When I think of bull sharks, I think of…
The bull shark, Carcharhinus leucas, is a widelydistributed species known for its stocky torpedo-like shape, broad snout, aggressiveness and unpredictability – just like a bull. They prefer warm shallow coastal waters, but also commonly reside in brackish and freshwater systems connected to the ocean. Because these sharks are amazing osmoregulators, they can survive in both fresh and saltwater, and thus it facilitates more frequent human contact relative to other shark species. Bites from bull sharks make up the majority of reported attacks across the globe when examining by species.
Bull sharks travel widely along the east coast of North America from the Gulf of Mexico to as far north as New Jersey. Most sharks and fishes are ectothermic, cold-blooded and their body temperature matches the temperature of the water around them. This feature of their physiology largely governs where species geographically reside. With few physical boundaries in the marine realm this is one of the major determinants of their distributions. This is why in unusually warms summers you can often find tropical species in more temperate oceans. If the anthropogenic changes to our climates continue, these occasional visitors could become permanent residents.
This brings us back to bull sharks with research recently published in Nature Scientific Reports with co-authors from the North Carolina Division of Marine Fisheries (NCDMF), Simon Fraser University and East Carolina University Study. They have found this apex predator on the East coast of the United States could be expanding its distribution northwards. As adults, bull sharks have a wide range as discussed earlier travelling as far north as New Jersey. However, juveniles seem to prefer the warmer brackish estuaries of the Gulf of Mexico like Mobile Bay, Alabama and Charlotte Harbor, Florida. The authors examined NCDMF gillnet survey data spanning 2003 through to 2016 from North Carolina and the survey captured 70 bull sharks within the juvenile size range for the species. Interestingly, all but six of those had been captured since 2011. According to he data prior to 2012, juvenile bull sharks in North Carolina were relatively rare, but transitioned to becoming a regular presence. Secondly, the size range has expanded gradually, with juveniles in 1stand 2ndyears of life, up to larger juveniles pushing >5 m. Not only does the presence of juveniles within their first year of life suggests they are being born along the North Carolina coast, seeing larger juveniles indicates that they could be returning repeatedly.
So, what has changed? The authors threw environmental data (depth, water temperature, salinity, and dissolved oxygen), spatial data (distance from the nearest inlet (the only openings to the ocean in the estuary) and distance from the nearest submerged aquatic vegetation (SAV) bed) together with the bull shark catch data into a model to identify which variables were significant. Basically, the model showed that bull shark catch had a significant relationship with temperature and salinity. Increases in bull shark catches increased during the first half of the summer coinciding with increasing water temperature and salinity in the study region.
What does this all mean? What does it mean for the sharks? What does it mean for the ecosystems? What does it mean for us? These incredibly adaptive sharks are just doing what their physiology tells them to do. We must also adapt.
For more details please see the publication
Bangley, CW, Paramore, L, Shiffman DS, Rulifson RA (2018) Increased Abundance and Nursery Habitat Use of the Bull Shark (Carcharhinus leucas) in Response to a Changing Environment in a Warm-Temperate Estuary. Scientific Reports.8. Article number: 6018 doi:10.1038/s41598-018-24510-z
https://www.nature.com/articles/s41598-018-24510-z
For a summary of this really interesting study please see:
http://www.southernfriedscience.com/is-a-changing-environment-bringing-baby-bull-sharks-to-north-carolina/
General Info on Bull Sharks from National Geographic
https://www.nationalgeographic.com/animals/fish/b/bull-shark/
