Modelling of larval dispersal of Baltic flounder (Platichthys solemdali) revealed drifting depth as a major factor determining opportunities for local retention vs large-scale connectivity

Abstract

For flatfishes, transportation of larvae to nursery areas is regarded a key mechanism for recruitment, with adaptations in larval behaviour to reach a suitable habitat. Here we model different possibilities of larval drifting of coastal spawning Baltic flounder Platichthys solemdali (recently identified as a species from European flounder P. flesus) to reveal opportunities for local retention vs large-scale dispersal to ensure settling in coastal nursery areas. Drifting depth, duration of drifting and effects of year and time during season were modelled using 1) a high-resolution local dispersal model, and 2) a large-scale connectivity database. The outcome revealed drifting depth as a major factor affecting larval dispersal.Drift at 10-22 m depth involved retention along the coast with the majority of larvae (≥94% or 69-93% according to 1 and 2, respectively) with end points ≤20 km from the coast enabling further successful migration to nursery habitats. Contrary, larval drift close to the surface resulted in advection with end points in the open sea (72-76%), i.e. loss of larvae, but with a small fraction (5-12%) displaying cross-basin connectivity. The results suggest, in agreement with depth distribution of spawning, a larval behaviour promoting drift in the lower part of the water mass, favouring retention close to coastal nursery areas. Obtained dispersal patterns may sustain both local recruitment but also connectivity with other areas, potentially explaining the low genetic diversity between areas for P. solemdali. Low interannual variability in dispersal patterns when drifting at 10-22 m depth suggests that larval drift is not a major bottleneck explaining recruitment variability in P. solemdali in the area.The study highlights the differences in life-history strategies of the species pair of flounder in the Baltic Sea; P. flesus spawning in the deep basins with extensive larval dispersal, and coastal spawning P. solemdali with, according to the model outcome, mainly local larval dispersal for sustaining a viable population, i.e. request for different management strategies.

Ecological connectivity of the marine protected area network in the Baltic Sea, Kattegat and Skagerrak: Current knowledge and management needs

“…Furthermore, local adaptations particularly prominent in the two endemic species, Baltic flounder Platichthys solemdali and the brown algae Fucus radicans , result in contrasting dispersal and connectivity patterns in the two different organismal groups (Momigliano et al 2018 ). The Baltic flounder has developed heavier eggs, different larval behaviour and spawn in coastal areas allowing shorter dispersal compared to its closest relative European flounder (Nissling et al 2017 ; Corell and Nissling 2019 ). In contrast, F. radicans relies on asexual reproduction and disperses farther than its closest relative bladderwrack, and therefore has a more northern distribution in the Bothnian Sea where the population is represented by one dominant clone (Bergström et al 2005 ; Tatarenkov et al 2005 ; Ardehed et al 2015 ).…”