- Top of page
Population abundance, distribution and habitat preference of the Mediterranean sympatric seahorses Hippocampus guttulatus and Hippocampus hippocampus were investigated in a semi-enclosed sea system (Apulian coast, Ionian Sea). A total of 242 individuals of seahorses were sighted in the 11 transects surveyed in summer 2011. Hippocampus guttulatus (n = 225) were 14 times more abundant than H. hippocampus (17). The mean abundance of H. guttulatus for all the pooled sites was 0.018 m−2 (SE ± 0.003) ranging from a maximum of 0.035 (SE ± 0.007) to a minimum of 0.008 (SE ± 0.002). The size structure of long-snouted seahorse shows a population ranging from 7 to 14 cm (SL) with a peak at 10 cm (TL). Juveniles (96.0 ± 8.0 mm) represent a significant fraction of the population, accounting more than 21% of the sighted individuals. In Mar Piccolo, H. guttulatus is able to shelter both in monotonous habitats, including the algal beds, and diversified ones, such as the rich filter-feeder communities that colonize hard substrates. By contrast, H. hippocampus is mainly associated with habitats of low complexity. Today, the Mar Piccolo di Taranto is among the most heavily polluted water bodies in South Italy, with trace metals, hydrocarbons, pesticides and organic wastes affecting both biotic and abiotic matrices. However, despite the high level of degradation, the presence of a large mussel farm has avoided the impact of towed fishing gears, and eutrophication of water bodies has ensured a high trophic level that supports large crustacean populations, potential prey for seahorses.
- Top of page
Worldwide, seahorse (Hippocampus Rafinesque 1810 spp.) populations and their relatives in the family Syngnathidae (pipefishes, seadragons and pipehorses) are threatened by (i) degradation of their estuarine, seagrass, mangrove and coral habitats (Olden et al. 2007), (ii) incidental capture in fishing gear (bycatch) (Vincent 1996), and (iii) over-exploitation for use in traditional medicines and in the aquarium trade (Salin et al. 2005). Moreover, seahorses have life histories, behaviour and ecology that make them particularly vulnerable to population decline (Foster & Vincent 2004): they provide lengthy and vital parental care for small broods, exhibit low mobility and high site fidelity, have low natural rates of adult mortality, and (in many species) maintain faithful pairbonds (Lourie et al. 1999; Vincent et al. 2011). In addition, seahorses inhabit shallow, coastal areas worldwide, where anthropogenic disturbances tend to be the most frequent and severe, and represent the main threat to their integrity. The decline of seahorse populations worldwide has therefore been brought to the attention of the international community, leading to their classification as threatened species, and their inclusion on the World Conservation Union Red List of Threatened Species (Vincent & Hall 1996; World Conservation Union 2002) and, in 2002, the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES 2002).
In particular, Hippocampus guttulatus Cuvier, 1829 and Hippocampus hippocampus Linnaeus, 1758 – the two Mediterranean species of seahorses – are also listed under OSPAR, European CITES (Curd 2009), the Bern Convention and the Barcelona Convention (Abdul Malak et al. 2011) and protected by the UK Wildlife and Countryside Act of 2008 (DEFRA 2008). However, as few data are available across most of their geographic range, neither species can be reliably classified in terms of global level risk according to IUCN categories, and they are listed, at present, as data-deficient (DD).
Along the Italian coasts, the trend of Hippocampus populations (H. guttulatus and H. hippocampus) is very similar to that described for other seahorse species in coastal areas worldwide (Vincent et al. 2011). Until 80, sightings of seahorses along Italian coasts were frequent, with individuals of both species often being accidentally taken as bycatch by trammel nets and trawlers. However, in the last few decades the degradation of sensitive habitats, the use of illegal fishing gear on coastal biocenoses, and the development of snorkelling and SCUBA diving have slowly but inexorably reduced seahorse population densities, structure and spatial distribution. As a result, in the last decade, seahorse sightings have been rare or scanty along most of the Italian coast and populations appear scattered, showing peaks of abundance in the Northern Tyrrhenian (Tuscany and Liguria) and Northern Adriatic Seas (Goffredo et al. 2004). Paradoxically, species which have been included on the World Conservation Union Red List of Threatened Species and are protected by several international conventions (CITES 2002) appear to have found suitable habitats in heavily anthropic areas. Indeed, Hippocampus spp. populations now appear to be mainly concentrated in harbours, mussel farms or artificial structures, which provide shelter from the direct impact of mechanical human activities (commercial fishing, dredging, dumping, SCUBA diving, snorkelling).
In this paper, we describe the Hippocampus populations (H. guttulatus and H. hippocampus) in the semi-enclosed sea system around the city of Taranto (Apulian coast, Ionian Sea). The first scientific report of seahorses in the Mar Piccolo of Taranto dates back to Cerruti (1938). Later, Parenzan (1969) reported the occurrence of Hippocampus in association with photophilous algal beds (mainly ‘Caulerpa, Rhytiphloea, Gracilaria spp., Chaetomorpha and Cladophora prolifera’). According to older local fishermen, in the last century, seahorses were frequently sighted and collected as bycatch during traditional fishing activities. Moreover, fishermen report a remarkable increase in seahorse sightings in the last 50 years, with the spread of mussel farming. According to this scenario, the increase in pools and hard structures utilized by farmers had strongly reduced the fishery, thus reducing sea horse captures. Purse seining, which up to the middle of the 20th century was the prevalent fishing activity within the Mar Piccolo, is now only very occasionally used, and is confined to a few tracts along the eastern coast of the inner inlet (Fig. 1).
Figure 1. Mar Piccolo di Taranto. Location of 11 underwater visual census sites: 1– 8 subtidal sites 0.5 – 5 m depth; A-C deep sites (5.1 – 10 m depth). Surface area per stratum is also indicated.
Download figure to PowerPoint
Today, the Mar Piccolo di Taranto is among the most heavily polluted water bodies in Southern Italy. Surrounding urban developments are known and presumed to have affected the semi-enclosed sea ecosystem with trace metals, hydrocarbons, pesticides and organic wastes (Cardellicchio et al. 2007; Petronio et al. 2012). However, despite the high level of degradation, the presence of a large mussel farm avoids any impact from towed fishing gears and the eutrophication of the water bodies ensures a high trophic level that supports large crustacean populations, mainly amphipods, which are potential prey for seahorses (Prato & Biandolino 2003).
In the present paper, our main objectives were to (i) assess the abundance of the local population; (ii) evaluate differences in habitat preference; (iii) describe the spatial distribution of Hippocampus populations in the semi-enclosed sea system of the Mar Piccolo. To achieve these goals, a visual census survey (during summer 2011) was carried out at 11 stations representative of the main benthic habitats and distributed along a confinement gradient. Abundance of the two species was then compared across different stations and substrates, and habitat preference was established.
- Top of page
A total of 242 individuals of seahorses were sighted along the 11 transects surveyed. Hippocampus guttulatus (n = 225) was 14 times more abundant than H. hippocampus (n = 17) (Table 1). Hippocampus guttulatus specimens were distributed at all sites surveyed, whereas H. hippocampus individuals were located only at sites 3, 4 and 6 (see Fig. 1). The mean abundance of H. guttulatus for all pooled sites was 0.018 per m2 (SE ± 0.003), ranging from a maximum of 0.035 (SE ± 0.007) at site 3 to a minimum of 0.008 (SE ± 0.002) at site 1.
Table 1. Indices of abundance for Hippocampus guttulatus and Hippocampus hippocampus
| ||Mar Piccolo subtidal sites||Mar Piccolo deep sites||pooled sites|
| Hippocampus guttulatus |
|Mean density (m−2)||0.008||0.033||0.035||0.031||0.033||0.023||0.008||0.010||0.006||0.011||0.006||0.018|
| Hippocampus hippocampus |
|Mean density (m−2)||0.000||0.000||0.001||0.003||0.000||0.001||0.000||0.000||0.000||0.000||0.000||0.0005|
In particular, H. guttulatus density was higher at sites in the central area of the Mar Piccolo (sites 2, 3, 4, 5) (Fig. 1, Table 1), and less abundant in the outer and inner parts of the semi-enclosed sea. The H. hippocampus mean density for all the pooled sites was, obviously, very low (0.0005 ± 0.0002 SE) with a peak at site 6 (0.003 ± 0.001 SE) located in the inner part of the Mar Piccolo (Fig. 1). The H. guttulatus sex ratio (females/total) was slightly female-biased (53.7%); sightings of H. hippocampus were too limited to calculate a proportion by sex.
The size (sexes combined) of long-snouted seahorse shows a population ranging from 7 to 14 cm (TL) with a peak at 10 cm (TL). Juveniles (with a size <96.0 ± 8.0 mm) represent a significant fraction of the population, accounting for more than 21% of the sighted individuals (Fig. 2). Unfortunately, there were too few sightings of H. hippocampus for a proper analysis of the population structure.
Figure 2. Standard length-frequency distribution (sex combined) of individuals of H. guttulatus sighted from June to August 2012 (10 mm size classes).
Download figure to PowerPoint
Hippocampus guttulatus density (data pooled across all sites) was broadly related to habitat characteristics. Ulva beds shelter the maximum density of long-snouted seahorse (0.058 ind.·m−2), whereas no sightings were recorded on Muds. The two-way ANOVA analysis performed on H. guttulatus density revealed significant differences between ‘Sites’ and ‘Habitats’ (Table 2). The SNK test performed on the ‘Habitat × Sites’ combination shows no differences in habitat preference in sites 1 and 7 (Table 2, Fig. 1). ULVA was the favourite habitat at sites 2 and 5, whereas artificial structures (AHS and PWSB) were the preferred habitats at sites 3, 4 and 8 (Table 2, Fig. 3).
Table 2. Summary of the results of the Student–Newman–Keuls post hoc tests. Only significant interaction terms (P < 0.01) are reported
|Habitat × Sites||42||0.9092||0.0216||16.74***|
|Total||223||2.0179|| || |
|Habitat × sites SNK test|
|2||ULVA > PWSB = AHS = SBSB > MUDS = CYNOD = CLIN|
|3||AHS = PWSB > all|
|4||AHS > PWSB = SBSB > all|
|5||ULVA > PWSB > all|
|6||PWSB > all|
|8||PWSB > all|
As reported in Fig. 1, the first inlet covers a total area of 8.01 km2, with 1.19 km2 within the bathymetric range 0–5 m, 1.93 km2 within the bathymetric range 5.1–10 m and 4.89 km2 deeper than 10 m. The second inlet covers a total area of 12.60 km2, with 3.98 km2 within the bathymetric range 0–5 m, 8.39 km2 within the bathymetric range 5.1–10 m and 0.25 km2 deeper than 10 m. Thus, considering the whole Mar Piccolo, the subtidal layer (0–5 m) covers a total of 5.17 km2, whereas the 5–10 m layer measures about 10.32 km2.
The estimate of population size shows a very high number of H. guttulatus individuals in the subtidal area [116,325 individuals, lower 95% confidence interval (CI) = 76,029; upper 95%CI = 156,621] and a less abundant fraction of the population in the intermediate stratum (5–10 m) (80,267 individuals, lower 95%CI = 47,316; upper 95%CI = 113,217). The H. hippocampus population is significantly small in the subtidal layer (3231 individuals, lower 95% CI = 1,878; upper 95%CI = 6,837) and completely absent in the intermediate layer.
- Top of page
It is well documented that several seahorse species worldwide are targeted by fisheries in several countries (e.g. Vietnam, India), where they are dried for traditional medicine markets (Zhang et al. 2003), for curiosities (Grey et al. 2005) or sold alive for the aquarium market (Martin-Smith & Vincent 2006). Moreover, several aspects of the seahorse's life strategies (i.e. small body size, paternal care, low mobility, sedentary adults and monogamous mating) (Foster & Vincent 2004) leave them particularly vulnerable to disturbance from human activities and sensitive to habitat degradation. European seahorse populations (Hippocampus hippocampus and Hippocampus guttulatus) in particular, which are not specifically targeted by fishing activities, are mainly threatened by accidental catch in towed fishing gears and by their sensitivity to habitat degradation (Woodall et al. 2011). According to the literature, H. guttulatus and H. hippocampus are considered to be organisms found in coastal habitats or ecosystems with good environmental status, very sensitive to environmental conditions and therefore a useful tool for analysing the environment health as a flagship species (Garrick-Maidment & Jones 2004). However, the present study describes seahorse populations (H. guttulatus and H. hippocampus) in a heavily polluted and human impacted semi-enclosed sea in Southern Italy. The estimated total abundance of seahorses in the two inlets of the Mar Piccolo di Taranto is really impressive, with values of 123,456–269,838 and 1878–6837 for H. guttulatus and H. hippocampus, respectively. Moreover, given the cryptic behaviour of seahorses and the poor water clarity in the study area, the mean density and population size of seahorses could be greatly underestimated. The long-snouted seahorse density recorded in the present study was comparable to that of similar sites in the Atlantic Sea (Curtis & Vincent 2005; Caldwell & Vincent 2012) and greater than that reported in the Mediterranean Sea (Goffredo et al. 2004), the latter being obtained by counting the number of seahorses per dive-hour. Length–frequency distribution (sex combined) of H. guttulatus shows a young population as whole, with a lack of large adults (more than 16 cm TL), and juveniles accounting for more than 21% of the population. Although a recruitment pattern is widely expected in the summer period, the juvenile fraction in the Mar Piccolo is abundant and, according to the literature (Woodall 2009), this would seem to reflect a good population status.
In the last three decades, the original landscape and ecology of the Mar Piccolo have been radically modified; soft bottoms have become dominant, varying from muds to mixed sands, whereas once-dominant sea grasses (Cymodocea nodosa) are now reported to be in regression, scattered in sparse patches and largely replaced by beds of unattached macroalgae such as Ulva spp. and Chaetomorpha linum (Cecere & Petrocelli 2009). Moreover, since the 1970s, mussel farms have colonized most of the sea bottom surface, increasing the availability of hard substrates and forming a mechanical obstacle to fishing activities with towed gears. Despite the critical environmental conditions, the Mar Piccolo di Taranto shelters a wide population of seahorses. In agreement with the data of Ria Formosa lagoon (Curtis & Vincent 2005), the population of H. guttulatus appears to be more numerous and distributed over a wider area, whereas sightings of H. hippocampus are scanty and concentrated in particular areas. According to Curtis & Vincent (2005), H. hippocampus in Mar Piccolo is mainly associated with habitats of low complexity (site 6; Fig. 1), whereas in other sites with the greatest habitat heterogeneity, its presence is rare and scanty. By contrast, H. guttulatus is able to shelter both in monotonous habitats, such as algal beds, and in diversified ones, such as the rich filter-feeder communities that colonize hard substrates. In addition, this seahorse can be found grasping both natural and artificial substrates, such as wooden poles, nets and ropes. The role of artificial substrates in sheltering seahorses has already been highlighted for both Mediterranean species (Curtis & Vincent 2005) and exotic species (Hellyer et al. 2011). Generally, seahorse density is patchy (Foster & Vincent 2004) and at most locations they are rare; however, they can be locally abundant (Curtis & Vincent 2005; Woodall 2009). Our data showed that seahorses are mainly concentrated in the central part of the Mar Piccolo (Sites 2, 3, 4, 5) (Fig. 1) where about 56% of the individuals were detected. The central part of the Mar Piccolo is served by tidal currents from the open sea (Capasso et al. 1989) and is unaffected by illegal activities of handling towed fishing gears practised in the eastern side of the second inlet. No confinement gradient has been observed, as the number of sighted specimens decreases from the central portion of the Mar Piccolo towards the innermost and the outermost portions of the sea system. The depth range seems to be more contracted than that reported in the literature for European seahorses. In fact, most of the sightings occurred in the subtidal zone, with only a small fraction of the population sheltered in the intermediate sites (5–10 m), where muds are dominant. This finding can be partially explained by the frequent dystrophic and anoxic events occurring in the deep layers of the Mar Piccolo (from about 7–10 m of depth) in summer time (Alabiso et al. 1997; Gaino et al. 2010). Moreover, the anoxia phenomenon seems to prevent the seasonal migration into deeper water (>10 m) described by Boisseau (1967) in cooler waters for both H. hippocampus and H. guttulatus (Corriero pers. obs.). In addition, most of the suitable seashore habitats, such as phanerogams and algal beds, but also hard substrates, are mainly concentrated within the first 5 m of depth.
Seagrasses (Posidonia oceanica, C. nodosa) are widely thought to be a common habitat for European seahorse species (Foster & Vincent 2004; Garrick-Maidment 2007). However, in the current study, only a very small number of seahorses were surveyed on the residual C. nodosa beds. Notwithstanding, in the Mar Piccolo di Taranto the seagrass has declined dramatically, and seahorse populations seem to prefer to shelter and feed in artificial habitats where they can find resources and safe holdfasts.
Hard complex substrates of human origin (mussel farm pools, rocky artificial substrates) are the most favourable habitats for seahorse populations in the Mar Piccolo. The abundant colonization of epifauna on artificial hard substrates offers shelter and camouflage and supports very abundant potential prey populations while increasing protection against predators (Wennhage & Pihl 2007). Both mussel farm pools and rocky artificial substrates increased structural complexity, maintaining a very high abundance of mobile crustaceans (mainly amphipods and copepods), which are considered to be common prey items for H. guttulatus and H. hippocampus (Kitsos et al. 2008). Moreover, this finding agrees with a recent study carried out by Hellyer et al. (2011) in Sydney Harbour, where artificial habitats support much higher abundances of the mobile epifauna (amphipods, copepods) that are consumed by seahorses. Among the algae, Ulva beds are a habitat of low complexity, giving rise to a rich detritus food chain due to its rapid turnover, thus supporting an abundant population of crustacean detritivores. By contrast, the presence of seahorse individuals on C. linum is rare or scanty; as suggested by Sundin et al. (2011), filamentous algae reduce visibility by obscuring objects in the water, thus decreasing the possibility of food intake.
Further study is required to determine whether habitat distribution changes on a seasonal basis or depending on the sex and life stages of seahorse individuals. In our opinion, in fact, whether prey item availability could be the main factor explaining the patchy distribution of seahorse habitat, ecological and behavioural aspects can affect the use and habitat preference during crucial and vulnerable stages of the seahorse life cycle (e.g. recruitment, courtship, coupling, male parental care).