The issues around photographing seahorses in the natural marine environment

Worldwide, seahorses (genus Hippocampus) populations and their relatives in the family Syngnathidae (pipefishes, seadragons and pipehorses) are threatened by degradation of their estuarine, seagrass, mangrove and coral habitats (Olden et al. 2007), incidental capture in fishing gear (bycatch) (Vincent 1996), and overexploitation for use in traditional medicines and in the aquarium trade (Salin et al. 2005).

As a result all species of seahorse are on Appendix II of CITES (the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES 2002), which restricts their international trade.

Seahorses have a number of traits that make them vulnerable to overexploitation including low mobility, small home range, pair bonding behaviour and extended parental care. They also live in coastal habitats that are increasingly facing human impacts leading to their destruction.

In particular, Hippocampus guttulatus Cuvier, 1829 and Hippocampus hippocampus Linnaeus, 1758 (two European 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 as "data deficient" (DD).

Although they are not closely related, genetically, both European seahorse species have similar life history traits with low mobility, small home ranges, low fecundity, lengthy parental care and mate fidelity. Seahorses produce pelagic young which can live in the plankton for up to eight weeks before settling in their adult habitat. Very few survive to this stage and little is known about individual populations of seahorses.

H. guttulatus (the spiny or long snouted seahorse) shows a marked preference for habitats with algae or seagrass coverage, but it is frequently found on the edges of seagrass habitat and it is believed there is a trade-off between the shelter of the seagrass and the availability of prey in more open water with better water exchange (OSPAR, 2009a). H. guttulatus inhabits shallow waters during summer months (as little as 1.5m depth) but migrates to deeper waters in winter, a journey which can take several weeks due to their ineffective fins.

Pixie, Hippocampus hippocampus, Copyright Gaye Rosier Photo taken by Gaye Rosier, Seahorse Project, without flash by adjusting camera settings to allow in more light.

H. hippocampus has a much less selective habitat preference than H. guttulatus and can be found on artificial structures. H.hippocampus generally inhabits deeper water than H. guttulatus (preferring minimum water depths of 10 m) but is believed to undertake a similar migration journey to deeper water over winter (OSPAR, 2009b).

Many species of seahorse are highly territorial with small home ranges and high levels of site fidelity (Garrick-Maidment et al. 2010). The size of home ranges appears to be smaller for males.

Seahorses are enigmatic species, with a crypto-benthic nature (live near the seabed and hide or are camouflaged) and unique characteristics where the male has a brood pouch and "gives birth" to the young. They are ofter found in easily accessible, shallow, near shore habitats, for example seagrass, and as a result they are a popular subject for professional and recreational underwater photography.

Photography is also being increasingly used to to identify individual seahorses as each has a unique pattern and set of projections on its skin. Photographs of seahorses, collated from both recreational divers and focused surveys can be used in conjunction with pattern matching software to assess populations and movements of these species (Goffredo et al. 2004).

However, using flash photography with seahorses is a contentious issue. Using the precautionary principle, it is best to avoid using a flash so as not to risk impairing, even temporarily, the visual acuity upon which a seahorse's survival depends.

Seahorses are believed to have a well developed acute visual system (Lee & O’Brien 2011). They are generally considered diurnal feeders (i.e. feed during the day), relying heavily on their vision to locate and capture prey.

Vision is thought to play an important role in crypsis (ability to avoid observation or detection) for seahorses (which can change skin colour and texture to match their surroundings) as well as in locating mates (Foster & Vincent 2004). In addition, vision is likely to play an important role in sexual selection and mating, for receiving visual ‘cues’ such as animal size, morphology and colour changes (Rosenqvist & Berglund 2011). Any damage to seahorse vision therefore has the potential to negatively impact their survival or reproductive potential.

Studies have shown that the visual systems of seahorses, like most fish, have evolved specific adaptations that can be related to the depth and colour of water in which they live, and to specific visual tasks such as predator avoidance and food acquisition (Bone et al. 1996).

Both European species are believed to be diurnal. In comparison with a human retina, the seahorse retina receives approximately ten times more illumination when exposed to an identical light source. Flash use at night or in low light conditions (twilight) would likely result in temporary visual impairment of any species due to the slow light adaptation response, and may modify behaviour or induce a stress response (MMO 2014).

Many seahorses pair-bond, if not for life, then on a seasonal basis, and it has been demonstrated that the seahorse reinforces its pair bonds by daily greeting (Vincent 1995). Disorientation or a behavioural response to a flash in terms of moving away from their current location may therefore disrupt pair bonding.

References:

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