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Empirical Length-at-Maturity for Northeast Atlantic Marine Fishes

This study presents a standardized dataset of 3,607 length-at-maturity (L50) estimates for 48 Northeast Atlantic marine fish species, derived from approximately 1.6 million individual records across 21 fishery-independent surveys to enable high-resolution analysis of maturation schedules across populations, sexes, and time.

Original authors: Federico Maioli, Francesca Vitale, Max Lindmark, Alessandro Orio, Casper W Berg, Martin Lindegren, Mikael van Deurs, Tobias Mildenberger

Published 2026-09-03

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CC BY 4.0

Original authors: Federico Maioli, Francesca Vitale, Max Lindmark, Alessandro Orio, Casper W Berg, Martin Lindegren, Mikael van Deurs, Tobias Mildenberger

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

In the ocean, the moment a fish grows old enough to reproduce is a turning point in its life. It is not just a biological milestone; it is a switch that redirects the animal's energy from growing larger to making more offspring. For the scientists who study fish populations and manage the oceans, knowing exactly when this switch flips is critical. If a fish is caught before it has a chance to reproduce, that individual is lost to the future of its population, and the entire stock can shrink. The standard way to measure this timing is by looking at size rather than age. Researchers ask a simple question: at what length does half of a group of fish become mature? This specific size, known as the length at maturity, acts as a ruler for the health of a fishery. It helps managers decide how big a fish must be before it can legally be kept, ensuring that most individuals get to reproduce at least once.

For decades, scientists have struggled to find a single, reliable ruler for this measurement across the vast waters of the Northeast Atlantic. The information that does exist is often scattered, borrowed from different studies, or based on broad guesses that do not fit the specific local conditions of every fish population. A fish living in the cold, deep waters of the North Sea might mature at a different size than its cousin living in the warmer, shallower waters of the Baltic Sea, yet old databases often treated them as the same. This lack of precise, local data makes it difficult to tell the difference between a young fish and an adult, which is essential for understanding how fish populations move, grow, and interact with their environment.

A new study by a team of researchers from Denmark and Sweden has finally provided a comprehensive map of these turning points. By gathering and analyzing a massive collection of data, they have created a standardized set of measurements for 48 different species of marine fish in the Northeast Atlantic. The team did not rely on old estimates or theoretical models; instead, they went directly to the source. They pulled raw biological records from 21 different scientific surveys that have been trawling the ocean floor since the year 2000. These surveys are independent of commercial fishing, meaning they are designed to count fish without the bias of fishing gear, providing a clear picture of what is actually out there. The researchers processed approximately 1.6 million individual records, checking the length, sex, and maturity stage of every fish they could find.

The work involved a careful and rigorous process of cleaning and organizing this data. The researchers had to translate different ways that various countries and laboratories had recorded fish maturity into a single, consistent language. They looked at the gonads, or reproductive organs, of the fish to determine if they were developing, ready to spawn, or had already spawned. To ensure accuracy, they focused their analysis on the specific times of year when fish are most likely to be reproducing, a period when these biological signs are clearest. They then used statistical methods to fit a curve to the data for each group of fish, calculating the exact length at which 50 percent of the individuals in that group were mature. They did this not just for the species as a whole, but broke the numbers down further by specific fishing stocks, distinct geographic areas, and even by sex, looking at males and females separately where the data allowed. They also tracked how these sizes changed over five-year periods, creating a timeline of maturation from 2000 to 2026.

The result is a dataset containing 3,607 specific estimates of maturity size. This is a significant leap forward because it covers 48 species, including many that are not currently monitored by commercial fisheries and for which little data previously existed. The study confirms that fish do not all mature at the same size, even within the same species. For example, the data shows that cod in the Baltic Sea mature at a much smaller size than cod in the North Sea, a difference that a single, broad average would completely miss. The researchers also found that these sizes can shift over time, responding to changes in the environment or the pressure of fishing. By providing these detailed, localized numbers, the study gives scientists and managers a powerful new tool. They can now classify individual fish in their surveys as juveniles or adults with much greater confidence, leading to better estimates of how many fish are available to reproduce. This clarity helps in understanding how fish populations are changing and provides a solid foundation for making decisions that keep the ocean's fish stocks healthy for the future.

Technical Summary: Empirical Length-at-Maturity for Northeast Atlantic Marine Fishes

Problem Statement
Size at maturity is a fundamental parameter in fish ecology and fisheries science, directly influencing reproductive output, population dynamics, and stock productivity. The length at which 50% of individuals are mature ( $L_{50}$ ) serves as a standard metric for maturation schedules and informs management measures such as minimum conservation reference sizes. However, existing $L_{50}$ estimates suffer from significant limitations:

Fragmentation: Estimates are often scattered across individual stock assessment reports or borrowed from broad compilations like FishBase.
Lack of Standardization: Available values vary widely in spatial, temporal, and sex-specific resolution and are rarely derived within a unified framework.
Validity Concerns: Applying maturity estimates from one population or time period to another can lead to systematic misclassification of life stages (e.g., confusing juveniles with adults), particularly in studies of population structure, trophic interactions, or distribution dynamics.
Data Gaps: Non-assessed or data-limited species often rely on proxy values (e.g., proportional to asymptotic length) or broad geographic averages that do not reflect local conditions.

Methodology
The authors developed a standardized dataset of $L_{50}$ estimates for marine fishes in the Northeast Atlantic using a rigorous, reproducible workflow:

Data Source: Raw biological records were extracted from the ICES Database of Trawl Surveys (DATRAS), covering 21 fishery-independent bottom-trawl surveys from 2000 to 2026. The dataset includes approximately 1.6 million individual maturity records for teleost and elasmobranch species.
Data Standardization:
- Taxonomic Validation: Species names were validated against the World Register of Marine Species (WoRMS).
- Length Filtering: Implausible lengths were removed by comparing records against species-specific maximum lengths ( $L_{max}$ ) from FishBase (excluding records >1.2 $\times$ $L_{max}$ ).
- Temporal Filtering: Data were restricted to the spawning and pre-spawning seasons to ensure gonads were sufficiently developed for reliable maturity staging. Spawning windows were derived from literature and ICES reports, with a hierarchical fallback to FishBase if specific area data were missing.
- Maturity Harmonization: The diverse maturity coding systems used by different national surveys (e.g., ICES, SMSF, BITS, RIVO) were harmonized into a binary classification (Immature $y=0$ vs. Mature $y=1$ ). Special handling was applied to ambiguous stages (e.g., Stage II in BITS surveys) based on validation against unambiguous keys.
Statistical Analysis:
- Maturity ogives were fitted using binomial generalized linear models (GLMs) with a logit link function.
- Models were fitted at three spatial resolutions: Global (entire species range), Stock (ICES stock unit), and Area (ICES statistical area).
- Estimates were further resolved by sex and five-year time periods (e.g., 2000–2004) where sample sizes permitted.
- A minimum threshold of 15 immature and 15 mature individuals was required for a unit to be included, ensuring stable logistic fits.
- $L_{50}$ (and $L_{25}$ , $L_{75}$ ) were calculated from model coefficients, with 95% confidence intervals derived via the delta method.
Quality Control: Estimates were filtered to ensure biological plausibility (monotonic increase with length, positive coefficients) and statistical reliability (AUC > 0.5, confidence intervals not excessively wide).

Key Contributions and Results
The study produced a comprehensive, open-access dataset containing 3,607 $L_{50}$ estimates for 48 species (including both teleosts and elasmobranchs). Key characteristics of the dataset include:

Resolution: Estimates are provided at global, stock, and area levels, with temporal resolution in five-year bins (2000–2024) and sex-specific breakdowns where data allowed.
Coverage: The dataset covers 48 species, including 38 species across 79 ICES stock units and 41 species across 27 ICES areas. It includes species that are not currently part of formal stock assessments.
Model Performance: The fitted models demonstrated high discriminatory ability, with median AUC values of 0.88 (global), 0.90 (stock), and 0.91 (area).
Validation: Global estimates showed strong agreement with FishBase medians (Pearson $r = 0.92$ ), confirming the magnitude of the estimates. Deviations were noted as biologically sensible (e.g., lower $L_{50}$ for Baltic cod compared to global averages).
Data Availability: The final dataset (l50_estimates.csv) and the analysis code are publicly available via Zenodo and GitHub under a Creative Commons license.

Significance and Claims
The authors position this work as a resource to support survey-based life-stage classification and the direct study of maturation variability. The significance of the dataset lies in its:

Standardization: Providing a consistent framework for comparing maturation across populations, sexes, and time, overcoming the fragmentation of existing literature.
Granularity: Enabling researchers to distinguish between juveniles and adults in distribution and dynamics studies with greater accuracy than broad, borrowed estimates allow.
Temporal and Spatial Insight: Facilitating the investigation of how maturation shifts over time and space, which can offer insights into the drivers of change such as size-selective fishing and environmental conditions.
Utility for Non-Assessed Species: Offering empirically derived maturity parameters for species lacking formal stock assessments, reducing reliance on proxies.

The paper explicitly notes limitations, such as the fact that estimates are derived from survey catches (which may under-sample small/juvenile fish due to gear selectivity) and rely on macroscopic staging rather than histology. Consequently, the authors advise users to interpret the values as survey-based parameters rather than absolute population-level truths and to prioritize stock- or area-level estimates over global averages where spatial variation is known to exist.

Technical Summary: Empirical Length-at-Maturity for Northeast Atlantic Marine Fishes

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