Length frequency of grey mullet obtained from Cross River Estuary, Nigeria (January 2023 – December 2024)

1. Data Composition and Size RangeThe dataset comprised 2,146 individual measurements of Mugil cephalus collected from artisanal fish landings across different sites in the Cross River Estuary. Each record represents a single fish, with the total length (TL) measured to the nearest 0.1 cm using a calibrated measuring board. The observed lengths ranged from 6 cm (smallest juvenile) to 35 cm (largest adult), showing a broad size range that covers multiple life stages of the population. 2. Frequency Distribution PatternThe length–frequency distribution displayed a unimodal pattern, meaning there was a single dominant size group. The modal class—the length range with the highest number of individuals—occurred between 21 and 23 cm, corresponding to mid-sized fish. This suggests a strong recruitment of juvenile and sub-adult fish into the fishery, while fewer large individuals were observed. Such a pattern typically reflects continuous fishing pressure that removes larger individuals, allowing smaller cohorts to dominate the population. The high number of mid-sized fish implies active recruitment, while the scarcity of larger size classes indicates that fishing mortality increases with fish size. 3. Data Structure and VariablesEach entry in the dataset likely includes the following key variables: Variable Description Measurement Unit Specimen ID Unique identification code for each fish — Total Length (TL) Measured from snout to caudal fin tip cm Frequency (N) Number of fish within each 2-cm size class Count Estimated Population (Nᵢ) Computed abundance per length class (extrapolated) Individuals Fishing Mortality (F) Estimated fishing mortality rate for each length class per year Biomass (B) Estimated steady-state biomass for each length class tonnes This structured format allows integration with length-based stock assessment models, such as those implemented in FiSAT II or R-based length–frequency tools. 4. Data Summary Statistics Mean length: Approximately 20.5 cm ± 4.7 SD Modal length: 21–23 cm Length range: 6–35 cm Sample size: n = 2,146 Skewness: Right-skewed distribution, indicating dominance of smaller fish Kurtosis: Moderate, showing a peaked central tendency with a gradual decline toward larger size classes These descriptive statistics confirm a truncated length structure, characteristic of overexploited fish stocks where older and larger individuals are rare. 5. Ecological and Fishery ImplicationsThe dataset reveals that the Mugil cephalus population in the Cross River Estuary is dominated by young and mid-sized fish, reflecting intense fishing activity and continuous recruitment. The unimodal shape indicates limited size diversity, likely resulting from selective gear use (such as gill nets and beach seines) that preferentially capture certain size ranges. The data also serve as the foundation for estimating key biological indicators—such as the optimum length (Lopt), length at first capture (Lc), and fishing mortality (F)—which were used in the study to assess stock status and exploitation level. 6. Analytical RelevanceThis length–frequency dataset forms the empirical basis for all subsequent analyses in the study, including: Estimation of population abundance and biomass distribution across length classes. Modeling the exponential decay in population numbers with increasing body length. Deriving biological reference points such as Lopt, LF50, and Lpeak. Assessing fishing mortality and growth overfishing patterns. Its detailed structure allows for robust length-based population modeling, which is particularly valuable in data-limited fisheries such as those in the Cross River Estuary. Summary In summary, the length–frequency dataset for Mugil cephalus in the Cross River Estuary provides a detailed, size-based snapshot of the fish population. It captures the full spectrum of individuals from juveniles to adults, but the dominance of mid-sized fish and scarcity of large ones highlight heavy fishing pressure. The dataset is fundamental to understanding population dynamics, supporting stock assessment, and guiding sustainable management of the mullet fishery in southeastern Nigeria.1. 数据组成与尺寸范围 本数据集包含从克罗斯河河口（Cross River Estuary）不同站点的手工渔业上岸量中采集的2146条鲻鱼（Mugil cephalus）的个体测量数据。每条记录对应单尾鱼类，使用校准量鱼板测量其总长度（Total Length, TL），精度可达0.1 cm。观测体长范围为6 cm（最小幼鱼）至35 cm（最大成鱼），覆盖了该种群多个生活史阶段，尺寸跨度广泛。 2. 体长频率分布模式 体长频率分布（Length-Frequency Distribution）呈现单峰模式，即仅存在一个优势尺寸组。众数组（modal class）——个体数量最多的体长区间——为21~23 cm，对应中等体型鱼类。这表明幼鱼和亚成鱼大量补充进入渔业种群，而大型个体的观测数量较少。 该分布模式通常反映持续的捕捞压力移除了大型个体，使得小型年龄组成为种群优势类群。中等体型鱼类占比高意味着种群补充活跃，而大型尺寸组的稀缺则表明捕捞死亡率随鱼类体型增大而升高。 3. 数据结构与变量 本数据集的每条记录包含以下关键变量： | 变量 | 描述 | 测量单位 | |------|------|----------| | 样本编号（Specimen ID） | 每尾鱼类的唯一识别码 | 无 | | 总长度（Total Length, TL） | 从吻部至尾鳍尖端的长度 | 厘米（cm） | | 频率（N） | 每个2 cm尺寸区间内的鱼类数量 | 计数 | | 估算种群数量（Estimated Population, Nᵢ） | 各体长区间的推算种群丰度 | 个体 | | 捕捞死亡率（Fishing Mortality, F） | 各体长区间的估算捕捞死亡率 | 每年 | | 生物量（Biomass, B） | 各体长区间的稳态估算生物量 | 吨 | 该结构化格式可与基于体长的资源评估模型（Stock Assessment Models）兼容，例如FiSAT II或基于R语言的体长频率工具。 4. 数据汇总统计 - 平均体长：约20.5 cm ± 4.7 标准差（SD） - 众数体长：21~23 cm - 体长范围：6~35 cm - 样本量：n = 2146 - 偏度：右偏分布，表明小型鱼类占据种群优势 - 峰度：中等峰度，呈现中心峰值突出、向大型尺寸组逐渐衰减的分布特征 上述描述性统计证实了种群体长结构存在截断现象，这是过度捕捞种群（overfished stocks）的典型特征——老龄及大型个体较为稀有。 5. 生态与渔业意义 本数据集显示，克罗斯河河口的鲻鱼种群以幼鱼和中等体型鱼类为主，反映了高强度捕捞活动与持续的种群补充。单峰分布表明种群尺寸多样性有限，这可能源于选择性捕捞渔具（如刺网和岸曳网）优先捕获特定尺寸区间的鱼类。 本数据同时为估算关键生物学指标提供了基础，例如最适体长（Lopt）、首次捕捞体长（Lc）以及捕捞死亡率（F），这些指标被用于本研究评估种群现状与开发水平。 6. 分析相关性 该体长频率数据集为本研究后续所有分析提供了实证基础，包括： - 估算各体长区间的种群丰度与生物量分布 - 模拟种群数量随体型增大的指数衰减规律 - 推导生物学参考点（biological reference points），如最适体长（Lopt）、LF50与体长峰值（Lpeak） - 评估捕捞死亡率与生长型过度捕捞（growth overfishing）模式 其结构化的详细格式支持可靠的基于体长的种群建模，这对于克罗斯河河口这类数据匮乏渔业（data-limited fisheries）而言尤为重要。 总结 综上，克罗斯河河口鲻鱼的体长频率数据集提供了该鱼类种群基于体型的详细快照，覆盖了从幼鱼到成鱼的全部个体范围。但中等体型鱼类占优、大型个体稀缺的特征凸显了严重的捕捞压力。该数据集对于理解种群动态、支撑资源评估以及指导尼日利亚东南部鲻鱼渔业的可持续管理具有基础性作用。