While browsing my facbeook newsfeed the other day, I noticed many of my friends were posting a local news article that caught my attention. Bull Sharks were caught near Point Lookout State Park in the Chesapeake Bay. While most of my friends were just interested in posting these articles for the sole purpose of making comments like: “I’m never swimming in the Bay again!”, I thought this would be a great blog opportunity to explain the two of the possible reasons a somewhat rare catch like this would happen: 1) Salinity levels in the Chesapeake Bay, and 2) Bull Shark biology.
For those of you who are not familiar with Southern Maryland geography. This state park is located at the mouth of the Potomac River where it meets the Chesapeake Bay. I would estimate that this location is about half way between where the Chesapeake Bay meets the Atlantic Ocean and where the Susquehanna River (the northern most river in the estuary) drains into the Bay. The total length of the Bay is about 200 miles, meaning that these sharks must have traveled around 100 miles up the Bay.
The Chesapeake Bay is what is known as an Estuary, where freshwater from streams and rivers, meet the saltwater of the ocean. This forms a gradient of salinity levels as the water mixes, creating what is typically referred to as brackish water. However, even within the Chesapeake Bay, salinity levels differ greatly. (As I learned in my Natural History of the Chesapeake Bay class at UMD) there are four main salinity zones in the Bay including Polyhaline (18-30 ppt), Mesohaline (5-18 ppt), Oligohaline (0.5-5 ppt), and tidal freshwater (< 0.5 ppt). Ocean waters average a salinity of about 35 ppt.
These salinity zones are not stagnant. They are changing all the time based on the season, temperature, rainfall, and other natural occurrences. For example, in the Spring, there is much more freshwater added to the bay from snowmelt and rainfall, causing an increased amount of freshwater flowing towards the mouth of the bay. However in the drier seasons, rainfall is not as common and therefore waters are typically more saline, with increased additions from the ocean’s salt water.
All fish osmoregulate in order to survive in both fresh and salt water environments. This means they have mechanisms which enable them to control the concentration of water inside their bodies, even when their external environment is causing them to gain or loose water. Sharks maintain osmoregulation in two ways. First, in addition to the salt present in their blood, there are other dissolved solvents present that help to create an osmotic level similar to the seawater. Second, they have a special system that excretes excess salt in the blood stream through urine. This system is typical to most sharks and cannot be changed which is why most sharks to only occupy saline environments. Bull sharks however, have developed an adaptation that allows them to gradually move into freshwater by decreasing the amount of salt that is removed from the blood stream.
Bull sharks have the ability to migrate from their high saline ocean environment of 35 ppt to less saline environments such as the Chesapeake Bay because of their ability to manipulate osmoregulation. Levels of salinity in the Chesapeake Bay are much higher in the drier months of summer because there is less freshwater added from rainfall. Point Lookout State Park is located in the typically mesohaline zone, documenting salinity levels of between 5 and 18 ppt, however since salinity levels increase in summer months, the Bull Sharks could have been been occupying waters of much high salinities. The Chesapeake Bay and its abundant fisheries would be very attractive to Bull Sharks and serve as a vital food source for these sharks in summer months. Bull Sharks have historically been found in rivers and lakes of much lower salinity levels than the Chesapeake Bay. Although it is shocking for locals to hear about these sharks being found in the Bay, there is a possibility they will be seen again and for them to migrate even farther north in the Bay.