Abstract

The field of citizen science is flourishing, and although terrestrial projects are more visible, in recent years thousands of volunteers have actively participated in marine research activities. These volunteers (also termed 'citizen scientists') may have experience in the research in which they are participating, but they have no formal degree in marine science or related topics. The participation of large numbers of volunteers with variable educational or professional backgrounds poses particular challenges for the professional scientists coordinating such research. Knowledge about the structure of these projects, the research activities conducted by citizen scientists, and quality control of data collected by volunteers is essential to identify their contribution to marine science. We examined 227 published studies in which professional scientists collaborated with volunteers in a wide range of marine investigations. Most studies focused on a diverse assemblage of animals, followed by flora and other topics (e.g., contamination or beach dynamics). Seabirds, marine mammals, turtles, and fishes were the most commonly studied animals, but several studies also dealt with marine invertebrates. Many of the studied taxa were commercially important, emblematic, or endangered species. Surveys of invasive species took advantage of the extensive spatial scale that can be covered by large numbers of volunteers. As would be expected, the research activities of citizen scientists were concentrated in easily accessible coastal habitats, including sandy beaches, estuaries, coral reefs, and seagrass beds. Hot spots of marine citizen science projects (CSPs) were found not only in North America and Europe, but also in the Indo-West Pacific region. Contributions made by citizen scientists were equally based on incidental observations as on standardized surveys. Some of the research projects had been active for more than a decade, but most were midterm programmes, lasting a few years or less. Volunteer participants came from a wide range of demographic backgrounds. Usually, the participants were adults of both sexes, but a few studies considered either only men or only women (mainly in small fishing communities). Whereas several studies were based on schoolchildren as volunteers, no study worked specifically with senior citizens. The educational level of participants, often not explicitly mentioned in the publications, was also diverse. Some projects selected participants based on their experiences, skills, or profession, but in the majority of the studies, there was either no selection or no information was provided, suggesting that any interested citizen could participate. The preparation of participants ranged from brief written or oral instructions to extensive (weeks) training sessions with professional experts. In general, training effort increased with the complexity of the tasks conducted by volunteers, a crucial element being the adjustment of simple methodologies to the capabilities of participants. Studies for which volunteers needed to identify many different species and estimate their abundances were considered the most complex tasks, and subsequent analysis of such studies by professional scientists must consider inherent bias or shortcomings. About half of the examined studies included some type of quality control to ensure that the data collected by citizen scientists met the standards of rigorous scientific studies. Several authors emphasized that data quality increased with the duration of project participation. Efforts therefore should be made to retain experienced volunteers over time, which is facilitated when volunteers perceive that their efforts lead to something of practical use, such as publications, conservation initiatives, management decisions, or policy actions. Participants seemed to value personal satisfaction and public recognition, but learning about the ocean was also important. The coordinators of marine CSPs often collaborated with organizations such as conservation groups, birdwatchers, dive associations, or fishermen's cooperatives to recruit volunteers, but media campaigns, personal communication, social media, and functional websites were also important. Some studies were based on small numbers of participants (e.g., artisanal fishermen); others involved thousands of volunteers (e.g., coral or litter surveys). Volunteer-generated data contributed information about population dynamics, health, or distribution of marine organisms and supported long-term monitoring programmes of marine protected areas, harmful algal blooms, or marine litter, among others. In general, the contribution of citizen scientists greatly enhances research capacity, providing an increased workforce over extensive spatial and intensive temporal scales at comparatively moderate costs. Citizen science is able to make significant contributions to marine science, where professional scientific activities are limited by the available human resources. Considering the vastness of the oceans and the diversity of habitats, communities, and species, proper understanding of this realm requires intensive research activities over time and space. This recognition should lead to increased consideration of citizen science as a powerful tool for the generation and spread of scientific knowledge. Furthermore, sharing knowledge between volunteer participants and professional scientists improves communication, trust, and capacity building, facilitating efficient collaboration in much-needed conservation initiatives.