October 9, 2014 at 3:22 pm
[caption id="attachment_346" align="alignright" width="300"]
A scale from a 5 year old landlocked salmon. Each year adds multiple rings and the space between each ring gives you an idea of how quickly the fish was growing.[/caption]
Understanding the age structure of a population is imperative in developing a management strategy for that population. Age structure refers to the number or percent of individuals in a population that are a given age. Age and size of individuals in that population can help determine the health of the population as a whole so it is not enough for biologists studying a population to measure just the size of a fish, they must also know the age of those fish in order to understand the structure of the population. For example, if you have five salmon from Lake X and five salmon from Lake Y and all of them are 16 inches long but the salmon from Lake X are three years old and those from Lake Y are six years old, then it can be understood that the salmon in Lake X have more food or less competition or better water quality than the salmon in Lake Y, allowing them to grow faster. The age and size structure of the population must be understood before any management suggestions can be made. Age structure, such as what percentage of the population is immature, what percentage is mature breeding age, and what percentage is past breeding age, is one of the first key pieces of knowledge that must be obtained when managing a population, but how do we determine the age of a fish?
Most fish can be aged using one of two body pieces. The first is by a small bone-like
[caption id="attachment_348" align="alignright" width="300"]
Looking at this scale from a landlocked salmon, can you determine the age of the fish when this scale was taken?[/caption]
structure called an otolith, which is found floating around in a fish’s inner ear, located behind the brain. An otolith is comprised of calcium carbonate and grows as the fish ages. There are three otoliths in each inner ear but only one of them is large enough to be used for aging. The otoliths act like the tiny bones in a person’s inner ear, giving the fish a sense of bodily orientation. The larger two of the six total otoliths can be retrieved from the fish’s head, cut, dyed, and viewed under a microscope where the compacted calcium carbonate rings can be counted to determine the fish’s age. While this method of aging is more accurate than the second method, it can only be done with deceased fish.
The second method is using a scale. While scales are not quite as accurate as otoliths, they allow the age of a fish to be determined without euthanizing the fish. A scale is taken by scraping off excess mucous, then gently pushing the point of a knife down and back toward the tail. This will dislodge the scale and if done gently will not cause damage to the scale or the fish. Multiple scales are taken from each individual fish in this manner, as some scales are easier to read than others and you cannot tell which are good scales for aging until they are under a microscope. Scales grown to replace those damaged from fighting with other fish or for escaping predation will have large unreadable patches and are not accurate for aging.
Once the scale is dried and placed on a slide on a microscope the age can be determined by counting ring patterns. Scales are made of connective tissue coated in calcium. This tissue is added to the edge of a scale as the fish grows. In spring and summer, fish typically grow faster due to high food availability, warmer water temperatures, and other contributing factors. Therefore, on the scale, the tissue rings appear spaced further apart. In the fall and winter when the fish is not growing as quickly the same amount of tissue is being added over a given period of time but the fish is not growing as quickly so these rings appear clustered closer together on the scale. Therefore you can see seasonal patterns in the rings on a fish scale. This is how reading a scale differs from reading tree rings. With a tree, each ring signifies a year of growth, however, with a fish scale, a year of growth may be signified by multiple rings spaced far apart (spring and summer growth) then multiple rings spaced closer together (fall and winter growth) then the following group of rings spaced far apart again thus separating one year from another. Although it sounds complicated, this growth pattern viewed with a practiced eye can determine how old the fish was when the scale was taken. By observing the pattern in which tissue was added to scales, it can sometimes even be determined at which age a fish spawned, if the fish is sexually mature.
[caption id="attachment_347" align="alignnone" width="300"]
If you said the age of the fish from which the last scale was taken was 4 years old you were correct.[/caption]
A scale from a 5 year old landlocked salmon. Each year adds multiple rings and the space between each ring gives you an idea of how quickly the fish was growing.[/caption]
Understanding the age structure of a population is imperative in developing a management strategy for that population. Age structure refers to the number or percent of individuals in a population that are a given age. Age and size of individuals in that population can help determine the health of the population as a whole so it is not enough for biologists studying a population to measure just the size of a fish, they must also know the age of those fish in order to understand the structure of the population. For example, if you have five salmon from Lake X and five salmon from Lake Y and all of them are 16 inches long but the salmon from Lake X are three years old and those from Lake Y are six years old, then it can be understood that the salmon in Lake X have more food or less competition or better water quality than the salmon in Lake Y, allowing them to grow faster. The age and size structure of the population must be understood before any management suggestions can be made. Age structure, such as what percentage of the population is immature, what percentage is mature breeding age, and what percentage is past breeding age, is one of the first key pieces of knowledge that must be obtained when managing a population, but how do we determine the age of a fish?
Most fish can be aged using one of two body pieces. The first is by a small bone-like
[caption id="attachment_348" align="alignright" width="300"]
Looking at this scale from a landlocked salmon, can you determine the age of the fish when this scale was taken?[/caption]
structure called an otolith, which is found floating around in a fish’s inner ear, located behind the brain. An otolith is comprised of calcium carbonate and grows as the fish ages. There are three otoliths in each inner ear but only one of them is large enough to be used for aging. The otoliths act like the tiny bones in a person’s inner ear, giving the fish a sense of bodily orientation. The larger two of the six total otoliths can be retrieved from the fish’s head, cut, dyed, and viewed under a microscope where the compacted calcium carbonate rings can be counted to determine the fish’s age. While this method of aging is more accurate than the second method, it can only be done with deceased fish.
The second method is using a scale. While scales are not quite as accurate as otoliths, they allow the age of a fish to be determined without euthanizing the fish. A scale is taken by scraping off excess mucous, then gently pushing the point of a knife down and back toward the tail. This will dislodge the scale and if done gently will not cause damage to the scale or the fish. Multiple scales are taken from each individual fish in this manner, as some scales are easier to read than others and you cannot tell which are good scales for aging until they are under a microscope. Scales grown to replace those damaged from fighting with other fish or for escaping predation will have large unreadable patches and are not accurate for aging.
Once the scale is dried and placed on a slide on a microscope the age can be determined by counting ring patterns. Scales are made of connective tissue coated in calcium. This tissue is added to the edge of a scale as the fish grows. In spring and summer, fish typically grow faster due to high food availability, warmer water temperatures, and other contributing factors. Therefore, on the scale, the tissue rings appear spaced further apart. In the fall and winter when the fish is not growing as quickly the same amount of tissue is being added over a given period of time but the fish is not growing as quickly so these rings appear clustered closer together on the scale. Therefore you can see seasonal patterns in the rings on a fish scale. This is how reading a scale differs from reading tree rings. With a tree, each ring signifies a year of growth, however, with a fish scale, a year of growth may be signified by multiple rings spaced far apart (spring and summer growth) then multiple rings spaced closer together (fall and winter growth) then the following group of rings spaced far apart again thus separating one year from another. Although it sounds complicated, this growth pattern viewed with a practiced eye can determine how old the fish was when the scale was taken. By observing the pattern in which tissue was added to scales, it can sometimes even be determined at which age a fish spawned, if the fish is sexually mature.
[caption id="attachment_347" align="alignnone" width="300"]
If you said the age of the fish from which the last scale was taken was 4 years old you were correct.[/caption]Categories