The rapid expansion of Great Cormorant populations during the second part of the 20th century has caused many conflicts, mostly with fisheries. Cormorants are often considered as important reason for depleted fish stocks, although evidences are often insufficient. This study of the Great Cormorant diet took place in the largest Lithuanian colony near Juodkrantė. Important for the impact evaluation cormorant nutrition aspects, including diet composition, its temporal variation and feeding selectivity, were determined. The reliability of pellet analysis for quantitative dietary assessments was evaluated for the first time using a stable isotope mixing model. Great Cormorant impact on spatial fish distribution was also assessed for the first time. Long-term fish community monitoring data were used for Cormorant impact assessment in the Curonian Lagoon. The results of our study are important for providing improved assessments of the long-term effects of Cormorants on fish communities in large, complex, highly productive aquatic systems. The practical significance of this study is its evaluation of competition between cormorants and commercial fishermen, which enables to substantiate the need to regulate cormorant population. Assessments of Cormorant impact on invasive Round Goby populations is also important. / XX amžiaus antroje pusėje didžiųjų kormoranų populiacijos Europoje staigus augimas sukėlė eilę konfliktų, daugiausia su žvejais ir žuvų augintojais. Kormoranai dažnai kaltinami dėl mažėjančių žuvų išteklių, tačiau moksliniais tyrimais pagrįstų įrodymų dažnai trūksta. Šiame tyrime buvo tiriama didžiųjų kormoranų mityba didžiausioje Lietuvoje Juodkrantės kolonijoje. Darbe buvo nustatyti svarbūs kormoranų poveikio vertinimui mitybos aspektai – raciono sudėtis ir jos kitimas laike, mitybos selektyvumas. Pirmą kartą buvo įvertintas atrajų analizės metodo patikimumas raciono sudėties kiekybiniam vertinimui, naudojant stabiliųjų izotopų sudėties analizės metodą. Taip pat pirmą kartą buvo vertinamas didžiųjų kormoranų poveikis žuvų populiacijoms erdvėje. Remiantis daugiamečio monitoringo duomenimis, buvo įvertintas kormoranų poveikis žuvų populiacijoms Kuršių mariose. Šio tyrimo rezultatai leidžia geriau įvertinti didžiųjų kormoranų ilgalaikį poveikį žuvų bendrijoms didelėse sudėtingose aukšto produktyvumo vandens sistemose. Praktinę vertę šiame darbe turi kormoranų ir žvejų verslininkų tiesioginės ir netiesioginės konkurencijos įvertinimas, leidžiantis pagrįsti kormoranų populiacijos reguliavimo priemonių tikslingumą. Svarbus praktiškai yra ir kormoranų poveikio invazinio juodažiočio grundalo populiacijai įvertinimas.

(I) Background. Conflicts emerge when populations of protected species grow to sizes that cause noticeable economic damage - like in the case of the fish consuming Great Cormorant (Phalacrocorax carbo sinensis). One possible approach for reconciliation is to regulate the size of the population in question. In doing so, regulation strategies have to meet multiple targets: 1) population size has to be reduced; 2) the viability of the population has to be maintained; 3) strategies have to adhere to the available budget. This thesis focuses on the regulation of populations that are structured into two groups - breeders and mature non-breeders. The pool of non-breeders provides a reserve for the breeders, whereby they may enable the population to resist regulation attempts. (II) Aims. 1) Development of a modeling framework and a conceptual model to provide an understanding of the functioning and effect of the population structure induced by non-breeders on population dynamics in a fluctuating environment. 2) Uncover the relation between non-breeder characteristics and the performance of regulation strategies. 3) Application of the modeling approach to the regulation of the Cormorant in order to evaluate the results from the conceptual model and find statements to support decisions on management strategies. (III) Methods. A conceptual stochastic time-discrete model, based on the logistic map with overlapping generations, is developed. Different types of threshold regulation strategies are applied. Strategies differed in which part of the model was affected by regulation. Resulting rules from the conceptual model are tested by applying them to a second age-structured model of a cormorant population, parametrized with data gained from a cormorant colony in Denmark. Analyzes of this model focus on the ecological-economic performance of regulation strategies and result in rankings of regulation options. Regulation performance is judged from different economic perspectives.

<p>The strong increase in number of Great cormorants <i>Phalacrocorax carbo</i> in Sweden in recent years has led to conflicts - particularly with fishery. This thesis focuses on the possible effects of cormorant predation on fish populations. In total, data from 15 lakes in South Sweden were included in this study while most studies were carried out in Lake Ymsen. The results suggest that the impact of cormorant predation on natural fish populations was small, and I observed no decline in fish mass after cormorants established. Cormorant predation on eel was difficult to evaluate because of several confounding factors.</p><p>Ruffe, roach and perch were the most important prey species to the cormorants and most fish taken were small. Cormorants do not seem to catch species and sizes in proportion to their occurrence in the fish community.</p><p>Total fish removal by cormorants varied considerably among lakes (0.2-15.0 kg/ha) and cormorant population sizes at the different lakes were significantly positively correlated with fishery catches, which in turn was significantly positively correlated with total phosphorous levels. Thus, cormorant densities in lakes, and perhaps elsewhere, seem to be governed chiefly by fish densities. The fact that cormorant predation appears not to reduce fish densities suggest cormorants to be regulated by other means than prey depletion. The mechanism behind population regulation could be a behavioural response of fish, making fish more difficult to catch for the cormorants.</p><p>In recent years, cormorant populations have been subjected to intensive legal and illegal actions with the aim to reduce cormorant numbers. However, the actions currently carried are well below the efforts needed to limit population sizes. To conclude, cormorants appear to compete little with fishery, with regards to free-living fish. The main problem is that cormorants sometimes damage and take away fish in fishing gears.</p>

The strong increase in number of Great cormorants Phalacrocorax carbo in Sweden in recent years has led to conflicts - particularly with fishery. This thesis focuses on the possible effects of cormorant predation on fish populations. In total, data from 15 lakes in South Sweden were included in this study while most studies were carried out in Lake Ymsen. The results suggest that the impact of cormorant predation on natural fish populations was small, and I observed no decline in fish mass after cormorants established. Cormorant predation on eel was difficult to evaluate because of several confounding factors. Ruffe, roach and perch were the most important prey species to the cormorants and most fish taken were small. Cormorants do not seem to catch species and sizes in proportion to their occurrence in the fish community. Total fish removal by cormorants varied considerably among lakes (0.2-15.0 kg/ha) and cormorant population sizes at the different lakes were significantly positively correlated with fishery catches, which in turn was significantly positively correlated with total phosphorous levels. Thus, cormorant densities in lakes, and perhaps elsewhere, seem to be governed chiefly by fish densities. The fact that cormorant predation appears not to reduce fish densities suggest cormorants to be regulated by other means than prey depletion. The mechanism behind population regulation could be a behavioural response of fish, making fish more difficult to catch for the cormorants. In recent years, cormorant populations have been subjected to intensive legal and illegal actions with the aim to reduce cormorant numbers. However, the actions currently carried are well below the efforts needed to limit population sizes. To conclude, cormorants appear to compete little with fishery, with regards to free-living fish. The main problem is that cormorants sometimes damage and take away fish in fishing gears.