| 1. |
- Ehrenberg, Sigrid Z., 1964-
(författare)
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The neglected ecology of the sand goby: Abundance and feeding in the Baltic sublittoral
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- I investigated the ecology and population density of the sand goby, Pomatoschistus minutus, at sub-thermocline depth in the north-western Baltic proper. Most previous research on the sand goby, and the co-occurring closely related common goby, P. microps, has focused on their stay in shallow waters, while important parts of their life cycles have been largely neglected.Photography was used to assess goby population density and size distribution, a basis for estimations of the potential annual food consumption of the goby community. Diet analysis of the most abundant goby, the sand goby, was combined with the population censuses to estimate the contribution of different prey to the goby consumption. Both benthic and pelagic prey were consumed, including two non-indigenous species. The amphipod Monoporeia affinis was by far the most important prey for the overall consumption, while pelagic and benthic copepods, and naidid oligochaetes were important prey for sand gobies < 3.5 cm. Goby predation is likely to influence the populations of naidid oligochaetes and the amphipod M. affinis, as large parts of their production may be consumed.The potential goby consumption was calculated to be about 72 kJ m-2 yr-1, which corresponds to 14-60 % of the annual production on soft bottoms in the area. In a laboratory study of the sand goby activity, active swimming occurred preferentially at night and burying in the sediment mostly at day. Both behaviours may lead to underestimates in photographic censuses of up to 40 %. The goby community in the sublittoral links the benthic and pelagic sub-systems in the area by being able to remove a substantial amount of energy from the benthic production, by inclusion of pelagic prey, and by holding an intermediate position in the coastal marine food web of the Baltic Sea. Consequently, their role in the flow of energy, elements and substances, some potentially harmful, between the pelagic and benthic ecosystem should be further investigated.
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| 2. |
- Holliland, Per B., 1984-
(författare)
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Trophic interactions and behaviour : Studies relevant to a Baltic Sea biomanipulation
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The main theme of this thesis is the interactions of animals with the environment and each other. The thesis was written within the framework of a biomanipulation project “Pikeperch in Himmerfjärden”. With the aim to investigate possible trophic pit-falls, give the manipulation the best possible start, and find ways to monitor the progression of the manipulation. In Paper I the diet of the invader cladoceran Cercopagis pengoi is analysed with stable isotopes; conducted prior to stocking. C.pengoi has a preference for large copepods, indicating possible competition with fish. Paper II investigates the behavioural differences between pikeperch fingerlings reared in different environments (pond vs. tank). Results suggest that fish reared in semi-natural ponds are more likely to survive directly after stocking. In Paper III and IV, the diel vertical migrations (DVM) of copepods are in focus. In Paper III the migrations of two copepod species: Acartia spp. and Eurytemora affinis are studied over season and life stage. The amplitude of migration was found to increase with ontogeny for both species, indicating evasion of visual predators. Paper IV examines the varying migratory patterns of adult female E. affinis finding that these animals migrate more actively when feeding conditions deteriorate and growth decreases. The overall conclusions of the thesis are that behavioural, not only direct trophic interactions are key when studying ecosystems.
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| 3. |
- Martinsson, Jesper, 1981-
(författare)
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Ecology of juvenile turbot and flounder in the Central Baltic Sea : Implications for recruitment
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Our understanding of turbot and flounder ecology in the Baltic Sea is insufficient for sound management decisions. This thesis aims to fill some gaps in current knowledge by providing information of the ecology of turbot and flounder within their juvenile habitat, and to relate these findings to issues assumed relevant for recruitment variation. Main focus is on turbot due to its relatively low abundance and high variability in recruitment. The distribution of both species was studied on different scales, as was environmental effects on food consumption in 0-group turbot. The 0-group turbot display a relatively restricted spatial distribution compared to flounder. This is possibly due to a more specialized diet, which may make them more vulnerable to habitat degradation, especially eutrophication as a strong negative correlation was found with the organic content in the sediment. The species show high temporal and spatial overlap when settling in July-September, with peak abundances in August, and at depths <1 m. Both species display sedentary behavior within the nursery ground. Compared to flounder, turbot was more mobile potentially due to its restricted diet calling for extended searches. For turbot, feeding conditions appear to vary between size groups, which potentially could cause variations in survival between years through size-selective mortality. But, the predation may be low in central Baltic Sea as the abundance of the main predator, brown shrimp are comparatively low during flatfish occupancy. A significant positive relationship was found between the recruitment of turbot and flounder, which suggests that no inter-specific interactions during the juvenile stage affect recruitment. This co-variation also suggests that the recruitment of the species is determined by the same phenomena, potentially by large scale abiotic factors during the egg- and larval stage. For turbot, additional variability is potentially generated during the juvenile stage due to its relatively restricted food and habitat requirements. The specific habitat demands of turbot revealed in this thesis may be used to protect and restore essentially nursery grounds.
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| 4. |
- Ahlbeck, Ida, 1981-
(författare)
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Living in a predation matrix : Studies on fish and their prey in a Baltic Sea coastal area
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis was written within the framework of a biomanipulation project where young-of-the-year (YOY) pikeperch (Sander lucioperca) were stocked to a Baltic Sea bay to improve water quality through a top-down trophic cascade. The aim of my doctorial studies was however focused on a broader ecological question, namely predation (the main driving force in a biomanipulation). Hence, this thesis consists of four papers where we study the interactions between predator and prey using fish and zooplankton and how these interactions can be measured. In paper I we evaluated the performance of different diet analysis methods by individual based modelling and found that when having a nutritional gain perspective, mass based methods described diets best. Paper II investigated how the explorative, foraging and anti-predator behaviour of the YOY pikeperch used for stocking were affected by their rearing environment (pond vs. tank rearing). The more complex and varied environment in the semi-natural ponds seemed to promote a more flexible and active behaviour, better equipping young fish for survival in the wild. For paper III we studied the diel vertical migration in the six copepodite stages of the zooplankton Acartia spp. and Eurytemora affinis in relation to fish biomass, phytoplankton abundance and temperature. Both species migrated and in addition showed increased migration range with size within species, indicating evasion from visual predators. Paper IV addressed the movement of littoral Eurasian perch (Perca fluviatilis) via stable isotope signatures (13C and 15N) and body condition. We found clear indications of sedentarity and intra-habitat dietary differences. Interactions between predators and prey are complex and affected by both physiological and environmental characteristics as well as behavioural traits. The results in this thesis suggest that different species and even different life stages pursue different strategies to survive.
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| 5. |
- Axenrot, Thomas, 1954-
(författare)
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Pelagic Fish Distribution and Dynamics in Coastal Areas in the Baltic Sea Proper
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Pelagic fish distribution and diel behaviour patterns were studied in coastal areas in the north-western Baltic Sea Proper to understand more about how fish distribution and behaviour might affect planning and analyses of results of hydroacoustic surveys (Papers I and II). The vertical distribution of fish at night from spring to autumn showed seasonal and annual trends that could be explained by predictable and consistent seasonal changes, e.g., in temperature and stratification. Horizontal fish distributions did not show any trends probably owing to a lack of such seasonal characteristics. The observed vertical fish distribution over the diel cycle showed that hydroacoustic surveys at night were to be preferred over daytime surveys. At night, fish did not school and were generally less aggregated resulting in less variable hydroacoustic backscattering values and a higher percentage of single echo detections. By starting the surveys one hour after sunset and stopping one hour before sunrise, confusion between day- and nighttime behaviour in fish could be avoided. At night, fish occupied mid-water layers to a higher extent than surface and bottom layers, which was beneficial for the quality of the hydroacoustic data, particularly with respect to the hydroacoustic blind and dead zones (i.e. surface and bottom, respectively).To quantify seasonal changes in pelagic fish abundance, densities and size distributions, nighttime hydroacoustic surveys were done every second week from spring through autumn in 2000 and 2001 (Paper III). There was a drastic increase in fish abundance and densities that started in early July and peaked in mid-August in both years. Analyses of the hydroacoustic data in relation to gillnet and trawl catches showed that the increase was caused mainly by young-of-the-year (YOY) herring. This age class is commonly not well represented in catches using traditional sampling methods like gillnets and trawling. Consequently, hydroacoustic data that have high precision and accuracy may improve quantitative estimates and our understanding of the biology in coastal nursery areas.Baltic herring spawn in coastal areas and the density of metamorphosed YOY individuals may provide an early estimate of year-class strength. By analysing the relationship between parameters known to affect recruitment success and year-class strength in age 2 herring (YCS) a model that predicted herring recruitment was developed (Paper IV). The model explained 93 % of the variation in the number of age 2 herring over the period 1985-2000 and included the parameters YOY densities, climate (North Atlantic Oscillation index) and spawning stock biomass (SSB). Thus YCS could be predicted two years earlier than today and three years before entering the fishery. Up to the present, three new years (2001-2003) have become available for testing the model. For one of these years the predicted YCS was notably different from the assessed YCS. The reason for this is not fully understood, but for all three years SSB was outside the range used in the original model. Including the three new years into the data series resulted in a poorer explanation of the observed recruitment variation (55 %). A comparison of the standardized regression coefficients of both models showed increased significance for the parameter YOY (from 0.47 to 0.61).
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| 6. |
- Hogfors, Hedvig, 1980-
(författare)
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Summer cyanobacterial blooms in the Baltic Sea - implications for copepod recruitment
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- During summer, the Baltic Sea is subjected to the world’s largest cyanobacterial blooms. These blooms are linked to eutrophication and raise many questions concerning their effects on the ecosystem. To understand their impacts on the food web dynamics, it is essential to assess growth responses of grazers to these cyanobacteria. In the northern Baltic proper, copepods are the most important herbivores providing an essential link between the primary producers and higher trophic levels. In this Thesis, Papers I & II evaluate methods to estimate copepod growth in response to feeding conditions in situ. The most conspicuous diazotrophic filamentous cyanobacterium in the Baltic Sea is Nodularia spumigena, a producer of nodularin which is highly toxic to vertebrates, yet its ecological role is largely unknown. In Paper III, reciprocal interactions between cyanobacteria, sympatric algae and copepods are studied. The results suggest that nodularin is likely involved in allelopathic interactions, but it is not an inducible defense against grazers. Furthermore, the results of Papers IV & V, indicate that natural assemblages of N. spumigena and Anabaena spp. may support copepod reproduction and that total diazotrophic filamentous cyanobacteria appear to provide a beneficial feeding environment for the feeding stages of copepod nauplii, most probably by stimulating the microbial communities that nauplii feed upon. Since cyanobacterial blooms are projected to increase due to global climate change, the combined effects of toxic cyanobacteria, ocean acidification and global warming predicted for year 2100 are further investigated on copepods in Paper IV. Taken together, these studies indicate that filamentous diazotrophic cyanobacteria contribute to sustaining secondary productivity and have potential implications of management practices with respect to combating eutrophication, global climate change and sustaining fish feeding conditions.
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| 7. |
- Ogonowski, Martin, 1980-
(författare)
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Studies on Baltic Sea mysids
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Mysid shrimps (Mysidacea, Crustacea) are efficient zooplanktivores in both marine and freshwater systems as well as lipid rich prey for many species of fish. Although some efforts have been made to study the role of mysids in the Baltic Sea, very few studies have been carried out in recent time and there are still knowledge gaps regarding various aspects of mysid ecology. This thesis aims to explore some of these gaps by covering a mixture of topics. Using multifrequency hydroacoustics we explored the possibility to separate mysids from fish echoes and successfully established a promising and effective method for obtaining mysid abundance/biomass estimates (paper I). An investigation of the current mysid community in a coastal area of the northern Baltic proper (paper II) demonstrated that the formerly dominant, pelagic mysid Mysis mixta had decreased substantially (~50%) in favor for phytoplanktivorous, juvenile Neomysis integer and Mysis relicta sp. By examining different aspects of mysid behavior, we studied the vertical size distribution of mysids in the field and found that size increased with depth/declining light, irrespective of temperature; indicating that their vertical size distribution primarily is a response to predation (paper II). In paper III, a combination of ecological and genetic markers was used to investigate intraspecific differences in migratory tendency. Both marker types indicated that some part of the Mysis salemaai population is sedentary on the bottom and that this strategy is a phenotypically plastic but persistent trait, analogous to the partial migrations seen in many birds and fishes. In paper IV a temperature and weight specific respiration model was developed for the littoral Praunus flexuosus. Routine respiration was moreover elevated by post-prandial effects (specific dynamic action) for longer times than previously suggested. Consequently, ignoring such effects could significantly bias respiration measurements.
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| 8. |
- Sjöberg, Niklas B., 1972-
(författare)
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Eel migration - results from tagging studies with relevance to management
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In response to the drastic decline of the European eel (Anguilla anguilla (L.)) fisheries have been reduced and elvers are stocked in areas where natural abundances are low. Are these measures adequate? To answer different aspects of this question, we have analysed more than a century of eel tagging, using both traditional and more novel capture – recapture analyses. Based on these long-term data, we have evaluated the impact of the Swedish eel coastal fisheries using Survival analysis. Our analysis indicates that the fishing mortality just prior the 2009 fishing restrictions were in the order of 10%.More recent tagging programs have focused on issues related to the fate of stocked fish. If and how they migrate out of the Baltic Sea and further on towards the Atlantic Ocean. Both earlier and our new studies reveal that all eels recaptured on the Swedish East Coast, no matter of their origin, migrate at a reasonable speed and direction towards the outlets of the Baltic Sea. Even though it is sometimes difficult to determine their origin, our analyses indicate that stocked fish were scarce among the recaptures. In an experiment on the Swedish West Coast, we knew the individuals’ origin (stocked or wild) and they had similar migration patterns.In contrast, silver eel in Lake Mälaren – assumed to have been stocked as elvers or bootlace eels – seemed to have difficulties in finding the outlets. Instead they overwintered and lost weight. However, weight losses are also significant among non-stocked individuals in the Baltic Sea, both if they overwinter and if they appear to be on their way out from the area. It remains an open question whether eels from the Baltic region in general, and whether the overwintered fish in particular, manage to reach the spawning area in the Atlantic Ocean.Based on current knowledge, I advocate invoking the precautionary approach and to concentrate Swedish eel stockings to the West Coast and allow the young fish to spread out on their own.
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| 9. |
- Österblom, Henrik, 1973-
(författare)
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Complexity and Change in a Simple Food Web : Studies in the Baltic Sea (FAO Area 27.IIId)
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- An influence at one trophic level can result in dynamic impacts also on other components of a food web. These dynamics are known as trophic cascades, and can be both top-down and bottom-up. After a near-collapse of the Baltic cod Gadus morhua stock in the 1980s, its main prey sprat Sprattus sprattus increased dramatically. The main food of sprat, marine copepods, decreased during the same time period, likely a combined effect of increased predation pressure from sprat and decreasing salinities. This shortage of food for sprat resulted in decreasing quality of sprat as a food source for common guillemots Uria aalge. However, a recent increase in fishing for sprat has again resulted in better feeding conditions for guillemots.Human impacts on this simple food web can be complex. In the early 20th century, marine mammals were abundant and nutrient levels were low in the Baltic Sea. This thesis illustrate that this situation corresponded to lower fish biomass. A reduction of seals early in the century led to reduced top-down control, which resulted in increasing fish stocks. Later, in the 1950s, the largest inflow of salt water during the century mobilized accumulated phosphorus from the deep sediments, which stimulated nitrogen fixation. Combined with increasing anthropogenic nutrient loads, this led to increased primary production and a rapid change from an oligotrophic to a eutrophicated state. This change can be termed a regime shift, which also stimulated fish production. Subsequent over-fishing of cod likely caused a second regime shift, from a cod- to a clupeid- dominated state, which led to the described effects on the common guillemots.Several factors affect the life-history of this long-lived seabird. Bycatches in gillnet fisheries is one factor directly affecting guillemot survival, and the proportion of bycatches increased during a period of increasing fishing effort. Surprisingly, avian cholera, a previously undocumented disease in common guillemots, was found at times to cause considerable adult mortality. Common guillemot life-history information can communicate the diversity of factors influencing marine ecosystems – hopefully this can increase our understanding of how complex even "simple" food webs are.
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| 10. |
- Holmborn, Towe, 1979-
(författare)
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Zooplankton growth and trophic linkages : Implications for fish feeding conditions in the Baltic Sea
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aim of this Thesis was to improve our understanding and assessment of feeding conditions for zooplanktivorous fish in the Baltic Sea.We investigated (papers I, II) the usefulness of biochemical proxies for assessments of growth and metabolic rates in the dominant Baltic copepod Acartia bifilosa. A predictive model (paper I) for egg production rate (EPR), based on body size, RNA content, and water temperature, was established using females of different geographical origin. This model demonstrates the usefulness of RNA content as a proxy for growth in zooplankton and, together with abundance data, it could be used to evaluate fish feeding conditions. Further (paper II), using A. bifilosa exposed to a food gradient, we evaluated responses of physiological rates and other biochemical proxies for growth and established correlations between physiological and biochemical variables. EPR and ingestion rate were most significantly correlated with RNA content. As assayed variables saturated at different food concentrations, food availability may affect assessments of physiological rates using proxies. In paper III, we explored the effect of high EPR and ingestion rate on astaxanthin content in A. bifilosa. We found that the astaxanthin content decreased at high feeding rates, most likely due to decreased assimilation efficiency. This may impact the quality of zooplankton as prey.The invasion of Cercopagis pengoi, a zooplanktivorous cladoceran, has altered the trophic linkages in the Baltic Sea food web. In paper IV, we evaluated the feeding of zooplanktivorous fish on C. pengoi and found that irrespective of size both herring and sprat feed on it, with large herring being more selective. In turn, C. pengoi feeds mainly on older copepods (paper V), which are acknowledged important in fish nutrition. These results indicate that C. pengoi may compete with fish due to the diet overlap.
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