖面数据（若透射计可用）。所有样本均使用70%乙醇进行保存，以保留鱼类头部的耳石（otoliths），后续可从中提取部分耳石，通过解析其微观结构中的每日生长环，测定鱼类的年龄及生长历史。

本研究采用光诱捕器采样的目的包括：
(i) 绘制大堡礁中部160km跨陆架断面上鱼类幼体的分布格局；
(ii) 对比礁体附近与远岸区域的鱼类幼体分布及丰度；
(iii) 量化选定的商业鱼类种群在日、月、年尺度上的丰度时间变异；
(iv) 探明水质及近海环流对鱼类幼体分布与丰度的影响。