Author: Eric Bovee, Fisheries and Aquatic Sciences M.S. Student, University of Florida; Advised by Dr. Debra Murie
You feel a heavy tug on your line, WHOA! As you crank on your reel, you see an enormous dorsal fin surface. You exclaim: “That’s a big fish!”. Once you land it and finally catch your breath, your eyes start to examine the amazing size of this fish, and one of the first questions that usually pops into your head is: I wonder how old this fish is?
This is the one of the most frequent questions that fishers ask me when I go on headboats to tag or collect fish. When discussing age, most fishers have heard about aging fish using an “ear stone”. This “ear stone” is called an otolith. Otoliths are paired calcareous structures located in the inner ear of bony fish. There are three different pairs of otoliths: sagittae, lapilli and asterisci. The otolith’s main function is to provide hearing and balance for the fish (Whitledge 2017). Usually, the sagittae and the lapilli are used for aging, because they are developed when the fish are larvae. Sagittal otoliths are usually the largest, and the easiest to extract and age. The popular exception to this is the catfishes, where the lapilli are used. You might ask yourself “Doesn’t this mean that the fish must perish to determine how old it is?” Sadly, using this methodology, the fish must be sacrificed.
Luckily, researchers have developed non-lethal methodologies. Some of the most popular non-lethal methodologies include using scales, spines, and rays. These methodologies are more appropriate when dealing with fish that are threatened/endangered or located in no-take Marine Protected Areas (MPAs). Aging fish using scales has been predominately focused on relatively short-lived, important recreational and commercial fish species (McInerny 2017). The most prevalent flaw with using scales is that they can underage fish that have moderate to old maximum ages. When this happens it can give fisheries managers the wrong impression on the sustainability of the fish population. A famous example of this was with the Lake Superior Cisco (Coregonus artedi) fishery collapse. Cisco were predominately aged with scales, which led to managers believing that they were a short-lived fish but, when researchers used otoliths to age them, they realized the scales were under aging the fish. This aging error was linked as one of the contributing factors to the collapse of the fishery (Yule et al. 2008). The scale methodology is therefore usually limited to fish that don’t live very long (i.e., <6-10 years) compared to a fish that lives much older. Luckily, researchers have used other hard structures to non-lethally age fish that are more precise and accurate and can be used on at least moderately older fish. This methodology uses fin spines and rays.
Spines and rays are more precise and accurate than scales as a methodology of aging fish nonlethally that get too old to use scales. Spines and rays are both dermal bones, and the growth of these structures are heavily influenced by the fish’s metabolism (Whitledge 2017). The dermal bone of the fish grows during periods of high metabolism. This growth is shown in the opaque zone in Figure 1. The large opaque zone indicates high growth that occurs during the summer in most areas in the U.S. The translucent zones show periods of slow growth or winter growth due to lower metabolic rates in colder water temperatures. A combination of a translucent and an opaque zone usually takes a year (summer and winter growth). In spines and rays, the thin translucent zones are counted as the age.
The drawbacks to using fin spines and rays for aging fish are that the core of the structure can undergo resorption and the annuli can show crowding on the edge (VanderKooy et al. 2020) . Resorption is when the first couple of annuli are covered up by the core of the spine or ray, which can lead to underestimating the age of the fish by a couple of years (VanderKooy et al. 2020). Crowding on the edge is when the narrow translucent lines are stacked so close together on the edge of the structure that they cannot visually be distinguished from one another, which can potentially lead to under-aging of the older fish. This piling up of annuli is caused by a lower growth rate as the fish grows older. There can also be problems of over-aging a fish, and this is due to translucent lines known as “false annuli” or “checks.”
These are usually due to short periods of slow growth that occur during the life of the fish, for example due to temperature shifts (e.g., during an El Niño event) or spawning. Readers who are aging fish can potentially count these “checks” as annuli. To determine if ages are accurate, ages from spines/rays are compared to otolith ages from the same fish for a subsample of fish over the age range of the species. Other alternatives include chemically marking the hard structure or recapture studies (Vanderkooy et al. 2020).
As a first-year master’s student at the University of Florida in the Murie Lab, part of my project involves non-lethally aging Gray Snapper (Lutjanus griseus) that are tagged and released from offshore spawning aggregations. To do this, I am collecting spines and rays from tagged-and released fish. I will make sure that these ages are accurate by comparing ray/spine ages to otolith ages from Gray Snapper also collected from charterboat and headboat fisheries. Non-lethally aging Gray Snapper in this manner will allow us to determine whether the dispersal and aggregation movement patterns differ between younger and older fish at offshore spawning aggregations off the West Coast of Florida.
References
McInerny, M.C. 2017. Scales. Pages 127-159 in M.C. Quist and D.A. Isermann, editors. Age and growth of fishes: Principles and techniques. American Fisheries Society, Bethesda, Maryland.
VanderKooy S, Carroll J, Elzey S, Gilmore J, Kipp J. 2020. A Practical Handbook for Determining the Ages of Gulf of Mexico and Atlantic Coast Fishes. Gulf States Marine Fisheries Commission and Atlantic States Marine Fisheries Commission, Ocean Springs, Mississippi.
Whitledge, G.W. 2017. Morphology, Composition, and Growth of Structures Used for Age Estimation. Pages 9-33 in M.C. Quist and D.A. Isermann, editors. Age and growth of fishes: Principles and techniques. American Fisheries Society, Bethesda, Maryland.
Yule, D.L., Stockwell, J.D., Black, J.A., Cullis, K.I., Cholwek, G.A. and Myers, J.T. (2008), How Systematic Age
Underestimation Can Impede Understanding of Fish Population Dynamics: Lessons Learned from a Lake
Superior Cisco Stock. Transactions of the American Fisheries Society, 137: 481-
Reefs to Rivers: Florida Fisheries Science 