Seahorses in focus: local ecological knowledge of seahorse-watching operators in a tropical estuary

Seahorse-watching has been performed by men, mostly former fishers, with low education level. The relatively long time in the current occupation, averaging 9 years, reflects the economic dependence and suggests this touristic activity has remained profitable over the years. Management strategies to ensure the existence of seahorses should consider the socioeconomic needs of dependent users [23], such as Project Seahorse approach towards seahorse fishers in the Philippines. Seahorse-watching in Maracaípe has high socioeconomic relevance, being the primary income source for all respondents, and accounting for over 50 % of the income of those jangadeiros who had complementary activities [12]. Our informants’ social organization into an Association of Jangadeiros reflects the valorization of their image, reinforcing their cultural identity and their importance as a professional group that should be included in the discussions on sustainable use of seahorses, as they are willing to it and are also concerned about the species conservation.

Biological data simply confirm the occurrence of seahorses in Maracaípe mangrove and adjacent reefs [20]. Informants added that inside the local estuary, seahorse habitat preference is driven by habitat structural complexity instead of depth, in agreement with literature [2, 29, 30]. In Maracaípe, the shallow estuarine area suffers intense daily tidal depth variations and thus the seahorse’s vertical movement follows these natural dynamics. Mangrove structures (e.g. roots, fallen wooden branches) provide a complex habitat with plenty of holdfasts, shelter and feeding opportunities, which are determinant factors for seahorse occurrence and habitat selection on our study site.

Seahorses use diverse natural and artificial holdfasts, demonstrating their adaptive ability, although often exposing themselves to the risk of human exploitation [31, 32]. Mangroves play an important role as habitats for seahorses during all their life stages [29, 30, 33]. Thus, seahorses can be used as flagship species for coastal zone protection [34], highlighting the need for mangrove conservation, which can benefit several other species.

Salinity and water transparency decrease concomitantly during the rainy season (austral winter) at Maracaípe estuary [19, 20], coinciding with the migration period reported by jangadeiros. However, poor water visibility could distort this perception, according to a respondent who stated that seahorses remain in the mangrove during winter, but are not seen because of water turbidity. Additionally, greater seahorse abundance in clear waters may be biased by conditions of better visibility during dry season. Seahorses may have seasonal migrations, although little is known about this movement pattern [2, 25]. Fishers along northeastern and northern Brazil have also reported seasonal seahorse migrations to saltier waters in the austral winter months [3]. Although the minimum salinity recorded for the Maracaípe estuary in the rainy season was 1 ppm [19], seahorses were found in points with salinity limits from 5 to 40 ppm, focusing on locations with an average of 26 ppm [20]. Euryhaline fishes inhabiting estuarine areas, such as H. reidi, are able to withstand salinity changes [2]. However, in some cases they might not survive the extreme variation caused by the rain freshwater inflow and flooding rivers [35].

Maracaípe informants reinforced the ethnobiological evidences of seasonal migrations possibly being related to abrupt changes in salinity. This potential migratory pattern should be further investigated to understand seahorse population dynamics and to implement more contextualized conservation measures that consider local variations in habitat use by H. reidi when designing marine protected areas [3, 25].

The consensual informants’ description follows scientific research. Seahorses are indeed generalists and voracious ambush predators, feeding on live moving preys [2, 36], from zooplankton to small fishes, especially microcrustaceans [37, 38]. The “mud worms” quoted by jangadeiros are possibly nematodes, polychaetes and oligochaetes. These food items were found in H. reidi stomach contents in the Mamanguape estuary, northeastern Brazil [37].

There are no biological data considering Sargassum spp. on seahorse diet, although several jangadeiros mentioned it. We believe our informants made this trophic association because they often see seahorses drifting attached to these algae. In fact, drifting on Sargassum spp. is one of the ways by which seahorses reach inside the estuary in strong current events, as reported by jangadeiros, and this corroborates dispersal and movement pattern studies on other Hippocampus species [39‐41]. Respondents may have quoted sludge as a food item on seeing seahorses foraging in the sediment or inspecting the substrate, probably seeking prey, similar to the behavior described for H. reidi in a natural environment [2, 42].

There is evidence that Syngnathidae are not targeted by specialized predators, but by generalist or opportunistic species [40], such as the fishes quoted as seahorse predators in the Maracaípe estuary and also reported by seahorse fishers along Brazilian coast [3]. Partial predation by crabs on seahorses have also been reported by biological and ethnoecological studies (e.g. [3, 43, 44]). This interaction is probably an agonistic behavior, as both species share the same habitat [3].

Despite the seahorses’ unusual reproductive characteristics [25], jangadeiros showed remarkable knowledge, corroborating the scientific data. Our respondents accurately recognized seahorse’s sexual dimorphism and brood size, information reportedly acquired through educational training. This shows that empirical knowledge alone would not allow for an accurate conclusion about seahorse sexual dimorphism.

At birth, the male performs muscle contractions to expel the offspring (resembling miniatures of the adults) out of the brood pouch [45]. Such behavior was observed by most jangadeiros when displaying seahorses to tourists in glass containers. Thus, males in advanced pregnancy stages are captured, posing potential harm to this H. reidi population if the handling stress significantly interferes with reproductive success. Studies demonstrate that stress affects fish reproduction (negatively or positively) in many ways depending on the nature and severity of the stressors [46, 47]. Therefore, we highly recommend further studies on the effects of such touristic activities on seahorse reproduction. Knowledge of how these stressors affect the physiology of fish species can inform critical conservation strategies [47].

Studies have reported brood size of seahorse species ranging from 100 to approximately 2000 newborns [2], closely matching the information provided by jangadeiros. The informants acknowledged that seahorses release many neonates at each reproductive event, but recognized that only few reach the adult phase due to high predation in early life stages, befitting biological data [24]. However, there are no accurate values for seahorse survival rate, because it is difficult to assess its initial planktonic phase [25]. For conservation purposes, it is an important fact that the informants acknowledged these aspects of the species survival fragility.

The reproductive period mentioned by the informants agrees with biological data (e.g., [20, 33, 44, 48]) and with perception of fishers from north and northeastern regions of Brazil [3] (Table 4). The temporal variation in different sites suggests the reproductive peak is linked to site-specific environmental conditions [24, 48]. In Maracaípe, the intensification of reproduction in the rainy season could be related to the increased strength of currents and floods, facilitating the dispersion of newborns in the planktonic phase out of the estuary. It could also be related to the increased water turbidity, hindering the action of predators on H. reidi and reducing jangadeiros’ catch pressure.

On another estuary of northeastern Brazil, it was suggested that H. reidi prefers conditions of higher salinity and better visibility of the dry season to intensify courtship rituals, as a larger number of egg-bearing males were observed in the beginning of the rainy season [33]. The author raised a hypothesis that newborns and juveniles could benefit from a greater supply of nutrients in the estuary during the rainy season. Nevertheless, the decrease in salinity levels in the rainy season could be a limiting factor for survival, according to the juvenile mortality reported in Maracaípe [20]. The consensual response among jangadeiros was that, in winter, the seahorses in the mangrove move to the sea. The information given in the reproductive biology and the influence of environmental variables should be further investigated. Contradictions between LEK and biological data can elucidate facts not yet perceived and suggest new investigative approaches and working hypothesis [49, 50].

The medicinal use of seahorses as described by the interviewees is documented in several regions of Brazil and Latin America [51, 52]. Nevertheless, jangadeiros argued not to believe in its effectiveness and they consider it an obsolete and unnecessary practice, because today there are drugs available for such diseases. This perception is positive for conservation and reduces fishing pressure for medicinal purposes. Thus, the respondents have no involvement with seahorse fishing or trade, contrasting to other Brazilian communities [3]. However, seahorses are still overexploited in Brazil for ornamental trade [53].

Older fishers generally have the longest baseline of changes in the abundance of marine organisms [54, 55]. However, seahorse-watching in Maracaípe is a relatively recent activity. Thus, possibly the jangadeiros still do not perceive short-term changes in the abundance of seahorses or such changes have not occurred. In this context, we expected that perceptions of changes in abundance would not vary according to the stakeholder’s age. Nevertheless, despite the non-significant statistically tested result, a decrease in abundance was mentioned by four older informants. Thus, the hypothesis of long-term declines in abundance should not be dismissed.

The informants reported mostly human-related impacts encompassing main global threats to seahorses, such as, fishing, habitat loss, collecting for ornamental and dried specimen trade [6, 24]. By considering the handling of seahorse specimens as a threat, the jangadeiros were concerned about the sustainability of their economic activity and assumed that it could have a negative impact. Along with pollution and sewage discharge, motor boat chemical impacts (fuel, oil, petroleum products, solvents, paint and resins waste from boat repair) can potentially be detrimental to marine habitats [56, 57] and also to seahorses, once they bioaccumulate xenobiotics [58].

Regarding habitat loss, in Maracaípe it is specially driven by illegal blast fishing, deforestation and anthropization of mangrove areas, besides sedimentation (referred to sand deposition upon the muddy sediment). According to jangadeiros, the geological phenomenon of sand deposition in the estuarine area of Maracaípe river has drastically increased in recent years, covering the submersed roots of mangrove vegetation and thus decreasing the availability of holdfasts for seahorses. Sites where the original sediment was mangrove mud were covered by sand, decreasing the river channel depth and hindering navigation in some points (informants’ comments by January 2016). This perceived increase in sand sedimentation could be related to the intensification of deforestation [59].

The environmental human-related pressures reported by jangadeiros for the Maracaípe mangrove were also registered in another study [59], where the overall index of impact on the Maracaípe estuary was considered high, caused by the lack of urban planning and disregard for environmental laws. Part of these alterations were linked to the uncontrolled growth of tourism and the Industrial Port of Suape, one of the main economic centers of Brazil, located 23 km away from the study site [59].

Although anthropogenic action and environmental changes represent a threat to marine biodiversity, fish populations can also be affected by unrevealed natural cycles [5]. Hence, for conservation assessments, seahorse populational trends in Maracaípe need to be investigated considering: a) the fact that seahorses exhibit natural population variations even in the absence of human exploitation [60, 61]; b) considering the predominating complexity of anthropogenic interactions at Macaraípe estuary, i.e., given the existence of other major potential driving causes of a decline (e.g., Fig. 5), and c) the hypothesis of insignificance of handling impacts by seahorse-watching operators in a given population health. Given the Data Deficient conservation status of many seahorse species, including H. reidi, there is an urgent need for more research, such as long-term monitoring of wild populations [5].