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Climate change and biological invasions are rapidly reshuffling species distribution, restructuring the biological communities of many ecosystems worldwide. Tracking these transformations in the marine environment is crucial, but our understanding of climate change effects and invasive species dynamics is often hampered by the practical challenge of surveying large geographical areas. Here, we focus on the Mediterranean Sea, a hot spot for climate change and biological invasions to investi- gate recent spatiotemporal changes in fish abundances and distribution. To this end, we accessed the local ecological knowledge (LEK) of small-scale and recreational fish- ers, reconstructing the dynamics of fish perceived as "new" or increasing in different fishing areas. Over 500 fishers across 95 locations and nine different countries were interviewed, and semiquantitative information on yearly changes in species abun- dance was collected. Overall, 75 species were mentioned by the respondents, mostly warm-adapted species of both native and exotic origin. Respondents belonging to the same biogeographic sectors described coherent spatial and temporal patterns, and gradients along latitudinal and longitudinal axes were revealed. This information pro- vides a more complete understanding of the shifting distribution of Mediterranean fishes and it also demonstrates that adequately structured LEK methodology might be applied successfully beyond the local scale, across national borders and jurisdic- tions. Acknowledging this potential through macroregional coordination could pave the way for future large-scale aggregations of individual observations, increasing our potential for integrated monitoring and conservation planning at the regional or even global level. This might help local communities to better understand, manage, and Email: eazzurr@gmail.com Funding information European Regional Development Fund, Grant/Award Number: MPA-Adapt 1MED15_3.2_M2_337; Albert II of Monaco Foundation 1 | INTRODUCTION The redistribution of Earth's species is among the most evident con- sequences of global warming (Parmesan & Yohe, 2003; Poloczanska et al., 2016) and a critical aspect for the health of both natural eco- systems and human populations worldwide (Pecl et al., 2017). These changes are usually greater for marine environments, because of their high environmental connectivity (Burrows et al., 2011) and because of the pivotal role of water temperatures, which strongly influence growth, survival, and reproduction in marine animals (Crozier & Hutchings, 2014; Reusch, 2014). In fact, even apparently modest changes in water temperature might trigger a rapid cascade of multiple pressures over marine organisms. Some species, unable to cope with these environmental alterations, or benefit from them, may change their abundances accordingly. However, mobile marine organisms also have another option: they can move to new areas where they were formerly absent (Cheung et al., 2009; Fogarty, Burrows, Pecl, Robinson, & Poloczanska, 2017). These two dynamics are not mutually exclusive, as they can be considered as different be- havioral and demographic responses that might coexist in the same species or population. Specifically, in the northern hemisphere, seawater warming has been associated with both the northward expansion of species and their increasing abundances (Fossheim et al., 2015; Perry, Low, Ellis, & Reynolds, 2005; Pörtner & Knust, 2007; Sabatés, Paloma, Lloret, & Raya, 2006). Yet, many studies provided evidence for the causal relationship between temperature, species distribution, and abun- dance (Cheung, Watson, & Pauly, 2013; Pinsky, Worm, Fogarty, Sarmiento, & Levin, 2013; Poloczanska et al., 2013) as well as their interplay with other global drivers, such as biological invasions, ma- rine overexploitation, and pollution (Stergiou, 2002; Walther et al., 2009). These changes, which are taking place across many differ- ent taxa and through different regions of the globe, have significant implications for biodiversity, ecosystems, and society (McGeoch & Latombe, 2016) and are considered to be particularly apparent in the Mediterranean, a semi-enclosed sea, which is warming faster than any other marine region in the world (Schroeder, Chiggiato, Bryden, Borghini, & Ben Ismail, 2016; Vargas-Yáñez et al., 2008). In addition, maritime traffic, mariculture, aquarium trade and above all, entries through the Suez Canal (Edelist, Rilov, Golani, Carlton, & Spanier, 2013; Parravicini, Azzurro, Kulbicki, & Belmaker, 2015) contribute to the introduction of a large number of nonindigenous species (hereafter referred as NIS) to this basin (Galil, Marchini, Occhipinti- Ambrogi, & Ojaveer, 2017; Golani et al., 2018; Zenetos et al., 2017), reshaping the structure of biological communities (Albouy et al., 2013, 2015, 2014; Katsanevakis et al., 2017) and impacting biodi- versity and fishery resources (Edelist et al., 2013). Despite the magnitude of these changes and their relevance for conservation and adaptation policy (Givan, Parravicini, Kulbicki, & Belmaker, 2017; Marras et al., 2015), observational studies are often fragmented in space (Elmendorf et al., 2015) and methodologically heterogeneous (Coll et al., 2010). This also applies to the northward expansions of warmwater species, a phenomenon that has been mostly described in the northwestern sectors of the Mediterranean basin, probably due to the uneven distribution of research ef- forts (Boero et al., 2008; Lejeusne, Chevaldonné, Pergent-Martini, Boudouresque, & Pérez, 2010; Marbà, Jordà, Agustí, Girard, & Duarte, 2015; Sabatés, Martín, & Raya, 2012). This fragmentation, together with the lack of coherent depictions of change, hampers the availability of reliable information to stakeholders and decision-mak- ers (Grafton, 2010; Pauly & Zeller, 2016). Indeed, in light of profound impacts that have already affected both people and the ecosystems they depend on, many national and transnational authorities and agencies are engaged in efforts to build adaptive capacity, seeking reliable information to enable people to anticipate and appropriately respond to the ongoing change (Coulthard, 2012). This explains the growing need of integrated monitoring and assessment systems to capture the ongoing transformations of marine ecosystems (includ- ing the effects of a changing climate) and to bring them into the pol- icy agendas (Creighton, Hobday, Lockwood, & Pecl, 2016). Certainly, our observational potential grew steadily during the last few years and increasing efforts are devoted to conceive global observation systems for up-to-date information on the state of biodiversity and the threats it faces (Tittensor et al., 2014). To achieve this, the use of standardized and cost-effective procedures is needed to underpin a large-scale observation strategy that can accommodate countries across a range of baseline knowledge levels and capabilities (Bélisle, Asselin, LeBlanc, & Gauthier, 2018; Latombe et al., 2017). These are key principles for collecting and integrating information from stake- holders across national boundaries. In this, fishers are a particularly interesting group of stakeholders, as they spend a considerable proportion of their lives in close contact with the marine environ- ment and they become familiar with local species. Therefore, their personal experience gained through individuals' observations over adapt to the ongoing biotic transformations driven by climate change and biological invaders.