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Gone to the Gulls: Predation on Threatened Steelhead

January 6, 2014


Squawking, squabbling gulls - those known raiders of garbage cans and stealers of sandwiches - are also taking a hefty bite out of threatened steelhead populations on the California coast. So report researchers in a paper published in the journal Ecosphere last fall: they calculated that an out-migrating juvenile steelhead's chances of being eaten by a Western gull can be as high as 30-45% on average in central California's coastal streams. That's some serious carnage for a population already on the edge. Although the gulls don't specialize on salmonids, and eat everything from trash to carrion, steelhead numbers are low enough (as few as a couple hundred in a given watershed) that even periodic snacking by Western gulls can quickly pick away at their populations, driving them closer to extirpation.


Coming up with such an estimate of overall predation is no simple task, since predation events are rare and hard to observe directly. Scientists can gauge predation to some degree by detecting PIT tags from fishes that turn up in gull roosting or nesting sites - but these tags only represent the minimum extent of predation, since an unknown portion of tagged, consumed fish go undetected. Each tag that the scientists detect at a gull colony (in this case, Aņo Nuevo Island off the central California coast) is the product of three events that each had to occur: 1) a tagged fish was eaten by a bird, 2) the bird transported and regurgitated or defecated the tag on Aņo Nuevo island, and 3) the scientists detected the tag with a mobile PIT tag antenna. Each of these events is a probability, which when multiplied together equal the probability of recapturing a given tag. By modeling different pieces of this equation, scientists could come up with an estimate of overall predation beyond just the tagged fish - and revealed that the detected tags are just a small piece of the predation story.


The scientists studied three watersheds within 7 km of Aņo Nuevo Island, and three that were 30-40 km away. They created a statistical model using data from three different studies - including an experiment in which they embedded tags into small hot dogs, flung them into a gaggle of hungry gulls with a slingshot, then calculated the probability that a consumed tag was transported to Aņo Nuevo Island. They also used a dataset of recovered PIT tags from steelhead tagged between 2005 and 2011.


The team found that nearly all the tags detected on the island, both from steelhead and the experimental hotdogs, came from the three closest watersheds (Waddell Creek, Gazos Creek, and Scott Creek), and virtually none from the creeks farther away. The transportation experiment revealed that only 7-16% of consumed tags were actually deposited on the island, meaning that the recovered tags from steelhead were indicative of much higher actual predation rates. While gull predation was high, it also varied considerably among years: from 7-67% at Scott Creek, and 11-82% at Waddell Creek. The researchers postulate that access to landfills has contributed to a population explosion (or a tenfold increase) of Western gulls on Aņo Nuevo Island over the last 25 years, highlighting the hand humans may play in subsidizing this opportunistic predator. Studies like this one stress the importance of estimating the full impact of predation on vulnerable fish populations in order to better manage them in the face of such threats.
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Happy Aņo Nuevo 

The start of the year is a particularly exciting time at Aņo Nuevo State Park, a coastal region 55 miles south of San Francisco. The park is a cacophony of life as lumbering elephant seals birth their pups and joust for new mates. The windswept Aņo Nuevo Point received the name "New Year's Point" when it was sighted by Spanish explorers in January 1603. The park is also home to Aņo Nuevo Island, a tiny predator-free refuge marked by an abandoned Victorian house, and home to hundreds of squawking sea birds that roost and make their nests offshore.


Elephant seals are by far the park's biggest attraction at this time of year, and adult males can weigh in at a whopping two and a half tons. The blubbery bulls first haul ashore in December, and fight each other for the right to mate with harems of smaller females. Females arrive later in December, and soon give birth to the pup they've been carrying since mating the previous year. They are then ready to mate in a matter of weeks, while still nursing their new pups. The pups are weaned in just a few months, once the adults depart in mid-March. Elephant seals spend most of their life at sea, swimming and feeding in the northern Pacific near Washington and British Columbia.



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