Frequently asked questions from anglers are: How old are fish? How do you tell their age? How does this help the fishery managers? Well, as the 5th Dimension song goes, "Let the sun shine in," hopefully in understandable layman's terms in answering these important questions.
Fisheries biologists and scientists use a variety of body parts to determine a fish's age. Some of them are scales, fin rays, dorsal and pectoral spines, vertebrae, cleithrum, and otoliths. All of these structures lay down annuli or rings similar to tree growth rings, as fish grow slower or faster seasonally, depending on water temperatures. Some of these require sacrificing the fish to obtain the sample, as we will see, and are not always used.
Scales are used when the fish are to be returned to the water and are better for some species than others. They are also best for younger fish, as fish grow faster, like people, when young. They are good for spiny ray fish like bass and bluegill. Typically at least 10 scales will be removed (they regenerate) and placed in a scale envelope, on which other pertinent information will also be recorded. Ten samples per inch class are obtained, as with humans, not all grow equally. They will be taken back to the lab, dried, and an impression made on plastic. This will allow the scale image to be placed in a micro fish reader, where the annuli or rings will be counted. This gives the biologist a look at the age structure of the population, survival rates of stocked fish, relative numbers and species composition, and growth rates, which will then be compared to other regional lakes and lakes statewide. In analyzing these data, they can then make management decisions like a recommendation to stock or cease stocking, stock a predatory fish to control other fish populations, manually remove a certain species, create new regulations, do habitat manipulations, or maintain the status quo if things look good and there is a good fishery.
Fin rays, both dorsal (back) and pectoral (near head) are used for fish like brook trout, pike or suckers. The removal of a fin ray does not impact the overall health of the individual fish. It has also proven to be more accurate, but it is more time consuming to prepare the fin ray for aging. The fin ray must be dried and then encased in epoxy before a cross-section is made to finally place the sample under a microscope for aging.
Dorsal spines are typically used for walleye, large and smallmouth bass, especially larger, older specimens. A scale from a 17-year-old walleye is almost impossible to age accurately. This method also requires a mounting in epoxy, but gives a much more accurate age estimate than scales. Pectoral spines are used for bullheads and catfish, which have no scales. These are prepared in a similar manner as fin rays and dorsal spines.
Vertebrae can be used, but require sacrificing the specimen. The spinal section is simply cleaned and placed under a microscope, where the annual rings are counted. A cleithrum, which is a pike boney structure under the gill, is used in a similar manner.
Otoliths are the most accurate and intriguing, but also require sacrificing the fish. They are simply ear stones (Oto=ear Lith=stone) found behind the fish's eyes in the brain cavity. They sit on microscopic sensory hairs and are sensitive to gravity (balance organ), acceleration and sound (hearing devices). Otoliths can determine the slightest sound wading fishermen make. There are three pairs, with largest (sagittae) used. They are calcium carbonate (limestone), which the fish extracts from the water. They can vary in size from the 1 on a penny (minnows) to over an inch in groupers. They are the single most important tool for determining the age and growth of fish worldwide. Otoliths do not get reabsorbed during decreased food times, as do scales, yielding a true age.
Otolith cross-sections under a microscope show concentric circles, similar to the feeding rings of a rising bluegill. It is the most accurate method for aging fish, period. Otoliths have been used since the late 19th century and their use continues to expand. They not only reveal the age of fish, but can be used to determine past climatic conditions.
Otoliths are also used by archeologists to determine what fish were in the diets of past cultures when they find them in old human dumps.
Finally, they are being used in art in the form of jewelry. Otoliths make interesting conservation pieces when made into earrings or pendants.
Your horoscope? Go Fish!