Novel estimation of gear selectivity using a concurrent mass mortality event: A case study using red drum (Sciaenops ocellatus) in the northern Gulf of Mexico

Abstract

Fishery-independent surveys are commonly used in modern stock assessment models to inform trends in abundance and these surveys may become more important when there are gaps in other data sources, such as harvest data. As a result of the federal harvest moratorium in the late 1980’s, Gulf of Mexico red drum (Sciaenops ocellatus) remains a data-limited species with little known about its post-escapement (6 – 46-year-old fish) abundance in offshore waters, which encompasses the spawning biomass of the stock. Historically, age and growth estimates were derived from purse seine collections, which was the industry’s preferred harvest technique. Recently, the addition of fishery-independent surveys, i.e. bottom longline surveys, sought to provide a potential alternative to purse seines; however, their efficacy in sampling the breadth of the offshore red drum population has not been widely evaluated. Here, we compared the age composition and selectivity of red drum collected with purse seine and bottom long line in offshore coastal waters of Mississippi and Alabama. Red drum collected in the purse seines ranged from 561 to 1018 mm total length (2–26 years old) and 770 − 1090 mm (2–36 years old) in bottom longlines. Additionally, an opportunistic sampling of red drum from a large fish kill in 2015 was used to estimate selectivity of red drum sampled by purse seine and bottom long line. Red drum selectivity generally decreased with age for the purse seine, while there was an increase in selectivity for the bottom long line survey. This novel approach using a mass mortality event to derive gear selectivity may allow fisheries scientists to refine selectivity measurements in stock assessments. Characterization of selectivity for different survey gears will allow for a more informed comparison of historic and current surveys when gear type effects change.