Salinity Gradient of the Baltic Sea Limits the Reproduction and Population Expansion of the Newly Invaded Comb Jelly Mnemiopsis leidyi

The recent invasion of the comb jelly Mnemiopsis leidyi into northern European waters is of major public and scientific concern. One of the key features making M. leidyi a successful invader is its high fecundity combined with fast growth rates. However, little is known about physiological limitations to its reproduction and consequent possible abiotic restrictions to its dispersal. To evaluate the invasion potential of M. leidyi into the brackish Baltic Sea we studied in situ egg production rates in different regions and at different salinities in the laboratory, representing the salinity gradient of the Baltic Sea. During October 2009 M. leidyi actively reproduced over large areas of the Baltic Sea. Egg production rates scaled with animal size but decreased significantly with decreasing salinity, both in the field (7–29) and in laboratory experiments (6–33). Temperature and zooplankton, i.e. food abundance, could not explain the observed differences. Reproduction rates at conditions representing the Kattegat, south western and central Baltic Sea, respectively, were 2.8 fold higher at the highest salinities (33 and 25) than at intermediate salinities (10 and 15) and 21 times higher compared from intermediate to the lowest salinity tested (6). Higher salinity areas such as the Kattegat, and to a lower extent the south western Baltic, seem to act as source regions for the M. leidyi population in the central Baltic Sea where a self-sustaining population, due to the low salinity, cannot be maintained

Seasonal changes and population dynamics of the ctenophore Mnemiopsis leidyi after its first year of invasion in the Kiel Fjord, Western Baltic Sea

We analyzed the seasonal variations of the ctenophore Mnemiopsis leidyi weekly collected since its first record in the western Baltic Sea in October 2006. The distribution pattern together with the seasonal dynamics and population outbreaks in late summer 2007 indicate recent successfully establishment of M. leidyi in this area. Seasonal changes showed two periods of high reproductive activity characterized by a population structure dominated by small size classes, followed by an increase of larger ones. These results further revealed that the bulk of the population remains in deep layers during the periods of low population density, whereas it appeared situated in upper layers during the proliferation of the species. We further emphasized the strength of the population outbreaks, which can reach abundances >10-fold higher in time periods shorter than a week. The predatory impact this species may have in pelagic ecosystems warns on the importance of its recent range of expansion

Potential pathways of invasion and dispersal of Mnemiopsis leidyi A. Agassiz 1865 in the Baltic Sea

The rapid spread of Mnemiopsis leidyi across the entire Baltic Sea after its first observation in 2006 gave rise to the question of its invasion pathway and the possible vector of its transport. To investigate pathways of M. leidyi invasion, the years 2005–2008 have been simulated by a three-dimensional coupled sea ice-ocean model of the Baltic Sea. In addition, a Lagrangian particle-tracking model has been utilized to test possible transport routes of this invader for 2006/2007. Based on the model, we exclude advection from the Kattegat as the main area of origin of M. leidyi and further spreading through the entire Baltic Sea. To explain the dispersion of M. leidyi in 2007 an earlier invasion already in 2005 is most probable. Alternatively, an invasion originating from main harbors with high ship traffic could also be a potential pathway. Drift simulations with drifter release in the main harbors are in good agreement with the observed distribution pattern of M. leidyi

Ocean current connectivity propelling the secondary spread of a marine invasive comb jelly across western Eurasia

Aim: Invasive species are of increasing global concern. Nevertheless, the mechanisms driving furtherdistribution after the initial establishment of non-native species remain largely unresolved, especiallyin marine systems. Ocean currents can be a major driver governing range occupancy, but this hasnot been accounted for in most invasion ecology studies so far. We investigate how well initialestablishment areas are interconnected to later occupancy regions to test for the potential role ofocean currents driving secondary spread dynamics in order to infer invasion corridors and thesource–sink dynamics of a non-native holoplanktonic biological probe species on a continental scale.Location: Western Eurasia.Time period: 1980s–2016.Major taxa studied: ‘Comb jelly’ Mnemiopsis leidyi.Methods: Based on 12,400 geo-referenced occurrence data, we reconstruct the invasion historyof M. leidyi in western Eurasia. We model ocean currents and calculate their stability to match thetemporal and spatial spread dynamics with large-scale connectivity patterns via ocean currents.Additionally, genetic markers are used to test the predicted connectivity between subpopulations.Results: Ocean currents can explain secondary spread dynamics, matching observed range expansionsand the timing of first occurrence of our holoplanktonic non-native biological probe species,leading to invasion corridors in western Eurasia. In northern Europe, regional extinctions after coldwinters were followed by rapid recolonizations at a speed of up to 2,000 km per season. SourceJASPERS ET AL. | 815areas hosting year-round populations in highly interconnected regions can re-seed genotypes overlarge distances after local extinctions.Main conclusions: Although the release of ballast water from container ships may contribute tothe dispersal of non-native species, our results highlight the importance of ocean currents drivingsecondary spread dynamics. Highly interconnected areas hosting invasive species are crucial forsecondary spread dynamics on a continental scale. Invasion risk assessments should considerlarge-scale connectivity patterns and the potential source regions of non-native marine species

The influence of temperature on the development of Baltic Sea sprat (Sprattus sprattus) eggs and yolk sac larvae

In spring 2004 and 2005 we performed two sets of experiments with Baltic sprat (Sprattus sprattus balticus Schneider) eggs and larvae from the Bornholm Basin simulating ten different temperature scenarios. The goal of the present study was to analyse and parameterise temperature effects on the duration of developmental stages, on the timing of important ontogenetic transitions, growth during the yolk sac phase as well as on the survival success of eggs and early larval stages. Egg development and hatching showed exponential temperature dependence. No hatching was observed above 14.7°C and hatching success was significantly reduced below 3.4°C. Time to eye pigmentation, as a proxy for mouth gape opening, decreased with increasing temperatures from 17 days post hatch at 3.4°C to 7 days at 13°C whereas the larval yolk sac phase was shortened from 20 to 10 days at 3.8 and 10°C respectively. Maximum survival duration of non-fed larvae was 25 days at 6.8°C. Comparing the experimental results of Baltic sprat with existing information on sprat from the English Channel and North Sea differences were detected in egg development rate, thermal adaptation and in yolk sac depletion rate (YSDR). Sprat eggs from the English Channel showed significantly faster development and the potential to develop at temperatures higher than 14.7°C. North Sea sprat larvae were found to have a lower YSDR compared to larvae from the Baltic Sea. In light of the predictions for global warming, Baltic sprat stocks could experience improved conditions for egg development and surviva

Ecological commonalities among pelagic fishes: comparison of freshwater ciscoes and marine herring and sprat

Systematic comparisons of the ecology between functionally similar fish species from freshwater and marine aquatic systems are surprisingly rare. Here, we discuss commonalities and differences in evolutionary history, population genetics, reproduction and life history, ecological interactions, behavioural ecology and physiological ecology of temperate and Arctic freshwater coregonids (vendace and ciscoes, Coregonus spp.) and marine clupeids (herring, Clupea harengus, and sprat, Sprattus sprattus). We further elucidate potential effects of climate warming on these groups of fish based on the ecological features of coregonids and clupeids documented in the previous parts of the review. These freshwater and marine fishes share a surprisingly high number of similarities. Both groups are relatively short-lived, pelagic planktivorous fishes. The genetic differentiation of local populations is weak and seems to be in part correlated to an astonishing variability of spawning times. The discrete thermal window of each species influences habitat use, diel vertical migrations and supposedly also life history variations. Complex life cycles and preference for cool or cold water make all species vulnerable to the effects of global warming. It is suggested that future research on the functional interdependence between spawning time, life history characteristics, thermal windows and genetic differentiation may profit from a systematic comparison of the patterns found in either coregonids or clupeids

Ocean current connectivity propelling the secondary spread of a marine invasive comb jelly across western Eurasia

Publication history: Accepted - 15 February 2018; Published - 16 May 2018.Aim: Invasive species are of increasing global concern. Nevertheless, the mechanisms driving further distribution after the initial establishment of non-native species remain largely unresolved, especially in marine systems. Ocean currents can be a major driver governing range occupancy, but this has not been accounted for in most invasion ecology studies so far. We investigate how well initial establishment areas are interconnected to later occupancy regions to test for the potential role of ocean currents driving secondary spread dynamics in order to infer invasion corridors and the source–sink dynamics of a non-native holoplanktonic biological probe species on a continental scale. Location: Western Eurasia. Time period: 1980s–2016. Major taxa studied: ‘Comb jelly’ Mnemiopsis leidyi. Methods: Based on 12,400 geo-referenced occurrence data, we reconstruct the invasion history of M. leidyi in western Eurasia. We model ocean currents and calculate their stability to match the temporal and spatial spread dynamics with large-scale connectivity patterns via ocean currents. Additionally, genetic markers are used to test the predicted connectivity between subpopulations. Results: Ocean currents can explain secondary spread dynamics, matching observed range expansions and the timing of first occurrence of our holoplanktonic non-native biological probe species, leading to invasion corridors in western Eurasia. In northern Europe, regional extinctions after cold winters were followed by rapid recolonizations at a speed of up to 2,000 km per season. Source areas hosting year-round populations in highly interconnected regions can re-seed genotypes over large distances after local extinctions. Main conclusions: Although the release of ballast water from container ships may contribute to the dispersal of non-native species, our results highlight the importance of ocean currents driving secondary spread dynamics. Highly interconnected areas hosting invasive species are crucial for secondary spread dynamics on a continental scale. Invasion risk assessments should consider large-scale connectivity patterns and the potential source regions of non-native marine species.Danish Council for Independent Research; Grant/Award Number: DFF-1325-00102B; FP7 People: Marie-Curie Actions, Grant/Award Number: MOBILEX, DFF - 1325-00025; EU, BONUS, BMBF, Grant/ Award Number: 03F0682; Excellence Cluster “Future Ocean”, Grant/Award Number: CP153

Seasonal variability of fecundity and spawning dynamics of Baltic sprat

11 páginas, 11 figuras, 5 tablas.-- PreprintSeasonal variability in fecundity has been observed in a number of marine fishes and is important towards understanding the reproductive potential of a fish stock. However, the seasonal dynamics of egg production of Baltic sprat (Sprattus sprattus balticus S) have not been well described to date. We present data on the timing of spawning, and the seasonal variability in batch fecundity, number of developing oocytes, oocyte dry weight, spawning fraction, fish condition and atresia for this species in the Bornholm Basin, Baltic Sea. Histological techniques in combination with image analysis were applied to investigate those variables based upon material sampled in 2005 and 2008. Sprat were reported to be in spawning condition from January to June in each year plus in 2008 signs of ovarian maturation were also observed in November. Relative batch fecundity was found to vary two-fold with 85 eggs g−1 ovary free body weight observed early in the spawning season (January 2005) and 165 eggs g−1 ovary free body weight late in the spawning season (June 2008). Variability in batch fecundity during peak spawning was rather low. A seasonal decrease in oocyte dry weight was related to an increase in batch fecundity. Spawning fraction varied over the course of the spawning period with values ranging from 0.29 in March to 0.18 in June. Stereometric analyses confirmed the indeterminate fecundity of Baltic sprat. Prevalence of atresia was low during peak spawning in April to June (1.0–4.0%) but considerably higher during the early spawning period in March (16.4%) and highest in November (38.5%). Female sprat condition was low during the spawning period and increased sharply after spawning ceased. Our study provides a better understanding of fecundity and spawning dynamics of Baltic sprat which will aid to improve the assessment of reproductive potential of this ecologically and economically important fish species.This study was partly conducted during a Short Term Scientific Mission (STSM) at the Institute of Marine Research Vigo, Spain (IIM-CSIC) supported by the COST action Fish Reproduction and Fisheries (FRESH, FA0601). This research was also supported by the German Science Foundation (DFG) cluster project RECONN2 #CL126/3-3 (Resolving Trophodynamic Consequences of Climate Change) within the priority program 1162 AQUASHIFT.Peer reviewe