Age and Growth, Reproductive, and Genetic Studies
During fish sampling, most specimens are identified, measured, and weighed. However, a set number of specimens from each length group or size class are further sampled—biologists collect otoliths (fish ear bones), gonads (reproductive organs), and genetic information from these fish. These samples are then sent to LDWF and partner labs for analysis. In the lab, biologists study age and growth relationships, reproduction, and genetics. This information helps create a species’ life history (information about how fast it grows, how often it reproduces, if it migrates, and its natural life span), which plays a key role in assessing the status of a fish population as changes in these parameters can indicate changes in the sustainability of the population. Understanding these factors also allows managers to set regulations that ensure fish can grow and reproduce before they are caught and removed from the population.
Otoliths are bones that sit just below a fish’s brain and help with equilibrium.
Age and Growth
In LDWF’s Age and Growth Lab in Baton Rouge, biologists work with otoliths to determine the age of sampled fish. Field biologists collect otoliths from standardized sampling as well as at docks from recreational anglers’ catch. They continuously grow throughout a fish’s lifetime, absorbing material in the water and forming rings. Fish grow quickly during the spring and summer creating clear or translucent zones bordered by dark or opaque rings created during the slow growth of the colder winter months. One ring equals one year (similar to rings on a tree), so the total number of rings equals the total age of the fish in years.
When biologists remove an otolith from a fish, they also record the fish’s length. They plot the age from each sampled fish against its corresponding length to estimate growth rates. Because growth rates are variable from fish to fish and species to species, biologists must look at a large range of sizes to determine the average size-at-age and the variability in growth rates of each species.
Determining age of sexual maturity (the age at which a fish can reproduce) is important for managing a fish population to ensure that a fish has time to reproduce and replace itself before it is caught.
Biologists at LDWF’s Reproductive Biology Lab in Grand Isle examine the reproduction of major recreational fish species. They collect gonads from sampled fish to determine when each species reaches sexual maturity.
For most species, the limiting factor for population growth is the number of eggs produced by females vs. the number of males in the population. As a result, our biologists focus on studying female ovaries. To study ovaries, they remove tissue samples from the ovaries, infuse them with wax, chill the samples in special trays, finely slice them, and then mount the slices on slides. They slides are put in a machine that stains the specimen by dunking it in a sequence of chemicals to make the tissues more visible under a microscope. Biologists use a microscope to examine the tissue cells and determine at what stage in the spawning cycle a sampled fish was.
With this information, biologists can determine how long spawning season is, how old spawning fish are, how often a fish spawns each season, and how many eggs they produce. All of this analysis requires a number of techniques and time—as a result the Reproductive Biology Lab focuses on one species per year. So far, they have worked on spotted seatrout, red drum, amberjack, bluefin tuna, red snapper, and several grouper species.
LDWF biologists collaborate with a number of universities and state and federal agencies to create genetic databases on several species found in Louisiana such as tarpon, sharks, snapper, and American eel. For example, to better understand Louisiana’s American eel population, biologists take fin clips from American eel specimens for genetic analysis to compare eels found in Louisiana with others collected around the country.
Genetic studies also allow LDWF to test the effectiveness of management techniques. For over 30 years, LDWF has stocked freshwater bodies with the Florida strain of largemouth bass to create larger, faster growing bass for recreational anglers to enjoy. LDWF biologists collect fin and liver samples from bass and send them to a lab at Louisiana State University for genetic analysis. From the analysis, biologists are able to see what percentage of a population is Northern, Florida, or a hybrid of the two. With this information, managers can focus future stocking efforts where the population shows higher levels of Florida or hybrid strains versus those with higher Northern strains where the previous years of stocking have been ineffective.