Gulf darter

Gulf darter
Conservation status
Least Concern  (IUCN 3.1)[1]
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Actinopterygii
Order:	Perciformes
Family:	Percidae
Genus:	Etheostoma
Species:	E. swaini
Binomial name
Etheostoma swaini (Jordan, 1884)

The Gulf darter, Etheostoma swaini, is a small freshwater fish in the family Percidae. It is one of the 324 fish species found in Tennessee in the United States.

Characteristics

E. swaini is known for its laterally compressed, robust body, small, conical head, slightly joined gill membranes and a wide frenum on its upper lip. The back has seven to 9 diffuse, square saddles and a distinctive light predorsal stripe. Horizontal light and dark banding is evident along the sides. Along their sides, breeding males have alternating red-orange and blue-green vertical bars that are most developed near the caudal fin.^[2]

Distribution

The Gulf darter generally inhabits small- to moderate-sized creeks, and occurs over a sand or sandy mud bottom, often in association with aquatic vegetation or a layer of organic debris. It can be found from Lake Pontchartrain, Louisiana east to the Ochlockonee River drainage, Florida, and in many eastern tributaries of the Mississippi River from Buffalo Bayou, Mississippi, north to the Obion River system of Tennessee and Kentucky. E. swaini is frequently encountered in extremely shallow locations, often foraging in water less than 5 cm deep.^[3]

In most creeks, the microhabitat of E. swaini is characterized by moderate to heavy amounts of aquatic vegetation, primarily Sparganium americanum. These fish actively forage in and among clumps of vegetation and they also use the areas of reduced current downstream from these clumps as resting sites. In larger, predominately vegetated creeks, it inhabits quiet streamside areas of sand and sand-silt substrates. In the smallest creeks, however, E. swaini may occasionally be found in shallow, swift riffles formed by logs, rocks or vegetation.^[3]

Ecology

In its preferred microhabitat, this darter is usually associated with Noturus leptacanthus and Percina nigrofasciata, and often with Ichthyomyzon gagei larvae. P. nigrofasciata appears to be the most ecologically similar species to E. swaini; however, the extent to which they may compete is not known.^[4] P. nigrofasciata forages in a much wider variety of microhabitats and does not exploit vegetation or organic debris to the extent E. swaini does.

Gulf darters are classified as insectivores, feeding on small invertebrates; for example, black flies, mayflies, and dragonflies. One studied found larval dipterans are the most important food items in fish of all sizes, in which chironomids were found in 71-100% of the stomachs examined. The primary predators of E. swaini probably are larger freshwater fish, such as Lota lota (burbots), Notorus flavus (stonecats), and Micropterus dolomieu (smallmouth bass). E. swaini, like many other darter species, have the ability to maintain position on the substrate in flowing water.^[5] This unique characteristic plays a key role in its microhabitat preference.

In regards to microhabitat, E. swaini has been shown to prefer creeks and rivers with rocky substrates and swift-moving riffles. This preference may be due to oxygen levels in the water during seasonal changes, or other factors such as feeding- or shelter-related habitat preferences. Because E. swaini has such a low tolerance for brackish water, human-induced changes such as pollution or sewer drainage could cause a huge negative impact on E. swaini abundance.^[6]

Lifecycle

Darters have a wide range of lifecycles, and size correlates with most characteristics. For example, larger darters grow faster, live longer, produce bigger clutches, and have longer reproductive spans.^[7] Mate selection by female darters is assumed to be common. In summary Gulf darters are: average size 5.3 cm, maximum growth 7.8 cm, maximum age 35 months, and average clutch size 39. E. swaini mates during mid-February to late March, typically when water temperatures are between 5.5 and 17.0°C; they congregate on gravel shoals where the stream leaves a pool to mate. Once mates are selected, the fish mate repeatedly for several days until the female lays about 90 eggs.^[7] Once the eggs are laid, females will begin burrowing into the gravel, submerging herself and the eggs, as a form of protection. Males tend to exhibit territorial behavior during breeding season.

Management

Currently, the largest threat to E. swaini is run-off and pollution due to urbanization. For instance, one study found the development of an interstate negatively affected the abundance of several fish species, including E. swaini, because of decreasing quality of water of the nearby creek.^[8] Therefore, it is extremely important to monitor run-off and pollution draining into rivers and streams because of the adverse effects they could have on fish communities. Currently, management plans consist of reducing nutrient, pesticide, and sediment loadings within such streams.^[9] This same study also suggests conservation practices should be a combination of both physical habitat monitoring and water chemistry monitoring because it would benefit fish communities within headwater streams more than just implementing one conservation practice or the other.^[9] Although broad management plans are in place for many rivers and streams and their fish communities as a whole, no current management plans specifically designed for E. swaini are in place. The lack of protection is probably because the Gulf darter is one of the most abundant darter species. Although E. swaini may be abundant now, many other darter species have become endangered or extinct, so better management plans for E. swaini should be implemented.

Recommendations

Due to its sensitivity to water purity and because it is a primary food source for many larger freshwater fish, it would sa management plan is needed to help maintain healthy freshwater ecosystems it occupies. To monitor and manage E. swaini, it would be ideal to have a good estimate of the species abundance.

References