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Mortality in eiders (Somateria mollissima) on the Ythan estuary, Aberdeenshire, and elsewhere, has been claimed to be associated with the spiny-headed worm, Profilicollis botulus. Eiders acquire the disease by eating crabs (Carcinus maenas) infected with the cystacanth stage of the parasite. A detailed quantitative study was undertaken to elucidate some of the factors which control the abundance of the parasite in its intermediate and definitive hosts. During 1979-81 the P. botulus burdens in eiders and crabs were regularly monitored. Eiders were either taken alive off the estuary, or off the nest, and sacrificed, or found dead on the shore line. Crabs were caught in traps at two sites on the estuary. Female P. botulus in eiders were assigned to one of five categories according to the state of development of the eggs contained within their pseudocoelom. To estimate the size range and numbers of crabs eaten by eiders, crab chelipeds, or parts of chelipeds, were recovered from eider faeces collected monthly from roost sites. Regression equations relating certain cheliped dimensions to crab carapace width were used to estimate the size range eaten. The numbers of chelipeds per unit weight of faeces, equivalent to the quantity excreted per eider per day, was used to estimate the numbers of crabs eaten. Daily P. botulus infection rates in eiders were calculated by combining the quantitative data on crab intake by eiders with the cystacanth burdens in the crabs. Captive eiders, reared from eggs, were used to determine the age, days post-infection, of the five categories of female parasites. These development times were combined with the burdens found in the eiders sampled from the estuary to produce a static life-table of the mortality of female P. botulus. From this, estimates of the parasite's establishment rate, mortality rate and lifespan were obtained.

Egg weight and fecundity of Clyde spring-spawning and autumn-spawning herrings, Clupea harengus L., in the nothern North Sea and North Minch in different years were analysed. For Clyde spring-spawning herring high variation in egg weight was found and it was attributed to more than one spawning group spawning in the area. Annual variation in fecundity between 1963-1984 was found. Fecundity in 1984 was found to be about 10% higher than previous years. For autumn-spawning herring, fish spawn in the North Minch produce about 1.18 times heavier eggs than those spawning in the northern North Sea. Regional annual variation in fecundity between 1971-1984 was found. In 1984 fecundity of samples collected from Shetland was 1.12 higher than those of North Minch. Fecundity indices for Shetland samples were found to be 1.23 to 1.77 higher in 1984 than 1971-1973. Annual variation between 1971-1973 was also found. It was concluded that fecundity is a potential contributor to population regulation not necessarily related to population density. Monthly analysis of energy content of soma and gonad was carried out for Clyde spring-spawning herring. Differences in the magnitude and timing of energy allocated to the gonad was found between sexes. Energy budget was prepared for each sex at ages 2 and 3. No deficient of energy during winter was apparent due to gonad maturation and metabolic activity. Routine metabolic rate was measured in a group of herring in a respirometer. Oxygen consumption was estimated to be 31.3, 93.3 and 278.6 mgO2 kg-1 h-1 at 5, 10 and 15oC respectively. Standard oxygen consumption was derived from oxygen consumption and theoretical swimming speed to be 20.4, 59.5 and 164.8 mgO2 kg-1h-1 at 5, 10 and 15oC.

Orientador: Roberto André Kraenkel / Banca: André M. de Roos / Banca: Marcus A.M. de Aguiar / Resumo: Predação intraguilda é um tipo de interação muito comum entre as espécies e pode influenciar fortemente na composição das comunidades ecológicas. Ela ocorre quando dois consumidores que compartilham de um mesmo recurso, e portanto competem, também apresentam comportamento predatório entre si. O consumidor que preda o seu competidor é frequentemente chamado de predador intraguilda, enquanto aquele que é predado é conhecido como presa intraguilda. Nesse trabalho nós investigamos alguns aspectos teóricos sobre esse tipo de interação. Primeiramente analisamos um experimento com ácaros predadores que foi realizado para testas as predições da teoria sobre padrões de exclusão em um gradiente de produtividade. Embora esse experimento foi cuidadosamente projetado para testar afirmações da teoria, seus resultados não concordam com ela. Utilizando um modelo bem simples para predação intraguilda que serve como uma representação daquele sistema, nós mostramos que: se levarmos em conta a dinâmica durante os transientes, e não apenas os resultados no equilíbrio, em que a teoria usual se baseia, e considerarmos que quando uma população atinge níveis muito baixos corresponde a uma extinção na realidade, os resultados experimentais concordam com a teoria. Em seguida nós estudamos questões que dizem respeito a influência dos diferentes estágios de vida dos indivíduos para a dinâmica da predação intraguilda. Nós introduzimos um modelo com estrutura de estágio em ambos os consumidores e consideramos a predação ocorrendo apenas dos adultos do predador intraguilda sobre os juvenis da presa intraguilda. Tem-se acreditado que essa interação dependente de estágio pode ter grandes efeitos sobre a dinâmica, uma vez que a pressão predatória sobre a presa intraguilda é reduzida... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Intraguild predation is a widespread interaction between species and can strongly influence communities composition. It occurs when two consumers which share a common resource, and hence compete, also engage into predation. The consumer which can prey on its competitor is often referred to as the intraguild predator while the other is called intraguild prey. In this work we investigate some theoretical aspects about these interactions. First we analyse an experiment with predatory mites which was carried to test patterns of exclusion along a productivity gradient, predicted by theory. Although this experiment was carefully designed to test the theoretical assertions, their results do not agree with theory. Through a very simple model for intraguild predation which serves as a representation of that system, we show that: if the short-term dynamics is taken into account rather than only equilibrium states, in which the usual theory is based, and we consider that populations that attain levels very close to zero are extinct, experimental results meet theory. Then we study questions concerning the influence of different life stages of individuals on the dynamics of intraguild predation. We introduce a model with stage structure in both consumers and consider predation occurring only from adults of the intraguild predator on juveniles of the intraguild prey. This stage dependent interaction was believed t ohave great effects on the dynamics, once predation pressure on the intraguild prey is reduced, and has been proposed as one feature that could promote coexistence. We check the outcomes of the system along a productivity gradient and show that stage structure do not induce great qualitative changes on the dynamics and the more likely resulting dynamics continues being the extinction of one of the consumers... (Complete abstract click electronic access below) / Mestre

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