Sanctuary Integrated Monitoring Network
Monitoring Project

Management program for the invasive kelp Undaria pinnatifida

Principal Investigator(s)

  • Steve Lonhart
    Monterey Bay National Marine Sanctuary


  • SIMoN
  • NOAA Catalyst Grant
  • NOAA Restoration Center
Start Date: September 01, 2002
End Date: December 30, 2009

A native of Japan, Undaria pinnatifida was first observed in southern California in early 2000. By August 2001 it had been noted in the Monterey Harbor. Since then the alga has been detected by others, including Dr. Pamela Roe, who in the summer of 2002 collected many large, fully grown individuals. However, until recently a concerted effort to study, and if possible, remove the seaweed was lacking. SIMoN and its partners (CIty of Monterey Volunteer Services and Harbormaster's Office) are leading an effort to determine whether manual removal is an effective means of reducing the population within the harbor. We are also mapping the distribution of the alga in order to measure its rate of spread and identify areas that are more susceptible to future invasion.

There are three main components to the monitoring effort in Monterey Harbor. First, in September 2002 UCSC undergraduate divers conducted a completely randomized survey of all floating docks in the harbor. The divers searched all sides of the floating docks and the pier pilings that supported them. Based on the results of this survey, several “hot spots” were noted, which became the focal points of subsequent efforts.

Second, volunteer divers recruited by the City of Monterey and under the direction of Dr. Steve Lonhart targeted Undaria hot spots for manual removal. Divers searched the floating docks and along the entire column of the cement pier pilings, removing all Undaria encountered.

And third, SIMoN staff measured and observed the following for all of the removed Undaria: 1) total length; 2) damage; 3) reproductive status (e.g., no sporophylls, sporophylls present but spores not released, and spores released); and 4) location.

This is a collaborative effort that involves MBNMS staff, the California Department of Fish and Game, University of California and California State University researchers and students, and the City of Monterey Harbor Office and Volunteer Program.

Summary to Date

After the initial effort to map Undaria in the Monterey Harbor, divers from the City of Monterey Volunteer Program began to remove Undaria manually. Since our first efforts in December 2002, at least 11,000 individuals have been removed, and many of those were pre-reproductive.

A survey of 85 percent of the harbor in September 2003 indicated that Undaria had spread further within the harbor. Although most of the individuals removed at this time were spent adults, and heavily grazed, we also noted the appearance of new recruits. As an annual alga, Undaria typically recruits in the winter, then dies back in summer. However this pattern has changed in Monterey Harbor, where it now appears that the alga recruits almost year-round.

Removal efforts were dramatically reduced in 2010 and will remain low in 2011 due to a lack of funding.

Scuba divers and dockside volunteers are needed for future monitoring and removals. For more information on volunteering with the monitoring program, please email or call 831/420-3661.

Monitoring Trends

  • The density and distribution of Undaria pinnatifida within Monterey Harbor varies each year. In some years, recruitment is high, while in others it is low or late. A predictable seasonal pattern is not apparent.
  • In 2002, new recruits did not appear until late November. In 2003 new recruits were already apparent in October, and in some areas even earlier. In the fall/winter of 2006, recruits were low in number.
  • Sporophylls are usually not apparent until the thallus reaches at least 25 cm. Large individuals (>50 cm) are usually heavily grazed. The herbivorous kelp crab Pugettia producta is common in the harbor, and juvenile crabs are often on the thallus of removed Undaria.
  • By February 2005, when Undaria should be recruiting in large numbers, fewer than 100 individuals were found and removed from Tiers A through K (the majority of the harbor). In contrast, a small part of D-tier had over 250 individuals removed in February 2004. It is not clear what caused the large decline.

Study Parameters

  • Growth
  • Range/Biogeography
  • Dispersal & Recruitment
  • Habitat association
  • Non-indigenous species
  • Abundance
  • Distribution
  • Density
  • Migration/movement patterns
  • Size structure

Figures and Images

Figure 1. Photograph of the invasive Asian kelp Undaria pinnatifida removed from Monterey Harbor.

Figure 2. Native geographic range of the Asian kelp Undaria pinnatifida highlighted in green.

Figure 3. Red coastal areas indicate where the invasive Asian kelp Undaria pinnatifida has colonized beyond its native range.

Figure 4. Volunteer diver holding an adult individual of the invasive Asian kelp Undaria pinnatifida removed from a pier piling in Monterey Harbor.

Figure 5. Since December 2002 volunteers and sanctuary staff have removed and measured over 17,000 individuals of the invasive Asian kelp Undaria pinnatifida from Monterey Harbor. The figure shows the number removed per month. Months with no data were not surveyed.


  • Lonhart and Bunzel (2009)
    Final Report to NOAA Community-Based Restoration Program: Monterey Bay Undaria Removal Project
    1.1 MB PDF
  • Dietrich and Lonhart (2010)
    Undaria pinnatifida: testing different methods of removal and the re-growth potential of an invasive kelp
    990 KB PDF