Commercial fishing in the Baltic Sea has led to decrease in abundance of large predatory fish and as a result the predation pressure on smaller fish species has been reduced. Three-spined stickleback (Gasterosteus aculeatus) is among the species that have benefited from reduced predation pressure. Sticklebacks are a small fish species that spends its juvenile stage in coastal habitat and its adult stage in the open sea with a yearly migration from the sea to coastal habitat and lakes for spawning. Sticklebacks have been shown to have a negative effect on fry of coastal species, such as perch (Perca fluviatilis). Therefore an increase in stickleback abundance, as a consequence of reduced predation pressure, can potentially further reduce abundance of fry of coastal species, such as perch. On the other hand sticklebacks could be subject to predation from adult perch. This two-species interaction is studied in the perspective of general ecological IGP (intra guild predation) theory. In order to determine how the observed increase of three-spined sticklebacks negatively or positively affects the growth of different stages of perch, a time series of operculum bones from perch collected by the coastal monitoring program prior to the increase of sticklebacks and up until present was analyzed. Three areas along the Swedish coast showing increased stickleback abundance (Gaviksfjärden, Norrbyn and Holmön) were compared to areas that had no sticklebacks or only a small increase in abundance (Kinnbäcksfjärden, Råneåfjärden, Långvindsfjärden) prior and after the general increase in stickleback density. In addition to the growth analysis, a dietary analysis of perch was carried out in order to determine to what extent perch utilize sticklebacks as a food source. I found that there were significant differences in regard to perch growth between the two time periods; however there was no significant difference in growth between the control areas and the stickleback areas except for perch of medium size. Further, this difference in growth between the control and stickleback areas was present both before and after the stickleback increase and could therefore not be tied directly to stickleback increase. Consequently, results from a multiple regression showed that stickleback abundance was not able to significantly explain the observed changes in perch growth. The dietary analysis showed that larger and intermediate perch tend to consume sticklebacks as well as benthic fauna and other fish species while smaller perch tended to feed exclusively on smaller invertebrates. This study concluded that the increase in stickleback abundance had only a very weak effect on perch growth.

Climate change is a global issue and the effects on fish populations remain largely unknown. It is thought that climate change could affect fish at all levels of biological organisation, from cellular, individual, population and community. This thesis has taken a holistic approach to examine the ways in which climate change could affect fish from both tropical, marine ecosystems (Great Barrier Reef, Australia) and temperate, freshwater ecosystems (non-tidal River Thames, Britain). Aerobic scope of coral reef fish tested on the Great Barrier Reef was significantly reduced by just a 2°C rise in water temperature (31, 32 and 33°C, compared to the current summer mean of 29°C) due to increased resting oxygen consumption and an inability to increase the maximal oxygen uptake. A 0.3 unit decline in pH, representative of ocean acidification, caused the same percentage loss in aerobic scope as did a 3°C warming. Interfamilial differences in ability to cope aerobically with warming waters will likely lead to changes in the community structure on coral reefs with damselfish replacing cardinalfish. Concerning Britain, there is evidence of gradual warming and increased rainfall in winter months over a 150 year period, suggesting that British fish are already experiencing climate change. It was evident from an analysis of a 15 year dataset on fish populations in the River Thames, that cyprinid species displayed a different pattern in biomass and density to all the non-cyprinid fish population, suggesting that there will be interfamilial differences in responses to climate change. Using a Biological Indicator Approach on the three-spined stickleback, Gasterosteus aculeatus, a 2°C rise in water temperature resulted in a stress response at the cellular and whole organism level. A 6°C rise in temperature resulted in a stress response at the biochemical level (higher cortisol and glucose concentrations), cellular level (higher neutrophil: lymphocyte ratio) and whole organism level (higher ventilation rate and lowered condition factor, hepatosomatic index and growth). G. aculeatus is considered to be temperature tolerant; therefore these results indicate that climate change may prove to be stressful for more temperature-sensitive species. This study has demonstrated that climate change will have direct effects on fish populations, whether they are in temperate regions such as Britain or in tropical coral reefs, but with strong interfamilial differences in those responses.

The costal populations of perch (Perca fluviatilis) in some parts of the Baltic Sea have been in decline for about two decades. Recruitment failure in the early larval stages has been put forward as a possible cause and the decline has also been suggested to coincide with increases in three-pined stickleback (Gasterosteus aculeatus) densities. The aim of this study was to study the effects of growth and survival of newly hatched perch larvae in the presence of the three-pined stickleback, and if possible determine the main mechanism behind any negative effects the perch may suffer under such conditions. Using large scale experimental ponds as a controlled habitat, an experiment was conducted where perch larvae were being exposed to sticklebacks under four different stages of their development. Results showed that the three- spine stickleback can have a strong negative effect on the survival of young perch. This effect was strongest in the earliest stage of perch development and decreased as they grew bigger. The zooplankton densities didn't differ between the controls and stickleback treatments, suggesting that the young perch didn't suffer from food limitation. Instead predation was identified as the main mechanism behind the high mortality. The results of this study highlight the potential danger of the observed patterns of decreasing predator populations in conjunction with increasing populations of smaller prey species in the Baltic Sea.

The seasonal reproduction of the three-spined stickleback is stimulated by long day photoperiod. As in other vertebrates, the reproductive system of stickleback is regulated by the brain-pituitary-gonadal (BPG) axis which is largely controlled by feedback effects. Both negative and positive feedback effects on the BPG axis have been found in fish. So far, the roles feedback effects on the BPG axis play in the photoperiodic regulation of seasonal reproduction are still unclear. This thesis focused on the photoperiodic regulation and gonadal feedback effects on the gene expressions of gonadotropin (GtH) and gonadotropin-releasing hormones (GnRH) in the brain and pituitary, and how gonadal feedback regulated the steroid homeostasis in stickleback.Both GnRH2 and GnRH3 mRNA was found in the hypothalamus. Higher expression levels of both GnRH2 and 3 in breeding than in post-breeding males suggested that they are both involved in seasonal reproduction. There was no evidence for a role of GnRH3, which may be the dominating form, in the photoperiodic control of reproduction. However, the polarity of the feedback effect on gnrh3 gene expression may turn from positive to be negative when the males went into post-breeding state. Tapeworm, Schistocephalus solidus, infection inhibited the reproduction of sticklebacks. However, the infection caused higher expression levels of both GnRHs and GtHs genes, which may be due to feedback effect on the BPG axis.Under short day, both lh-β and fsh-β were suppressed by low androgen levels. This negative feedback may inhibit maturation completely, unless a rise of androgens triggers positive feedback under long day. The change in feedback polarity may result in all or nothing maturation. Furthermore, the androgen inhibitory effect on lh-β and fsh-β under short day could be abolished by aromatase inhibitor, which means the estrogen may cause negative feedback in males under short day.There was no compensation effect on plasma androgen level in fully mature hemi-castrated fish. However, both testosterone and 11-ketoandrostenedione treatments increased plasma levels much less in sham-operated fish than in castrated ones, indicating that homeostatic mechanisms are nevertheless present. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows<strong>:  </strong>Paper 1: Submitted. Paper 3: Submitted. Paper 4: Submitted.</p>

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Many chemicals present in the aquatic environment can interfere with physiological functions in fish. Exposure to chemicals can be revealed by the use of biomarkers. Induction of 7-ethoxyresorufin O-deethylase (EROD) activity is a commonly used biomarker for exposure to CYP1A inducers such as dioxins and polyaromatic hyrdrocarbons. Vitellogenin is a frequently used biomarker for estrogenic compounds in various fish species whereas a biomarker for androgens, spiggin, is only found in sticklebacks. The main objectives of this thesis were to evaluate gill EROD activity as a biomarker and the three-spined stickleback as a model species in ecotoxicological studies. EROD activities were measured in gill, liver and kidney in rainbow trout (Oncorhynchus mykiss) caged in urban areas in Sweden. EROD induction was most pronounced in the gill. Also in fish caged at reference sites, with an expected low level of known CYP1A inducers, a marked gill EROD induction was found. One suggested inducer in rural waters is humic substances (HS). To evaluate the EROD-inducing capacity of HS, three-spined sticklebacks (Gasterosteus aculeatus) were exposed to HS of natural or synthetic origin. Both kinds of HS caused significant EROD induction. Gill EROD activities were also induced in sticklebacks exposed to ethynylestradiol (EE2) and β-naphthoflavone (βNF), alone and in combinations. Production of vitellogenin was induced in sticklebacks exposed to ≥50 ng EE2/l and a significant decrease in spiggin production was observed in individuals exposed to 170 ng EE2/l. Results from this thesis further strengthen the contention that gill EROD activity is a very sensitive biomarker for CYP1A inducers and that the stickleback is a suitable biomonitoring species, especially for exposure to CYP1A inducers. The finding that not only classical CYP1A inducers but also HS and high EE2 concentrations stimulate gill EROD activity is of significance for the interpretation of biomonitoring data.

Both temperature and terrestrial dissolved organic carbon (TDOC) have strong impacts on aquatic food web dynamics and production. Temperature affects vital rates of all organisms and terrestrial carbon has been shown to alter the dynamics of phytoplankton and bacterial production and affect the trophic structure of planktonic food webs. As climate change predictions for the Baltic Sea suggests future increases in both terrestrial carbon run-off and increases in temperature, the aim of thesis was to adopt a system-ecological approach and study effects of these abiotic variables, not only on interactions within planktonic food webs, but also on the growth and consumption rates of one of the most common zooplanktivorous fish in the Baltic Sea, the three-spined stickleback Gasterosteus aculeatus. Results showed that three-spined sticklebacks display a high degree of resilience against increasing temperatures, as both growth rates as well as consumption rates on zooplankton were high at temperatures well over 20 °C. Furthermore, it was shown that the minimal resource densities required to sustain individual and population growth, actually decreased with increasing temperatures, implying that sticklebacks around their optimum temperature for growth at 21 °C will actually have an increased scope for growth. As stickleback population densities have increased over the last decade in the Baltic Sea and are now suggested to out-compete other coastal fish species for shared zooplankton resources, the results presented in this thesis suggest that increased water temperatures would only serve to increase sticklebacks competitive advantage. As the structuring role of this small zooplanktivore on pelagic communities might be considerable, further studies investigating competitive interactions as well as patterns of population abundances are definitely warranted. TDOC was overall shown to stimulate bacterial production and the microbial food web. Because of the longer trophic pathways required to transport carbon from bacterial production to higher trophic levels, the addition of TDOC always reduced food web transfer efficiency. However, it became apparent that the full effect of TDOC additions on pelagic food webs was complex and depended heavily not only on the existing trophic structure to which the carbon was introduced, but also on ambient temperature levels. When three-spined sticklebacks were part of food webs with significant TDOC inputs, the presence of fish, indirectly, through predator release of lower trophic levels, amplified the magnitude of the effects of carbon addition on bacterial production, turning the base of the system significantly more heterotrophic, which ultimately, impacted negatively on their own production. However, when a pelagic food web containing sticklebacks was simultaneously subjected to realistic increases in temperature and TDOC concentrations, food web efficiency and fish production increased compared to present day conditions. These results were explained by a temperature dependent increased production potential of zooplankton, sustained by an increased production of heterotropic microzooplankton via TDOC additions, which lead to higher fish production. Although the increased number of trophic linkages in heterotrophic food webs should have reduced energy transfer efficiency, these negative effects seem here to have been overridden by the positive increases in zooplankton production as a result of increased temperature. These results show that heterotrophic carbon transfer can be a viable pathway to top-consumers, but also indicates that in order to understand the full effects of climate change on trophic dynamics and fish production, abiotic variables cannot be studied in isolation.

L’extraction de l’uranium a eu pour conséquence une remobilisation de cet actinide au niveau des écosystèmes avoisinants les mines. L’utilisation de sels métalliques lors de la réhabilitation des sites miniers et la présence naturelle de métaux ont accentué les niveaux de la contamination métallique dans les hydrosystèmes soumis aux rejets miniers uranifères.Des expériences in situ ont été conduites au niveau de deux anciens sites miniers uranifères français. L’encagement de l’épinoche à trois épines a été employé pour connaître les effets sublétaux de ce mélange métallique, ainsi que sur la susceptibilité de ce poisson envers un stress biologique.Cette pollution, caractérisée par de plus importantes concentrations métalliques (notamment en uranium), a entraîné un stress oxydant chez l’épinoche visible sur plusieurs biomarqueurs, ainsi que d’autres effets dépendants du site d’exposition.La contamination polymétallique a occasionné une augmentation de la susceptibilité des épinoches au stress biologique, en empêchant leurs réponses phagocytaire antioxydante à ce stress. Ces travaux ont permis de renforcer l’intérêt de la technique d’encagement lors d’une étude environnementale, et celui des immunomarqueurs au sein d’une approche multi-biomarqueurs. / Uranium extraction has resulted in a remobilisation of this actinide into mine surrounding ecosystems. Uses of metal salts during mining site rehabilitation, and the natural presence of metals have increased the metal contamination in hydrosystems submitted to mine tailings.In situ experiments were conducted in two former French uranium mining sites. Three-spined stickleback caging was used to determine the sublethal effects of this metal mixture on this freshwater fish, as well as its effects on fish susceptibility to a sudden biological stress.This pollution, characterised by higher metal concentrations (especially for uranium), has led to an oxidative stress in sticklebacks visible through several biomarkers, and other effects dependent on the study site. The polymetallic contamination has modified the stickleback responses to the biological stress, by preventing their phagocytic and antioxidant responses. This work has reinforced the interest of the caging technique during environmental studies and that of immunomarkers in a multi-biomarker approach.

Reproduction in vertebrates, including fishes, is under control of the brain-pituitary-gonad (BPG) axis. The female three-spined stickleback, Gasterosteus aculeatus, produces egg clutches at intervals of a few days and spawns them in a nest built by male. Following ovulation, eggs are stored in the ovarian cavity surrounded by the ovarian fluid (OF). If spawning or spontaneous release do not occur, the eggs can undergo overripening, a phenomenon occurring both in nature and captivity. In this PhD thesis, the changes of reproductive hormones and vitellogenesis were studied at overripening of eggs and over the natural spawning cycle. OF properties were also examined at overripening of eggs and after treatment with sex steroids. Plasma levels of steroids: testosterone (T), estradiol (E2), 17,20β-dihydroxypregn-4-en-3-one (17,20β-P) and 17,20β,21-trihydroxypregn-4-en-3-one (17,20β,21-P) were measured by radioimmunoassay, and relative mRNA levels of the pituitary gonadotropins (fsh-β/lh-β), brain gonadotropin-releasing hormones (gnrh2/gnrh3) and kisspeptin and its receptor (kiss2/gpr54) by qPCR. Overripening of eggs was accompanied with a significant reduction in most of endocrine parameters of BPG axis (T, E2, 17,20β-P; lh-β; kiss2, gpr54). Low level of hormones could be advantageous for the overripe egg-bound females, since this would reduce further ovulations giving higher chances to survive and reproduce again. Over the 3-day spawning cycle, T and E2 were highly correlated, showed cyclicity with low levels at ovulation and increasing from 24 and 6 hours post-spawning (hps), respectively. Spawning may give rise to this increase as these rises did not occur if release of the eggs does not happen (overripe females). A peak at pituitary lh-β mRNA levels appeared 48 hps, a day before the next ovulation. No significant changes were found for the other studied hormones. Vitellogenesis was studied by measurement of the vitellogenin mRNA levels in the liver by qPCR. The levels were highest at 24 and 48 hps and were positively correlated to both E2 and T over the cycle. However, changes were small suggesting a rather continuous vitellogenesis over the stickleback spawning cycle which could be an advantage for a multiple spawner with a limited spawning season. Overripening reduced vitellogenin mRNA levels but did not abolish it. OF amount was diminished in overripe females and had a lower viscocity but higher dry weight and protein levels than in non-overripe ovulated females, suggesting that changes in OF properties are related to the egg overripening. The effects of steroids were studied using Silastic capsules. T and 17,20β-P induced an increase of OF amount, but protein levels were only increased in 17,20β-P-treated females, proposing a role of this steroid in the control of OF secretion. 1-D SDS-PAGE showed that OF contained several proteins, some of them came from eggs, but no consistent differences between groups. Concluding, the knowledge of the reproductive physiological changes is important for understanding their essential roles in the production of viable eggs in this species but also in the reproductive physiology of female fishes in general. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.</p>