Fish Sampling

Using different types of fishing methods, LDWF biologists regularly sample fish and shellfish populations, as well as the water itself, in fresh and saltwater environments. Each type of sampling targets a different size of various species and provides samples across age groups. Each sampling method is standardized—the same gear, technique, time of year, and stations are typically repeated over time. Sampling stations are selected to represent different habitats throughout a waterbody.

Biologists record important information about the samples they catch, counting and measuring their samples and sometimes dissecting and studying them to determine their sex, age, diet, and reproductive information . They then use this information to assess the status of fish and shellfish populations  and in other research.

Freshwater Sampling

Louisiana’s diverse inland waters include manmade impoundments, backwater complexes, river systems, and brackish coastal lakes. The state is divided into nine inland districts —each district is responsible for creating management plans that set up sampling schedules for the major commercially and recreationally-fished waterbodies within its borders.

Freshwater sampling methods include electrofishing, hoop nets, lead nets, gill nets, and seines. As the name suggests, electrofishing applies an electric current to the water through a pole or boom placed into the water, stunning fish. The stunned fish float to the surface where biologists collect them in fiberglass landing nets. Biologists primarily conduct electrofishing from a boat but can also use a backpack unit for sampling smaller areas such as streams. LDWF conducts electrofishing sampling twice a year—during the spring, biologists sample black bass and crappie, and during the fall, they focus on black bass and forage species (small fish species that larger fish eat).

LDWF biologists use hoop nets to sample catfish species during late spring and mid-summer. Hoop nets are tube-shaped nets separated by a set of hoops spaced along the length of the net to keep it open. Each hoop is tapered to a small opening called a throat, which prevents fish from swimming out of the net. Biologists fish two hoop nets in tandem with both ends anchored to prevent the nets from moving. They bait the nets with cheese or soybean chips placed in bait bags behind each throat to increase catch.

LDWF biologists use lead nets during a couple days in the fall to sample crappie and sunfish species. Lead nets are similar to hoop nets, but the openings facing each other are connected by a weighted panel of net that forces the fish to swim into one of the hoop nets.

LDWF biologists use gill nets to sample striped and hybrid striped bass and commercial species such as gizzard shad, carp, and buffalo fish during overnight sets in the winter. Gill nets are a set of vertical panels of varying mesh size often set out and fished in a straight line. As fish try to swim through the net, they get stuck between the mesh openings. Biologists collect fish from the net as the net is picked up. Each panel is made of a different size mesh to target different size and species of fish.

Through shoreline seining, LDWF biologists anchor a vertical net to the shoreline, stretch it perpendicular to the shore, and swinging it back around toward the shore to capture fish inside the net. The net is equal to or higher than the waterbody and weighted on the bottom to keep it on the water bottom and prevent fish from escaping. Biologists use shoreline seines to sample black bass and sunfish species at night during the late summer.

LDWF also samples freshwater mussel species around the state. Mussels are an indicator species for the health of freshwater systems. Three-person crews of biologists sample mussels by wading in small streams and performing timed tactile searches for live mussels and shells. They also use SCUBA gear in waters deeper than one meter.

Saltwater Sampling

Inshore and Nearshore

For inshore and nearshore waters, LDWF has divided the state’s coast into five coastal study areas (CSAs) along the boundaries of the five major basins along the coast. In each of these areas, biologists conduct sampling focused locally on shrimp, crab, oysters, and coastal finfish like red drum and speckled trout.

Throughout the year, biologists use different sizes and types of trawls to sample shrimp  in various habitats to determine season openings and closures. They also use otter trawls to sample blue crabs across the state’s inshore waters and territorial sea to determine their abundance.

Every July, LDWF biologists conduct square meter sampling to assess oyster populations on the state’s public oyster areas. They SCUBA dive on designated sampling stations within each CSA. At each station, they randomly place a square meter frame on the oyster reef and collect all live and dead oysters, reef-associated organisms, and exposed reef material from the surface layer within the frame. They replicate this process five times at each sampling station. Once the samples are back on the boat, biologists identify, separate, and count live and dead oysters, spat, fouling organisms, oyster predators, and hooked mussels; they also measure and sort the oysters. Biologists use the data from square meter sampling in yearly oyster stock assessments.

LDWF biologists also use oyster dredges to monitor the state’s oyster reefs throughout the year. An oyster dredge is a metal frame with 10 teeth on one side that break up the ground as the dredge is pulled across the water bottom. Dredged material is collected in a net or bag that is attached to the frame. Biologists examine the material for recent oyster spat (larva) settlement and signs of mortality.

LDWF samples saltwater finfish year-round along the coast. Biologists use gill nets, like those used in freshwater, to sample juvenile fish such as spotted seatrout and other species for abundance, year class strength (how many young of year are present), and species’ movements. They also use bag seines to capture smaller prey species in marsh-edge and shoreline habitat—the seine is anchored to the boat and a biologist enters the water to open and close the seine. They use otter trawls to sample finfish in larger inshore bays and waterways. During October through March, biologists use trammel nets to sample species such as red and black drum. Trammel nets are a type of gill net. A panel of smaller mesh is sandwiched between panels of larger mesh—this design allows for sampling of a large range of sizes and species. In addition, biologists in some of the CSAs have been experimenting with electrofishing in the marsh and shoreline habitats to sample these same populations. Early results have shown that electrofishing may be a better sampling gear in certain locations.

Offshore

Offshore sampling is often conducted as part of regional and national efforts, such as the Southeast Area Monitoring and Assessment Program (SEAMAP), a cooperative state, federal, and university program to collect, manage, and disseminate fishery independent biological and environmental data from the marine waters of the southeastern United States. LDWF biologists participate in a several SEAMAP projects, including sampling plankton, groundfish, and species caught with bottom longline and vertical line fishing gear. 

Biologists collect plankton by towing fine mesh nets that taper into a collection tube next to a moving boat. (Plankton is the collective name for microscopic eggs and larvae of fish and crustaceans that live adrift in the ocean currents.) Neuston nets collect surface plankton, and bongo nets collect plankton found lower in the water column. Biologists retrieve the nets after a set time, then sort and count the collected samples.

Biologists tow a standard commercial shrimp trawl across muddy bottoms to sample an assortment of groundfish, other juvenile finfish, and invertebrates such as shrimp. After each trawl, all of the specimens are collected, measured, weighed, and catalogued. By using a shrimp trawl, biologists are able to sample the juveniles of important species that cannot be caught in other surveys.  

Biologists set bottom longlines—long mainlines with baited droplines attached at even intervals—to sample larger animals along the bottom of the ocean. After a set amount of time referred to as soak-time, they retrieve the lines and measure, weigh, and typically release each catch. The soak-time is short, increasing the likelihood that released fish will survive. Some samples are kept for further study such as determining age and growth information. Various species of sharks make up the bulk of the catch, but red snapper, grouper, and other bottom dwellers occasionally show up.

In vertical line sampling, biologists use bandit reels—large spooled commercial fishing reels—to lower a line of hooks suspended above a large weight to the bottom around structures such as oil rigs. Hook sizes are varied to sample different sizes of fish. Bait type and soak-time are standardized, and biologists document all aspects of the catch for further analysis. Red snapper is one of the most common catches in vertical line samples.

Water Quality Sampling

In both fresh and saltwater habitats, biologists gather information about the current conditions at each sample site using a handheld meter with a digital probe that is lowered to various depths in the water column to read the water. Based on sampling needs, these meters can read temperature, turbidity (the amount of particulate matter in the water), dissolved oxygen, pH, and conductivity. By understanding the chemical and physical characteristics of the water, biologists can determine how certain conditions affect fish populations.