The Sclerochronology Lab
It is not often that university graduates fall into a job right out of school and stay there for the rest of their careers. But that is exactly what transpired for Shayne MacLellan. After receiving her Bachelor of Science degree from the University of British Columbia in 1977, within a year she found herself working in what was then known as the fish-ageing lab at DFO's Pacific Biological Station (PBS) in Nanaimo, BC. As Ms. MacLellan notes, “At the time I didn’t know you could have a career in fish ageing. I have been there ever since.”
She adds, “It was Dr. Richard Beamish, our Senior Scientist at PBS, who formed the lab in 1977 when our extended jurisdiction to 200 miles came into being. He had a vision of creating a lab where people worked and focused on age determination. He recognized that it really wasn’t being done in a standard and accurate way and such a lack had to be corrected because age is a key piece of data that is used by biologists to assess stocks.”
The program, now called the Sclerochronology Lab, and which Ms. MacLellan heads up, supplies age data for almost all commercial species in the Pacific region, ageing 115,000 or more fin/shellfish per year. The samples they age come from a variety of sources that include port and observer samples from the commercial fishery, survey samples from scientific field trips, from first nations and sports fisheries, and even occasionally from foreign fisheries.
The state of fish ageing science has become a lot more refined since Ms. MacLellan's university days when her professor projected a picture of a fish scale and said if you just squint, and count the fuzzy lines, you know how to age fish.
These days, fish agers, or readers as they are called, are concerned with identifying and counting three types of growth zones – the wide summer zone, the time of fast growth – followed by the narrow winter zone called the annulus, and a third zone called a “check”. Similar to tree rings – a wide and then a narrow zone alternating, that is what they look for. The summer and the winter zone together equal an annual zone, one year's worth of growth. The summer zone will be opaque and the annulus will be translucent on most ageing structures, although that will vary depending on the structure being examined. For each structure the lab has developed a method that has a series of rules or criteria that readers must learn and apply as consistently as possible. “Checks” can be particularly difficult to interpret correctly because sometimes they can be as prominent or nearly as prominent as an annulus. That's part of the reason it takes years of experience to be able to produce accurate age data.
Transparency: cross-section of a clam shell in the hinge area. The arrows point to "signature years," which correspond to calendar years in which relatively fast or slow growth occurred and help to validate the established age.
Judy McArthur, PBS
The growth rings are burnt brown, which makes them easier to identify. The otolith was determined to be 27 years old.
Laboratory photo: Karen Charles, PBS
The otolith surface is from a much younger fish than the cross-section. The otolith is white with straight, dark and translucent areas that correspond to growth rings.
Laboratory photo: Karen Charles, PBS
The readers age hard tissue – such as fish scales, otoliths (ear bones), fin rays, and shells in the case of shellfish. Much can be ascertained by a visual inspection as they section the tissue with a variety of saws so they can see internal annual layers to age. As MacLellan notes: “Our work is both macroscopic and microscopic and we use a variety of microscopes and projectors. Much depends on the structure involved. For instance with an otolith you can think of a drawing in two dimensions, like a plate. You can see annual growth zones if you just look down on the surface of the flat plate; but as the fish mature less material is deposited concentrically on the perimeter of the otolith while it thickens only and at some stage you have to look at a cross-section to see the mature growth.”
Cross-sections of the dorsal fin of a Pacific cod
Each fin ray consists of two elements, as shown above. The yellow dots indicate narrow, translucent growth rings. The large, dark, opaque areas are summer zones. The fish was determined to be 6 years old.
Laboratory photo: Darlene Gillespie, PBS
The end goal is getting a better understanding of fish demographics. Ms. MacLellan: “We want to understand the fish’s biology at various stages of their lives. How big they are at certain ages. How old they are when they reproduce for the first time. How long they reproduce for. And of course how long they live. Understanding rates of growth, reproduction, and mortality through the collection of age data are all important factors that get plugged into statistical models that biologists use to assess stock and population.”
Scientists and other end users marry up the age data to other biological data such as length, weight, sex, and maturity - which in turn are matched to where the sample came from when captured. The data is rolled up into multiple years and even decades – allowing them to determine age classes of fish that are moving through the population. One of the things they want to know is how many of what age fish are out there in the fishery.
So how can age data be used to monitor the health of fish stocks? Notes Ms. MacLellan, “In an area of heavy fishing we can look at the age compositions. In the case of rockfish for example, you might find instead of having populations that have a healthy age range of anywhere from the time of maturity which may be around age 10 up into the 80s and 90s and all the decades in between, you might find only teenagers there; or early 20s at the best. So age is a good indicator of the overall health of the stock because there is a reason for longevity and representation - in all age classes.”
Needless to say, getting the data right is critical. To that end, the Lab has developed standard training methods and procedures to monitor the quality of their data. “There are two things that we measure,” says MacLellan “accuracy and precision. If you look up those terms in the dictionary they use each other to describe themselves; but in science there is a difference. Precision means repeatability. Accuracy means how close to the truth you are. For example, we precision test every single sample that we age. That means that we have a reader who goes through the entire box of structures and produces ages. Then we write up a random test that picks out anywhere from 10 – 20 percent of the sample that has been aged and a second reader is given that sheet and they have to independently age these fish again. So that is where the precision/repeatability comes in.”
Training fish agers takes a long time, and is very much a hands-on and in-house process, since there is no school in Canada that provides comprehensive fish age determination courses. Ms. MacLellan: “I am lucky to have over 150 years experience amalgamated in my group. We train staff to learn how to age all species using all methods as their careers advance. But that takes time. After two or three years they are still a novice, after five years they are considered to be at an intermediate level, and are really starting to contribute more fully. After ten years they have grasped most everything and have probably worked with all species and all aging methods.”
And therein lies a bit of a challenge for the Lab in the future. All that collective expertise and memory will soon disappear because many of the agers are nearing retirement. That's why Ms. MacLellan and her staff have been documenting their methods, procedures, and standards, and drawing up an extensive manual of what they do and how they do it for the next generation of sclerochronologists. They have also put into action a succession plan to hire new staff as retirements occur.
Good thing too, since the Lab has an international reputation for the unique work they do and how they do it. And of course the scientists and policy folks up the line have grown pretty dependent on the usefulness of the age data they have been receiving over the years. And the Pacific Region fishery is all the better for it.
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