The Art and Science of Sea Horse Tagging
by Simon Dakin
I've been assisting Colin Piper with his survey work on the seahorses at Balmoral shark-proof nets. I wanted to see whether the information we were collecting could be improved and so I got in touch with the people at Project Seahorse: (http://www.seahorse.mcgill.ca)
I received a phone call from Dr Keith Martin Smith: (http://ww2.mcgill.ca/biology/labs/vincent/whoweare/kmart.htm) who invited me to join him at a seahorse tagging dive on 10th January 2003.Keith works for Project Seahorse which is centred at a Canadian university but he is based at the University of Tasmania. I believe Keith has previously given a presentation to the URG about the Project Seahorse work. The person who has been monitoring the seahorses here in Sydney on a regular basis over the past few years is Jonathan Clark-Jones though much of the pioneering work in Sydney was done by Dr Amanda Vincent:(http://ww2.mcgill.ca/biology/faculty/vincent/) who heads up Project Seahorse. I've been asked not to mention the dive site as there is some concern that people might be collecting seahorses at some locations (this is believed to be the case at Balmoral).
After introductions and gearing up we started searching the nets for seahorses. At this site, which has no stinger nets, it was necessary to examine both sides of the net as some can only be spotted from the appropriate side. There are two typical places on the nets that are favoured by seahorses, either directly on the mesh with their tail wrapped around the V formed at the bottom of each diamond shape in the mesh, or on clumps of vegetation that have formed on the net.
At Balmoral we generally only survey from the inside of the net as the remaining stinger nets make it difficult (and potentially dangerous) to survey from the outside. It's my belief at Balmoral that the majority of the seahorses will be spotted just by searching from the inside as the high density population areas are next to the wharf and have stinger nets which prevent access to the net from outside. An outside survey at Balmoral will certainly fail to find many of the seahorses in these regions. At some times of day it will be difficult to survey some sections from the inside at Balmoral because the sun will be shining through the net into the divers' eyes and only seahorses silhouetted will be readily spotted and under these circumstances a diver outside the net would be useful. At Balmoral we've tended to find the bulk of the seahorses at the bottom portions of the net, possibly because these are less susceptible to wave action and also quite likely because these are most heavily vegetated areas.
On finding a seahorse it was inspected with the help of a blue LED dive light which makes the various coloured marks shine brightly. Even in ambient light the marks can be seen but the blue LED light makes it easier to see them and especially to differentiate between any yellow and orange tags. If the seahorse was unmarked then it was gently removed from the net and placed in an individual numbered mesh bag which was in turn placed in a catch bag. The location where the seahorse was removed was marked on the net with a peg which was unique by virtue of a combination of its own colour and a coloured plastic tie attached to the peg. The peg details were marked on a slate along with the number of the mesh bag.
After all the mesh bags were used the catch bag was taken ashore and placed in a tub of seawater. Each mesh bag in turn was removed from the tub and the seahorse placed in a small tank that contained seawater with clove oil, a natural fish anaesthetic. After the seahorse had calmed down they were removed and injected with a thermosetting polymer called VIFE (Visual Implant Fluorescent Elastomer) that is made by Northwest Marine Technology, an American company. The polymer is made up in advance and stored at a low temperature until it's ready to be used. In our case it was kept on ice in an esky until required. Checks are made on a sample kept after a tagging session to ensure that the polymer is setting correctly. The polymer is injected using a very small syringe or a specialist hand injector that has a fine needle but a larger body to make dispensing easier. The company also sell or lease powered injectors for mass marking projects. One of the accompanying pictures shows the 4 places on each side of the seahorse that are used as tag positions. Using combinations of these positions and different VIFE colours (red, yellow, green, orange) gives a large enough number of combinations for individuals to be uniquely marked. In our case we used just green VIFE and applied the marks to only one side of the seahorse.
After the marks were applied, the species of seahorse, sex and head and body length were recorded. Two species of seahorses were found, white's (Hippocampus whitei) and big-belly (Hippocampus abdominalis). A knobbly crown on the head of the seahorse indicates a whitei, an absence a big-belly. Big-bellies at Balmoral are usually larger, much paler and sometimes marked with largish dark spots though, as many seahorses can change colour to match surroundings or during courtship, colour is not an infallible means of discriminating between the species. The sex of the animal is readily determined in adults by the presence of a prominent brood pouch on the belly of the males, less so with juveniles.
The length of the animal from tip of snout to end of straightened tail can be taken at this point though when subsequent net surveys are performed the length of an individual can only be estimated as they should not be removed from the net.
Previously we'd assumed that all of the seahorses at Balmoral where white's but a subsequent census has shown that there are also a small number of big-belly. The tagging site also has predominantly white's but this balance used to be reversed before the nets were replaced. It's thought that after the nets were replaced, some seahorses in the surrounding environment (typically seagrass meadows) moved to the net. It's not known what effect the replacement of nets would have on the population densities of seahorses in the adjacent areas but clearly if the net replacement was too frequent there must be some impact.
It was found that marking the big belly seahorses was fairly straightforward whilst it was difficult to inject some of the positions on the smaller White's simply because they are so tiny compared to big-bellies, a problem exacerbated in juveniles.
The home range of seahorses is very small, perhaps as little as 1 metre radius for males and so after marking great care has to be taken to return the individuals to their original positions. Of course, after marking it's important to monitor the nets on a regular basis to yield information on aspects such as each individuals home range and growth rates. Surveys at both day and night need to be made though on our one subsequent visit to the site at night we found it very difficult to spot any seahorses. Seahorses are diurnal and seem to be much more secretive at night though with a blue LED torch we'd perhaps have found it easier to spot those that were tagged.
If the group was interested we could extend the data we collect on our Balmoral survey to match that used by Project Seahorse. Colin and myself have an excellent PDF file with the data collection protocol used by Project Seahorse. However if we did so we'd need to tag and track individuals over an extended period of time and this would be a major commitment. Certainly it can do no harm to continue our general population survey perhaps with attention now being paid to note the relative proportions of white's and big-belly seahorses.
We also have a copy of some of Dr Amanda Vincent's original papers on the Whites seahorses in Sydney. These contain some fascinating insights into seahorse behaviour such as the dance each pair perform every morning soon after dawn. This lasts on average 6 minutes and afterwards the couple split up and go their own way for the remainder of the day. Seahorses are monogamous and the dance is believed to maintain the bond and possibly to permit them to gauge whether they're both in a state to mate.
If anyone would like to join me for a survey dive at Balmoral I'd be delighted to show them around. By the way, there is also an excellent selection of other marine life on view and all within about 150m of a post dive cafe:
We also take the opportunity whilst surveying to tidy up and on the past two dives we've removed 7 pairs of swimming goggles, a number of plastic spoons and forks, a driving licence, a toy car and a mobile phone.
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