Diagram of a mature female winter skate (Leucoraja ocellata) showing the measurements taken onboard ship and the tagging and injection locations
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Diagram of a mature female winter skate (Leucoraja ocellata) showing the measurements taken onboard ship and the tagging and injection locations |
Tagging and recapturing fish can reveal lots of interesting information about a species’ biology and ecology. For example, recaptures can yield information about a species’ growth rate as well as reveal possible migration patterns. At the Bedford Institute of Oceanography (BIO), a skate mark-recapture program was started in the summer of 2004 with the Fishermen and Scientists Research Society (FSRS) to help determine age and growth, migration pathways, stock discreteness, and population abundance of all species of skate on the eastern Scotian Shelf off Nova Scotia, Canada.
On board ship, skates were deemed in relatively good shape for tagging based on the following criteria:
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No visible or extensive trawl wounds |
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Minimal abrasions on the skin from the thorns and spines of other fish |
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Active spiracles |
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A fair amount of pectoral fin flapping and jaw gaping |
Skates were then
tagged with either a T-bar or dart-tipped tag, with the size of the tag
dependent on the size of the skate. A
single tag was inserted dorsally into the fleshy part of the pectoral fin of
each skate, towards the posterior end. Efforts
were made to tag and inject juveniles and adults of each species.
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A large female winter skate (Leucoraja ocellata) being measured onboard the MV Teleost |
A large female winter skate (Leucoraja ocellata) being tagged with a dart-tipped tag onboard the MV Teleost |
Tagged skates were also injected intra-peritoneally with a veterinary antibiotic called oxytetracycline (OTC). This is a chemical commonly used in age and growth studies on fish, as it deposits at sites of active calcification (i.e. in vertebral centra, for example) and can remain distinct for some time. More specifically, in the event of a recapture, the calcified structures of the individual can be examined and the time at liberty can be compared with the number of bands deposited distal to the mark, allowing one to validate age estimates obtained from vertebral band counts. Validation is a crucial part of any age and growth study, and a mark-recapture of chemically-tagged wild fish is one of the most rigorous, indirect means of validating the frequency of formation of growth increments.
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An oxytetracycline (OTC) injection gun and a conventional T-bar tagging gun used for the mark-recapture of Northwest Atlantic skates |
A maturing female winter skate being injected ventrally with oxytetracycline (OTC) onboard the MV Teleost |
Unfortunately, to date there have been no recaptures from over 800 skate injected, tagged, and released on the eastern Scotian Shelf. Captive rearing of skates within the laboratory at BIO has revealed that OTC is incorporated in the cartilaginous structures of each species, suggesting that mark-recapture is a suitable validation method in northwest Atlantic skates. However, the failure of the wild mark-recapture to date raises some interesting questions, such as what the discard mortality of northwest Atlantic skates is and what their temperature and pressure sensitivities are.
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Photographs taken under ultraviolet (UV) light (340-350 nm) of the vertebrae of an immature male winter skate (Leucoraja ocellata) injected with oxytetracycline (OTC) and held in captivity for 12 months. The deposition of the chemical OTC tag is clearly visible in the growing edges of the corpus calcarea (red arrows) and the intermedialia (yellow arrows), as is growth distal to the mark (red bar). |
If you should happen to catch a skate tagged with a yellow T-bar (Floy) or dart tag, please return the whole frozen fish with the following information:
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Date of capture |
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Name/ phone # / address of person returning tag and fish |
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Position caught (latitude and longitude) |
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Depth and water temperature at location of capture |
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Method of capture |
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Tag number and skate length |
Contact the one of the following individuals below so that the fish can be picked up by a fisheries technician.
Canadian
Shark Research Lab
Population
Ecology Division
Bedford
Institute of Oceanography
P.O. Box
1006
Dartmouth
,
B2Y
4A2
Phone:
(902) 426-6382
E-mail:
mcphier@mar.dfo-mpo.gc.ca
or
Carl MacDonald
Fishermen and
Scientists Research Society
P.O. Box
25125
Halifax
,
B3M 4H4
Phone: (902)
461-8119
Toll free:
1-800-226-3777
E-mail:
macdonaldcd@mar.dfo-mpo.gc.ca
People who return
tagged skates will receive more specific information about where the fish was
tagged, and other biological information. If
a return address is included with the fish, you will be provided with a ball cap
for your efforts.
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