Sandbar sharks

The movement patterns and resource use of the sandbar shark, Carcharhinus plumbeus, in the western north Atlantic.

LOCATION: Jupiter, Florida, USA.

Overview

Sandbar sharks are globally distributed in nearshore waters, including the western north Atlantic where they exhibit seasonal north-south migratory behavior. Adults use coastal marine waters while juveniles rely on bays and estuaries as nurseries because of their high productivity and protection from predators. Consequently, their use of shallow waters puts them in close contact with humans, who severely overfished them in the late 20th century in the western north Atlantic, leading to declines of up to >70%. In 2008, sandbar sharks were listed as a prohibited species in the United States under the Atlantic Fishery Management Plan for Consolidated Highly Migratory Species, which requires fishers to immediately release sandbar sharks upon capture. But they are still threatened by fisheries bycatch and habitat deterioration across the region. Sandbar sharks are managed as a single stock in the United States Atlantic and Gulf of Mexico based on genetic analysis, but their distribution across a wide geographic range suggests there may be regional differences in behavior and resource use patterns. Research on sandbar sharks has largely focused on relative abundance, growth rates, age of maturity, and reproduction, with research on behavior limited to a few highly studied areas like Chesapeake Bay and Delaware Bay. Thus there is a need to better understand the movements and resource use of sandbar sharks, including 1) their proclivity for partial migration in which only some individuals migrate; 2) key prey groups prior to, during, and after migrations; 3) the tendency for sharks to associate with the same individuals; and 4) how this varies between males and females, and if any behaviors place them at risk for continued overlap and interactions with fisheries.

Conservation Concern

Like most shark species, sandbar sharks are found in nearshore waters where food resources are more abundant than the open ocean, but where overlap with humans is substantial. Seasonal shifts in temperature and productivity lead sandbar sharks to migrate to warmer, more productive waters to avoid metabolic and thermal stress. These migrations are also tied to the reproductive behavior of sandbar sharks, with females giving birth in shallower, protected waters away from large coastal sharks. Considering the substantial declines of sandbar sharks in the western north Atlantic attributed to fishing (up to >70% declines), the late age at which they reach maturity (>10 years of age), and the limited number of offspring they produce (<15) every other year, their population is still recovering and the species is listed as endangered under the IUCN Red List. Sandbar sharks are no longer experiencing overfishing in the United States, but most research on sandbar shark behavior was conducted prior to or immediately after their listing as a prohibited species. Thus, our understanding of how management and conservation measures have impacted this species beyond changes in abundance are limited. Sandbar sharks are still harvested across much of their geographic range, thus our understanding of their behaviors in a more protected environment (e.g. western north Atlantic) offers insight into how healthier sandbar shark populations function, which may serve as conservation goals for populations that are experiencing overfishing.

Research Questions

• What are the seasonal distribution patterns of sandbar sharks in south Florida?

• Do male and female sandbar sharks exhibit different migration and residency patterns?

• What food resources do sandbar sharks rely on and does this change seasonally?

• Do sandbar sharks consistently aggregate with the same individuals?

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