Decline of flounder (Platichthys flesus (L.)) at the margin of the species' distribution rangeJournal of Sea Research


Henri Jokinen, Håkan Wennhage, Antti Lappalainen, Kaj Ådjers, Martti Rask, Alf Norkko
Aquatic Science / Ecology, Evolution, Behavior and Systematics / Oceanography


an ltura

Accepted 17 August 2015

Available online 22 August 2015 large-scale environmental change due to eutrophication, climate change, oversterblom et al., 2007). as indicators of enviElliot, 2002), marginal l due to their complex

Journal of Sea Research 105 (2015) 1–9

Contents lists available at ScienceDirect

Journal of Se e lsdients that determine themix of marine and freshwater species present (Bonsdorff, 2006; Nohrén et al., 2009; Olsson et al., 2012; Villnäs and

Norkko, 2011). Due to the demanding conditions, many species form life-cycle covering most of the coastal ecosystem compartments. However, a complex life-cycle also makes it difficult to disentangle the driving forces behind an observed seas, being exposed to persistent eutrophication, global climate change and intensive fishing (Elmgren, 2001; Korpinen et al., 2012;

Niiranen et al., 2013).

The brackish Baltic Sea is characterized by strong environmental gra(Ljunggren et al., 2010; Möllmann et al., 2009; Ö

Among the many advantages in considering fish ronmental and ecological change (Whitfield and flatfish populations reflect change especially welMany coastal seas are undergoing large-scale environmental change due to eutrophication and climate change combined with overexploitation and direct human-induced habitat destruction (Airoldi and Beck, 2007; Jackson et al., 2001; Lotze et al., 2006). These ongoing changes and the consequent degradation of essential habitats for fish have resulted in habitats that are no longer able to fulfill nursery, feeding and reproductive functions (Seitz et al., 2014; Thrush and Dayton, 2010). The Baltic Sea is no exception. It is one of themost affected coastEuropean flounder (Platichthys flesus (L.) — hereafter referred to as ‘flounder’) in the northern Baltic Sea is one such species.

Based on anecdotal evidence and commercial landings, flounder have shown signs of a recent decline in the northern Baltic Sea (Fig. 1). This development, together with concurrent changes also in other fish species (see Ådjers et al., 2006; Lappalainen et al., 2001;

Lehtonen et al., 2009; Köster et al., 2003; Nilsson et al., 2004), are likely to be symptoms of a larger ongoing shift in the Baltic Sea ecosystemmarginal populations with unique genes and these populations exceedingly valuable but ⁎ Corresponding author.

E-mail address: (H. Jokinen) 1385-1101/© 2015 Elsevier B.V. All rights reserved.pressure from environmental change (Johannesson and André, 2006;

Kirkpatrick and Barton, 1997; Lesica and Allendorf, 1995). The1. IntroductionKeywords:

European flounder

Population change


Marginal population

Environmental change

Northern Baltic SeaSigns of this unfavorable development are seen as population declines in species affected. We provide the first synthesis and characterization of the recent population decline in European flounder (Platichthys flesus (L.)) in the northern Baltic Sea, using available fishery-independent data from the Finnish coast, which is at the margin of the distribution range of this species. The objective was to document recent changes in the flounder population, identify the onset and quantify the magnitude of the change. The results showed substantial decreases in flounder numbers and biomass across the whole study area during recent decades. Based on the time series data available, flounder abundances declined by 46–97% from the 1990s to the 2000s, with top values in the mid-1990s and current low levels reached in the early 2000s. Additionally, signs of decreasing flounder size and condition were also observed. We discuss potential reasons for the development in terms of environmental change and fishing, and identify potential bottlenecks for populationmaintenance andmechanisms behind population change, thereby contributing to our general understanding of marginal flounder populations. © 2015 Elsevier B.V. All rights reserved.Received 18 September 2014

Received in revised form 5 August 2015 exploitation and habitat destruction, with subsequent degradation of living conditions for many fish species.

Article history: The Baltic Sea is undergoingDecline of flounder (Platichthys flesus (L.)) distribution range

Henri Jokinen a,⁎, Håkan Wennhage b, Antti Lappalaine a Tvärminne Zoological Station, University of Helsinki, Hanko, Finland b Department of Aquatic Resources, Institute of Marine Research, Swedish University of Agricu c Nature Resources Institute Finland (LUKE), Helsinki, Finland d Fiskeribyrån, Ålands Landskapsregering, Mariehamn, Åland, Finland e Nature Resources Institute Finland (LUKE), Jyväskylä, Finland f Marine Research Centre, Finnish Environment Institute (SYKE), Helsinki, Finland a b s t r a c ta r t i c l e i n f o j ourna l homepage: www.genotypes, which make also very susceptible to .t the margin of the species' c, Kaj Ådjers d, Martti Rask e, Alf Norkko a,f l Sciences, Lysekil, Sweden a Research ev ie r .com/ locate /searesPresent along the coastal areas of northeast Atlantic and the Mediterranean, the European flounder is also the most widely distributed flatfish species in the Baltic Sea (ICES, 2014). Flounder are well adapted to brackish conditions and found throughout the Baltic except for the

Bothnian Bay and the inner part of the Gulf of Finland, where salinity 2 H. Jokinen et al. / Journal of Sea Research 105 (2015) 1–9is too low (Nissling et al., 2002; Nissling and Dahlman, 2010). In the northern Baltic Sea, at the south and southwest coast of Finland, this species lives on the edge of its distributional range, but has nevertheless been one of the main marine coastal species in the area (Ådjers et al., 2006; Lappalainen et al., 2000). Flounder exhibit two different spawning strategies in the Baltic. In themore saline western and southern central partsflounders are offshore (pelagic) spawners,while flounders in the low saline northern and central areas, including the Finnish coast, are coastal (demersal) spawners. In contrast to the pelagic eggs, eggs from the demersal population are smaller and thicker, able to get