LDWF Fisheries Biologists Interested In More Than Size When Making Assessments Of Fish Stocks

story by LDWF Staff

When a recreational angler catches a fish, typically they are only worried about its size. After all, that’s what you brag to your friends about.

When Louisiana Department of Wildlife and Fisheries biologists catch fish, they care about how big they are also. However, they look at factors other than size, all so that recreational anglers and commercial fisherman can continue fishing long into the future.

At LDWF’s Fisheries Age and Growth Lab in Baton Rouge and the Fisheries Reproductive Biology Lab in Grand Isle, biologists produce information vital to assessments of the fish stocks in Louisiana waters.

In the Age and Growth Lab, LDWF biologists examine the ages, lengths and weights of many species of fish such as red snapper, southern flounder, red drum, black drum, striped mullet, spotted seatrout, sheepshead, gray triggerfish, gray snapper, vermilion snapper, king mackerel, cobia, tripletail, yellowfin and blackfin tuna and wahoo among saltwater species. Freshwater species such as largemouth bass, crappie, American eel and channel catfish are also analyzed.

Getting the lengths and weights is easy. Simple and accurate measurements can do the trick. However, finding the precise age of the fish is a little more complex for many species.

In the case of some fish, like spotted seatrout, finding the age isn’t that difficult. Most don’t live longer than 10 years. They grow at a rapid rate for the first three to four years after which growth slows. But those growth rates are variable from fish to fish and females grow faster overall than males. So a 14-inch fish might be anywhere from 1 to 4.5 years old. Or, considered the other way, a 2-year-old might be anywhere from 8 to 23 inches long.

However, different species grow at different rates. Red snapper, for example, can live more than 50 years but growth slows drastically after 10 to 15 years. Red snapper also exhibit a lot of variability in size at age. Therefore, a 10-year-old red snapper might look just like a 50-year-old red snapper. It’s up to LDWF biologists to make the distinctions to better guide fishery management decisions.

Scales, spines, rays and vertebras can be taken from some species and have varied results of success in accurately telling the fish’s age. However, otoliths, or ear bones, provide biologists with the best opportunity to determine the age of the fish.

The otolith is a bone that sits just below the fish’s brain and helps with its equilibrium in the water column, kind of like our ears help us with our balance. Unlike the fish’s body, the otolith continues growing until the fish dies and it constantly absorbs material in the water, forming rings. These rings can be counted to estimate the age of the fish, similar to the way counting tree rings can estimate the age of a tree.

“Throughout the spring and summer fish exhibit a period of fast growth resulting in a clear, or translucent zone, on the fish’s otolith,” explained LDWF Biologist Program Manager Nicole Smith, who manages the Age and Growth Lab. “Then in the winter months when the water is colder, fish exhibit a period of slower growth resulting in a dark or opaque zone on the otolith. Those opaque rings can be counted to estimate the age of the fish.”

Biologists sample fish for their otoliths from marinas, commercial fish docks and recreational fishing locations throughout the state. The otoliths are taken to the Age and Growth Lab in Baton Rouge for analysis. Biologists may also take gonads for analysis.

LDWF biologists view the rings in the otolith through a microscope by examining a small cross section of the otolith. The otolith must be cut by a saw at a very precise angle into a very thin section in order to count rings accurately.

Once a year, LDWF biologists, along with those of the four other Gulf States, attend the Gulf States Marine Fisheries Commission Otolith Processors meeting. The intent of this annual meeting is to compare ageing techniques between biologists and between state laboratories to ensure accuracy and precision of fisheries age data being generated throughout the Gulf States. This is accomplished by calculating an average percent error (APE) between individual biologist otolith readings or between laboratories. An APE of five percent or less indicates that the two sets of readings are consistent in their ageing technique with a high degree of precision between readings.

“Reading otoliths and obtaining an accurate estimate of age is very difficult to do,” Smith said. “It seems like it would be an easy concept, you just count rings. But what’s hard is that each species’ otoliths differ from one another in size and shape, requiring specific ageing methodologies unique to each species. It’s a skill that requires a lot of training.”

It might not be easy but once LDWF biologists get the ages and lengths of many individual fish of a population, they can then put that data into a model to determine growth rates for the species. This analysis also helps them determine the average size-at-age as well as the variability in sizes and growth rates at a given age.

So why is determining the age of a fish being harvested so important? It’s up to LDWF fishery managers to make sure that a fish has had ample time to reproduce and replace itself before it ends up in a fisherman’s boat. Otherwise, fish populations could become depleted and the species that you grew up bringing out of the water might not be there for your children and grandchildren.

By determining the size and age at which a fish species becomes sexually mature, LDWF can guide fisheries management decisions to ensure that enough mature fish are able to reproduce before they are harvested. However, determining when a species becomes sexually mature takes a lot of work.

The LDWF maintains a Fisheries Reproductive Biology Lab at our Grand Isle Research Facility specifically geared toward examining the reproductive strategies of recreationally important fish species in Louisiana waters. In order to investigate reproduction for a given species, biologists sample gonadal tissues from as many individual fish as possible for histological and macroscopic examination. Because most of the species’ populations that are examined are more limited by the number of eggs the female produces than the number of male fish in the population, LDWF biologists concentrate their research on the female ovaries.

When conducting this research, biologists seek to answer the questions: How long is the spawning season? How old are the fish that are spawning? How often do they spawn during the spawning season? How many eggs do they spawn during each spawning?

To answer these questions, biologists must utilize a number of lab techniques and methodologies that require a bit of effort. Therefore, the lab focuses on one species per season/year.

By incorporating both the age and growth data along with the reproductive biology data into a stock assessment, managers can better estimate the status of a given stock and take appropriate steps to ensure that the species remains healthy with the ability to replenish itself for future generations to enjoy.

So the next time you return from a successful fishing trip and you brag to your friends about the size of your fish, remember all the research that LDWF conducts each day which supports important conservation and management decisions. The vital research and subsequent decisions ensure those fish are available to the fishermen for years to come.

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