Visible Implant Elastomers in gilthead seabream (Sparus aurata L.) for experimental research: Preferred injection sites to optimize tag retention and minimize histological effects

Fish tagging is an important tool for individual or group identification and is commonly used for biological, ecological and fisheries management studies. The key for ensuring the quality of these studies lies primarily in the tag design, which should be economically affordable, easy to handle and provide long-term, efficient detection, without compromising fish health and welfare. A great variety of tags exists (e.g., natural marks, T-anchor tags, fin-clipping, passive integrated transponders), but all have their limitations. Visible Implant Elas-tomers (VIE) tags are often used in experimental studies, where small fish are often used due to space limitations. VIE tags are coloured, biocompatible silicone marks that are injected under the skin. The colour is externally visible, which allows for efficient tag identification and reduces handling of the fish. In this study, we aim to validate VIE tags on gilthead seabream, an important model in marine parasitology research. Fish of different sizes were tagged on the dorsal and/or caudal fin and monitored weekly for up to 180 days. Overall, VIE tags were visible and no indicators suggested an impaired welfare of the tagged fish, e.g., no mortality or reduced growth rate of the fish, no morphological abnormalities or injuries, nor any conspicuous swimming or feeding activity, similar to untagged fish. Tag retention analysis (Beta regression model) demonstrated that injection site and fish size were critical for tag optimization. Histological analysis revealed no severe tissue response when the silicone was in connective tissue; however, the silicone sometimes migrated to organs such as the kidney and spleen. Although this phenomenon is known in tadpoles, it had not yet been observed in fish. It was particularly frequent when tagging the caudal fin, likely related to their well-developed vascularization, where the silicone probably reaches the circulatory system. This migration had no effect on fish health, as melanomacrophage centers involved in phagocytosis were the only response in the kidney and spleen when this occurred, and this could probably be avoided by reducing the volume of VIE material injected into the caudal fin. Considering the low toxicity of VIE tags, and despite the migration of the material in the fish, we recommend these tags as a safe option for use under laboratory conditions at two tagging sites in a wide range of sizes of gilthead seabream. In addition to providing the most up-to-date information on the use, retention and effects of VIE tags on fish for research purposes, we also suggest testing VIE tags prior to experimentation to avoid compromising the robustness of the study.