Onions are widely produced within the tropics, but little scientific research has been done specifically on the Eritrean cultivars, like Hagaz Red 1 and 2 (HR I, and HR 2). Many onion cultivars are limited in their range of adaptation due to the combined effects of photoperiod and temperature. A priority for research on the crop was to elucidate the local crop's growth response to environmental conditions, particularly temperature and daylength. The Eritrean cultivars HR I and 2 and an American (Louisiana) cultivar Red Creole (RC) grown in South Africa were grown in growth rooms under all combinations of three daylengths (11.5h, 12h, 12.5h) and three day/night temperatures (25/12°C, 30/15°C and 35/18°C). Growth responses were determined at 108 days and by using a growing degree day (GDD) base. A broken-stick regression model was used to determine the points of inflection, indicating the initiation of bulbing. Based on leaf area and plant height data, mathematical differentiation equations and coefficient of determination (R2) were applied to determine the base temperature (6.4°C) for these particular cultivars. All three cultivars needed at least 12 h daylength for bulb initiation when assessed by a bulbing ratio >=2.0. A bulbing ratio >=2.0 characterizes the onset of bulbing. Under a 11.5 h daylength, a temperature higher than 25/12°C decreased vegetative growth. Temperature in this region may be a supra-optimal condition for the growth of these cultivars at this daylength. However, the 25/12°C and 30/15°C temperatures were found to be ideal for onion bulb production under 12 hand 12.5 h daylengths. The three cultivars (HR I, HR 2 and RC) showed very similar growth response to the daylength and temperature interactions. The thermal presentation of plant growth indicated that there were relationships between bulb initiation and rate of leaf area growth under inductive conditions (12 hand 12.5 h). Under the 12 h daylength, cultivars needed 343, 482, and 597 GDD units before bulb initiation and 405, 432, and 431 GDD to increase the rate of leaf area development at 25/12°C, 30/15°C, and 35/18°C, respectively. Under a 12.5 h daylength, these cultivars needed 344, 423, and 432.2 GDDs to initiate bulbing and 140, 411, and 579 GDDs to increase leaf growth rates at 25/12°C, 30/15°C, and 35/18°C, respectively. In the 12 h daylength, bulbing was initiated and followed by an increased rate of growth of leaf area. However, the reverse happened for the 12.5 h daylength. Overall, where plant response to temperature can be expressed as the rate of progress towards a morphogenetic change, GDD values can be used to predict a plant developmental stage at a particular temperature. It must be concluded that temperatures induced significant variations in growth components (leaf number, plant height, leaf area), and affected bulbing response. The findings in this study confirmed that the cultivars require only a certain fixed amount of thermal time for their development at a particular temperature, and that, if anything, the slow growth rate at the higher temperature must be due to supra-optimal temperatures. They also require a minimum 12 h photoperiod for bulb formation. / Thesis (M.Sc.Agric.)-University of KwaZulu-Natal, Pietermaritzburg, 2005.

No description available.

Ritmos biológicos e sua sincronização por ciclos ambientais geralmente são estudados em roedores, mantidos em laboratório. Nosso grupo vem estudando ritmos de atividade/repouso de roedores subterrâneos sul-americanos conhecidos como tuco-tucos (Ctenomys aff. knighti) em laboratório e campo. Na natureza, sabemos que os tuco-tucos emergem à superfície diariamente em episódios de até uma hora. Registros curtos da atividade de superfície dos tuco-tucos, anteriormente obtidos pelo grupo, haviam sugerido que o padrão diário das saídas variava sazonalmente, sendo crepusculares durante o verão e concentrados no meio do dia, durante o inverno. Esse mesmo padrão sazonal já foi observado em populações de outros roedores de deserto e foi atribuído a reações dos roedores às variações diárias de temperatura, no verão e inverno. Por outro lado, a sazonalidade dos padrões de atividade de várias espécies de mamíferos e aves também pode ser explicada pela sincronização diária do ritmo de atividade a diferentes fotoperíodos. O objetivo desse trabalho é caracterizar a variação sazonal dos ritmos diários de atividade dos tuco-tucos, com uso de bio-logging, e verificar a contribuição dos fatores ambientais para a sazonalidade desses padrões. Ao longo do ano, acelerômetros e luxímetros foram presos a 29 animais recentemente capturados (15 fêmeas e 14 machos). Cada animal foi mantido individualmente em arenas semi-naturais por um mês. Variáveis ambientais foram registradas simultaneamente. Outros 18 animais (9 fêmeas e 9 machos) foram mantidos em laboratório, expostos ao fotoperíodo natural e em ambiente com temperatura constante. Os resultados confirmaram a sazonalidade do padrão temporal de atividade de superfície dos tuco-tucos. Os acelerômetros se mostraram mais precisos e sensíveis a pequenos movimentos corporais do que os registros de telemetria encontrados na literatura. Esses sensores também revelaram padrões de atividade subterrânea e noturna, em ambiente semi-natural. Durante o verão, a atividade de superfície diária dos tuco-tucos foi melhor prevista pelo horário do dia (fator endógeno) do que pelas variações de temperatura ambiental (fator exógeno). Durante o inverno, entretanto, a temperatura ambiental teve maior contribuição no padrão de saídas dos tuco-tucos, indicando complexas contribuições dos fatores endógenos e exógenos no delineamento dos padrões de atividade / Biological rhythms and their synchronization by environmental cycles are generally studied in rodents, inside laboratory. Our group studies activity/rest rhythms of South American subterranean rodents known as tuco-tucos (Ctenomys aff. knighti), in laboratory and field. In nature, it is known that tuco-tucos emerge daily to the surface, in brief episodes. Short records of tuco-tucos\' individual surface activity, previously collected by our group, had suggested that daily temporal pattern of emergence varied seasonally, being crepuscular in summer and concentrated around midday in winter. The same seasonal pattern was observed in populations of other desert rodents and was attributed to daily variations of environmental temperature, in summer and winter. At the same time, seasonality of activity patterns of various species of mammals and birds can be explained by daily synchronization of the activity rhythm by different photoperiods. Our aim is to characterize seasonal variation of daily activity rhythms of tuco-tucos, using bio-logging, and to verify the contribution of environmental factors in the seasonality of these patterns. Throughout the year, accelerometers and luximeters were tied to 29 freshly caught animals (15 females and 14 males). Each animal was kept inside semi natural enclosures for one month. Environmental variables were recorded simultaneously. Other 18 animals (9 females and 9 males) were kept inside laboratory, exposed to natural photoperiod, in a room with constant temperature. We confirmed the seasonality of temporal pattern of surface activity of tuco-tucos. The accelerometers resulted to be more precise and sensitive to small body movements than telemetry records found in literature. They also revealed nocturnal and subterranean activity patterns inside the enclosures. During summer, daily surface activity of tuco-tucos was better predicted by time of day (endogenous factor) than by environmental temperature (exogenous factor). During winter season, however, environmental temperature has a stronger contribution on the emergence pattern in tuco-tucos, indicating complex contributions of endogenous and exogenous factors that outline activity patterns

Em 2007 o complexo de soja foi responsável por 19 por cento do total das exportações do agronegócio brasileiro, gerando mais de U$11 bilhões em divisas. Projeções indicam que até 2020 o Brasil deverá produzir 105 milhões de toneladas de soja, ou 81 por cento a mais do que a atual produção. Apesar de incidir sobre a produtividade a maior expectativa de crescimento, espera-se que a expansão significativa em área ocorra nas regiões de baixa latitude dos Cerrados, e em especial, no estado do Mato Grosso. É fundamental a determinação de estratégias que considerem produção, dimensões econômicas e ambientais. O uso da modelagem agrícola é a única forma de explorar sistematicamente o potencial produtivo em escala global e predizer produtividades futuras em determinadas condições. A utilização de computadores e banco de dados torna-se uma ferramenta cada vez mais necessária, no sentido de disponibilizar rapidamente informações de qualidade a um custo relativamente inferior, quando em comparação a experimentos de campo. O modelo LINTUL, desenvolvido pela Universidade de Wageningen, foi extensamente utilizado para predizer a produtividade potencial de culturas baseado no uso eficiente da radiação solar, sob condições ótimas de desenvolvimento. A cultura de soja, porém, nunca foi testada por este modelo. Sendo assim, os objetivos deste trabalho foram: (i) adaptação, por intermédio da parametrização e da calibração do modelo LINTUL para estimação da produtividade potencial da cultura de soja; (ii) disponibilizar um algoritmo em Visual Basic (Microsoft), validando e replicando os procedimentos do LINTUL, tornando este modelo acessível a uma ampla gama de usuários. Para a parametrização do modelo, foram consideradas características específicas da cultura, como por exemplo, a sensibilidade ao fotoperíodo. Para calibração, foram usados dados de experimento de campo conduzido em Planaltina-DFe Sorriso- MT. Os parâmetros não avaliados no experimento foram extensivamente revisados com base na literatura. Os principais componentes do processo de crescimento e desenvolvimento da cultura foram analisados individualmente através de comparativos de respostas. Posteriormente, um estudo de adequação do modelo foi feito, com objetivo de viabilizar uma comparação de comportamentos, quando em diferentes situações. As variáveis climáticas usadas neste modelo foram temperatura máxima e mínima e radiação solar. A simulação dos estádios fenológicos é baseada na soma térmica. O modelo LINTUL-soja demonstrou excelente desempenho quanto à predição de tendências, com a variação dos valores dos parâmetros. Todavia, o modelo necessita de ajustes quanto a introdução de módulos, que contemplem o balanço de carboidratos. O uso de uma linguagem de programação aberta, como o Visual Basic, replicou a linguagem usada pelo modelo LINTUL com sucesso. / In 2007 the soybean complex was responsible for more than 19% of the total Brazilian agribusiness exports, which corresponded to US$ 11 billion generated in currencies. According to projections, soybean production in Brazil will increase significantly till 2020, reaching 105 million tons, or an addition of 81 percent to the current crop season. That huge difference is expected to come from a significant increase in yield, while the increase in acreage should come from the low-latitude regions of Cerrados, mainly in Mato Grosso state. Thus, it becomes extremely important to set out strategies that consider production, economic and environmental dimensions. The usage of agricultural modeling is the only mean of systematically exploring the production potential on global scale and predict future yield in certain conditions. Computers and database application becomes an implement even more efficient in respect of turn quickly and quality information available with lower costs, when compared to the field experiments. The LINTUL model was developed by the Wageningen University, and extensively used to predict crop potential production based on radiation use efficiency concept, under optimal conditions of growth. The soybean crop has never been tested by this model. The goals of this research are: (i) to adapt - through parameterizing and calibration - the LINTUL model to estimate the potential production of soybean; (ii) to provide an algorithm in Visual Basic language, in order to validate the calculations and replicate the procedures executed by LINTUL, making this model accessible to a wide range of users. In order to parameterize the model, crop-specific characteristics were taken into account, such as the sensibility of soybean to photoperiod. In respect to calibration, it was used field data collected in Planaltina-DF and Sorriso-MT. The mainly components of the growth and development process were analyzed based on response comparatives. Afterward, it was done a study of adequacy of the model, in order to enable a comparison of behaviors, when in different situations. The climatic variables used in this model were maximum and minimum temperature and solar radiation. The simulation of phenological stages was based on thermal unit concept. The LINTUL-soybean model has demonstrated an excellent performance to predict trends, with the change in the parameters values. However, the model needs to be adjusted in respect to an introduction module to calculate the carbohydrates balance into the canopy. The use of an open programming language, such as Visual Basic, replied the language used by LINTUL model successfully.

Em 2007 o complexo de soja foi responsável por 19 por cento do total das exportações do agronegócio brasileiro, gerando mais de U$11 bilhões em divisas. Projeções indicam que até 2020 o Brasil deverá produzir 105 milhões de toneladas de soja, ou 81 por cento a mais do que a atual produção. Apesar de incidir sobre a produtividade a maior expectativa de crescimento, espera-se que a expansão significativa em área ocorra nas regiões de baixa latitude dos Cerrados, e em especial, no estado do Mato Grosso. É fundamental a determinação de estratégias que considerem produção, dimensões econômicas e ambientais. O uso da modelagem agrícola é a única forma de explorar sistematicamente o potencial produtivo em escala global e predizer produtividades futuras em determinadas condições. A utilização de computadores e banco de dados torna-se uma ferramenta cada vez mais necessária, no sentido de disponibilizar rapidamente informações de qualidade a um custo relativamente inferior, quando em comparação a experimentos de campo. O modelo LINTUL, desenvolvido pela Universidade de Wageningen, foi extensamente utilizado para predizer a produtividade potencial de culturas baseado no uso eficiente da radiação solar, sob condições ótimas de desenvolvimento. A cultura de soja, porém, nunca foi testada por este modelo. Sendo assim, os objetivos deste trabalho foram: (i) adaptação, por intermédio da parametrização e da calibração do modelo LINTUL para estimação da produtividade potencial da cultura de soja; (ii) disponibilizar um algoritmo em Visual Basic (Microsoft), validando e replicando os procedimentos do LINTUL, tornando este modelo acessível a uma ampla gama de usuários. Para a parametrização do modelo, foram consideradas características específicas da cultura, como por exemplo, a sensibilidade ao fotoperíodo. Para calibração, foram usados dados de experimento de campo conduzido em Planaltina-DFe Sorriso- MT. Os parâmetros não avaliados no experimento foram extensivamente revisados com base na literatura. Os principais componentes do processo de crescimento e desenvolvimento da cultura foram analisados individualmente através de comparativos de respostas. Posteriormente, um estudo de adequação do modelo foi feito, com objetivo de viabilizar uma comparação de comportamentos, quando em diferentes situações. As variáveis climáticas usadas neste modelo foram temperatura máxima e mínima e radiação solar. A simulação dos estádios fenológicos é baseada na soma térmica. O modelo LINTUL-soja demonstrou excelente desempenho quanto à predição de tendências, com a variação dos valores dos parâmetros. Todavia, o modelo necessita de ajustes quanto a introdução de módulos, que contemplem o balanço de carboidratos. O uso de uma linguagem de programação aberta, como o Visual Basic, replicou a linguagem usada pelo modelo LINTUL com sucesso. / In 2007 the soybean complex was responsible for more than 19% of the total Brazilian agribusiness exports, which corresponded to US$ 11 billion generated in currencies. According to projections, soybean production in Brazil will increase significantly till 2020, reaching 105 million tons, or an addition of 81 percent to the current crop season. That huge difference is expected to come from a significant increase in yield, while the increase in acreage should come from the low-latitude regions of Cerrados, mainly in Mato Grosso state. Thus, it becomes extremely important to set out strategies that consider production, economic and environmental dimensions. The usage of agricultural modeling is the only mean of systematically exploring the production potential on global scale and predict future yield in certain conditions. Computers and database application becomes an implement even more efficient in respect of turn quickly and quality information available with lower costs, when compared to the field experiments. The LINTUL model was developed by the Wageningen University, and extensively used to predict crop potential production based on radiation use efficiency concept, under optimal conditions of growth. The soybean crop has never been tested by this model. The goals of this research are: (i) to adapt - through parameterizing and calibration - the LINTUL model to estimate the potential production of soybean; (ii) to provide an algorithm in Visual Basic language, in order to validate the calculations and replicate the procedures executed by LINTUL, making this model accessible to a wide range of users. In order to parameterize the model, crop-specific characteristics were taken into account, such as the sensibility of soybean to photoperiod. In respect to calibration, it was used field data collected in Planaltina-DF and Sorriso-MT. The mainly components of the growth and development process were analyzed based on response comparatives. Afterward, it was done a study of adequacy of the model, in order to enable a comparison of behaviors, when in different situations. The climatic variables used in this model were maximum and minimum temperature and solar radiation. The simulation of phenological stages was based on thermal unit concept. The LINTUL-soybean model has demonstrated an excellent performance to predict trends, with the change in the parameters values. However, the model needs to be adjusted in respect to an introduction module to calculate the carbohydrates balance into the canopy. The use of an open programming language, such as Visual Basic, replied the language used by LINTUL model successfully.

Bibliography: leaves 131-150.

Photoperiodism refers to the organism’s ability to detect and respond to seasonal changes in the daily duration of light and dark and thus constitutes one of the most significant and complex examples of the interaction between the organism and its environment. This thesis attempts to describe the prevalence of variation in a photoperiodic response, its adaptive value, and its putative genetic basis in a common cruciferous weed, Capsella bursa-pastoris (Brassicaceae). Furthermore, the thesis presents a first comprehensive comparative overview of the circadian clock mechanism in an early land plant, Physcomitrella patens (Bryophyta), thus providing insights into the evolution of the plant circadian system. In an introductory survey of global gene expression changes among early- and late flowering accessions of C. bursa-pastoris we found an enrichment of genes involved in photoperiodic response and regulation of the circadian clock. Secondly, by phenotyping circadian rhythm variation in a worldwide sample of accessions with known flowering time, we detected robust latitudinal clines in flowering time and circadian period length, which constitute strong indications of local adaptation to photoperiod in the shaping of flowering time variation in this species. In an attempt to elucidate putative genetic causes for the correlated variation between circadian rhythm and flowering time, we found that sequence variation and diverged expression in components regulating light input to the clock, PHYTOCHROME B (PHYB) and DE-ETIOLATED 1 (DET1) make them strong candidate genes. Finally, we present a comparative study of circadian network topology in the moss P. patens. Phylogenetic analyses and time series expression studies of putative clock homologues indicated that several core clock genes present in vascular plants appeared to be lacking in the moss. Consequently, while the clock mechanism in higher plants constitutes at least a three-loop system of interacting components, the moss clock appears to comprise only a single loop. We conclude that C. bursa-pastoris is a highly suitable model system for the further elucidation of the molecular variation that influences adaptive change in natural plant populations. Furthermore, we believe that the continuing study of the seemingly less complex circadian network of P. patens not only can provide insights into the evolution of the plant circadian system, but also may help to clarify some of the remaining issues of the circadian clock mechanism in higher plants.

Thesis (Ph. D.)--University of Oregon, 2002. / Typescript. Includes vita and abstract. Includes bibliographical references (leaves 174-182).

Despite teleosts being the largest and most diverse group of vertebrates, fish models currently used to study photoperiodic effects on fish physiology have been limited to a few species, most of which are temperate seasonal breeders. The overall aim of this work was to expand our knowledge on circadian biology and environmental physiological effects in Nile tilapia (Oreochromis niloticus niloticus), a continuous breeding species of tropical-subtropical origin. The circadian light axis of Nile tilapia is described with regards to melatonin production. Circadian melatonin profiles of fish under 12L:12D photoperiods were observed to be low at day and high at night, suggesting melatonin to be an entraining signal as observed in all other vertebrates. When constant light (LL) was used, such day and night fluctuations where abolished. However when fish where exposed to constant darkness (DD) a strong robust endogenous melatonin rhythm was found, suggesting the presence of circadian oscillators in this species. Importantly, this endogenous rhythm was observed to be maintained for at least three weeks under darkness and proved to be circadian in nature. Moreover, although the melatonin system was able to produce day and night melatonin rhythms when exposed to a different (6L:6D) photocycle, the oscillator appeared to not be entrainable to such a short photo cycle when exposed to DD, as melatonin levels remained high. When comparing the circadian organization of different teleost species including Nile tilapia, preliminary studies showed at least three divergent circadian light organizations in teleosts. Nile tilapia was characterised by a pineal gland far less sensitive than in other fish species as demonstrated through in vitro studies. Furthermore, pineal melatonin production was clearly dependent on the light perceived by the eyes as ophthalmectomy resulted in basal plasma melatonin levels during the dark period. These findings are the first to be reported in a teleost and could be comparable to the circadian light organization of higher vertebrates such as mammals. The onset of puberty of Nile tilapia was studied with regards to the newly discovered Kiss1/GPR54 system. Such a system has recently been discovered in mammals and found to be the primary switch of the brain-pituitary-gonadal (BPG) axis. The results of this study not only suggest a link between the Kiss1/GPR54 system and the onset of III puberty in this tropical batch spawning teleost, that would be a highly conserved feature across vertebrates, but also that the transcriptional mechanisms regulating GPR54 expression could be directly or indirectly influenced by light. Finally, a study was conducted on the effects of different intensities of continuous light (LL) on the growth and sexual development of Nile tilapia up to first maturation. The results showed a significant growth response of fish in all LL treatments compared to control fish. Importantly, this confirmed that LL enhances growth in this species and suggests that it is the light regime more than the intensity which is having an effect. This work thus provides important basic knowledge of the light entrainment pathway and circadian melatonin rhythms in Nile tilapia. Of special importance is the discovery of a strong endogenous melatonin oscillator and a novel circadian organization in fish which would seem to be homologous to that observed in higher vertebrates. Moreover, this work provides evidence that the newly discovered Kiss1/GPR54 system has a similar role in fish as has been found in mammals and that such a system could be directly or indirectly regulated by light. If so, Nile tilapia and other fish species could become important models in the chronobiology and reproduction fields. Finally, this work not only increases our basic and applied knowledge of this species, but also broadens our understanding of the circadian light axis in teleosts and its mediatory effects on reproduction.

Sexual maturation during commercial culture of Atlantic cod (Gadus morhua L.) represents a significant production bottleneck restricting the profitability of the industry. Such problems in other species have traditionally been addressed by artificial manipulation of photoperiod cycles, however little research exists in this field in cod. This thesis therefore investigates the interactions between artificial photoperiod manipulation, sexual maturation and somatic growth in this species. In the first experiment, populations of Atlantic cod (hatched, spring 1999) were maintained on either a simulated natural photoperiod (SNP) or continuous illumination (LL) from approximately 15 months post hatch (MPH) (July 2000) in an enclosed tank system. Growth performance was recorded monthly along with observations of reproductive activity over the subsequent 2 years (up to July 2002). At both 2 and 3 years of age the entire population raised under SNP matured and spawned, during which time mean weight reduced by 13% and 24% respectively. No spawning individuals were recorded at 2 years of age in the LL population and only 18% were observed to spawn at 3. However, observations of both changes in gonadal morphology (observed via ultrasound scanning) and a suppression in growth rate at 2 years of age in the LL population alluded to a maturation “dummy run” regulated by an endogenous clock. Despite this phenomenon, the LL treatment realised a 39% and 43% improvement in wet weight following 1 and 2 years of exposure to LL respectively. When the diel cycle of plasma melatonin was compared between the treatments in February 2001 (23MPH) the SNP population displayed an A-profile diel rhythm ranging between 20 and 50 pg/ml while the LL treatment did not display any rhythm. In the second experiment of this work, two populations of cod (hatched, spring 2001) were reared in commercial open cage systems, one of which experienced continuous additional artificial illumination between July 2002 (15MPH) and October 2003 (30MPH) provided by four, 400W submerged lighting units. Growth and maturation were assessed in both populations throughout. In March 2003 (24MPH) it was apparent that spawning individuals were present in both the SNP and LL populations though a significantly lower number of spawning individuals in the LL treatment suggested that the peak in spawning activity was delayed by about 1 to 2 months. With both populations apparently maturing at 2 years of age, there was no significant difference in weight between the populations at the end of the trial. In comparison to experiments I and IV of this work, these results would suggest that in comparison to salmonids for example, Atlantic cod appear to have a heightened sensitivity to light allowing individuals to differentiate the ambient photoperiod signal from the application of continuous artificial light. In the third experiment, 6 populations of approximately 20 tagged individuals (hatched spring 1999) were maintained, from December 2000 to July 2002, under either SNP, LL or one of four, out of season “square wave” photoperiod regimes (repeating cycles with a 12 month period, consisting of a 6 month window of LL followed by six months of short day lengths [SD, 7L:17D] which had been staggered to start over a six month period). Each individual was monitored monthly for maturation status. Out of season “square wave” photoperiods were demonstrated to successfully entrain maturation and hence significantly alter the spawning profiles in these populations. Application of LL from December 2000 failed to inhibit maturation in the spring of 2001 and, in fact, advanced the spawning season by 1 month while those that experienced SD from the same date showed significant extension of the subsequent spawning season. Interestingly, the males maintained on LL throughout the experiment matured both in the spring of 2001 and one year later in the spring of 2002 while females under the same treatment only matured and spawned in 2001. In the fourth experiment, a total of 830 tagged individuals were raised either under SNP or one of 7 photoperiod treatments, consisting of 5 groups transferred from SNP to LL at 3 monthly intervals between 6 and 18 MPH where they remained and a further two groups maintained on LL from 6 to 15MPH and 6 to 21 MPH respectively before being returned to SNP. Both the gonadic and somatic axes were monitored at the physiological and endocrinological level at three monthly intervals from 6 to 27 MPH. The results demonstrated that it is the falling autumnal photoperiod signal after the summer solstice, more specifically after October, that is responsible for recruiting individuals to enter the sexual maturation cycle. Furthermore, in all treatments where this signal was masked i.e. those which experienced LL starting at or prior to 15MPH, except for some restricted spermatogenic activity in the males testis observed at 27MPH, there was no significant reproductive activity and growth was improved by up to 60% at 27 MPH. While providing evidence for direct photic stimulation of somatic growth, the growth results were also correlated with the measurement of plasma IGF-I and demonstrated its potential as a tool to assess growth rates in the species. Plasma melatonin measured at 15MPH, as in experiment I, was suppressed in all populations which were under LL photoperiods. By identifying the photoperiod “window of opportunity” which recruits individuals into the sexual maturation cycle, this work was able to conclude that the application of LL from the summer solstice prior to maturation is the most efficient photoperiod strategy to be adopted by the aquaculture industry to realise maximum growth potential from their cultured stocks.