How fish are connected in the Mediterranean

The Mediterranean Sea is a hotspot for marine biodiversity, which is supported by around 185 MPAs – most of which are situated in its north-western part. The EU-funded project 'Larval connectivity for effective MPAs networks: A multidisciplinary approach' (MAPACO) examined connectivity patterns among MPAs and unprotected sites in the region in order to build effective MPA networks.

Project partners used ecology, dispersal model and genetic conductivity methodologies to answer two critical questions. These were: which areas of the Mediterranean Sea receive fish larvae issued from an MPA and where do the fish observed in an MPA originate from?

The Cabo de Palos – Islas Hormigas Marine Reserve (CPMR) off the south-eastern coast of Spain was chosen as the study site. The species examined was striped red mullet (Mullus surmuletus), which is of high economic value and distributed across the entire Mediterranean Sea.

A dynamic model based on currents and temperature data was developed to simulate the dispersal of M. surmuletus larvae. Connectivity between fish populations was assessed using population genetics analyses based on 10 microsatellite markers. Researchers analysed a total of 900 fish samples from 9 areas of the western Mediterranean Sea.

The high number of samples used by MAPACO gave an excellent picture of the genetic diversity of the M. surmuletus population, which appeared highly homogenous. This demonstrated a considerable level of gene exchange between areas, achieved through larval dispersal.

Genetics and modelling results suggest that the M. surmuletus population in the western Mediterranean Sea is a single one. This was demonstrated by the high levels of exchange among individuals and genes between sites. This finding has significant consequences for future management, since fish larvae drift across the borders of different countries but fish stocks are managed locally.

The success of MAPACO resulted in a better understanding of the relationship between distinct coastal areas through their exchange of fish. This highlights the role of MPAs as a source of fish stocks for Mediterranean-based fisheries.