Applications of underwater acoustics data in fisheries management for spotted seatrout, Cynoscion nebulosus, in estuaries of South Carolina
Roumillat, Myra Brouwer
The spotted seatrout, Cynoscion nebulosus, is an estuarine-dependent member of the family Sciaenidae, commonly known as "drums" or "croakers." Spotted seatrout are year-round residents of estuaries along the South Atlantic coast and spawning takes place inshore and in coastal areas. During summer months, male spotted seatrout produce "drumming" sounds, presumably to attract females that are reproductively active. By listening to these sounds using hydrophone equipment we determined the locations, seasonality and periodicity of spawning aggregations in Charleston Harbor, South Carolina.
Spotted seatrout are batch-spawning fish. That is, they release gametes in small batches over the course of the spawning season. The latter extends from April through September along the South Atlantic and Gulf of Mexico coasts. In this species, the total number of eggs a female produces in a season (annual fecundity) is dictated by the number of eggs released during each spawning event (batch fecundity) and the number of such spawning events during the course of the season (spawning frequency). Estimating the annual fecundity is necessary to determine reproductive potential, and is made even more useful for fisheries management purposes if separated by size or age class within a population.
Behavior patterns based on acoustic data enabled us to target females in imminent spawning condition, then carry out egg counts to estimate batch fecundity and spawning frequency for each of the three dominant age classes (ages 13) in our waters. Ultimately, our annual fecundity estimates for each age class will facilitate management of this species in South Carolina.
Over a decade of sampling the Charleston Harbor estuarine system we have observed that from mid to late afternoon female trout start leaving our sampling areas-- the shallow water adjacent to the edge of the marsh-- and head for deeper water to spawn. Our hydrophone surveys have indicated that spawning typically begins around 1800h and ceases around 2200h. Females then return to feeding grounds near the marsh. Knowledge of this reproductive behavior enabled us to target spotted seatrout in the mid-late afternoon specifically to capture fish that were readying for a spawn. Females that were back in the shallows after having spawned the previous evening were available for capture during daytime sampling.
We used trammel nets deployed from shallow water boats to capture trout at pre-selected sites in Charleston Harbor. Sites were chosen based on proximity to known spawning locales. We obtained batch fecundity estimates by directly counting the number of eggs in a small sample of ovarian tissue. By knowing the weight of the sample and the weight of the entire ovary, we obtained estimates of the total number of eggs in each ovary. To determine the frequency of spawning, we looked at histological preparations. The presence of specific structures in ovarian tissue indicates that the fish has spawned within the previous 24 hours. By obtaining the inverse of the proportion of females that spawned the previous evening, we could estimate how often the fish were spawning. Aging was accomplished by observing sections of the fishs earbones (otoliths)
under the microscope. Otoliths are bony structures in the animals head that help the fish orient properly in the water. They accumulate growth rings, much like a tree trunk does. By counting the growth rings we could confidently discern s the fishs age.
The main impetus behind this study was to establish realistic annual fecundity estimates by age class that could be used in predictive modeling of the spotted seatrout population in coastal South Carolina. We were able to develop equations relating fecundity to length and age that can be used to estimate the reproductive potential for each age class.