Understanding the life cycle and the demography of individual species is critical for the management of threatened populations and integral to understanding the ecology of various ecosystems and habitats (Kimirei et al., 2015; Perez & Munch, 2010). This knowledge is particularly critical for fish species, as the maintenance of populations and biodiverse assemblages is heavily influenced by larval supply, post-settlement processes, and demographic processes such as recruitment (Caley et al., 1996). The successful recruitment of a juvenile fish into the adult population is strongly determined by individual size, particularly in marine environments, where selection pressures favoring larger individuals are approximately five times the strength of the same pressure on terrestrial taxa (Perez & Munch, 2010). As such, habitats that provide the necessary resources to support high rates of juvenile growth are important for the survival of juveniles and their recruitment into the wider population (Lefcheck et al., 2019; Perez & Munch, 2010). Marine habitats that are able to support this can have a greater density and abundance of juvenile species (e.g., seagrass meadows, kelp forests, mangroves) and are termed “nursery habitats” (Lefcheck et al., 2019).

Achoerodus viridis (Eastern Blue Groper) is an iconic fish species in eastern Australia, and particularly in the state of New South Wales (NSW) where it is the state marine emblem due to its popularity and social importance. This species is listed as “Near Threatened” by the International Union for Conservation of Nature (IUCN) due to the loss of key habitat and historical overfishing, and is a protected species in the states of NSW and Victoria (Choat & Pollard, 2010). They belong to the Labridae family of fish and are a long-lived hermaphroditic species, which start out as female juveniles with an initial-phase color of pinky brown (Figure 1a), growing to sexually mature females (Figure 1b) before transitioning to an adult male form in which they take on their iconic blue color (Figure 1c; Gillanders, 1995). A. viridis have a biphasic life cycle, starting out in a presettlement planktonic larval phase before transitioning to a post-settlement juvenile/adult phase. A. viridis larvae often settle onto seagrass meadows, Ecklonia (kelp) forests, or in-shore rocky reef habitats and occupy these habitats throughout their juvenile phase (Gillanders, 1997b; Gillanders & Kingsford, 1996). Mature A. viridis often emigrate from these juvenile habitats and recruit into adult populations on exposed costal reefs or habitats near the entrance of estuaries (Gillanders, 1997b; Gillanders & Kingsford, 1996). This movement is most likely to be a response to post-settlement selection pressures, as the resources in the juvenile habitat become inadequate to support the individual, or other factors such as predation pressures become more prominent (Gillanders, 1997a, 1997b).

These in-shore estuarine habitats are particularly vulnerable to the increasing pressures of climate change and other anthropogenic impacts such as eutrophication and decreasing water quality; leading to the degradation or loss of these key habitats (Lotze et al., 2006). This poses a threat to the long-term stability of many A. viridis metapopulations, as the loss of key recruitment habitats may decrease the ability of estuarine and coastal systems to support an abundant population of A. viridis. As such, furthering our understanding of the life history of A. viridis by identifying critical recruitment and juvenile habitats will ensure the long-term stability of the A. viridis population. Here we report on the novel use of oyster reefs on the southeast coast of Australia by juvenile A. viridis (Eastern Blue Groper; hereafter referred to as blue groper).

Juvenile blue gropers were observed as part of a broader study that conducted seasonal (summer and winter) remote underwater video surveys of fish assemblages on four remnant oyster reefs in NSW (Bermagui, Crookhaven, Towra Point, Port Hacking; Pine et al. unpublished manuscript). For each video, the MaxN (the maximum number of individuals of a given species observed in a single frame throughout a video) of each species was recorded to obtain a measure of relative abundance (Schobernd et al., 2013). At the sites where A. viridis were recorded (Bermagui, Crookhaven, Towra Point), observations of juvenile blue groper were common, occurring in 29.8% of videos taken during summer (37 out of 124), although the frequency varied by location with observations in 56%, 65%, and 3% of all videos at Crookhaven, Bermagui, and Towra Point, respectively. Based on ranked MaxN values (with hyperabundant shoaling species Hyperlophus vittatus, Ambassis jacksoniensis, and Atherinomorus vaigiensis removed) juvenile A. viridis were moderately common at Bermagui and Crookhaven, ranking 7th out of 17 total ranks and 8th out of 23 respectively (Figure 2a,b). They were much less common at Towra Point, ranking 16th out of 17 ranks (Figure 2c).

The frequency of their occurrence at Bermagui and Crookhaven gives an indicator of their abundance on threatened oyster reefs and suggests that this is not just a one-off vagrancy event from traditional habitats (e.g., kelp forests, seagrass meadows, estuarine rocky reefs), but that oyster reefs may be playing a critical, yet to date underappreciated, role in the ontogenetic movement of this species in these regions. The lack of any individuals at Port Hacking and low numbers of individuals at Towra Point is interesting, given the fact that these two estuaries are more urbanized (located within Sydney) and more degraded than the other two systems (Reid, 2020). Urbanized systems are more likely to have limited recruitment due to increased levels of habitat fragmentation/loss and decreased water quality, in addition to lower levels of fish stock due to increased fishing pressures (Taylor & Suthers, 2021). This could potentially be why we did not observe the same level of abundance of A. viridis in these estuaries compared with Bermagui and Crookhaven.

In the majority of observations, individuals were directly interfacing with the oyster reef matrix, either using the reef for protection, or feeding on small invertebrates and epifauna found on the oyster shells (Figure 1d,e). The observation of individuals using the matrix for protection is particularly important as the three-dimensional complexity of oyster reefs and other hard-structured habitats can reduce predation pressures and the impacts of the hydrodynamic environment on individuals, both of which have deleterious impacts on the mortality rate and demography of juveniles (Forrester, 1990; Johnson, 2006). Furthermore, the observation of this frequent foraging behavior suggests that juvenile A. viridis are utilizing these threatened oyster reefs as a source of food, potentially enhancing their growth rate and recruitment success.

Notably, no adult blue gropers were observed during the study. The lack of adults present on these reefs is not surprising, as juvenile A. viridis are known to emigrate to more exposed coastal habitats as they mature due to the greater availability of resources and favorable conditions (Gillanders, 1997b). However, this highlights the key role that oyster reefs may play within the ontogenetic movements of A. viridis.

This observation of an iconic and threatened species using threatened oyster reefs as a juvenile habitat is significant, not only because it enhances our understanding of A. viridis and their ontogenetic movement, but because of the opportunity that this presents for the engagement and funding of ongoing and future oyster reef restoration projects. Over the past century, nearly 85% of all oyster reefs have been lost worldwide due to various anthropogenic impacts (Beck et al., 2011). In Australia, reefs consisting of the two endemic oyster species, Saccostrea glomerata (Sydney Rock Oyster) and Ostrea angasi (Southern Mud Oyster) are considered functionally extinct, and while there is significant momentum for restoration efforts within Australia, such work is costly and lengthy, and maintaining interest is likely to be difficult (Beck et al., 2011; Schrobback et al., 2014). The identification of threatened oyster reefs as a key habitat for threatened juvenile blue gropers is likely to buoy these efforts and increase public support, a phenomenon often observed when iconic species are used in support of conservation efforts (Lee et al., 2015).

The observations we report here introduce the possibility that the decline in a potential key habitat (oyster reefs) may have been an unforeseen barrier to efforts to stabilize blue groper populations in the past. If this is the case, restoration and conservation of these threatened oyster reefs may enhance ontogenetic movement opportunities for the species by providing key juvenile habitats, which could further aid in the long-term stability and protection of blue groper populations. Although beyond the scope of this paper, further studies into the recruitment and demography of juvenile A. viridis on threatened oyster reefs is needed to enhance our understanding of its life history and to better predict the impact that oyster reef restoration may have on the population status of this iconic species.

了解单个物种的生命周期和人口统计对于受威胁种群的管理至关重要，并且对于了解各种生态系统和栖息地的生态学不可或缺（Kimirei 等人，2015 年；Perez 和 Munch，  2010年 ）。这种知识对鱼类特别重要，因为种群和生物多样性组合的维持在很大程度上受到幼体供应、定居后过程和人口统计过程（如补充）的影响（Caley 等，1996 年 ）). 将幼鱼成功招募到成鱼种群很大程度上取决于个体大小，特别是在海洋环境中，在海洋环境中，有利于较大个体的选择压力大约是陆地类群相同压力的五倍 (Perez & Munch, 2010  )。因此，提供必要资源以支持幼鱼高速生长的栖息地对于幼鱼的生存及其在更广泛种群中的招募非常重要（Lefcheck 等人，2019 年；Perez 和 Munch，  2010 年）。能够支持这一点的海洋栖息地可以拥有更大密度和更丰富的幼年物种（例如，海草草甸、海带森林、红树林），并被称为“苗圃栖息地”（Lefcheck 等人， 2019 年）。

Achoerodus viridis (Eastern Blue Groper) 是澳大利亚东部的一种标志性鱼类，尤其是在新南威尔士州 (NSW)，由于其受欢迎程度和社会重要性，它是该州的海洋标志。由于主要栖息地的丧失和历史上的过度捕捞，该物种被国际自然保护联盟 (IUCN) 列为“近危”物种，并且是新南威尔士州和维多利亚州的受保护物种（Choat & Pollard，2010 年 ）). 它们属于 Labridae 鱼科，是一种长寿的雌雄同体物种，最初是雌性幼鱼，初始阶段颜色为小指棕色（图 1a），在过渡到成年雄性形态，它们呈现出标志性的蓝色（图 1c；Gillanders，  1995 年）。A. viridis有一个双相生命周期，从定居浮游幼虫阶段开始，然后过渡到定居后幼年/成年阶段。A. viridis幼虫通常栖息在海草草甸、海藻（海带）森林或近岸岩石礁栖息地，并在整个幼年阶段占据这些栖息地（Gillanders，  1997b; 吉兰德斯和金斯福德，  1996 年）。成熟的A. viridis通常会从这些幼年栖息地迁出，并在暴露的沿海珊瑚礁或河口入口附近的栖息地招募到成年种群 (Gillanders,  1997b ; Gillanders & Kingsford,  1996 )。这种运动很可能是对定居后选择压力的反应，因为幼年栖息地的资源变得不足以支持个体，或者其他因素，如捕食压力变得更加突出（Gillanders，1997a  ， 1997b ）。

这些近岸河口栖息地特别容易受到气候变化和其他人为影响（如富营养化和水质下降）日益增加的压力的影响；导致这些重要栖息地的退化或丧失（Lotze 等人，  2006 年）。这对许多A. viridis种群的长期稳定性构成威胁，因为关键补充栖息地的丧失可能会降低河口和沿海系统支持丰富的A. viridis种群的能力。因此，通过确定关键的补充和幼年栖息地来进一步了解A. viridis的生活史，将确保A. viridis的长期稳定性人口。在这里，我们报告了幼年A. viridis（东部蓝摸鱼；以下简称蓝摸鱼）对澳大利亚东南海岸牡蛎礁的新颖使用。

作为更广泛研究的一部分，该研究对新南威尔士州四个残余牡蛎礁（Bermagui、Crookhaven、Towra Point、Port Hacking；Pine 等人未发表的手稿）的鱼类组合进行了季节性（夏季和冬季）远程水下视频调查。 ). 对于每个视频，记录每个物种的 MaxN（在整个视频的单个帧中观察到的给定物种的最大个体数）以获得相对丰度的度量（Schobernd 等人，2013  ）。在A. viridis的地点被记录（Bermagui、Crookhaven、Towra Point），对幼年蓝摸鱼的观察很常见，在夏季拍摄的视频中占 29.8%（124 个视频中有 37 个），尽管频率因位置而异，观察结果分别为 56%、65%、以及 Crookhaven、Bermagui 和 Towra Point 的所有视频的 3%。根据排名的 MaxN 值（去除了过度丰富的浅滩物种Hyperlophus vittatus、Ambassis jacksoniensis和Atherinomorus vaigiensis）幼年的A. viridis在 Bermagui 和 Crookhaven 相当普遍，分别在 17 个总排名中排名第 7 位，在 23 个排名中分别排名第 8 位（图 2a， b). 它们在 Towra Point 不太常见，在 17 个排名中排名第 16（图 2c）。

它们在 Bermagui 和 Crookhaven 出现的频率表明它们在受威胁的牡蛎礁上的数量丰富，并表明这不仅仅是传统栖息地（例如海带森林、海草草甸、河口岩礁）的一次性流浪事件，而是牡蛎礁可能在该物种在这些地区的个体发育运动中发挥着重要但迄今为止未被充分认识的作用。Port Hacking 没有任何个体，Towra Point 的个体数量很少，这很有趣，因为这两个河口比其他两个河口城市化程度更高（位于悉尼境内）且退化程度更高（Reid，  2020). 由于栖息地破碎化/丧失程度增加和水质下降，以及捕捞压力增加导致鱼类资源水平下降，城市化系统的补充更有可能受到限制（Taylor & Suthers，2021 年 ）。这可能是为什么我们没有在这些河口观察到与 Bermagui 和 Crookhaven 相比相同水平的A. viridis的原因。

在大多数观察中，个体直接与牡蛎礁基质接触，要么利用珊瑚礁进行保护，要么以牡蛎壳上发现的小型无脊椎动物和表栖动物为食（图 1d、e）。使用矩阵进行保护的个体观察尤为重要，因为牡蛎礁和其他硬结构栖息地的三维复杂性可以减少捕食压力和水动力环境对个体的影响，这两者都对死亡率产生有害影响青少年的比率和人口统计（Forrester，  1990 年；Johnson，  2006 年）。此外，对这种频繁觅食行为的观察表明，幼年的A. viridis正在利用这些受威胁的牡蛎礁作为食物来源，有可能提高它们的生长速度和招募成功率。

值得注意的是，在研究期间没有观察到成年的蓝色摸索者。这些珊瑚礁上缺乏成虫并不奇怪，因为众所周知，由于资源和有利条件的更大可用性， 幼年的A. viridis在成熟时会迁移到更暴露的沿海栖息地（Gillanders， 1997b）。然而，这突出了牡蛎礁可能在A. viridis的个体发育运动中发挥的关键作用。

这种使用受威胁的牡蛎礁作为幼鱼栖息地的标志性和受威胁物种的观察具有重要意义，不仅因为它增强了我们对A. viridis及其个体发育运动的理解，而且因为这为正在进行的参与和资助提供了机会以及未来的牡蛎礁恢复项目。在过去的一个世纪里，由于各种人为影响，全世界近 85% 的牡蛎礁已经消失（Beck 等人，  2011 年）。在澳大利亚，由两种地方性牡蛎组成的珊瑚礁，Saccostrea glomerata（悉尼岩牡蛎）和Ostrea angasi（Southern Mud Oyster）被认为在功能上已经灭绝，虽然澳大利亚的恢复工作势头强劲，但此类工作成本高昂且耗时长，而且可能很难保持兴趣（Beck 等人，2011 年；Schrobback 等 人，  2014 年）。将受威胁的牡蛎礁确定为受威胁的幼年蓝摸鱼的主要栖息地可能会支持这些努力并增加公众支持，这种现象在使用标志性物种支持保护工作时经常观察到（Lee 等人，2015 年 ）。

我们在此报告的观察结果表明，潜在关键栖息地（牡蛎礁）的减少可能是过去稳定蓝摸鱼种群努力的不可预见的障碍。如果是这种情况，恢复和保护这些受威胁的牡蛎礁可能会通过提供关键的幼鱼栖息地来增加该物种的个体发育运动机会，这可能进一步有助于蓝摸鱼种群的长期稳定和保护。尽管超出了本文的范围，但需要进一步研究受威胁的牡蛎礁上幼年 A. viridis 的招募和人口统计，以加强我们对其生活史的理解，并更好地预测牡蛎礁恢复可能对种群状况产生的影响这个标志性的物种。