The complete dataset named: “[Article dataset] Establishing a comprehensive host-parasite stable isotope database to unravel trophic relationships”18 (https://doi.org/10.6084/m9.figshare.28087397) is readily available on Figshare and on ISOBANK19 (https://isobank.tacc.utexas.edu/). Our data-gathering template is comprehensive and designed to encompass as much useful information about the parasite-host system as possible. There are 60 required metadata fields, all of which can also be found in the same Figshare repository, as “ParaSITE project_Legend key”. Collaborators were asked to fill in this template as accurately as possible. When some information was not known or was inaccessible, default or “n/a” parameters were selectable., Over the past decades, stable isotopes have been infrequently used to characterise host-parasite trophic relationships. This is because we have not yet identified consistent patterns in stable isotope values between parasites and their host tissues across species, which are crucial for understanding host-parasite dynamics. To address this, we initiated a worldwide collaboration to establish a unique database of stable isotope values of novel host-parasite pairs, effectively doubling the existing data in published literature. This database includes nitrogen, carbon, and sulphur stable isotope values. We present 3213 stable isotope data entries, representing 586 previously unpublished host-parasite pairs. Additionally, while existing literature was particularly limited in sulphur isotope values, we tripled information on this crucial element. By publishing unreported host-parasite pairs from previously unsampled areas of the world and using appropriate host tissues, our dataset stands unparalleled. We anticipate that end-users will utilise our database to uncover generalisable patterns, deepening our understanding of the complexities of parasite-host relationships and driving future research efforts in stable isotope parasitology., We would like to thank the IsoEcol 2021/2022 team for allowing us to run this project and for waiving our conference fees when we presented the preliminary results of this work. We are also very grateful to Isotrace, the IRMS Unit in Dunedin, New Zealand, for running part of these samples free of charge. This research was supported by the Royal Society Te Apārangi Marsden Fund (19-UOO-212) awarded to AJMS, which allowed her to study host-parasite interactions using stable isotopes for four years. JB was supported through a New Zealand Ministry of Business, Innovation and Employment Endeavor Grant (no CAWX2207). RGS was supported by a Juan de la Cierva-Formación post-doctoral research grant (FJC2019-040921-I) from MCIN/AEI/10.13039/501100011033 (Spain) and EU NextGenerationEU/PRTR programmes. RGS is currently supported by the European Union’s Horizon Europe research and innovation programme under a Marie Skłodowska-Curie Postdoctoral Fellowship 2023 (grant agreement No 101150001-PelCon). IGV was supported by a Juan de la Cierva-Incorporación post-doctoral research grant (IJC2019-040554-I) from MCIN/AEI/10.13039/501100011033 (Spain). TJW received support from the National Recovery and Resilience Plan (NRRP), Mission 4 Component 2 Investment 1.4 – Call for tender No. 3138 of 16 December 2021, rectified by Decree No. 3175 of 18 December 2021 of the Italian Ministry of University and Research funded by the European Union – NextGenerationEU; Award Number: Project code CN_00000033, Concession Decree No. 1034 of 17 June 2022 adopted by the Italian Ministry of University and Research, CUP C63C22000520001, Project title ‘National Biodiversity Future Center – NBFC., Peer reviewed

This dataset contains the values of natural abundance of stable carbon and nitrogen isotopes in bulk muscle and gut contents, and nitrogen isotopes in muscle amino acids measured in 1801 samples of 10 small pelagic fish species sampled during the PELACUS spring cruises in the northwestern Iberian shelf and in adjacent waters of the southern Bay of Biscay. Length, weight, sex and maturity stage data for each individual analyzed are also provided. Fishes were collected by midwater trawls of ca. 30 m2 pelagic net equipped with a graded-mesh netting (starting with 30 mm and ending with 4 mm). The duration of each trawl was between 25 and 40 min. Different species occurred in the same trawls. Samples of muscle tissue and gut contents were preserved dried (50°C, 48 h) or freeze-dried. No treatments for lipid removal were used prior to carbon analyses. Stable isotope determinations in bulk samples (also carbon and nitrogen contents) were made by isotope-ratio mass spectrometry coupled to an elemental analyzer (Bode et al. 2018). Stable nitrogen isotopes in amino acids were determined by isotope-ratio mass spectrometry couples to a gas chromatograph (Bode et al., 2021)., Agencia Estatal de Investigación (AEI), grant/award no. MCIN/AEI/10.13039/501100011033 FJC2019-040921-I: Postdoctoral research grant Juan de la Cierva-Formación
Agencia Estatal de Investigación (AEI), grant/award no. MCIN/AEI/10.13039/501100011033 IJC2019-040554-I: Postdoctoral research grant Juan de la Cierva-Formación
Agencia Estatal de Investigación (AEI), grant/award no. PID2020-115620RB-100: QLOCKS project
Agencia Estatal de Investigación (AEI), grant/award no. PID2023-147149NB-100: MTL-FISH
Fifth Framework Programme (FP5), grant/award no. QLRT-2001-00818: SARDYN (2002-2005)
Horizon Europe (HorizonEU), grant/award no. 101150001: A new approach to deciphering inshore-offshore connectivity of pelagic food webs
Instituto Español de Oceanografía (IEO), grant/award no. ECOPEL: ECOPEL (2006-2008)
Sixth Framework Programme of the European Commission (FP6), grant/award no. 8120-09-9: Understanding recruitment processes using coupled biophysical models of the pelagic ecosystem (REPROdUCE), Peer reviewed