Climate change, invasive species and the shifting distribution of Mediterranean fishes: a large scale assessment based on Local Ecological Knowledge

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