A global dataset of carbon and nitrogen isotopic signatures of the bluefish Pomatomus saltatrix and its potential fish prey

The bluefish Pomatomus saltatrix is a pelagic predator occurring in temperate and subtropical waters worldwide; in the Mediterranean Sea it was originally distributed in the southern and eastern sectors of the basin, yet in the last decade it has greatly extended its range towards the northward and westward sectors of the Mediterranean basin. Range-shifting marine species such as the bluefish have the potential to determine novel species interactions altering community structure and ecosystem processes analogously to what observed for non-indigenous species, thus acting as “native invaders”. It is to date acknowledged that the functional traits of non-indigenous species (NIS hereafter) may provide key information for predicting their impact within invaded habitats. Successful biological invasions are often facilitated for generalist species that can shift and adapt their trophic role in invaded food webs, ultimately affecting their structure and functioning. In this context, the analysis of carbon and nitrogen stable isotopes can provide important clues on NIS (and native invaders) functional impacts, as they can be used to investigate NIS trophic habits and, in turn, to detect post-invasion changes in food web structure. Here we provide a harmonized global compilation of 30 geo-referenced records of Pomatomus saltatrix δ13C and δ15N values taken from the published literature. To this end, on September 16th 2021 the online databases ISI Web of Science and Scopus were searched for publications by a multiple search criterion using the term “Potamotus saltatrix” in conjunction with “stable isotopes”. The results were complemented with those obtained from queries on Google Scholar (https://scholar.google.com/) and saved using the freeware Harzings’s Publish or Perish ver. 7.27.284953, using identical keywords. The literature search resulted in a total of 522 sources; subsequently, their titles and abstracts were screened in order to remove laboratory studies and field investigations not directly performed in marine environments. The full text of the remaining 247 studies was examined in detail to filter those where P. saltatrix occurrence was reported explicitly, together with information on the country, latitude and longitude, the year of the record, and at least the mean δ15N values of sampled specimens. Publications where the sampling location had no exact coordinates but was reported in maps were also included in the selection. In addition, the sources to be selected had to include isotopic information on potential fish prey of the bluefish, with the ultimate aim of providing the isotopic baseline for the estimation of the trophic position of the predator. To this end, the prey genera listed by FishBase (www.fishbase.org) for Pomatomus saltatrix were used as a reference. In addition, the trophic position of prey species was drawn from FishBase. 30 eligible publications matching the aforementioned criteria were eventually selected. Mean δ15N values were extracted from tables and figures together with mean δ13C values, δ15N and δ13C standard deviations and sample numerosity when available; figures were digitized after a fivefold enlargement and converted to numerical form using a graph capture software (https://automeris.io/WebPlotDigitizer/). Information on the country, location, season of the study, size of P. saltatrix specimens and its potential prey species were also extracted when available. For those studies where sampling locations had no coordinates but were reported in maps, all contextual information was used to locate the geographic area of the study in Google Earth. Maps were extracted using the freeware GIMP (ver. 2.10.20, https://www.gimp.org/), overlaid to the study area in Google Earth, and adjusted to match the background. Subsequently, the sampling area was georeferenced using placemarks; the coordinates of the sampling area was used to estimate the latitude and longitude in decimal degrees of its centroid using the centroid function from geosphere R package (https://cran.rproject.org/web/packages/geosphere/index.html). The dataset is contained in Table 1; complementary information regarding the description of the fields included in the dataset and of the literature sources used to extract the data including permanent identifiers (bibliographic Citation DOI) if available are reported in Table 2 and 3, respectively.