New Zealand's endangered yellow-eyed penguin have been found to follow the line of swooping seabirds - literally - when it comes to reaping the food stirred up by trawlers.
An Otago University research team have discovered the penguins, of which there were around 500 estimated pairs left on mainland New Zealand, forage in straight lines for several kilometres by following furrows in the seafloor scoured out by fishing trawlers.
Using GPS dive loggers the researchers monitored the penguins' movements over three years showing the birds use furrows scoured on the seabed by otter boards from trawl nets to find food, particularly blue cod.
"This research is unique as it shows for the first time that not only do flying seabirds follow fishing vessels, but also penguins, with the latter foraging after a trawler has gone through a particular area," lead research Professor Philip Seddon said.
The researchers said blue cod and other bottom feeders were likely to forage around the seafloor lines because they were attracted to the marine life stirred up and exposed by the action of the nets being dragged behind fishing trawlers.
The lines made by the otter boards, which keep the mouth of the trawl net open, are up to 15cms wide and two centimetres in depth on a north-east to south-west axis.
They can remain on the sea floor for a year or more and are clearly visible.
GPS dive loggers were attached to the back of the birds to determine the depth the penguins dive, their locations and line of travel and how far they swim in one foraging trip.
Lines on the seafloor were located by using video footage taken by a remote operated vehicle launched from the university's research vessel Polaris II.
Many penguins swim to a depth of between 60 and 70 metres to feed during multiple dives - up to 80 - over several hours before returning to shore.
The penguins can travel up to 120 kilometres in one trip, while foraging in the mid-shelf fishing grounds some 20 kilometres off the Otago Peninsula.
The study shows that the birds also revisit the lines on subsequent occasions and might develop a visual memory of the area, researchers say.
"It appears that using the lines for foraging is particularly related to bad breeding years when penguins are more likely to go further out to sea to find blue cod and other bottom feeders. This might also be due to the individual preference of some birds though," said Dr Thomas Mattern, the first author of the paper reporting the results.
But the researchers say that one of the downsides of foraging around the trawl lines might be that an exclusive diet of blue cod, which tends to be low in nutritional value, could affect breeding.
As yet, there was no confirmation of this hypothesis and further research was needed to determine if there is any relationship between foraging patterns, diet quality and breeding success in the penguin population.
The research was published in the journal PloS ONE and supported by the Yellow-Eyed Penguin Trust.