Opportunities for hybridization between two sympatric flounder (Platichthys flesus) ecotypes in the Baltic Sea

Abstract: The Baltic Sea flounder is separated into two ecotypes with different reproductive characteristics, one spawning off-shore at higher salinities and one spawning in coastal areas at lower salinities. However, coastal spawning flounder have also been found off-shore during the spawning season, especially in cold years, suggesting that the more stable temperatures in off-shore waters are more suitable for reproduction than in coastal areas during cold years. The occasional co-occurrence of both ecotypes during spawning indicates that hybridization may take place. My aim was to investigate whether low temperatures have negative impact on coastal spawning flounder viable hatch and larval size, i.e. may trigger coastal spawners tospawn off-shore in cold years, and if hybridization between the two ecotypes is possible. My results showed that egg and larval development, viable hatch and larval size was adversely affected at low temperatures (<4 °C). I also found that hybridization between the two ecotypes is indeed possible, as viable hatch and larval size did not differ between hybrids and non-hybrids. Therefore, I concluded that low temperatures could be an environmental driving force for the event of hybridization to take place. When putting my results in a broadercontext, beach seine surveys 2012-2015 indicated that flounder recruitment was successful in 2012 and 2014; both years with intermediate temperatures. However, recruitment for both ecotypes seems to have failed almost completely in 2013 when temperatures during spawning were low (compared to in 2012, 2014 and 2015) and 2015, when temperatures were above average. This indicates that temperatures may have been insufficient for successful recruitment, resulting in very few settled 0-gr flounder in general. Hence, I discuss the possibility that temperatures outside the normal range cause a mismatch with preferred food items, limiting food availability. In conclusion, low temperatures potentially lead to a higher number of hybridization events, but recruitment appears to be unsuccessful in years with temperatures above or below average, thus reducing the chances of finding hybrid individuals in the Baltic Sea.