# Population Dynamics of the Widespread Alien Decapod Species, Brown Shrimp (Penaeus aztecus), in the Mediterranean Sea

## TL;DR

This study examines the population dynamics of brown shrimp in the Mediterranean, revealing faster female growth, seasonal spawning, and the impact of parasitism on reproduction.

## Contribution

The study provides the first detailed analysis of brown shrimp population dynamics in the Mediterranean, including growth, reproduction, and parasitism patterns.

## Key findings

- Females grow faster and dominate larger size classes, with all individuals ≥ 46 mm CL being female.
- Parasitism by Epipenaeon ingens reduces mature female density, with 42% of potential spawners failing to develop gonads.
- Turtle excluder devices improve trawl efficiency by reducing bycatch of nontarget species like loggerhead turtles.

## Abstract

This study focuses on the brown shrimp, an Atlantic species that has spread rapidly into the Mediterranean since its first discovery in 2009 and has become an economically important crustacean. Despite increased distribution, landings, and aquaculture efforts, comprehensive information on its population dynamics, such as reproduction, recruitment, age, growth, selectivity, and mortality, is still lacking. Length–frequency distributions over a 27-month period from surveys and commercial bottom trawls reveal sexual dimorphism, with faster growth observed in females. These results improve understanding of reproduction, spatiotemporal migration, growth, mortality dynamics, and trawl selectivity of brown shrimp, providing valuable insights for sustainable fisheries management in the Mediterranean.

This study investigated the population dynamics, growth, reproduction, and parasitism of Penaeus aztecus over a 27-month period in Antalya Bay (Eastern Mediterranean). P. aztecus was the most abundant shrimp species, comprising 53.4% of the collected shrimp specimens. Abundance varied seasonally, with peak densities in summer. Males had a smaller mean carapace length (CL) of 25.8 mm compared to females at 30.2 mm. Females dominated larger size classes, with all individuals ≥ 46 mm CL being female. The sex ratio was balanced at 1:1, with no seasonal variations. Spawning occurred year-round, peaking in June, late summer, and November. Females displayed four ovarian stages, with the first mature size (FMS) at 36 mm CL. Parasitism by Epipenaeon ingens reduced the density of mature females, with 42% of potential spawners failing to develop gonads. The prevalence of parasitism showed seasonal variation and was inversely correlated with sea surface temperature (SST). Marine recruitment occurred from June to November, with a peak between July and September. Growth analysis revealed faster rates in females, while males reached a smaller maximum size. This study also identified inefficiencies in gear selectivity, with many juvenile shrimp (below recruitment size) being retained. Trawl efficiency improved with the use of turtle excluder devices (TEDs), which reduced bycatch of nontarget species, such as loggerhead turtles and cartilaginous fishes. The instantaneous total mortality rate (Z) ranged from 0.658 to 0.026 month⁻1 for male shrimp and from 1.00 to 0.014 month⁻1 for female shrimp, with survival sharply declining after recruitment, leaving only about 3.6% of individuals surviving beyond 10 months.