Largemouth bass abound in Mississippi’s coastal rivers and marshes. Unfortunately, growth is slow, and fish over 3 pounds are rare. Why, in these food-rich systems where redfish grow to several pounds in their fist year of life, is slow growth of marsh largemouth a reality and large bass a rarity?
Largemouth bass are freshwater fish, but they have a surprisingly high tolerance to salt water. Largemouth bass have been collected in salinities up to 24 parts per thousand (ppt), approximately two-thirds of full-strength seawater, and fisheries biologists commonly collect largemouth in salinities ranging from 1 to 12 ppt in Louisiana marshes.
Salinities below those that are lethal could stress the bass and reduce feeding or conversion of consumed food to bass flesh. To see if salt water affected marsh bass growth, Louisiana State University fishery researchers tested the growth of brackish-water marsh bass and those collected from fresh water. For both groups of bass, growth was greatest in fresh water and declined with increasing salinity up to 8 ppt. Both groups of bass quit feeding at 12 ppt.
The marsh bass had greater growth than freshwater bass at 8 ppt, but the growth of freshwater bass was double that of the marsh bass at 0 ppt salinity.
It appears that salinity can explain the reduced growth of marsh bass, but salinity is not constant in the marsh, and largemouth bass can move to areas of lower salinity. The next step in the LSU research was to evaluate salinity preference of largemouth bass.
Salinity of adult freshwater and marsh bass was tested in laboratory tanks where the fish could freely move among zones of different salinity. The marsh bass spent most of their time at 3 ppt salinity; they frequently occupied 6 ppt, but spent little time in fresh water or at 9 and 12 ppt.
The freshwater bass also spent most of their time at 3 ppt salinity, but equally occupied fresh water and 6 ppt salinity; higher salinities were avoided. Both freshwater and marsh bass preferred 3 ppt salinity.
To further assess salinity preference, adult largemouth bass captured in the marsh were implanted with ultrasonic transmitters, released near their capture sites and tracked. Fish released in March remained near their release location in water with 2 to 4 ppt salinity until May when salinity increased to 8 ppt. The fish were not detected after May, presumably evacuating the area.
A second group of marsh bass was implanted with transmitters and released at their capture site in July and August. These fish also remained near their capture and release sites at salinities of 3 to 5 ppt until late September when the salinity climbed to 12 ppt. Again, the tracking results suggest the sonic-tagged bass left the area when salinity increased above 8 ppt.
Together, the tracking and laboratory results indicate that marsh bass prefer salinities of 3 to 6 ppt and avoid higher salinities.
Slower growth of marsh bass is not unique to Louisiana or Mississippi. Throughout the Gulf and up the Atlantic coast, marsh bass have slower growth than inland populations.
The laboratory growth studies support slower growth at low salinities than in fresh water, but the bass demonstrated a preference for low-salinity environments. Fish typically select environments where they can optimize their growth. Why would the bass select a habitat where they grow more slowly?
The answer may lie in other studies that assessed the diets of marsh bass. Despite the abundance of forage fish, like Gulf killifish, menhaden and mullet, invertebrates predominate in the diets of marsh largemouth. Numerous studies have demonstrated that forage fish are needed for fast growth of juvenile and adult largemouth bass. Why, then, would marsh bass not eat the abundant, preferred and energetically desirable forage fish?
Unlike most inland systems where the largemouth bass is the top predator, the largemouth is just one of many predators — red drum, large gar, alligators, jacks and sharks to mention only a few — in the marsh. The abundant forage fish primarily occupy the open waters of the marsh, a habitat where the largemouth bass is not an effective predator but where it becomes vulnerable prey to a myriad of large predators. Survival instinct guides them to seek shelter in marginal vegetation and consume what they can capture.
The high likelihood of being consumed by a long list of predators may also explain why there are so few large bass in the marsh.
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