In population management, the e ective population size, N e , can be viewed in tandem with actual population size, N , as the main factors determining a population's long-term viability and sustainability. N e is the number of individuals in an observed population that would lose genetic variation at the same rate as an ideal population. Understanding which demographic factors that a ect N e /N , will make resource allocation and decision making more e ective, either if the management goal is to maximise, maintain or minimize N e /N . The goal of this thesis was to calculate the demographic parameters that determine N e /N , following the method of Engen et al. (2010), and then determine which of these parameters N e /N is most sensitive to. In other words, determine which parameters that contribute most to the total variation in N e /N . This was done, using data on 13 Norwegian populations of house sparrows (Passer domesticus), including more than 4000 individuals, and spanning up to 20 years. To find which of the demographic parameters (demographic variance, generation time, stable age distribution, reproductive values, individual fecundity and survival) that a ect N e /N most, sensitivity analyses were carried out. Using the global variance-based Sobol' method, it was found that demographic variance, especially of older individuals, was the parameter that N e /N was most sensitive to. Generation time was found to be less important than demographic variance, which includes all the other parameters. The demographic variance of a population is determined by fecundity and survival on the individual level. The individual reproductive values were found to be most sensitive to fecundity, followed by survival. In contrast, the stable sex-age distributions, and the sex-age specific reproductive values, were found to be of little importance. For population management purposes, the results from this study show that resources should be focused on the manipulation of demographic variance in older individuals, more specifically their fecundity and survival. Even though these results are from insular populations of house sparrows, they may also apply to fragmented populations of other species with similar life histories and demography. i SammendragInnen populasjonsforvaltning er e ektiv populasjonsstørrelse, N e , sammen med observert populasjonsstørrelse, N , hovedfaktorene som avgjør overlevelse og hvor baerekraftig en populasjon er på sikt. N e er antallet individer i en observert populasjon som ville miste genetisk variasjon med samme rate som i en ideell populasion. A forstå hvilken faktorer som er med påå påvirke N e /N , vil gjøre både ressursallokering og avgjørelser innen forvaltning mer e ektive. Dette gjelder både for mål omå maksimere, opprettholde eller minimalisere N e /N . Formålet med denne oppgaven varå beregne de demografiske parameterne som inngår i N e /N , vedå følge metoden i Engen et al. (2010), for deretterå finne hvilke av disse parameterne N e /N er mest sensitiv til. Det vil si at man ide...

Harvesting is often size‐selective, and in species with sexual size dimorphism, it may also be sex‐selective. A powerful approach to investigate potential consequences of size‐ and/or sex‐selective harvesting is to simulate it in a demographic population model. We developed a population‐based integral projection model for a size‐ and sex‐structured species, the commonly exploited pike (Esox lucius). The model allows reproductive success to be proportional to body size and potentially limited by both sexes. We ran all harvest simulations with both lower size limits and slot limits, and to quantify the effects of selective harvesting, we calculated sex ratios and the long‐term population growth rate (λ). In addition, we quantified to what degree purely size‐selective harvesting was sex‐selective, and determined when λ shifted from being female to male limited under size‐ and sex‐selective harvesting. We found that purely size‐selective harvest can be sex‐selective, and that it depends on the harvest limits and the size distributions of the sexes. For the size‐ and sex‐selective harvest simulations, λ increased with harvest intensity up to a threshold as females limited reproduction. Beyond this threshold, males became the limiting sex, and λ decreased as more males were harvested. The peak in λ, and the corresponding sex ratio in harvest, varied with both the selectivity and the intensity of the harvest simulation. Our model represents a useful extension of size‐structured population models as it includes both sexes, relaxes the assumption of female dominance, and accounts for size‐dependent fecundity. The consequences of selective harvesting presented here are especially relevant for size‐ and sex‐structured exploited species, such as commercial fisheries. Thus, our model provides a useful contribution toward the development of more sustainable harvesting regimes.

1. Humans are influencing animal and plant populations both directly (e.g.through harvest) and indirectly by altering environments. For many exploitedspecies, stocking with captive-bred individuals is a common strategy tomitigate negative human impacts and sustain populations over time. However,accumulating knowledge of negative side effects of stocking calls for quantificationof consequences and exploration of sustainable alternatives.2. Evaluating alternative management strategies using quantitative models iscentral to conservation. Here, we investigate the effects of several managementstrategies on a population of landlocked, migratory brown trout (Salmotrutta) inhabiting a large lake and spawning in a dammed river. We assess thepopulation level consequences of terminating a long-term stocking programmeand evaluate whether the loss of artificial recruitment may be compensatedby changes in harvest regulations and/or river habitat improvement.3. We build an integral projection model (IPM) classifying individuals bybody size, life history stage, and location relative to the hydropower damand parameterised it with 50 years of individual-based data supplementedwith literature values. We first analyse the model to assess size, structure,and relative importance of different mortality components across life stagesand locations in trout populations with and without stocking. We theninvestigate potential responses of an unstocked population to managementactions involving different sets of harvest rules, reductions in dam passagemortality, and improvements of spawning habitat below the dam.4. Our model predicts a strong population decline of 12–21% per year in theabsence of stocking. This decline is largely attributed to high harvest mortality,and drastic reductions in fishing pressure thus necessary to ensure populationviability without stocking. Reducing mortality associated with passage of thehydropower dam and restoring spawning areas has only small positive effectson population growth. Nonetheless, these mitigation measures can contributeto population viability when combined with changes in harvest regulations.5. Intensely harvested populations may rely heavily on the addition of captive-bredindividuals, and our results indicate that premature termination of stockingprogrammes can be detrimental without compensatory mitigation measuressuch as harvest reductions and habitat improvements. It is therefore crucialto collect necessary data and assess the impacts of alternative managementstrategies using quantitative models prior to making decisions.

Freshwater species are particularly vulnerable to emerging threats linked to climate change because they are often already heavily impacted by habitat destruction, pollution, and exploitation. For many harvested populations of freshwater fish, these combined impacts have been mitigated for decades through stocking with captive-bred individuals. However, stocking may lead to loss of genetic variation, which may be crucial for adaptation under climate change. Exploration of sustainable alternatives is therefore paramount. We used a female-based integral projection model (IPM) to assess the consequences of terminating a long-term stocking programme for a population of landlocked, migratory brown trout Salmo trutta, and to evaluate relative effectiveness of alternative management strategies involving harvest regulations and river habitat improvement. The IPM classified individuals by body size, life history stage, and location relative to a hydropower dam, and was parameterised with 50 yr of individual-based data, supplemented with literature values. Model simulations indicated a strong population decline of 22-29% per year without stocking, much of which was attributed to high harvest mortality. Consequently, drastic reductions in fishing pressure were predicted to be necessary to ensure population viability without stocking. Mitigation measures reducing mortality associated with the hydropower dam or restoring spawning areas could further contribute to population viability when combined with changes in harvest regulations. Our results thus emphasise that large changes in management strategies, such as termination of long-term stocking programmes, require a thorough assessment of potential consequences and alternative mitigation strategies using data and models, or, at the very least, a precautionary approach under consideration of on-going climate change.

For species with individual variation in reproductive success, experience in breeding and the distribution of different breeders is important for population productivity and viability. Human impacts, such as climate change and harvesting, can alter this distribution and thus population dynamics. Here, we investigated the effect of spawning experience on population growth in a population of migratory brown trout Salmo trutta subject to stressors including migration barriers, harvesting, and climate change. We described the population dynamics with a structured integral projection model that differentiates between first-time and repeat spawners. We then took a scenario-based approach to test to which extent spawning experience has a positive effect on the population growth of brown trout by running 3 different model simulations: a baseline scenario with no changes to the reproductive output of the population, a non-selective scenario in which the reproductive output of all spawners was reduced, and a selective scenario where the reproductive output of only first-time spawners was reduced. We found that the reproductive output of repeat spawners is more important than that of first-time spawners for population growth, in line with other studies. Moreover, the contribution of first-time spawners to the population growth through their own survival is more important than their contribution to growth through reproduction. To ensure the continued existence of the study population, survival of first-time spawners and reproductive success of repeat spawners should be prioritised. More generally, including breeding experience adds more mechanistic detail, which ultimately can aid management and conservation efforts.