Black Abalone Regain Lost Ground

Black abalone on the underside of a boulder in the rocky intertidal zone
A large black abalone, with a characteristic smooth, dark shell.

NPS / Jessica Weinberg McClosky

March 15, 2017 - For nearly 20 years, Channel Islands National Park biologists recorded the same number of black abalone (Haliotis cracherodii) at Cuyler Harbor: zero. Modern abalone monitoring began at Cuyler Harbor, located on San Miguel Island, in 1992. At that time, biologists counted 165 of the dark, smooth-shelled sea snails. After 1997, they found none. The black abalone had fallen victim to withering syndrome, a disease that may have been exacerbated by the warm-water El Niño events of the 1990s. Black abalone throughout the region suffered a similar fate. By 1998 less than one percent of southern California's historic black abalone population remained.

Fortunately, at Otter Harbor, just a few miles away on San Miguel Island, a depleted population of survivors persisted. In parts of the rocky intertidal zone that were once covered in layers of abalone five individuals deep, fewer than a hundred individuals remained. Small remnant populations hung on in other places as well, but mostly at sites on San Miguel. The survivors were typically older, larger abalone, and for several years, that was almost all biologists could find. There was little evidence of a successful younger generation, which didn’t bode well for the future.

Otter Harbor, San Miguel Island
Lots of abalone at Otter Harbor in 1986 Otter Harbor in 2008, with no clearly visible black abalone
View of Otter Harbor in 1986. NPS
View of Otter Harbor in 2008. NPS
Here, a view of Otter Harbor in 1986 was photographed again in 2008, after the abalone had been missing from the scene for about a decade. In the 1986 image, the black abalone are the numerous round forms covering the sides of the long rock crevasses. Worn or barnacle-encrusted shells cause many of the abalone to appear white.



Then, beginning around 2007, more young black abalone began turning up, mostly on Santa Cruz Island. At first, the sight of so many juveniles was strange and unfamiliar. The youngest abalone recruits, at less than an inch long, appeared miniature alongside some of their older, cereal bowl-sized counterparts.

Baby black abalone next to a camera lens cap for scale
New black abalone recruits (baby black abalone) in 1991, before they all but vanished for a number of years.

NPS

One particular site on the south side exemplified the trends on Santa Cruz Island. Where counts were once down to one, abalone numbers began climbing exponentially. Within a few years, the population numbered in the hundreds. Other sites across the islands also began seeing growing numbers of black abalone.

Black Abalone Counts at Channel Islands National Park, 1985-2014

Scatter plot with trendlines showing black abalone numbers plummeting followed by a rise in total abalone numbers, including juvenile numbers, beginning around 2007
Scatter plot of black abalone counts between 1985 and 2014, with trendlines. Early monitoring was done in a series of fixed plots, .9-11.2 square meters in size. Methods were updated in 1992 to include timed searches of entire sites (hundreds of square meters each) and measurements of individual abalone. Those measuring less than 45mm are considered juveniles. After abalone counts plummeted sharply in the 1980s and 90s, total counts and juvenile counts have begun trending upward in recent years.
Upside-down black abalone shell with the dead, withered black abalone body still inside
Black abalone are still found suffering from, or dead as a result of, withering syndrome. The disease affects the abalone's digestive tract, leaving them unable to digest their food. Diseased individuals that aren't eaten by predators in their weakened state ultimately die of starvation.

NPS

Although populations were on the rise, withering syndrome did not disappear. Diseased individuals and empty abalone shells have continued to turn up over the years. A dip in abalone numbers on some islands from 2013 to 2015 could be related to a resurgence of the disease.

San Miguel Island sites were the last to see a rise in juvenile abalone. Aside from a brief surge in 2005, juveniles remained rare there until last year. Now though, Otter Harbor abalone are looking younger than ever. A growing number of new recruits seem to be appearing among the older abalone. But what of Cuyler Harbor?

As changes in the abalone population played out throughout the park, abalone remained absent from Cuyler Harbor. That is, until 2016. After almost two decades of fruitless searches, biologists discovered a single adult black abalone! This year, however, their surprise was even greater. Where they would have been excited to find one or two, biologists counted 15 black abalone!

Panorama of Cuyler Harbor at low tide
The Cuyler Harbor monitoring site on San Miguel Island.

NPS

Part of the surprise for biologists at Cuyler Harbor is that they don't really know how the abalone got there. The site is small, with hardly any crevasses hidden from human eyes, so they doubt they missed any young abalone in recent searches. Yet most of the abalone were already several years old. Did they climb over the impressive ridge separating the site from good habitat to the East? Did they navigate a sandy subtidal area to get around the ridge? Biologists had not previously pictured the humble black abalone achieving such feats.

The other thrilling part of the Cuyler Harbor discovery is the sheer amount of time that had passed between abalone sightings. The ability to document such a cycle of disaster and potential recovery over the course of decades is a testament to the value of long-term monitoring efforts. As more decades of data accumulate, our understanding of the normal variability of nature grows. Over time we also become better able to detect when something unusual or concerning is taking place. For example, monitoring results showing a sharp population crash helped justify protecting the black abalone under the Endangered Species Act in 2009.

Multiple black abalone of different sizes sharing one section of rock
As of 2017, a mix of black abalone sizes/ages is becoming increasingly common to see at monitoring sites across Channel Islands National Park. Even so, the population remains at or below one percent of historic levels.

NPS

Now, biologists are hoping to continue learning about what a black abalone recovery looks like. As they do, they may be able to come up with measurable ways of describing a healthy population. One measure, for instance, could be the population’s percentage of juvenile abalone. If all goes well, such metrics could one day be used to justify delisting the black abalone as an endangered species.


Prepared by the Mediterranean Coast Inventory and Monitoring Network.

Last updated: March 21, 2017