Dead sharks typically sink after death due to their unique anatomy. However, under specific conditions, a dead shark may eventually resurface because of decomposition.
Initial Sinking: Shark Anatomy
Sharks generally sink immediately after death because they lack a gas-filled swim bladder, an organ present in most bony fish that regulates buoyancy. Unlike bony fish, sharks are cartilaginous fish and do not possess such a mechanism. This means sharks must actively swim to stay in the water column when alive.
A shark’s skeleton is composed of cartilage, which is lighter and more flexible than bone. Despite this, cartilage is still denser than seawater, contributing to the shark’s negative buoyancy. Without active movement, a shark will naturally descend.
Sharks also possess a large, oil-rich liver, which can constitute a significant portion of their body mass. This liver contains squalene, a low-density oil that provides some lift. However, this oily liver primarily serves as an energy reserve and aids in dynamic buoyancy, requiring the shark to swim continuously to generate lift with its pectoral fins. Upon death, this dynamic lift ceases, and the inherent density of the shark’s body, even with its oily liver, causes it to sink.
Later Buoyancy: Decomposition Gases
While a dead shark initially sinks, it can sometimes float to the surface later due to decomposition. After death, bacteria within the shark’s body break down its tissues through anaerobic decomposition. This activity generates various gases.
These gases accumulate within the shark’s body cavity, causing the carcass to bloat. As enough gas builds up, the shark’s overall density decreases, potentially becoming less than that of the surrounding water. When this occurs, the buoyancy becomes positive, and the shark may rise to the surface.
Several factors influence how quickly a dead shark will float. Water temperature is a significant factor, as warmer temperatures accelerate bacterial activity and gas production, leading to faster decomposition. The size of the shark also plays a role; larger sharks require more gas accumulation to achieve buoyancy. This floating state is temporary. Eventually, accumulated gases may be released as the body ruptures, or scavengers consume the carcass, causing the shark to lose buoyancy and sink back to the seafloor.