Contributions to the assessment of the status of, and the impacts of stressors on, marine mammal populations

Marine top predators such as common dolphins, bottlenose dolphins and harbour porpoises are subject to a variety of natural and anthropogenic threats, such as bycatch, pollution, parasites and disease. These threats can negatively affect the survival and fecundity of individuals, which in turn can be transmitted between individuals of the same or other species, causing consequences at the population level. In addition, it is important to keep in mind that the effects of these threats can also be transmitted through the food web affecting the ecosystem. Knowing and assessing the effects of threats at the individual, population and ecosystem levels of these species, which are protected by national and international legislation (e.g. European Habitats Directive 92/43/EEC, European Marine Strategies Framework Directive 2008/56/EC), is essential to estimate the extent of their impact and to be able to propose effective mitigation measures. This PhD thesis aims to contribute directly and indirectly to the main steps in the process of assessing the effects of different natural and anthropogenic threats on cetacean populations: monitoring, assessment and mitigation. The thesis focuses mainly on the Northeast Atlantic, and addresses both transferable and non-transferable threats, i.e. threats whose effects can be transferred through the food web, through close contact and from mother to calf. Given the relatively poor integration of transferable threats into the impact assessment process, we dedicate particular emphasis on transferable threats, and on the cumulative effects of multiple threats acting simultaneously on populations. Also, given the relevance of the impact that by-catch is causing on these populations and on the fisheries sector, the sustainable limits of the accidental catcher will be explored, taking into account possible effects of another major threat (contaminants, in particular, PCBs). Given that one of the main sources of information to be used in this thesis is stranding nets, their potential to contribute to the study of bycatch will be evaluated. Specifically, during this thesis a comprehensive literature review of the impact of the main transferable threats (pathogens, parasites, chemical contaminants and microplastics) on the most common small cetacean species in the Northeast Atlantic: common dolphin, bottlenose dolphin, striped dolphin and harbour porpoise will be carried out. It will also review available information on the cumulative effects resulting from the interaction between multiple threats acting simultaneously on these species and their populations. Then information from different stranding networks in European countries with an Atlantic coastline will be compiled on their structure, limitations, protocols and data collected in order to assess their potential to monitor marine mammal populations, as well as to understand and quantify the impact of the threats they face (with special emphasis on the impact of bycatch) through stranding information. Finally, we will theoretically explore the utility of applying an intermediate complexity ecosystem model (MICE) originally designed to assess fishery stocks of speices of commercial interest, such as the Gadget (Globally applicable Area Disaggregated General Ecosystem Toolbox) model framework, to assess the status and impact of two of the main threats to the Northeast Atlantic common dolphin population distributed in the north of the Iberian Peninsula: bycatch and contaminants.