The idea of sharks following cruise ships stems from a misunderstanding of shark behavior and vessel speed. Sharks do not intentionally track a moving cruise ship across the ocean. The sustained speed of a modern cruise liner, typically around 20 knots (23 miles per hour), is too fast for any shark species to maintain for an extended period. Instead, the presence of sharks near these large vessels is a product of opportunistic investigation. This interaction is localized and temporary, driven by immediate sensory cues.
Confirming the Behavior
The interaction between sharks and large vessels is better described as a localized attraction rather than continuous tracking behavior. A shark may appear in the immediate vicinity of a ship, often in the wake or underneath the hull, but it cannot keep pace with a vessel traveling at cruising speed. Sharks are capable of short bursts of speed, but their typical cruising speed is significantly slower than that of a cruise ship.
The phenomenon is transient, with sharks drawn to the momentary environmental changes a ship introduces to the water column. The vessel essentially acts as a temporary, moving aggregation point for marine life. This is fundamentally different from the idea of a shark “following” a ship, which implies intelligent tracking of the route over long distances. The shark is merely investigating a sudden, large-scale disturbance in its territory before moving on.
Why Sharks Are Drawn to Ships
Opportunistic Feeding
The primary mechanism attracting sharks is the potential for an easy meal, often created by the vessel’s waste discharge. Modern cruise ships generate vast amounts of wastewater, including graywater and treated sewage (blackwater). While regulations require treatment, the discharge still contains organic material and nutrients that create a localized scent trail, or “chum line,” especially in the nutrient-poor open ocean.
This discharge attracts smaller fish and invertebrates, which in turn draw scavenging sharks into the ship’s wake. Historically, this effect was much more pronounced with vessels that dumped raw galley waste. Even the nutrient-rich effluent from modern wastewater systems can signal a potential food source. The sheer volume of this discharge from a large vessel can be substantial.
Sensory Cues
Beyond the scent of organic matter, sharks are drawn in by the powerful sensory cues produced by the vessel’s machinery. The large engines and propellers of a cruise ship emit strong, low-frequency vibrations and acoustic noise into the water. Sharks possess a highly sensitive lateral line system, used to detect pressure changes and vibrations. These low-frequency sounds, particularly those below 1,000 Hertz, can travel long distances. The steady thrum of the engines can signal a substantial source of activity or a confused animal, prompting investigation.
Common Species Associated with Vessels
The sharks most commonly observed near large vessels are highly pelagic species, meaning they inhabit the open ocean far from coastlines. The Oceanic Whitetip Shark (Carcharhinus longimanus) is historically the species most associated with ships, earning the nickname “sea dog” from mariners due to its persistent, inquisitive nature. This species is a slow-moving, opportunistic scavenger that cruises near the surface, making it perfectly positioned to investigate any large, new object or scent trail.
Silky Sharks (Carcharhinus falciformis) are another pelagic species that frequently approach vessels. They often compete with oceanic whitetips for food and are known to aggregate near floating objects, which they naturally associate with baitfish schools. Coastal species, such as Great White Sharks or Bull Sharks, are far less likely to be found near cruise ships in the open ocean, as their feeding territories are confined to continental shelf waters or shallower coastal areas.