渤海湾鱼类早期资源群落结构特征及其时间变化研究

渤海湾历史上为黄渤海多种主要经济鱼类的重要产卵和育幼场。基于历史调查资料和补充调查,构建渤海湾鱼卵、仔稚鱼调查数据集,通过数理统计分析阐述1982年以来渤海湾鱼类早期资源(浮性鱼卵和仔稚鱼)群聚特性和演替过程。结果显示,渤海湾鱼类早期资源结构、产卵和育幼场重心位置处于持续更替过程中,不同时期早期资源的种类组成、资源丰度、优势种类和物种多样性水平等呈明显季节更替,118º30ʹE以西湾中底部海域一直为产卵和育幼场集中分布区。相较1980s,渤海湾鱼类早期资源已发生结构性改变,且渤海湾作为传统经济鱼类的产卵和育幼场所的功能下降明显。渤海湾鱼类早期资源种数和资源丰度均在2010s前期跌至历史低值,近年来又显著回升。鱼类早期资源种数由1980s的39种,1990s前期的31种、后期的34种,2000s—2010s初期的22 种,逐次下降至2010s前期的21种,2010s中后期开始逐步回升,至2020s初期已达40种。2020—2021 年鱼卵种数约为1980s的80%左右,资源丰度约为彼时的88%;仔稚鱼种数与1980s大致相当,资源丰度则为彼时的1.33倍。鱼卵、仔稚鱼物种多样性水平呈年际和年代际剧烈波动,鱼类早期资源种类年际更替现象明显,且近年来呈明显加快趋势。产卵类型、栖息类型和适温类型亲体种数均呈先下降后上升的变动趋势,全年综合浮性卵种数比例下降,黏性卵和具卵膜丝卵种数比例升高;陆架浅水中上层鱼类种数所占比例升高,中底层和底层鱼类所占比例降低。斑 (Konosirus punctatus)、青鳞小沙丁鱼(Sardinella zunasi)、小黄鱼(Larimichthys polyactis)、带鱼(Trichiurus japonicus)、花鲈(Lateolabrax maculatus)和半滑舌鳎(Cynoglossus semilaevis)等渤海湾传统经济鱼类早期资源密度显著降低;鳀(Engraulis japonicus)、蓝点马鲛(Scomberomorus niphonius)、赤鼻棱鳀(Thryssa kammalensis)和少鳞 (Sillago japonica)等鱼类早期资源密度显著增高。渤海湾鱼类早期资源发生量主要取决于产卵场的环境条件、捕捞强度和对鱼类早期资源的损害程度,渤海湾鱼类早期资源结构及其动态变化是环境和捕捞双重扰动下鱼类群落内生态位错位交替和结构性渔业资源衰退的具体体现。     

Selectivity matters: Rules of thumb for management of plate‐sized,sex‐changing fish in the live reef food fish trade

Effective management of fisheries depends on the selectivity of different fishing methods, control of fishing effort and the life history and mating system of the target species. For sex‐changing species, it is unclear how the truncation of age‐structure or selection of specific size or age classes (by fishing for specific markets) affects population dynamics. We specifically address the consequences of plate‐sized selectivity, whereby submature, “plate‐sized” fish are preferred in the live reef food fish trade. We use an age‐structured model to investigate the decline and recovery of populations fished with three different selectivity scenarios (asymptotic, dome‐shaped and plate‐sized) applied to two sexual systems (female‐first hermaphroditism and gonochorism). We parameterized our model with life‐history data from Brown‐marbled grouper (Epinephelus fuscoguttatus) and Napoleon fish (Cheilinus undulatus). “Plate‐sized” selectivity had the greatest negative effect on population trajectories, assuming accumulated fishing effort across ages was equal, while the relative effect of fishing on biomass was greatest with low natural mortality. Fishing such sex‐changing species before maturation decreased egg production (and the spawning potential ratio) in two ways: average individual size decreased and, assuming plasticity, females became males at a smaller size. Somatic growth rate affected biomass if selectivity was based on size at age because in slow growers, a smaller proportion of total biomass was vulnerable to fishing. We recommend fisheries avoid taking individuals near their maturation age, regardless of mating system, unless catch is tightly controlled. We also discuss the implications of fishing post‐settlement individuals on population dynamics and offer practical management recommendations.