Age and Growth, Reproductive, and Genetic Studies

During LDWF's 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.

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.

Life History and Reproduction

For many fish species, the limiting factor for population growth is the number of eggs produced by females and the number of young that survive, or are recruited, to the population. To estimate the reproductive potential of a population, biologists at the Grand Isle Fisheries Research Lab target spawning fish and sample ovaries from the females. The gonad tissues infused with wax, finely sliced, then mounted on slides for histological analysis. Biologists use a microscope to examine the tissue cells and determine at what stage in the spawning cycle a sampled fish was.

This data can be used to determine how long a spawning season is, how old spawning fish are, how often a fish spawns each season, and how many eggs they produce. Ultimately, information on reproduction can be used to estimate for how many offspring may be produced by the current population, allowing managers to set harvest limits that account for future recruitment.

Histological analysis requires a number of techniques and time. As a result, the Fisheries Research Lab rotating through species of interest, with spotted sea trout, sheepshead, red drum, and black drum being rotating annually.

Other Research

The Fisheries Research Lab also conducts targeted research to identify and address data gaps to better inform management of Louisiana’s important fisheries. Recent and ongoing research includes:

Testing the efficacy of modified fyke nets for monitoring southern flounder:
Southern flounder are an important species for Louisiana’s anglers, but adults are not often encountered in standard sampling conducted by the LDWF. To address this, Fisheries Research Lab biologists deploy modified fyke nets in passes between barrier islands during the annual flounder spawning migration. These nets are effective for capturing southern flounder, and additional work is ongoing to assess how this can be used for management.

Assessing post-release mortality of reef fish:
Red snapper and other reef fish are susceptible to barotrauma – a type of injury resulting from rapid decrease in pressure when captured from depth. Symptoms include distended stomach and eyeballs, the fish becoming positively buoyant and, ultimately, mortality.

Recent legislation states that anglers must have a fish descending device (FDD) or venting tool rigged and ready to use when targeting reef fish in the Gulf. Fisheries Research Lab biologists have been conducting research to assess how well FDDs work for minimizing depredation red snapper, greater amberjack, and gray triggerfish caught and released from Louisiana reefs. Biologists capture a fish, release it using an FDD with a video camera attached, and observe the outcome. Less than 2% of all fish released using a FDD have been depredated by sharks, demonstrating FDDs as a practical tool to minimize post-release mortality.

Genetics

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 Florida bass, a close relative of the native largemouth bass, to create larger, faster-growing bass for recreational anglers to enjoy. LDWF biologists collect fin clips from bass and send them to a lab for genetic analysis. From the analysis, biologists are able to see what percentage of a population is native largemouth bass, Florida bass, 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 levels of largemouth bass strains, where the previous years of stocking have been ineffective