Causes and consequences of the round goby invasion in the Baltic Sea and beyond

Species invasions have increased rapidly over time and pose negative effects on ecosystems and societies worldwide. The round goby, one of the most widespread invasive fishes in the northern hemisphere, established in the Baltic Sea in 1990. My results show that the round goby, in contrast to most native species in the coastal fish community, exhibits territorial behaviour and parental care, which may have given it a competitive advantage and facilitated its establishment in the Baltic Sea. I found the major diet components of the round goby to be macroinvertebrates, but DNA metabarcoding revealed feeding also on several fish species, likely in their early life stages. Further, the round goby constituted prey for cod, perch and pike. When round goby was abundant in the environment, the reliance on fish prey increased for both round goby and its predators. As a potential consequence of round goby expansion to freshwater, I further studied round goby interactions with spawning salmon. Presence of round goby delayed salmon spawning, and male and female salmon resided closer together in the presence of higher densities of round goby compared to lower densities, which could be interpreted as a protective behaviour. In order to minimise the negative effects of the round goby in and around the Baltic Sea, I recommend management measures to reduce its abundances and prevent further spread, for example by strengthening native coastal predatory fish populations, developing a round goby fishery and developing a system for early detection and eradication of the round goby in rivers and streams

The devil is in the details: exploring how functionally distinct round goby is among native fish in the Baltic Sea

Understanding the characteristics and conditions that make non-indigenous species (NIS) successful at establishing in recipient communities is a key in determining their potential impacts on native species, as well as to improve management actions such as prevention of future invasions. The round goby (Neogobius melanostomus) is one of the most widespread non-indigenous fish species in the Northern Hemisphere, including the coastal zones of the Baltic Sea. The impacts of round goby in the Baltic Sea are pronounced and multifaceted, yet our knowledge regarding the underlying assembly processes determining its establishment is limited. To overcome this knowledge gap, we applied a trait-based approach to assess the degree of niche overlap and functional (trait) similarity between round goby and native fish species in coastal areas from the Baltic Sea, based on the functional distinctiveness metric. Our results show that round goby is generally quite similar (or not dissimilar) to the native fish of the regional species pool, at least in terms of its overall trait composition. Conversely, round goby demonstrates pronounced differences compared to the native community in its display of parental care and territorial behaviour. Such differences in individual traits could play an important role in round goby’s invasion success in the Baltic Sea, including its interactions with native species (e.g. competition). Our results and their potential implications may be highly relevant for conservation and management if integrated within existing risk assessment tools for biological invasions in order to prioritise and enhance the effectiveness of preventative actions towards the expansion of round goby

Environmental abundances of the non-native round goby Neogobius melanostomus influence feeding of native fish predators

The authors assessed the importance of the round goby Neogobius melanostomus as prey for three native predatory fish species, Atlantic cod Gadus morhua, European perch Perca fluviatilis and northern pike Esox lucius, in a northern and southern area of the Baltic Proper, using a combination of visual analysis and DNA metabarcoding of predator stomach contents. To explore the influence of environmental abundances of N. melanostomus on predation, they related the occurrence of N. melanostomus in predator diets to its abundance in survey fishing. Gadus morhua and E. lucius in the southern area showed the highest tendency to feed on N. melanostomus when it was abundant, as N. melanostomus occurred in up to 100% of stomachs and constituted up to 88% of the total diet volume proportion. The diet contribution of N. melanostomus was associated with N. melanostomus abundances for G. morhua and E. lucius, and when N. melanostomus was abundant, these predators exhibited lower prey richness and a higher degree of piscivory. G. morhua and P. fluviatilis also fed less on crustacean prey when N. melanostomus was abundant. The high importance of N. melanostomus in diets of native fish predators may modify indirect interactions between N. melanostomus and native prey species in invaded coastal communities

New molecular methods to assess biodiversity. Potentials and pitfalls of DNA metabarcoding: a workshop report

This report presents the outcome of the joint work of PhD students and senior researchers working with DNA-based biodiversity assessment approaches with the goal to facilitate others the access to definitions and explanations about novel DNA-based methods. The work was performed during a PhD course (SLU PNS0169) at the Swedish University of Agricultural Sciences (SLU) in Uppsala, Sweden. The course was co-organized by the EU COST research network DNAqua-Net and the SLU Research Schools Focus on Soils and Water (FoSW) and Ecology - basics and applications. DNAqua-Net (COST Action CA15219, 2016-2020) is a network connecting researchers, water managers, politicians and other stakeholders with the aim to develop new genetic tools for bioassessment of aquatic ecosystems in Europe and beyond. The PhD course offered a comprehensive overview of the paradigm shift from traditional morphology-based species identification to novel identification approaches based on molecular markers. We covered the use of molecular tools in both basic research and applied use with a focus on aquatic ecosystem assessment, from species collection to the use of diversity in environmental legislation. The focus of the course was on DNA (meta)barcoding and aquatic organisms. The knowledge gained was shared with the general public by creating Wikipedia pages and through this collaborative Open Access publication, co-authored by all course participants

Why are fish in the Baltic Sea so small? : A study of somatic and gonad growth in relation to salinity in turbot (Scophthalmus maximus)

It has been shown that fish of both marine and limnetic origin display increased growth at intermediate salinities. Furthermore, it has been shown that fish in the brackish water Baltic Sea are smaller compared to their conspecifics in the Atlantic, where salinities are higher. Also, it has been suggested that fish produce more eggs at the edges of their distribution range as a response to inferior environmental conditions. In this study, I investigated if there is a trade-off in energy investment between somatic and gonad growth in relation to salinity. To do this, I performed a growth experiment and a literature review. In the growth experiment, juvenile turbot were reared in salinities of 6, 10.5, 15 and 30 ‰. I found that turbot juveniles from Gotland grew equally well in all salinities investigated. In the literature review, data from the Baltic Sea was tested against data from the North and Black Seas. Data of turbot total energy investment (somatic and gonad growth) was analyzed. I found that energy content at age differed significantly between the populations investigated with lower energy content for the Baltic Sea populations. Also, growth rate in relation to energy content (size) was analyzed for the different populations, but no difference for growth rate in relation to energy content occurred. The result of the analysis of growth rate indicates that the change in allocated energy is the same, regardless of population, and thus that fish from the Baltic Sea display growth rates similar to those of other populations. It was also established that energy investment in gonads increased along with decreasing salinities. The smaller size of turbot in the Baltic Sea is therefore probably the result of a difference in size at maturity, possibly because less energy is allocated to somatic growth and more energy to start producing eggs. It is probably also the consequence of that the Baltic Sea turbot, post sexual maturity, continue to invest more energy in egg production

Why are fish in the Baltic Sea so small? : A study of somatic and gonad growth in relation to salinity in turbot (Scophthalmus maximus)

It has been shown that fish of both marine and limnetic origin display increased growth at intermediate salinities. Furthermore, it has been shown that fish in the brackish water Baltic Sea are smaller compared to their conspecifics in the Atlantic, where salinities are higher. Also, it has been suggested that fish produce more eggs at the edges of their distribution range as a response to inferior environmental conditions. In this study, I investigated if there is a trade-off in energy investment between somatic and gonad growth in relation to salinity. To do this, I performed a growth experiment and a literature review. In the growth experiment, juvenile turbot were reared in salinities of 6, 10.5, 15 and 30 ‰. I found that turbot juveniles from Gotland grew equally well in all salinities investigated. In the literature review, data from the Baltic Sea was tested against data from the North and Black Seas. Data of turbot total energy investment (somatic and gonad growth) was analyzed. I found that energy content at age differed significantly between the populations investigated with lower energy content for the Baltic Sea populations. Also, growth rate in relation to energy content (size) was analyzed for the different populations, but no difference for growth rate in relation to energy content occurred. The result of the analysis of growth rate indicates that the change in allocated energy is the same, regardless of population, and thus that fish from the Baltic Sea display growth rates similar to those of other populations. It was also established that energy investment in gonads increased along with decreasing salinities. The smaller size of turbot in the Baltic Sea is therefore probably the result of a difference in size at maturity, possibly because less energy is allocated to somatic growth and more energy to start producing eggs. It is probably also the consequence of that the Baltic Sea turbot, post sexual maturity, continue to invest more energy in egg production

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

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

Swimming behavior and mortality of the indigenous amphipod Monoporeia affinis in presence of the invasive polychaete Marenzelleria spp. in the Baltic Sea

The Monoporeia affinis population has declined drastically in the Baltic Sea since 1999. In the late 1980’s, an invasive polychaete, Marenzelleria spp., established in the Baltic Sea. This experiment aimed to investigate if M. affinis’ swimming behavior and mortality was affected by competition from Marenzelleria spp., and if so the competition was size- or densitydependent. One control series without Marenzelleria spp. and four series with different sizes and densities of Marenzelleria spp. were studied during 20 days, to determine potential effects of Marenzelleria spp.  No statistically significant results were found neither for differences in swimming behavior between treatments, or in amphipod mortality between treatments. However, a statistically significant difference in swimming behavior over time within treatments was found in one treatment. The experiments did not show any negative impact on M. affinis from the presence of Marenzelleria spp

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

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

Swimming behavior and mortality of the indigenous amphipod Monoporeia affinis in presence of the invasive polychaete Marenzelleria spp. in the Baltic Sea

The Monoporeia affinis population has declined drastically in the Baltic Sea since 1999. In the late 1980’s, an invasive polychaete, Marenzelleria spp., established in the Baltic Sea. This experiment aimed to investigate if M. affinis’ swimming behavior and mortality was affected by competition from Marenzelleria spp., and if so the competition was size- or densitydependent. One control series without Marenzelleria spp. and four series with different sizes and densities of Marenzelleria spp. were studied during 20 days, to determine potential effects of Marenzelleria spp.  No statistically significant results were found neither for differences in swimming behavior between treatments, or in amphipod mortality between treatments. However, a statistically significant difference in swimming behavior over time within treatments was found in one treatment. The experiments did not show any negative impact on M. affinis from the presence of Marenzelleria spp