Mapping Seahorses Along the Spanish Coasts: Preliminary Insights from Citizen Science

Seahorses (Hippocampus spp.) are charismatic fish with unique morphological adaptations. Their vulnerability to habitat loss, overfishing, and pollution has made them a focus of conservation efforts worldwide [10,1]. Unfortunately, due to a lack of sufficient data, numerous species are still categorized as Data Deficient by the IUCN. Understanding seahorse distribution is crucial for effective management and protection. CS initiatives are valuable tools for collecting large-scale data on species [2-4], including seahorses [5-7]. We used CS data to investigate the occurrence of seahorses along the Spanish coast, including the Balearic and Canary Islands. We provide preliminary insights into the spatial patterns of seahorse distribution. Our findings will contribute to a better understanding of seahorse ecology and inform conservation efforts in these regions.

We used Citizen Science (CS) data (period 2006-2023) from the Observadores del Mar (OdM) platform to examine seahorse distribution along the Spanish coast, including the Balearic and Canary Islands. The observations were validated by expert review. ArcGIS 10.8 software [8] was employed for mapping.

A total of 767 observations were analysed, including H. algiricus Kaup, 1856 (n=629), H. hippocampus (n=137), and H. guttulatus Cuvier, 1829 (n=1) (Figure 1). The data analysis revealed a significant disparity in observational coverage across different regions of the Spanish coast (Figure 2). The Cantabrian Arc and some parts of the Mediterranean coast were identified as areas with limited data. The species H. guttulatus was more abundant than H. hippocampus, particularly in the Mediterranean (Table 1). This pattern is consistent across European coastal regions [9- 12]. Although both species often co-occur, H. guttulatus prefers shallower complex habitats that are sheltered from strong currents [11]. Nearly half of all observations (42.1% for H. hippocampus; 47.7% for H. guttulatus) were recorded at depths between 6 and 10m. Beyond 15m, the number of observations decreased significantly (12.4% and 10.1%, respectively). At depths greater than 30m, only a single individual of H. hippocampus was observed (38m), in a sandy area devoid of vegetation. The presence of H. hippocampus (27.3%) in the shallowest zone (0-5 meters) was almost double that of H. guttulatus (14.6%). Considering the habitat types (sandy, rocky, seagrass meadow-macroalgae, and others) available for selection on the OdM platform, H. hippocampus showed a greater preference for sandy bottoms (44.8% of total observations) compared to H. guttulatus (37.9%) and a lower preference for seagrass meadows (17.1% in H. hippocampus and 28.2% in H. guttulatus). When assessing the number of specimens in relation to depth and habitat type, a higher relative occurrence of H. hippocampus was noted in sandy areas over seagrass meadows and at greater depths, compared to H. guttulatus (Figure 3). Beyond 10 meters, the presence of specimens decreased considerably, with a greater preference for rocky areas.

Figure 1:Species on the Spanish coasts:
a) H. algiricus (male) ©Sabina López
b) H. hippocampus (male) ©Fernando Quintela
c) H. guttulatus (female) ©Xaime Beiro

Table 1:Seahorse observations grouped by geographical region.

Figure 2:Occurrence of seahorse species along the Spanish coast (Source: OdM). Substrate data (standardized EUNIS format) from EMODNET. Red squares indicate key seahorse hotspots. Base map: Service of the National Geographic Information Center

Figure 3:Percentage distribution of seahorse specimens across habitat types, categorized by depth range.

Conclusion

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• Abstract
• Introduction
• Materials and Methods
• Results and Discussion
• Conclusion
• Acknowledgements
• Conflict of interest
• References

The data provided valuable information on the spatial patterns of seahorse occurrence and identified areas with limited information. Hence, the study demonstrates the effectiveness of CS in contributing to marine conservation research and informing future conservation initiatives for the long-term survival of seahorses. However, the limited availability of CS data restricted the scope of this study. Besides this, while CS data has greatly enhanced our ability to collect population data, it comes with certain limitations. For seahorse observations made by scuba divers, depth is a significant constraint, as most dives do not go deeper than 15-20 meters, with only rare exceptions reaching 30 meters. Consequently, populations at greater depths are often overlooked. To expand the database and gain a more comprehensive understanding of seahorse distribution, we must actively engage the public and encourage their involvement in data collection, especially in understudied regions. Additionally, a recent study on the distribution of seahorses in the Gulf of Cadiz and nearby areas [13] highlighted the need for an inclusive approach engaging a diverse range of contributors (e.g., other CS portals, fishermen, and fisheries/oceanographic campaigns) to generate additional data.

Acknowledgements

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• Abstract
• Introduction
• Materials and Methods
• Results and Discussion
• Conclusion
• Acknowledgements
• Conflict of interest
• References

Study funded by the Ministry for the Ecological Transition and Demographic Challenge (MITERD) through the Hippo-DEC Project (Ref. 20233TE008). Seahorse photos are from validated observations reported by the OdM community. The authors thank the OdM staff, observers, staff of the Spanish Autonomous Communities and collaborating entities for their support.

Conflict of interest

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• Abstract
• Introduction
• Materials and Methods
• Results and Discussion
• Conclusion
• Acknowledgements
• Conflict of interest
• References

All authors declare that they have no conflicts of interest.

References

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• Abstract
• Introduction
• Materials and Methods
• Results and Discussion
• Conclusion
• Acknowledgements
• Conflict of interest
• References

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