Some of the most important information in managing living natural resources comes from learning the ages of individuals within population. Age information is important for determining when an organism can reproduce, the rate at which organisms perish, and the success of reproduction within a given year. Age information is also extremely useful to management of fisheries when coupled with size information to provide estimates of growth and mortality.
While age information is extremely beneficial to resource managers, it can be difficult information to obtain. Humans are the only creatures with birth certificates (excluding purebred dogs, horses, etc.) and consequently, scientists have developed several approaches to estimate ages of organisms. Often in mammalian research, age is estimated by looking at deterioration of teeth over time. Invertebrates, such as mollusks, can be aged by looking at their shells, whereas crustaceans (crabs and lobsters) can be difficult or impossible to age. A variety of methods have been developed to age fish, but aging via hard, calcified structures is most commonly used.
When talking to people about aging fish, scientists often provide the analogy of counting the rings present on a stump after felling a tree. This analogy works well as both trees and fish experience differential growth patterns seasonally due to differences in temperature. As a result, both trees and fish exhibit slow growth during the winter followed by faster growth during the summer. This leads to growth rings called annuli forming on the hard parts of a fish, as calcium is deposited at different rates throughout the year. This only works in temperate regions where seasonal differences in temperature lead to changes in growth rates. In tropical regions, the use of hard parts to age fish doesn't usually work.
Catfish have historically been aged using several structures, including otoliths, spines, vertebrae, and probably other bony structures as well. Pectoral spines and otoliths are the most commonly used structures. Pectoral spines are the sharp, serrated projections protruding from the pectoral fins. These "spines" have stabbed many unsuspecting catfish anglers, and many species even contain venom within their spines (ichthyocrinotoxins), but this is a good subject for a future blog post. Spines are still a useful aging tool as they can be removed from a fish non-lethally. However, research has shown that annuli within catfish spines often erode in larger, older fish, and this can lead to aging errors. Consequently, otoliths have become a more popular choice for scientists to estimate ages of catfish.
A right pectoral spine from a blue catfish.
Otoliths (Greek ot: "pertaining to the ear" and lithos: "stone") are calcified structures inside the heads of fish. They are often called “ear bones”, but are not actually bony structures. Otoliths are important to fish as they provide sensory information on balance and detection of sound. Fish have three pairs of otoliths that are called the asterisci, lapilli and sagittae. Different species of fish are aged
using different structures. For instance, black basses (largemouth, smallmouth, and spotted basses) are aged using the sagittae, while catfishes are aged using the lapilli. Generally, the structure selected is the largest present to make the process of aging the fish a little easier.
Aging fish using otoliths generally requires some processing of the structures to observe the rings or annuli. With basses, this may only requires breaking the structure in half, but for catfishes this is impractical due to the shape of the lapilli. To age catfishes, the otolith is first glued to a glass microscope slide to hold it in place while the scientist grinds away some of the otolith to view the annuli inside. Once the annuli are visible, scientists polish the viewing surface with a fine grit sandpaper and/or mineral oil to polish the surface, making discernment of annuli easier. Once the otolith is polished, it can be viewed under a microscope and the annuli are counted. While this is a relatively straightforward process, it can be time consuming and requires some time to gain proficiency in the technique.
Otolith from a 19" blue catfish. Note that it takes them several years to grow to 19".
So what have fisheries biologists learned from aging blue catfish? While blue catfish are one of the largest freshwater fish species in North America, they grow slowly and may take decades to reach large sizes. They are a long-lived species with lifespans up to 20-30 years. Growth rates are highly variable, and some individuals may never get big. Studies have shown that a 15 year old blue catfish could weigh as little as 5 lbs or as much as 100 lbs. In Virginia, blue catfish growth rates are slowing down in the James River, and the popular trophy fishery within this river may never be as good as it was in the early 2000's.