Variation in Temperature, Change in Barometric Pressure, and Salinity Explain Trends in the Acoustic Backscatter of Fishes Within a Large Ship Channel Inlet in the Gulf of Mexico

Abstract

Human-fortified ship channel inlets can represent the only connection between estuarine and coastal waters for tens of kilometers in some areas of the Northern Gulf of Mexico, making them bottlenecks for fish movement. A variety of fishes associate with the jetties that fortify ship channel inlets, and the deep depths of channel inlets relative to surrounding waters may provide a unique type of habitat. To understand variability in fish biomass in these vital areas, 49 hydroacoustic surveys of the Aransas Channel Inlet, Texas were conducted and paired with environmental (e.g., temperature) and meteorological (e.g., barometric pressure) data between January 2018 and February 2020. Fishery-independent and fishery-dependent data from the surrounding area were employed to provide context to variability in fish backscatter (i.e., volume backscattering strength, s v , a proxy for fish biomass). Generalized additive models indicated that variation in fish backscatter was best explained by variation in temperature, change in barometric pressure, and salinity (adj.-R 2 = 0.71). There were two dimensions to these effects. In the first, variation in temperature tracked seasonal shifts in the relative abundance of pelagic fishes, which were well sampled by acoustic technologies relative to demersal fishes. In the second, episodic cold fronts (i.e., high pressure systems) were associated with high values of fish backscatter -likely because the Aransas Channel Inlet is far deeper than surrounding areas and protected by jetties, making it less affected by physical disturbances and rapidly changing air temperatures or precipitation. Thus, disturbances in the Aransas Channel Inlet (e.g., dredging and construction, shipping traffic) are likely to be most impactful to non-demersal fishes when temperature and salinity are low (< c.a. 15 °C and < c.a. 28 psu) and frontal systems are likely to affect the area (barometric pressure increase > c.a. 3 mb or decrease > 7 mb over 24 h).