DOI: 10.5281/zenodo.17268092
title: Length frequency of grey mullet obtained from Cross River Estuary, Nigeria (January 2023 – December 2024)

description: 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.