The threespine stickleback (Gasterosteus aculeatus L.) is a highly varied and widely distributed species of small fish. Throughout most of its range there are two distinct forms. One form (trachurus) is anadromous and the other form (leiurus) is sedentary and lives entirely in fresh water. Hagen (1967) concluded that these two forms were re productively isolated and that both leiurus and trachurus should be considered as species. Hagen's conclusion was based on differences he found in ecological habitats, breeding times, and the apparent absence of introgression between, leiurus and trachurus. The purpose of this study was to determine if ethological factors or mate selection contribute to reproductive isolation. Mate selection tests were conducted in which (1) courting males were simultaneously exposed to receptive females of both types, and (2) receptive females were simultaneously exposed to both types of courting males. The results of both these tests clearly demonstrate that matings between similar phenotypes occur significantly more often than matings between dissimilar phenotypes. Additional experiments attempt to document the specific behavioural mechanisms responsible for mate selectivity. The orientation of males to females, and females to males, was observed and quantified. Males were found to orient, or direct most of their courtship, to females of their own phenotype. Females also tended to orient to males of their own phenotype. Certain behavioural patterns of male courtship were quantified and compared between leiurus and trachurus. Significant differences were found in a number of courtship activities: leiurus males perform significantly more zig-zags per minute than trachurus males, the first courtship response of leiurus males is usually a zig-zag while the first response of trachurus males is usually a bite. Non-random mating between leiurus and trachurus may contribute greatly to their reproductive isolation. Non-random or selective mating may be explained by difference in behaviour between the two forms, particularly during courtship. / Science, Faculty of / Zoology, Department of / Graduate

Intensive field observations were conducted on a small British Columbia coastal stream for two years, supplemented by laboratory rearing and experiment. Stress was placed upon possible isolating mechanisms, as outlined by Mayr, that might serve to keep the marine stickleback (trachurus) and the freshwater form (leiurus)distinct. Lelurus permanently occupies the upper reaches of the stream; trachurus is anadromous, but enters the lower reaches to breed in "pure" freshwater. Between the breeding ground of the two, where some coexistence occurs, hybridization between leiurus and trachurus is extensive, but it is restricted to very narrow zones both in Little Campbell River near Vancouver and in certain Vancouver Island streams. Morphological analysis provides firm circumstantial evidence that hybrids are plentiful and that back-crossing occurs. Hybridization is confirmed by rearing experiments in the laboratory with crosses in all combinations. Most backcrossing is to the freshwater race and this is considered in detail. Behavioural experiments demonstrated the absence of mate preference; random mating occurs with respect to ethological isolation and hybrids perform courtship and parental care activities normally. Rearing hybrids uniformly and in all conbinations demonstrated the absence of genetic incompatibility, and vigorous offspring were produced. Fertile hybrids are abundant in the wild, but the sexes are seasonally isolated from one another. Behavoiral and genetic blocks to hybridization are absent, and thus there is no means to prevent hybridization where leiurus and trachurus come together. But evidence from the field and from preference tests demonstrated that ecological isolating mechanisms are very powerful barriers to hybridization. This is enforced to some extent by partial seasonal isolation, but early spawning migrants make a major contribution to hybridization in the Little Campbell River. Several factors probably operate to cause very narrow hybrid zones in stickle-backs, including a sedentary habit, habitat preference, and hybrid inferiority outside the hybrid zone. The latter could not be detected inside the hybrid zone. The complex of Isolating mechanisms found may well produce stable hybrid zones over long periods of time, and is discussed. Reinforcement of ecological isolating mechanisms probably occurs, and Moore's criticism of the spread of reinforced genotypes would not apply in this instance. Adaptation and consequent ecological isolating mechanisms are the cause of hybrid inferiority. In this instance pre and postmating mechanisms cannot be separated, for they go hand in hand. Geographic speciation is possible but problematical, and sympatric speciation cannot be excluded as a plausible explanation of divergence between sticklebacks as a result of disruptive selection. Distributional patterns of leiurus can best be explained by repeated and independent origins from trachurus. The two forms fulfill the species definition by Mayr and remain reproductively isolated, have well developed isolating mechanisms, and show considerable genetic divergence, much of it polygenic. There is no evidence that, introgression occurs, but a reverse cline in leiurus as well as very narrow hybrid zones demonstrates that selection severely restricts gene flow. Reproductive isolation between marine and freshwater forms seems to be widespread around the hemisphere. / Science, Faculty of / Zoology, Department of / Graduate

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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<p>In the fifteen-spined stickleback, <i>Spinachia spinachia</i>, males provide females with direct benefits by fanning, cleaning and guarding the offspring. Males announce their parental skills through intense body shakes during courtship. Females preferred to mate with more intensely shaking males. As a result, females got better fathers for their offspring, as such males achieved a higher hatching success. Not only did male behavioural cues attract females, but males also used their nests as extrabodily ornaments. The nest is held together with shiny secretional threads consisting of a glycoprotein. Females chose to spawn in nests with more secretional threads. A likely reason for this is that the threads are metabolically costly for the male to produce and the amount of secretion indicates a male's nutritional status, which is of great importance as parental duties are energetically costly. Moreover, females preferred nests built high up in the vegetation, as such nests were safer from egg predators. Competition with other males for females favoured males building higher nests than did their neighbours, probably because females preferred high nests. Male-male interactions, such as sneaking and egg stealing, caused decreased paternity among males in nature as assessed by a microsatellite analysis. Males adjusted their paternal effort according to their previous investment in the brood, but not according to paternity. Thus, female choice is based on multiple cues and results in better paternal care. Males invest in courtship, male-male competition, nest construction and paternal care, the outcome determining their success in mate attraction.</p>

In the fifteen-spined stickleback, Spinachia spinachia, males provide females with direct benefits by fanning, cleaning and guarding the offspring. Males announce their parental skills through intense body shakes during courtship. Females preferred to mate with more intensely shaking males. As a result, females got better fathers for their offspring, as such males achieved a higher hatching success. Not only did male behavioural cues attract females, but males also used their nests as extrabodily ornaments. The nest is held together with shiny secretional threads consisting of a glycoprotein. Females chose to spawn in nests with more secretional threads. A likely reason for this is that the threads are metabolically costly for the male to produce and the amount of secretion indicates a male's nutritional status, which is of great importance as parental duties are energetically costly. Moreover, females preferred nests built high up in the vegetation, as such nests were safer from egg predators. Competition with other males for females favoured males building higher nests than did their neighbours, probably because females preferred high nests. Male-male interactions, such as sneaking and egg stealing, caused decreased paternity among males in nature as assessed by a microsatellite analysis. Males adjusted their paternal effort according to their previous investment in the brood, but not according to paternity. Thus, female choice is based on multiple cues and results in better paternal care. Males invest in courtship, male-male competition, nest construction and paternal care, the outcome determining their success in mate attraction.