The distribution of species, and the interactions of those species to their environment, is a primary focus of ecology. Ecological questions regarding predatory prey relations, competition, or evolutionary pressures rely on organisms being within proximity to one another. Understanding the limitations on a species distribution is therefore crucial. This project focuses on improving distribution maps for one marine predator, the Sixgil Shark, Hexanchus griseus.
Sixgill sharks are the world’s third largest predator shark. They are generalist predators with a global distribution along continental shelfs. Like many sharks, sub adult sixgills may inhabit brackish as well as oceanic water, but unlike other species the adults are also known to utilized low saline systems. One such system is the Pacific Coast’s Salish Sea, the inland waterways on the border of western Canada and the United States. Within the sea, and specifically within Washington’s Puget Sound, adult sixgills have been found at sizes and depths previously believed to be strictly oceanic: sharks as large as 16 feet and as shallow as 30 meters have been studied by NOAA Fisheries scientists up and down the sound. These studies have revealed a seemingly seasonal migration of these sharks, but more investigations are needed to take the raw movement data and pair it with the drivers of shark distribution.
Possible drivers of shark distribution are not merely the physical habitat and resources therein, but also climactic variables such as temperature. As ectotherms, water temperature limit sharks on the extreme ends due to biological processes such as circulation. Behaviorally however, water temperature can impact sharks as different temperature waters have different densities. Sharks detect prey using olfactory and electric signals. As water density changes, detection of such factors alters and can influence realized temperature ranges. Similar to temperature, changes in salt content can greatly change water density. Salinity variations also alter the ion ratios within animal cells so marine organisms need to remain within higher salinity waters. This limitation is also true of their prey.
Within a coastal environment such as Puget Sound, temperature and salinity can vary greatly. Seasonal rains greatly reduce the salinity of nearshore habitats, and can often be cooler than surface waters. Conversely, the summer drought leads to higher nearshore salinities as evaporation increases salt concentrations. Puget Sound is further complicated due to its great number of rivers and large oceanic water conduit though the Strait of Juan de Fuca. This variety provides an opportunity however to examine shark distribution across a wide variety of accessible habitats and conditions. Using this local as a study area, and publically available salinity and temperature data, we can begin to examine sixgill shark distributions.