Climate change is causing water temperature to rise, and many lakes in the boreal zone will experience browning of waters (brownification) due to increased input of dissolved organic carbon (DOC). In fish, warming may cause resource limitation and decrease both fish size and population abundance. Many fish species display ontogenetic niche shifts during their lifetime, shifting to larger prey as they grow. Brownification may change the timing for, the benefits from or prevent individuals from displaying ontogenetic niche shifts by decreasing large prey abundance in the benthic zone or making fast-moving prey harder to see. This can cause resource limitations, suppressing growth and population growth. This study investigated the effects of increasing DOC and water temperature on ontogenetic diet shifts, size structure, and population abundance in nine-spine stickleback (Pungitius pungitius) in an experimental pond system with a warming treatment and a gradient of DOC concentration. Warming had a negative effect on population number, biomass, maximum fish size, stomach fullness, and consumption of large prey. Contrary to expected outcomes, increasing DOC input resulted in higher population, biomass, and larger maximum sized fish as DOC increased. DOC did not negatively affect ontogenetic diet shifts. In the relatively shallow enclosures, the highest DOC concentration may not have reached the threshold where the shading effect of DOC overturns the benefits of extra nutrients associated with DOC. Hence, in shallow lake ecosystems, climate change induced DOC increase may support fish production, while warming may have strong negative effects on fish population abundance and size.

Este trabalho teve como objetivo o estudo dos hábitos alimentares de três espécies de peixes da família Carangidae, da zona de surf da enseada de Caraguatatuba, São Paulo. Amostras de Selene setapinnis, Selene vomer e Oligoplites saliens foram obtidas com rede de arrasto de porta, mensalmente, entre maio de 2003 e outubro de 2004, em duas áreas previamente selecionadas na enseada, com profundidade variando entre 1 e 5 metros. Foram medidos e pesados 3022 exemplares, e 1367 estômagos foram retirados para a análise de conteúdo estomacal. A composição da dieta foi analisada por meio das frequências de ocorrência, numérica, gravimétrica, e volumétrica, e de índices alimentares. Todos os exemplares eram jovens e a sua dieta foi composta principalmente por Crustacea. Outros grandes grupos presentes foram Chaetognatha e Teleostei. Dentre os crustáceos, destacaramse as larvas de Decapoda, os misidáceos, os camarões Acetes americanus e os copépodes calanóides Labidocera fluviatilis e Acartia lilljeborgii. As variações intraespecíficas da dieta, bem como as relações interespecíficas, foram avaliadas através de análises de agrupamento. Foi observada uma tendência de aumento do tamanho da presa com o aumento do tamanho do peixe. Embora A. americanus tenha sido importante na dieta em todos os tamanhos, nas três espécies, houve maior destaque desse item nos peixes maiores, enquanto Lucifer faxoni e larvas de Decapoda foram mais importantes nos menores. Avaliando-se o comportamento alimentar, há indicações de que as três espécies tendem a ser generalistas e a explorar o hábitat de maneira semelhante. Em relação à variabilidade temporal, as espécies diferiram entre si: S. vomer, não apresentou diferença intra-anual significativa, ao contrário de S. setapinnis e O. saliens. / The purpose of this work was to study the food habits of three Carangidae juvenile fish species, from the surf zone in Caraguatatuba sound, São Paulo. Samples were taken monthly, with an otter trawl, from May 2003 to October 2004, in two areas previously selected in the bay, between 1 and 5 meters deep. Weight and length of 3002 specimens were taken, and 1367 stomach contents were examined. The diet composition was analyzed through frequencies of occurrence, number, weight and volume, and feeding indexes. The main food item was Crustacea, particularly Decapoda larvae, Mysidae, the shrimp Acetes americanus, and the calanoid copepods Labidocera fluviatilis and Acartia lilljeborgii. Chaetognatha and Teleostei were also present. Seasonal and ontogenetic variations of the diet and interspecific interactions were performed by similarity measures. Ontogenetic changes in diet were recognized; smaller fish consumed smaller prey, and the prey size increased with the body size. Though Acetes americanus were the main item of all sizes, it was more important in bigger fish, while Lucifer faxoni and Decapoda larvae were more important in smaller ones. Selene setapinnis and Oligoplites saliens showed temporal food variability, but Selene vomer did not. The three species were considered as generalists showed similarity in feeding habits.

Este trabalho teve como objetivo o estudo dos hábitos alimentares de três espécies de peixes da família Carangidae, da zona de surf da enseada de Caraguatatuba, São Paulo. Amostras de Selene setapinnis, Selene vomer e Oligoplites saliens foram obtidas com rede de arrasto de porta, mensalmente, entre maio de 2003 e outubro de 2004, em duas áreas previamente selecionadas na enseada, com profundidade variando entre 1 e 5 metros. Foram medidos e pesados 3022 exemplares, e 1367 estômagos foram retirados para a análise de conteúdo estomacal. A composição da dieta foi analisada por meio das frequências de ocorrência, numérica, gravimétrica, e volumétrica, e de índices alimentares. Todos os exemplares eram jovens e a sua dieta foi composta principalmente por Crustacea. Outros grandes grupos presentes foram Chaetognatha e Teleostei. Dentre os crustáceos, destacaramse as larvas de Decapoda, os misidáceos, os camarões Acetes americanus e os copépodes calanóides Labidocera fluviatilis e Acartia lilljeborgii. As variações intraespecíficas da dieta, bem como as relações interespecíficas, foram avaliadas através de análises de agrupamento. Foi observada uma tendência de aumento do tamanho da presa com o aumento do tamanho do peixe. Embora A. americanus tenha sido importante na dieta em todos os tamanhos, nas três espécies, houve maior destaque desse item nos peixes maiores, enquanto Lucifer faxoni e larvas de Decapoda foram mais importantes nos menores. Avaliando-se o comportamento alimentar, há indicações de que as três espécies tendem a ser generalistas e a explorar o hábitat de maneira semelhante. Em relação à variabilidade temporal, as espécies diferiram entre si: S. vomer, não apresentou diferença intra-anual significativa, ao contrário de S. setapinnis e O. saliens. / The purpose of this work was to study the food habits of three Carangidae juvenile fish species, from the surf zone in Caraguatatuba sound, São Paulo. Samples were taken monthly, with an otter trawl, from May 2003 to October 2004, in two areas previously selected in the bay, between 1 and 5 meters deep. Weight and length of 3002 specimens were taken, and 1367 stomach contents were examined. The diet composition was analyzed through frequencies of occurrence, number, weight and volume, and feeding indexes. The main food item was Crustacea, particularly Decapoda larvae, Mysidae, the shrimp Acetes americanus, and the calanoid copepods Labidocera fluviatilis and Acartia lilljeborgii. Chaetognatha and Teleostei were also present. Seasonal and ontogenetic variations of the diet and interspecific interactions were performed by similarity measures. Ontogenetic changes in diet were recognized; smaller fish consumed smaller prey, and the prey size increased with the body size. Though Acetes americanus were the main item of all sizes, it was more important in bigger fish, while Lucifer faxoni and Decapoda larvae were more important in smaller ones. Selene setapinnis and Oligoplites saliens showed temporal food variability, but Selene vomer did not. The three species were considered as generalists showed similarity in feeding habits.

When intraspecific individuals differ in resource intake, scramble competition occurs among inferior individuals growing food-dependently. Scramble can be released through predation mortality. As a consequence of this release, production rates in inferior individuals increase and biomass overcompensation in the subsequent life-stages may occur. When intraspecific individuals do not differ in their resource intakes biomass overcompensation does not occur. If an individual changes its resource intake over ontogeny, the balance of intake and losses, its energetics, will change over ontogeny. Furthermore, differences will arise between the energetics of different life-stages. The predominant volume of interspecific competition theory is based on studies assuming no stage-specific differences in energetics, neglecting the influence of ontogeny on community dynamics altogether. We study how an stage-specific differences in energetics affect expectations from conventional competition theory. We use a stage-structured biomass model consistently translating individual life history processes, in particular food-dependent growth in body size, to the population level. The stage-structured population can be reduced to an unstructured population, if the energetics of all individuals are assumed to be equal.  The stage-structured model, however can also describe population dynamics when this equality is broken. We use the stage-structured biomass model to contrast the stage-specific differences resulting in a stage-structured population model, with an unstructured population model assuming no differences between stages. We show that stage-specific differences in energetics can affect competition on various trophic levels. I: In stead of outcompeting each other, a predator can be facilitated by another preying a scrambling prey life-stage of the same prey population. II: In coexistence with their prey, omnivores with an ontogenetic diet shift, where juvenile omnivores feed on resource and adults on prey, affect community structure only as predators, not as competitors to their prey. We show coexistence of omnivore and prey is not possible if the dominating interaction is competition. Feeding on prey, however, alleviates competition with prey and facilitates the introduction of omnivores. III: An ontogenetic diet shift creates niche partitioning, where without it this would result in neutral coexistence of two consumers competing for two resources. IV: Furthermore, predators can change resource requirements of diet shifters such that diet shifters can reduce resources to lower equilibria and sustain higher predator biomass than consumers without stage-specific differences in energetics. / LEREC