MSc student Clayton Manning (@claytongmanning) studied how different habitat, prey, and predator variables affect the abundance and size distributions of seahorses in Australia.  He teased out which variables are most important among different seagrass beds, and within a single seagrass bed. 

Targeted fishing, incidental capture, and habitat degradation have led to the decline of seahorse populations around the world. Of these three pressures, the effect of habitat degradation on seahorse populations is the least understood. This makes it difficult for conservationists to accurately assess the status of seahorse populations or make appropriate action plans for their conservation. 

Clayton's work will help conservationists and resource managers around the world to make better decisions about how to manage seahorse populations and shallow seas habitats.

(Banner photo: Guylian Seahorses of the World)

Project details

Searching for seahorses in Port St. Stephens, Australia

Searching for seahorses in Port St. Stephens, Australia

Seahorses are cryptic, sedentary fish that live in a diverse range of habitats - including seagrasses, coral, macroalgae and mangroves - but we are largely unaware of what it is about specific seagrass beds or coral reefs that support either large or small numbers of seahorses. This limits the ability of conservationists to accurately assess seahorse conservation status or make appropriate action plans. Pinpointing how different elements of habitat affect seahorse populations will better equip conservationists to inform management decisions.

First things first - seahorses must avoid predation. Although seahorses are not particularly appetizing because of their strong, bony frame, some opportunistic fish and octopus are known to feed on seahorses (especially juveniles) when they're hungry. The pressure is great enough that seahorses spend a considerable amount of time hiding, often camouflaged with their background. You will rarely find a seahorse swimming in the open for this reason.

If not preyed upon, seahorses must then be able to eat the small crustacean prey that they love (amphipods, copepods, isopods, decapods). Although you will find many of these critters on any seagrass leaf you look at, the amount and suite of prey vary between and within habitats. Making things even more confusing (and fun), prey also vary according to other habitat characteristics such as complexity, substrate type, depth, patch size and patch connectivity. 

The objective of my thesis was to disentangle which of these three variables - predator abundance, prey availability, and patch characteristics - are most important when it comes to seahorse abundance and population dynamics. My research was co-supervised by Dr. David Harasti  - a research scientist with the New South Wales government. He has over 15 years of experience working with Hippocampus whitei, the seahorse species I worked with. My fieldwork was spent searching for and comparing the seahorse populations and habitat characteristics we find between different seagrass (Posodonia) beds, and between different soft coral (Dendronephthya australis) patches along the southern shore of Port Stephens, NSW, Australia.

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Further reading

Bergert, B.A. & P.C. Wainright. 1997. Morphology and kinematics of prey capture in the syngnathid fishes Hippocampus erectus and Syngnathus floridae. Marine Biology, 127: 563-570.

Caldwell, I.R. 2012. Habitat use, movement, and vulnerability of sedentary fishes in a dynamic world. PhD dissertation, University of British Columbia, Vancouver, Canada.

Curtis, J.M.R & A.C.J. Vincent. 2005. Distribution of sympatric seahorse species along a gradient of habitat complexity in a seagrass-dominated community. Marine Ecology Progress Series, 291: 81-91.

Foster, S.J. & A.C.J. Vincent. 2004. Review Paper: Life history and ecology of seahorses: implications for conservation and management. Journal of Fish Biology, 65: 1-61.

Harasti, D., K. Martin-Smith & W. Gladstone. 2012. Population dynamics and life history of a geographically restricted seahorse, Hippocampus whitei. Journal of Fish Biology, 81: 1297-1314.

Harasti, D. K. Martin-Smith & W. Gladstone. 2014. Does a no-take marine protected area benefit seahorses. PLoS ONE, 9(8): e105462.

Harasti, D., K. Martin-Smith & W. Gladstone. 2014. Ontogenetic and sex-based differences in habitat preferences and site fidelity of White's seahorse Hippocampus whitei. Journal of Fish Biology, 5: 1413-1428.

Howard, R.K. & J.D. Koehn. 1985. Population dynamics and feeding ecology of pipefish (Syngnathidae) associated with eelgrass beds of Western Port, Australia. Austrailian Journal of Marine and Freshwater Resources, 36: 361-370.

Kendrick, A.J. & G.A. Hyndes. 2005. Variations in the dietary compositions of morphologically diverse syngnathid fishes. Environmental Biology of Fishes, 72: 415-427.

Rosa, I.L., T.P.R. Oliveira, A.L.C. Castro, L.E. de Souza Moraes, J.H.A. Xavier, M.C. Nottingham, T.L.P. Dias, L.V. Bruto-Costa, M.E. Araujo, A.B. Birolo, A.C.G. Mai & C. Monteiro-Neto. 2007. Population characteristics, space use and habitat associations of the seahorse Hippocampus reidi (Teleostei: Syngnathidae). Neotropical Ichthyology, 5: 405-414.

Vincent, A.C.J., K.L. Evans & D. Marsden. 2005. Home range behaviour of the monogomous Australian seahorse, Hippocampus whitei. Environmental Biology of Fishes, 72: 1-12.