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Commercial Fisheries News
Volume 37 Number 6
February 2010
Monkfish aging study may alter old thinking
NEW BEDFORD, MA – Like many other fish species, monkfish can be aged by reading the number of “rings” on certain bony body parts such as otoliths, vertebrae, and dorsal spines. It’s like counting the circles on a slab of tree trunk. Three rings means the fish is three years old. Ten rings means it’s 10 years old.
Or does it?
Scientists are beginning to question whether this long-accepted aging technique is completely suitable for monkfish, especially the older ones.
“We could be underestimating age. We’re not sure if a ring is laid down every year when growth slows down,” said Crista Bank, a technician at the University of Massachusetts-Dartmouth School for Marine Science and Technology (SMAST) who is also working on a master’s thesis about the subject.
Yet knowing the age of monkfish is key to understanding their basic biology. So that’s why researchers have embarked on a new series of studies to get to the root of the matter.
“We want to validate that the aging method is accurate, which will give us more confidence in our stock assessments,” said Bank.
While the primary purpose of Bank’s monkfish work at SMAST is to determine whether the current aging methodology is producing good results, she also is investigating whether one particular aging tool is preferable over another.
Is it better to count rings on otoliths – those tiny, calcified inner ear bones – or on vertebrae, which distinctly show rings without even being sliced? Or what about the first dorsal spine – the one with the “fishing lure” dangling from it? That’s what some European scientists use to age monkfish.
Graham Sherwood of the Gulf of Maine Research Institute (GMRI) has been deeply involved in the monkfish tagging program and its related studies. He emphasized the importance of all of the various aging work.
“There’s a real difficulty in aging monkfish,” he said. “We really need to know how long they live.”
Oxytetracycline
Some of the aging work is being carried out during monkfish tagging trips.
Researchers, working aboard commercial fishing vessels, have been surgically implanting data storage tags into monkfish tails and then injecting the fish with oxytetracycline.
The tagging project is designed to produce more information about monkfish migration patterns and north/south mixing rates, but the oxytetracycline injections are part of the aging studies.
The oxytetracycline “marks” the fish. It works by leaving a light glow or stain on the specific ring that was being laid down at the time the fish was caught, tagged, and injected.
Then, when the fish is recaptured, researchers will be able look for the marked ring with ultraviolet light, which will help them determine how much growth occurred between tagging and recapture, assuming the fish isn’t recaptured before it has time to grow.
Crista Bank also is keeping live monkfish in holding tanks at SMAST.
“Growth in the lab won’t be like actual growth in the wild, but we should be able to see seasonal growth patterns,” she said. “This also will help us confirm whether the oxytetracycline method is working.”
Bank participated in all but one of the 2009 tagging trips and is one of four researchers who have mastered the surgical procedure used to implant the high-tech electronic data storage tags into monktails. She also has taught other researchers to do oxytetracycline injections so the work can continue even when she’s not onboard.
Oxytetracycline is an antibiotic. Anne Richards of the Northeast Fisheries Science Center, one of the project leaders in the monkfish tagging program, said using the antibiotic for aging studies might have an ancillary benefit following tag surgery.
“It should help with healing by reducing the chance of infection at the tag site,” she said.
Microchemical work
In another phase of the aging work, researchers plan to save otoliths from recaptured monkfish for future microchemical analyses that may help them better interpret growth rings.
The people involved hope to age fish chemically by looking at the ups and downs of certain microchemical components in the otoliths, such as with the element strontium. Researchers may be able to track seasonal variations in temperature, for example, by looking at differences in strontium concentrations.
It’s complicated and highly technical work for sure, but it has the potential to provide amazing results.
Graham Sherwood said GMRI researchers ran a single otolith through the process at Memorial University in St. John’s, Newfoundland.
“We thought it was an eight-year-old monkfish,” he said. “But after the analysis, it looked like it might have been more like 13 years old.”
Thanks to industry
The aging studies will be years in the making and won’t produce any immediate results, but fishermen are supportive of the effort, knowing that better aging information will lead to better stock assessments, which then will lead to better management.
With this in mind, industry has been working hand-in-hand with researchers, taking them to sea when needed and supplying them with fish.
Bank, who was readying to meet a fisherman at the dock one Friday night in mid-January to pick up yet another live monkfish for her laboratory holding tanks, expressed deep appreciation for industry’s support.
“Without them, we couldn’t do any of this,” she said. “We rely on their help.”
Janice M. Plante
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