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How Old Is That Fish?
Figure 1. Photograph of a striped bass scale with annuli numbered. Age of this scale is estimated to be 5+ years old.
I typically receive several questions each month from anglers who want to know the age of a fish they have caught. When I tell them the fish's approximate age, they usually seem quite surprised. Therefore, I thought it would be good to share some basic information on how and why fish are aged, as well as provide you with the knowledge for estimating the age of the fish you catch. Hopefully, this will aid you in answering this age old question, pun intended.
Fish are composed of a variety of hard bony structures that contain natural growth rings (think rings on a tree trunk) which can be used for aging, such as scales, vertebrae, fin spines, and gill plates. For many years scales were the standard hard structure used for determining a fish's age because they were easy to collect and you did not need to sacrifice the fish to get them. Aging fish this way is done by counting the distinct bands called annuli. Because fish typically grow much faster in the warmer months than they do in the cooler ones, distinct rings are formed from the uneven seasonal growth (Figure 1). However, this method often leads to underestimating the age of older fish due to difficulties in clearly seeing annuli, which become more compact as growth slows down after reaching maturity. Another problem with using scales is that fish regenerate lost scales, and if a regenerated scale is used to age the fish it will only reflect the age from when the scale started growing.
More recently, otoliths (hard calcium carbonate structures located behind the brain of bony fishes and commonly referred to as ear bones) have become the standard for aging fish. Otoliths also contain annuli, but are generally much easier to see even in older fish, making this method more accurate and precise for aging fish. However, for rare or endangered species scales are still preferred for aging since removing otoliths requires sacrificing the fish.
Identifying a fish's age is important for effective fisheries management strategies. Knowing how old a fish is allows fisheries biologists to determine growth rates, age at maturity, and maximum age. These parameters are used to construct age-based stock assessment models that estimate the age and structure of fish populations. Stocks are geographic subgroups of a fish population that can be managed as a discrete unit (e.g., Lower Laguna Madre spotted seatrout). Stock assessments are used to estimate the general health of stocks, which in turn greatly influence fishing bag and size limits. For example, if a stock is composed primarily of small younger fish then a new minimum size limit may be needed to allow those fish to reach larger sizes in order to give them a chance to spawn and replace themselves before being harvested.
I have included two graphs constructed from TPWD data plotting total Length at Age for redfish and spotted seatrout to give you an idea of the age of a particular fish at a given length. Taking a look at the redfish's length at age plot, you can see a period of rapid growth during the first five years of the fish's life before slowing down and leveling off. Based on this graph, a 12-inch redfish is around age 1, an 18-inch fish is around age 2, a 24-inch fish is around 3 years old, and so on. Once a redfish reaches lengths greater than 35 inches, it becomes difficult to estimate its age based solely on length.
Spotted seatrout data reveals a much straighter line (more linear growth) than that of redfish. After reaching age 1, trout undergo a fairly steady growth rate throughout their relatively short lifespan of 10 years. Also notice the significant differences in growth rates between sexes, with males growing considerably slower than females. This graph shows that a female trout 15-16 inches is around age 2, while it takes a male 3 years to reach that size range. You can also see that it is fairly rare to find a male trout over 20 inches in length in Texas coastal waters.
I do want to point out that there is quite a bit of variability in length at age for both species due to differences in factors such as genetics, diet, and habitat. Therefore these graphs are simply age estimates at a given length, but will give you a good approximation of the age of redfish and spotted seatrout. Now you can impress your fishing buddies by telling them the age of the redfish or spotted seatrout you catch the next time you are out on the water. A word of caution, doing this may result in a fish nerd classification as many of my friends refer to me.
Fish are composed of a variety of hard bony structures that contain natural growth rings (think rings on a tree trunk) which can be used for aging, such as scales, vertebrae, fin spines, and gill plates. For many years scales were the standard hard structure used for determining a fish's age because they were easy to collect and you did not need to sacrifice the fish to get them. Aging fish this way is done by counting the distinct bands called annuli. Because fish typically grow much faster in the warmer months than they do in the cooler ones, distinct rings are formed from the uneven seasonal growth (Figure 1). However, this method often leads to underestimating the age of older fish due to difficulties in clearly seeing annuli, which become more compact as growth slows down after reaching maturity. Another problem with using scales is that fish regenerate lost scales, and if a regenerated scale is used to age the fish it will only reflect the age from when the scale started growing.
More recently, otoliths (hard calcium carbonate structures located behind the brain of bony fishes and commonly referred to as ear bones) have become the standard for aging fish. Otoliths also contain annuli, but are generally much easier to see even in older fish, making this method more accurate and precise for aging fish. However, for rare or endangered species scales are still preferred for aging since removing otoliths requires sacrificing the fish.
Identifying a fish's age is important for effective fisheries management strategies. Knowing how old a fish is allows fisheries biologists to determine growth rates, age at maturity, and maximum age. These parameters are used to construct age-based stock assessment models that estimate the age and structure of fish populations. Stocks are geographic subgroups of a fish population that can be managed as a discrete unit (e.g., Lower Laguna Madre spotted seatrout). Stock assessments are used to estimate the general health of stocks, which in turn greatly influence fishing bag and size limits. For example, if a stock is composed primarily of small younger fish then a new minimum size limit may be needed to allow those fish to reach larger sizes in order to give them a chance to spawn and replace themselves before being harvested.
I have included two graphs constructed from TPWD data plotting total Length at Age for redfish and spotted seatrout to give you an idea of the age of a particular fish at a given length. Taking a look at the redfish's length at age plot, you can see a period of rapid growth during the first five years of the fish's life before slowing down and leveling off. Based on this graph, a 12-inch redfish is around age 1, an 18-inch fish is around age 2, a 24-inch fish is around 3 years old, and so on. Once a redfish reaches lengths greater than 35 inches, it becomes difficult to estimate its age based solely on length.
Spotted seatrout data reveals a much straighter line (more linear growth) than that of redfish. After reaching age 1, trout undergo a fairly steady growth rate throughout their relatively short lifespan of 10 years. Also notice the significant differences in growth rates between sexes, with males growing considerably slower than females. This graph shows that a female trout 15-16 inches is around age 2, while it takes a male 3 years to reach that size range. You can also see that it is fairly rare to find a male trout over 20 inches in length in Texas coastal waters.
I do want to point out that there is quite a bit of variability in length at age for both species due to differences in factors such as genetics, diet, and habitat. Therefore these graphs are simply age estimates at a given length, but will give you a good approximation of the age of redfish and spotted seatrout. Now you can impress your fishing buddies by telling them the age of the redfish or spotted seatrout you catch the next time you are out on the water. A word of caution, doing this may result in a fish nerd classification as many of my friends refer to me.