Field studies of seahorse population density , structure and habitat use in a semi-closed north-eastern Mediterranean marine area ( Stratoni , north Aegean Sea )

The present study was carried out in the marine area of Stratoni, Greece, where two seahorse species are present (Hippocampus hippocampus and H. guttulatus). Two surveys were conducted (September 2016, May 2019) to gather information regarding seahorse species’ abundance, distribution and habitat characteristics. Four different seahorse natural and artificial habitat types were identified. Results revealed that the abundance of H. hippocampus was relatively high, especially at sites with artificial structures, while the presence of H. guttulatus was rare. Data collected can provide baseline information for future population assessments.

by sparse distribution, low mobility, small home ranges, low fecundity, lengthy parental care and mate fidelity (Foster and Vincent, 2004). Seahorse life history and behaviour renders them vulnerable to population decline (Vincent et al., 2011), which lead to the inclusion of many seahorse species in the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES Convention) and in the International Union for Conservation of Nature (IUCN) Red List of Threatened Species.
The short-snouted seahorse Hippocampus hippocampus Linnaeus, 1758 and the longsnouted seahorse Hippocampus guttulatus Cuvier, 1829 are two species present in the Mediterranean Sea. Although both species have been assessed as 'Data Deficient' at a global level, they have recently been re-assessed as 'Near Threatened' in the Mediterranean Sea (Pollom, 2014;2017). They are both typically present in coastal environment, and associated with habitats of different complexity (Correia et al., 2015a;Correia et al., 2018;Woodall et al., 2018). Despite the extended geographical distribution of both species there are only a few inshore locations where seahorse abundance, distribution and habitat use have been studied within the Mediterranean Sea (Louisy, 2011;Gristina et al., 2015;Ape et al., 2019). To our knowledge, ecological data has been rare in Greek waters and especially in the Aegean Sea focusing on wild seahorse population structure (Kitsos et al., 2008;Woodall et al., 2018) including ichthyofaunal assessments (Koutrakis et al., 2005;Lamprakis et al., 2008).
The present study was carried out in the marine area of Stratoni (Chalkidiki Peninsula, north Aegean Sea) which seems to be a refuge for seahorse populations, initially reported by professional divers during fieldwork activities in November 2007. The objectives of the present study were to: (i) describe the seahorses' habitat types in the Stratoni marine area; (ii) estimate the seahorses' population density and describe the population structure; (iii) investigate seahorses' habitat use.
The marine area of Stratoni ( Figure 1, latitude from 40°30' to 40°32.5' and longitude from 23°45' to 23°32.5'), lies within Ierissos Gulf, a semi-closed water body, affected by small river inputs, characterized also by soft sediments and high biodiversity (Koutrakis et al., 2003). A first pilot survey of the area was conducted in September 2016 to gather information regarding seahorse abundance, distribution and habitat characteristics using SCUBA underwater visual census. A total of 15 100m-long and 4m-wide transects were laid parallel to the shoreline, at three different depths (5m, 7.5m and 10m), covering a total area of 6,000 m 2 . Whenever an individual of a seahorse was found, species, sex and size (height), was recorded along with four environmental variables: water depth and water temperature at depth, holdfast at first sighting (structure grasped by a seahorse's tail) and the predominant habitat (i.e. benthos observed within a 1m 2 quadrat centred around the seahorse). This data was used to identify and classify the most representative habitats found according to the European Nature Information System (EUNIS) habitat code (http://eunis.eea.europa.eu). In May 2019, four sites were chosen per habitat type and registered in a GPS unit: (a) sand  In 2016, a total of 19 seahorses were sighted (Table I)   hippocampus varied in size from 6.0 to 11.5 cm (mean size of 8.9 ± 1.9 cm), while H. guttulatus size ranged from 6.5 to 13.5cm (mean size of 8.6 ± 3.0 cm). Temperature This study is the first one to describe population density and structure as well as investigate habitat use of the two seahorse species in Greek seas. Results of the two field surveys and previous underwater observations ( Mentogiannis, pers. comm.) indicate that the population of H. hippocampus appears to be present over time in the marine area of Stratoni, while individuals of H. guttulatus are rare. Considering that both species are classified as "Near Threatened" in the IUCN Red List, it is of the utmost urgency that this baseline information is made available to contribute to appropriate location-specific conservation strategies.
The seahorse H. hippocampus mean size is within the range documented for other areas of the Mediterranean Sea (Woodall et al., 2018), while the juvenile fraction seems to reflect a good population status (Woodall, 2009). Furthermore, the high density value of H. hippocampus estimated from the second survey at this site is the highest described in the Mediterranean (Woodall et al., 2018), which indicates that the species might find the necessary environmental conditions to settle even in relatively limited areas. Both seahorse species occur in the study area most probably due to high availability of food resources observed (e.g. dense populations of hyperbenthic crustaceans; Koulouri, pers. comm.). The presence of artificial structures (ropes) deployed a few months prior to the 2016 survey might have contributed to aggregate seahorses as they provide much needed holdfast. Seahorse species have been reported as typically associated with seagrass habitats (Foster and Vincent, 2004). However, no seahorse was found in the Posidonia oceanica meadows of the study area, which might indicate that other important environmental variables are at play. Considering that seagrass meadows have been found to be a very important habitat for seahorses as they provide shelter and food availability (Woodall et al., 2018), it is worth further exploring the presence/absence of seahorses in Posidonia beds of the study area, while investigating the associated biotic and abiotic factors. Unlike previous studies (Woodall et al., 2018), H. hippocampus was much more abundant than H. guttulatus in both surveys. This might be explained by the type of low complexity habitats in the study area, muddy substrate and water depth (Woodall et al., 2018). The use of artificial structures as habitat enrichment for seahorses, firstly observed in 2016, has apparently contributed to seahorse settlement in the deployment area, as also documented in other studies (Correia et al., 2013;Correia et al., 2015b;Simpson et al., 2020). In fact, most of the seahorses of both species were found grasping artificial structures (e.g. ropes) in the study area. While these structures may act as fish aggregation devices (Correia et al., 2015b), they provide holdfasts and enhance the habitat complexity, providing a long-term beneficial effect on the recovery of seahorse populations, particularly as a major component of a wider rehabilitation and management plan.
The marine area of Stratoni seems to support seahorse populations by providing a substratum of artificial holdfasts and relatively high food sources. The results of our study demonstrate that the population of seahorses is well adapted to this particular environment. However, there is an urgent need of substantially further research on the local population distribution, structure and ecology of H. hippocampus and H.
guttulatus through a long-term monitoring plan. Population genetic studies may also contribute to assess the genotypic health of local seahorses' population, which might be facing genetic isolation. Future research will contribute to inform decision-makers and stakeholders in order to protect and conserve these charismatic fishes in the marine area of Stratoni, used also for other coastal areas of the Aegean Sea.