The sight of a fishing vessel surrounded by a swirling cloud of seabirds is a common observation in marine environments. This predictable gathering is an ecological phenomenon driven by human activity in the open ocean. Seabirds have learned that fishing boats provide a reliable, concentrated food source, fundamentally altering their natural foraging patterns. This association creates an artificial link between human industry and marine wildlife, establishing a consistent food subsidy that attracts birds from vast distances. Understanding this relationship requires examining which birds are involved, what they are scavenging, and how different fishing techniques influence the interaction.
Identifying the Common Seabird Followers
The birds most frequently seen congregating around fishing vessels favor opportunistic feeding and long-distance travel. Highly adaptable gulls are constant followers, often patrolling coastal waters and harbors where they readily exploit discarded material. Their generalist diet and tendency to form large flocks make them efficient at consuming waste from fishing operations.
A second major group includes the Procellariiformes, which encompasses albatrosses, petrels, and shearwaters. These pelagic species possess a highly developed sense of smell, allowing them to detect the odor plume of a working vessel and its associated food from many miles away. Albatrosses are particularly drawn to large vessels, utilizing their immense wingspans to track boats over vast oceanic distances.
Shearwaters and skuas are also regular attendees. Skuas are known for their aggressive, kleptoparasitic behavior, often harassing smaller birds to steal their scavenged meals. This opportunistic feeding behavior primes these species to exploit the easy pickings offered by a moving fishing operation.
The Scavenging Opportunity: Why Birds Follow Boats
The primary motivation for this persistent following behavior is the reliable and energy-efficient food source generated by the fishing process. Seabirds are mainly attracted by two categories of waste: fishery discards and processing offal. Fishery discards consist of the non-target catch, such as smaller fish or invertebrates, that are thrown back into the sea because they are non-commercial or regulatory constraints require their disposal.
The second major attractant is offal, which includes the internal organs, heads, and other waste material from target fish processed on board. This offal is rich in fats and proteins, representing a substantial, high-energy meal that is easier to obtain than catching live prey. This consistent availability creates an artificial food subsidy, allowing some scavenger populations to increase beyond what the natural environment could sustain.
This readily available resource significantly influences the birds’ natural diet and movement patterns. Satellite tracking data reveals that some shearwaters adjust their foraging trips to match the predictable work schedule of trawler fleets. The influx of human-derived food effectively concentrates bird activity, shifting their foraging effort from natural sources to the wake of a vessel.
How Different Fishing Methods Influence Interaction
The type of fishing gear deployed strongly dictates the nature and intensity of the interaction between the birds and the vessel. Trawling, which drags a large net through the water, results in the quick release of large amounts of discards and offal when the catch is sorted. This sudden release creates intense feeding frenzies, attracting massive numbers of birds who risk colliding with the heavy trawl cables, known as warp strike.
Longline fishing presents a different and more dangerous hazard, particularly for deep-diving birds like albatrosses and petrels. This method involves setting miles of line with thousands of baited hooks exposed at the surface before they sink. Birds are attracted to the bait and can become hooked, subsequently being dragged underwater and drowning. This process is responsible for killing hundreds of thousands of seabirds annually.
Pelagic longlines (targeting tuna and swordfish) and demersal longlines (targeting groundfish) both pose significant threats. Other methods, such as purse seining or pole fishing, generally result in less direct interaction and bycatch. These operations can still provide scavenging opportunities from spillover or minor discards, but the risk profile is lower compared to trawling or longlining.
Protecting Seabirds from Fishing Gear
To reduce the accidental mortality of seabirds, a range of practical mitigation measures has been implemented across various fisheries globally. For longline vessels, one of the most effective tools is the use of bird-scaring lines, often called Tori lines, which are brightly colored streamers towed behind the boat. These lines deter birds from accessing the baited hooks while they are exposed on the water’s surface.
Another common technique involves adding weight to the longlines to ensure the baited hooks sink rapidly, moving them out of the foraging depth of most seabirds. Fishing vessels can also implement night setting, which takes advantage of the fact that many seabirds are diurnal and less active in the dark. This reduces the visibility of the bait during the most vulnerable period of line deployment.
Fishing practices can also be modified to manage the attractive food waste. This includes avoiding the discharge of offal and discards while setting or hauling gear, reducing the number of birds attracted to the vessel at that high-risk moment. The combination of these technological and operational solutions has proven successful in reducing seabird bycatch, with some fisheries reporting reductions of over 90% in incidental mortality.