Visible implant fluorescent elastomer tagging of the big-bellied seahorse, Hippocampus abdominalis
Introduction
The ability to identify animals, either individually or as part of a group, is of fundamental importance in many biological studies, e.g. for assessing population size, movement, growth, age or fecundity. Common techniques in fishes involve the use of external or internal tags or marks (Parker et al., 1990, Dewey and Zigler, 1996, Haines et al., 1998, Winner et al., 1999, Baras et al., 2000, Hammer and Blankenship, 2001). Ideally, such tags should have minimal effects on mortality, growth and reproduction of the target species so that the results from tagging studies accurately reflect the parameters of the study population (Wydoski and Emery, 1983, Frederick, 1997, Willis and Babcock, 1998). As well, tags should be retained at a rate that allows confidence of unambiguous recognition (Wydoski and Emery, 1983, Frederick, 1997, Willis and Babcock, 1998); tag loss or tag non-recognition will bias data leading to inaccurate conclusions (Winner et al., 1999).
Seahorses and their relatives (family Syngnathidae) are charismatic fishes that are a focus for marine conservation efforts. Concerns about unsustainable levels of exploitation and loss of their habitat have prompted research into their biology and trade (Vincent, 1996). However, management of wild populations has been hampered by lack of fundamental biological data in many areas. Many of these data will be obtained only if reliable marking techniques can be developed. Furthermore, seahorses are candidate species for aquaculture development (Wilson and Vincent, 1998, Payne and Rippingale, 2000; Woods, 2000a, Woods, 2000b; Adams et al., 2001). Effects of aquaculture on wild populations, identification of wild and captive-bred animals and routine husbandry (e.g. brood identification) all make it imperative to develop a reliable marking technique for seahorses.
Seahorses have proved particularly difficult to mark because of their unusual morphology (external bony plates and reduced fins) and susceptibility to disease (Vincent and Sadler, 1995). Field studies have been successfully conducted using numbered plastic discs attached with cotton around the ‘neck’ region (Vincent and Sadler, 1995) but care must be taken to adjust these tags as the animals grow and the effects of the tags on mortality and growth have not been determined. One technique that shows considerable potential is visible implant fluorescent elastomer (VIFE). VIFE is a brightly coloured, fluorescent polymer that is supplied in liquid form and will cure to a flexible solid with the addition of a suitable curing agent (Northwest Marine Technology, Shaw Island, Washington, USA). It can be mixed, then injected subcutaneously using a small-bore needle in a variety of body or fin locations where it should form a permanent, visible, non-toxic mark. Although employed successfully in many fish species (e.g. Bailey et al., 1998, Haines et al., 1998, Malone et al., 1999, Olsen and Vøllestad, 2001), VIFE has been used to tag syngnathids in only two studies. An M.Sc. study by Schannel van Dijken (School of Environmental and Marine Science, University of Auckland, NZ, 2001) monitored 43 VIFE-tagged Hippocampus abdominalis in the wild over 12 months, but effects of tagging were only examined in two individuals held for 3 weeks at Leigh Marine Laboratory. There were no untagged controls and no replication. Similarly, Jodie Le Cheminant conducted an M.Sc. study on Knysna seahorses, H. capensis, at London Zoo (School of Conservation Sciences, Bournemouth University, UK, 2000) but the study only ran for 9 weeks and there was only a single tank of VIFE-tagged animals together with a single control tank of untagged individuals. This design did not allow detection of long-term effects, nor partition of variance between-tank and tagging effects. Thus, we considered neither study to have rigorously investigated the effects of VIFE tagging on seahorses.
Our objective in this study was to assess the suitability of VIFE tagging in seahorses, specifically H. abdominalis. This species is primarily of interest to the ornamental aquarium market, although it is sometimes traded in the medicinal market when sourced from trawler bycatch (Woods, 2000a, Woods, 2000b). Occurring all around New Zealand and south-east Australia, there is little published information on the life history and ecology of this species, particularly in New Zealand waters. Specifically we tested the tag retention and visibility of VIFE tags in H. abdominalis, as well as any impact upon growth and survival.
Section snippets
Study species and tagging procedure
The big-bellied seahorse, H. abdominalis, is one of the largest species of seahorses reaching 30+ cm standard length (SL) and 60+ g in wet weight, and is reported from macroalgal habitats, rocky reefs and artificial structures such as jetties (Lourie et al., 1999, Kuiter, 2000). Colour/pattern is variable ranging from near white to yellow to variable brown/dark spots and splotches on head and trunk, with some tail-banding and spotted dorsal fins (Lourie et al., 1999, Kuiter, 2000).
H. abdominalis
Results
After being handled and placed into the initial holding tanks, all animals swam immediately to the bottom, attached themselves to the substratum and exhibited hiding behaviour (tail tightly coiled around the substratum, body close to the substratum and head tucked down). However, tagged animals exhibited this hiding behaviour longer than control animals; whereas control animals showed normal behaviour and feeding within a mean 9.9±0.7 min, tagged animals took a mean 85.5±4.9 min to return to
Effects of tagging
We conclude from this study that VIFE tags have limited biological effects on captive seahorses. Most importantly there were no mortalities associated with tagging. Successful tagging experiments generally require that mortality be unaffected by tagging. If mortality were increased by tagging potential problems would include: (a) lower probability of recapture/re-sighting of animals making studies less precise, less efficient and less cost-effective, (b) over-estimates of population size when
Conclusions
Given the results of this experiment, we suggest that VIFE tags are well suited for ecological and aquaculture studies of seahorses allowing non-intrusive visual detection of tags. The life span of most species of seahorse is not known but is believed to range from 1 to 5 years (Strawn, 1958, Lockyear et al., 1997, Lourie et al., 1999). Growth rate depression of 10% or less over 7 months and tag half-life under blue light of almost 2.5 years mean that seahorses would be minimally affected and
Acknowledgements
We would like to thank Jodie Le Cheminant and NorthWest Marine Technologies for helpful initial discussions. Thanks to Amanda Vincent and Alison MacDiarmid for critically reviewing earlier manuscript drafts. Thanks to all the Mahanga Bay staff for assisting in feeding the seahorses when we were required elsewhere. Funding for CW was provided by the Foundation for Research, Science and Technology (CO1X0002). KMS was supported by Chocolatérie Guylian N.V., Belgium. Experimental procedure was
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