Ocean destratification and fish evacuation caused by a Mid-Atlantic tropical storm

Abstract

Tropical and extratropical storms commonly occur in the Northwest Atlantic Ocean, sometimes causing catastrophic losses to coastal fisheries. Still, their influence on fish movements and range shifts is poorly known. We coupled biotelemetry observations of black sea bass in the U.S. Mid-Atlantic Bight with numerical modelling of the coastal ocean to evaluate the influence of Hermine (3–8 September 2016) on cold pool thermal destratification and fish evacuation. Spring through fall, black sea bass is a sedentary species, with movements focused on structure where they support important commercial and recreational fisheries. During summer 2016, we characterized the movements of 45 acoustically tagged black sea bass at three sites deploying acoustic receivers moored in shelf waters 18–31 km east of Ocean City, Maryland, and at depths 20–32 m in the southern Mid-Atlantic Bight. On 3 September 2016, cyclonic winds of Hermine caused rapid destratification of the water column. At experimental sites, bottom temperatures rose from 13 to 23°C in 10 h. An oceanographic model and observing data showed that the effects of this destratification dominated large portions of the Mid-Atlantic Bight and had long term effects on seasonal evolution of the shelf temperature. Nearly half of remaining black sea bass on 3 September (40%) permanently evacuated the experimental sites. Those that remained showed long-term depressed activity levels. Although the cause of this incomplete evacuation is unknown, it exemplifies partial migration, which may buffer black sea bass to regional impacts of changed timing or increased incidence of tropical storms.

Effects of intense storm events on dolphin occurrence and foraging behavior

“…Our study found that storms in the US Mid-Atlantic Bight caused sea surface temperature anomalies to vary, as was expected based on the mixing of the stratified water column with cooler bottom water, thus lowering the surface temperature, as found in other studies 14 . While lower water temperatures are unlikely to have directly affected the dolphins' behavior, their ectothermic prey species are influenced by these changes 14,39 . The resulting changes in prey distribution may have indirectly influenced dolphin movements and foraging.…”

“…During Tropical Storms Jose and Maria, intense wave action caused gray triggerfish (Balistes capriscus) in North Carolina to increase their emigration rates 44 . Concurrent research in our study region found that black sea bass (Centropristis striata) evacuated the region as a result of Tropical Storm Hermine in 2016 14 . Water column mixing and changing bottom temperatures initiated a migration of 40% of tagged black sea bass and caused reduced activity rates by those that remained 14 .…”

“…Fish species exhibit highly variable responses to storms. While environmental changes elicited permanent range shifts in some prey species 14 , short-term distributional changes (on the scale of days) were found in others 12,44 . Vertical and horizontal range shifts in fish to avoid hypoxic zones caused by storm events have also been observed 41 .…”

“…While bottlenose dolphins and other marine mammals are highly mobile, adaptable, and potentially less directly impacted by changes in water temperature than ectothermic species, they could be indirectly affected by alterations in prey behavior and distribution. We hypothesized that dolphin detections and foraging behavior in the Mid-Atlantic Bight would consequently be reduced during and following intense storm events such as tropical storms due to altered prey distribution 14 and avoidance of the storm. In the future, marine ecosystems may be increasingly exposed to disturbances such as storm events, which are expected to occur with higher frequency and intensity as the climate changes 8 .…”

Influence of thermal stratification and storms on acoustic telemetry detection efficiency: a year-long test in the US Southern Mid-Atlantic Bight

“…After the onset of strati cation, ΔT ranged from 0.8-12.9 °C at the nearshore site and 1.7-17.4 °C at the mid-shelf site until the Cold Pool rapidly collapsed from September 7-10; stronger thermal strati cation consistently occurred at the deeper mid-shelf site as compared to the nearshore site. Acute periods of partial-to-complete destrati cation of nearshore waters are often associated with large wind events or tropical storms [21], and were observed on May 16-19, June 2-4, July 5-9, and August 19-24 where ΔT decreased by 3.5-8.7 °C ΔT over the course of several hours to several days.…”