# Population Structure and Growth Dynamics of the Invasive Blue Crab Callinectes sapidus in the Loukkos Estuary (Morocco)

## TL;DR

This study examines the population structure and growth patterns of invasive blue crabs in a Moroccan estuary, revealing seasonal and sex-based differences.

## Contribution

This is the first detailed study of blue crab population dynamics in the Loukkos Estuary, providing baseline data for managing this invasive species.

## Key findings

- The blue crab population in the Loukkos Estuary is well established with strong seasonal and sex-based differences in size and maturity.
- Males are larger and more abundant than females, with immature females dominating most of the year.
- The estuary functions primarily as a nursery and growth area for young crabs.

## Abstract

The blue crab Callinectes sapidus is an invasive species that has recently established populations in several Mediterranean and Atlantic coastal ecosystems. In this study, we examined the biological characteristics of the blue crab population in the Loukkos Estuary using samples collected over one year. Our results showed that this population is well established and structured, with clear differences between males and females and strong seasonal changes in body size, condition, and maturity. Males were generally larger and more abundant than females, and immature females dominated most of the year. The estuary appears to serve mainly as a nursery and growth area, supporting young and developing crabs. These findings provide important baseline information that can support future monitoring and management strategies for this invasive species in a protected estuarine environment.

This study provides the first insights into the biology of the blue crab Callinectes sapidus in the Loukkos Estuary, based on 461 individuals collected between December 2022 and November 2023. Results indicate a well-structured invasive population. Carapace width ranged from 52 to 201 mm (mean ± SD: 121.7 ± 25.4 mm) and total weight from 12 to 512 g (128.2 ± 76.6 g). Morphometric analyses revealed pronounced sexual dimorphism, with males larger and heavier than females. Size structure shifted seasonally, with smaller crabs dominating spring–summer samples and larger crabs in winter. Biometric relationships were significant and indicated negative allometric growth in both sexes. The sex ratio was strongly male-biased (M/F = 2.72). Condition factor varied with season and sex, peaking in summer and reaching minima in autumn. Female maturity exhibited marked seasonality: immature females prevailed from spring to autumn, whereas mature females occurred mainly in winter. Logistic modeling estimated size at 50% maturity (L50) at 126.7 mm carapace width in females. Results suggest that Loukkos Estuary functions primarily as a nursery and growth area for C. sapidus and provide essential baseline information for future monitoring and management of this invasive species.