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年鱼类繁殖季，研究团队于大堡礁（Great Barrier Reef）中部开展了四次跨陆架巡航，分别于10月、11月、12月及1月实施，单次巡航时长均为10天。所有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）中断作业。

研究团队在基珀礁（Keeper Reef）、赫利克斯礁（Helix Reef）、法拉第礁（Faraday Reef）与米尔米登礁（Myrmidon Reef）四处珊瑚礁的紧邻下游区域锚定了光诱捕器（light-trap）：每处礁体布设4台诱捕器，其中3台布设于表层，1台布设于20m水深。所选四处礁体具备相近的规模、孤立性，且均位于磁海峡（Magnetic Passage）南侧，横跨汤斯维尔（Townsville）外礁体带的完整宽度。锚定光诱捕器每日采样一次，单台诱捕器的样品整合了前一晚三个时段的捕捞渔获：东部标准时间（EST）21:00-22:00、00:00-01:00与03:00-04:00。

该近礁采样辅以跨陆架断面采样：在大堡礁潟湖（GBR潟湖，站位CS0-CS4）、磁海峡（站位CS5-CS9）或西珊瑚海（站位CS10-CS14）的5个开阔水域站位布设漂流式光诱捕器。根据天气状况，每晚的断面采样（对应全长160km的完整断面）在单次巡航中需完成3次。在15个站位中，每站布设两组间距200-300m的浮标绳，每组浮标绳分别搭载一台表层光诱捕器与一台20m水深光诱捕器。所有诱捕器在布设1小时后回收，样品内容物将在船舶驶往下一站位的途中完成处理。

研究团队同时通过一系列锚定海流计收集海洋学数据：两套海流计布设于礁体带两侧，第三套布设于磁海峡内。该观测覆盖范围足以开展礁体带内部及单座珊瑚礁周边的水动力流场模拟。

在1991/1992年鱼类繁殖季，四处礁体的近礁采样强度得到提升：每处礁体额外增设一台20m水深的光诱捕器。该季对诱捕器进行了升级，同时加深了漂流式深水诱捕器的布设深度，使其在距海底5m范围内采样，最大布设深度达100m。受夜间在大陆架外作业难度增加、且该区域采样渔获量持续偏低的影响，珊瑚海区域的采样强度有所下调。大陆架外的采样缩减至单次巡航仅开展一晚，但研究团队尽可能通过在断面近岸端（该区域采样效率最高）增加采样强度来弥补。本季采样均安排在月球上弦至下弦时段，周期为9月至次年1月。此外，本季还在棕榈海峡（Palm Passage）增设了6个采样站点（PP14-PP18），并于10月、11月及12月开展了采样。

1992/1993年夏季（对应10月至次年2月的新月时段），研究团队在5次巡航中均于大堡礁潟湖（CS0-CS4）开展了3晚的采样作业。在1991/1992年与1992/1993年季，研究团队在每个开阔水域站位使用赛毕德（Seabird）品牌的温盐深剖面仪（CTD）采集水温、盐度与光透射率剖面数据，光透射率剖面的采集取决于光透射率仪（transmissometer）的可用情况。

所有样品均使用70%乙醇保存，以保留鱼类头部的耳石（otoliths），后续可通过解析耳石微观结构中的日生长纹，提取部分样品并测定其年龄与生长史。

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跨陆架断面上的鱼类幼体分布；
(ii) 对比礁体附近与远海区域的幼体分布与丰度；
(iii) 量化选定的经济鱼类种群丰度在日、月、年尺度上的时间变异性；
(iv) 阐明水质与近岸环流对幼体分布及丰度的影响。