Can Passive Acoustics be Used to Estimate the Length of Atlantic Croaker (Micropogonias undulatus) within the Pamlico Estuary?

The Atlantic States Marine Fisheries Council (AFMFC) attributes the decline in the Atlantic croaker (Micropogonias undulatus) stock to over-fishing practices. The majority of collected Atlantic croaker are small young of year (YOY) individuals which may lead to changes in age and size-class structure within the population. North Carolina and other southeastern states receive economic benefits from the fishing practices of Atlantic croaker and other important Sciaenid fishes. The goal of this research is to determine if Atlantic croaker populations can be monitored using passive acoustics by ascertaining if sound-production of a population will provide the listener with information such as length maturity or sex. An instrumented tripod (ITPod) was deployed to estimate the physical parameters (i.e. currents water quality waves and turbidity) of the water and a passive acoustic recording system recorded environmental sounds (0.05) for all months except June (ANOVA p=0.01). Next a comparison was made between the Atlantic croaker the predicted mean length from the passive acoustic recordings the mean lengths from the trawls and the mean length predicted from the active acoustic echosounder surveys. There was no significant difference between the mean Atlantic croaker total lengths from the trawl and all of the fish in the active acoustics (ANOVA p>0.05) except for June (ANOVA p=0.001). Therefore I conclude that the passive acoustic estimates of Atlantic croaker lengths (predicted from fundamental frequency) obtained in the laboratory did not correctly predict the observed length-structure of the Atlantic croaker population. This result may be due to the cutoff frequencies of the sites; in shallow water the dominant frequency of an acoustic signal can change as it propagates. If the wavelength of the low-frequency component of a signal exceeds the water depth (the cutoff frequency) than these frequencies are essentially filtered from the recorded sound. Another factor influencing cutoff frequency is that of the resonance of trapped bubbles under the surface of the sediment. These bubbles can cause their own echoes when the sound reverberates off of the surface of the bubble. The bubble reverberations will add higher frequency components to the original sound. Together these phenomena would have increased the dominant frequency at the hydrophone leading to an underestimate of length-structure for an Atlantic croaker population (the higher dominant frequencies would predict smaller fish observed). My research shows that Atlantic croaker length is inversely related to the fundamental frequency of the "croak" call but the acoustic environment and the effects of the cut-off frequency need to be better understood prior to using this method in the field to estimate Atlantic croaker lengths.