Kelp Forests: MBNMS

Kelp forests extend throughout most of the nearshore waters of the Monterey Bay National Marine Sanctuary. Hugging the coastline, they extend from just beyond the breaking waves to depths of <30 meters. Although kelp forests are a familiar and iconic habitat within the sanctuary, kelp canopy cover typically encompasses about 25 square miles, or less than 0.5% of the total surface area within the sanctuary. Although MBNMS encompasses 6,094 square miles of ocean, only 8% is within recreational diving depths (<130 feet).

Two species account for the bulk of physical structure and kelp biomass: the giant kelp, Macrocystis pyrifera, and the bull kelp, Nereocystis luetkeana. Although it is difficult to distinguish the two species using remotely sensed data (e.g., aerial photographs or hyperspectral images), long-term patterns of kelp canopy cover are persistent throughout the sanctuary.

Bull kelp, which tolerates high wave action, is typical along exposed rocky shores, whereas giant kelp is abundant in all areas except the most exposed sites. Both species co-mingle from Cambria to Monterey, but giant kelp is very rare north of Santa Cruz.

Kelp forests provide important structural features and ecosystem function to coastal marine communities. The physical structure of kelp provides vertical habitat similar to trees on land; it is used by numerous fishes and invertebrates. Kelp forests are considered an important nursery habitat for nearshore rockfishes and serve as a primary foraging area for many southern sea otters, Enhydra lutris nereis.

Although giant kelp was harvested commercially in southern California, the only commercial harvesting in the sanctuary is small-scale, limited to extraction by hand, and is used to feed farm-raised abalone. Management and leasing of kelp beds for commercial or other uses is administered by California Department of Fish and Game.

Giant kelp is a perennial but rarely lives longer than two to three years, and bull kelp is an annual. Both species recruit in late spring and early summer.

Rapid growth by giant kelp allows it to dominate nearshore habitats quickly. Kelp can also quickly recover from adverse environmental conditions (e.g., high wave action in winter, El Niño events). Drift kelp serves as in important ecological subsidy to intertidal and deeper shelf habitats, exporting significant amounts of carbon and nitrogen into communities via detritus.

In addition to the canopy-forming species, there are several kelp species (e.g., Pterygophora californica, Laminaria spp.) that form an understory below giant and bull kelp. This understory adds a tremendous amount of biomass to the kelp forest and provides additional habitat for fishes and invertebrates.

Because kelp forests serve such an important ecological role both structurally and functionally, they require monitoring and, when necessary, effective management. To understand and manage this critical habitat, we must first determine what drives the dynamic nature of the system, disentangling natural variation from human-induced change.

To date, our ability to tease apart natural from human-induced change has been poor. However, current and past monitoring efforts, including the Partnership for Interdisciplinary Studies of Coastal Oceans (PISCO) and the Cooperative Research and Assessment of Nearshore Ecosystems (CRANE) programs, will improve our understanding of kelp forest habitats in the sanctuary. These programs monitor and characterize fish, invertebrate and macro-algal assemblages and provide information to enhance resource managers’ abilities to implement ecosystem-based management and conservation.

Monitoring kelp forests at multiple spatial scales (e.g., within a single kelp bed vs. the Big Sur region) and linking trends and patterns to environmental processes (e.g., global climate change, decadal oscillations, El Niño events) help to create a more comprehensive view of the entire system. In addition, this information leads to testable hypotheses that will foster a better understanding of the mechanisms driving environmental change and how we can best alter human behavior to reduce our impact on these natural resources.

Monitoring

Information gathered from recent and ongoing kelp forest monitoring efforts includes the following:

Marine Resources Survey

Prior to SCUBA surveys, sites with persistent kelp canopy cover (based on California Department of Fish and Game remote-sensing data) were assumed to be relatively similar. Surveys showed that kelp cover was not tightly correlated with habitat quality and species richness. Some areas with extensive kelp cover were ranked lower (for richness, abundance, et al.) than those with little or no kelp. These results indicate that remotely sensed data must be ground-truthed before use in the process of resource management.

Juvenile Rockfish Abundance Surveys

From 2000 to 2004, eight species of juvenile rockfish (blue, black, yellowtail, canary, olive, widow, bocaccio and copper rockfish as well as the gopher /black and yellow complex) were observed by Tom Laidig at NOAA Fisheries at two sites offshore of Pacific Grove. In general, blue rockfish was the most abundant species, followed by yellowtail rockfish. Olive and black rockfishes were the next most abundant, and the remaining species were only seen occasionally. Blue and yellowtail rockfishes were abundant from 2001 to 2003, but their numbers dropped in 2004. Yellowtail rockfish were decreasing since their high in 2002. Olive rockfish were at low levels in all years, with the highest in 2001. Black rockfish increased in each of the four years.

Partnership for Interdisciplinary Studies of Coastal Oceans (PISCO) Subtidal Component

Since 1999, researchers from UC Santa Cruz, UC Santa Barbara and Stanford University have monitored intertidal and subtidal sites along central California, from Los Angeles to Half Moon Bay. Fish assemblage structure (relative abundance of species) differs regionally and inside versus outside marine reserves along the central coast. Significantly greater numbers were seen for five of 13 common species targeted by fishing (mostly rockfishes in the genus Sebastes), whereas only one of 10 common species not targeted by fishing was more abundant within reserves. No significant difference in either the average length or the proportion of larger fishes was seen between reserve and non-reserve sites. Based on estimates of size, density and length-specific fecundity for individual species, targeted fish populations have significantly higher biomass and larval-production capacity within reserve boundaries.