Reconstructing the Historical Density, Size, and Age Structure of the Noble Pen Shell (Pinna nobilis) Population: Insights from Malo Jezero Lagoon, Mljet National Park (Adriatic Sea)

Abstract

The noble pen shell (Pinna nobilis) is a key bivalve species found in the Mediterranean that has suffered dramatic declines due to mass mortality events (MMEs) caused by pathogens like Haplosporidium pinnae. This study looks at the historical population structure of P. nobilis in Malo jezero, a coastal lagoon in Mljet National Park, Croatia, using data collected before the MME. During a field survey in 2018, data on the population density, size, and age of 3800 individuals, using a grid-based transect method, were collected. The population density ranged from 7.50 to 55.83 individuals per 100 m2, with an average of 25.42 individuals per 100 m2, over 11 520 m2, reflecting a high abundance compared to other populations. All individuals were mature, with no juveniles or signs of recent recruitment. The estimated ages ranged from 8.00 to 44.34 years, with 20 individuals exceeding the expected maximum size. The population was comprised of older individuals, making it vulnerable to sudden events, due to a lack of younger individuals. The isolation of Malo jezero may limit larval exchange with other populations, contributing to recruitment challenges. This study provides important information for understanding P. nobilis populations and supports the potential of Malo jezero for future conservation and reintroduction efforts.