Every year, about one billion pounds of bird seed is provided to backyard birds in the United States. That’s a lot of food provided by humans.
But even that is a small serving compared to another source of bird food.
Move over sunflower seeds, thistle and suet cakes.
To find the biggest bird feeder of all, you have to go beyond your backyard – and out to sea.
Around the world, more than 60 billion pounds of fish per year are discarded by commercial fishing enterprises.
That’s a lot of waste. But it doesn’t exactly go to waste.
A large proportion of discarded fish is consumed by seabirds, many of which (like gulls) are adapted to a scavenging lifestyle. It makes discarded fish by far the largest human-generated source of bird food.
Some seabirds get as much as 50 percent of their calories from fish discards.
What does this mean for conservation, especially when conservationists argue for reducing discarded fish?
The Great Bird Feeder in the Sea
Fish may be discarded for several reasons. Many fishing techniques are not selective enough to catch only the desired species.
Because the bycatch has very low market value, it is tossed overboard. Legal requirements can also lead to discards. Fish that exceed a fishing boat’s catch quota, are too small, or otherwise cannot be legally brought to port are also discarded.1
Together, these discards create a huge source of bird food. The most-studied regions with regard to seabird use of fishery discards are the North Sea in northern Europe and the Mediterranean Sea off the coast of Spain.
Studies that track movements of seabirds find that discard-loving seabirds suspend their natural foraging strategies when there are fishing boats at work. The birds go where the boats are, feeding on the trail of discards left in their wake.2-4
In the North Sea, the cast of discard-eating seabirds includes northern gannet, northern fulmar and black-legged kittiwakes. In the Mediterranean, the players include Audouin’s gull, yellow-legged gull and Balearic shearwater.
These species take a big chunk of their calories from fishery discards. Northern fulmars, for example, meet 50 percent of their energy requirement with discards.5
The rarest and most threatened Mediterranean seabird, the Balearic shearwater, takes 41 percent of its calories from fishery discards during nesting season.6
Two Mediterranean gulls, Adouin’s gull (another rare species) and yellow-legged gull, consume the majority of their calories at the back of fishing boats.7
Most of the species mentioned above have experienced population booms over the course of the 20th and 21st centuries.
This is in part due to better conservation protection of birds – but fisheries discards also play a big role.
On the other hand, species that aren’t as keen on discards, such as terns, haven’t experienced the same steady increase in population.
What Happens When the Handouts Stop?
Research in the Mediterranean on two seabird species that make heavy use of fisheries discards is revealing.
A trawling moratorium allowed for an experiment to test the effect of removing the food subsidies provided by commercial fishing discards. The closed period corresponded with the breeding season of Audouin’s8 and yellow-legged gulls.9
As a result of the fishery being closed, the gulls laid eggs later than usual, had fewer eggs per clutch and smaller egg size. Nest abandonment increased and hatching success decreased.
The bottom line: the gulls had poorer reproductive success in years when there was no fishing.
It’s clear that removing the food subsidy results in tougher times for the population. These results also imply that the gull population is artificially high.
If fishing were to stop entirely, we might expect the gull population to decline and reach an equilibrium as the birds resume their natural foraging strategies.
But What if the Fishing Didn’t Stop but the Discards Did?
That is a distinct possibility as new regulations go into effect in Europe. The European Union, in response to considerable public pressure, has begun to implement new regulations to reduce the amount of fisheries discards.
The trick is that the reduction can come in two ways: either by bringing more of the traditionally discarded fish to port, or by using more selective fishing gear that would leave the waste fish alive in the water.
Having more selective gear is best for the birds (and the rest of the ecosystem) because it leaves more food for them to find in place of the easy pickings of discards.
But bringing all fish to market, including those that were formerly discarded, could spell trouble for European seabird populations.
Under this scenario, the previously discarded fish would be used to create products like fish oil, pet food and aquaculture fish-feed.
With no discards and less fish overall in the sea, we might see broad population and reproductive impacts as seen in the Adouin’s Gull experiment described above.
On top of that, there would be increased competition among seabirds for more limited food resources.
Adding to the troubles of smaller, more specialized seabirds is the likelihood that larger and aggressive scavenging seabirds which formerly used discards as their bread and butter would resort to the common behavior of bullying smaller seabirds and stealing food (known as kleptoparasitism).
Even more macabre, a massive predatory gull, the great skua, switches to eating other seabirds when there are fewer discards around and forage fish populations are low.10
Fortunately, fishermen would rather not fill their holds with less valuable fish, so there is hope that the discard-limiting regulations will drive innovations in more selective gear that would be better for fishermen and birds alike.
Such innovations would result in more value per trip for fishermen and more fish left in the sea to fulfill their role in the ecosystem (and fill seabirds’ bellies).