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Research Technical Report

Diver Disturbance in Kelp Forests

Schaeffer, T.M. and M.S. Foster
Coastal Solutions Group

Report to the Monterey Bay National Marine Sanctuary

January 30, 1998

Recreational SCUBA diving activity has greatly increased in the past 20 years, with the potential for causing important disturbance to subtidal reefs. While diver disturbance on coral reefs has been assessed in a number of studies, effects on temperate kelp forests have not been previously examined. We estimated diver disturbance in southern Monterey Bay giant kelp forests by following 42 divers in summer, 1997 and recording disturbances they caused. During a 0.5 hr. dive, the average diver contacted the bottom 43 times, touched four animals, and detached two algal blades. It is estimated that over 60,000 divers use these kelp forests every year. Our results suggest that the concentration of large numbers of divers in local, usually wave-protected kelp forests could lead to permanent alterations in community structure. Such impacts might be mitigated through more environmentally aware diving promotion, better training, and designation of ecologically resilient shore entry and exit points and underwater training areas. These impacts should be considered in proposals to establish and manage marine protected areas.

With increased human population growth, and increased interest in and access to the shore, there has been heightened concern about damage to the marine environment from recreational activities. The most well documented consequences of such activities are trampling on temperate shores (e.g. Beauchamp and Gowing, 1982; Brosnan and Crumrine, 1994) and coral reefs (e.g. Kay and Liddle, 1989; Hawkins and Roberts, 1993), and diver disturbance to coral reefs (e.g. Hawkins and Roberts 1992; Rouphael and Inglis, 1997). Much less is known about the effects of recreational diving activities on temperate subtidal reefs. In 1996, 728,295 divers were certified by the Professional Association of Diving Instructors (PADI), and this organization alone certified over 100,000 SCUBA divers per year since 1980 and over 400,000 per year since 1990 (PADI, 1997). The diving activity resulting from these high certification rates could have important effects on temperate reefs, particularly if concentrated in small areas.

What are the effects of this level of diving activity on the subtidal environment? Divers can damage the environment by intentionally or accidentally holding, trampling, kicking, kneeling, or standing on benthic organisms (Ward, 1990; Hawkins and Roberts, 1992). Most studies of the effects of these disturbances have been done on coral reefs. Talge (1990) followed divers on a coral reef and counted how often they physically interacted with the reef. Twenty six percent of divers observed had one or less interactions with corals, and divers wearing gloves accounted for 72~ of the interactions. In a study of the efficacy of visual fish surveys, Stanley and Wilson (1995) found that fish densities declined 41-77~ in the presence of divers. Conversely, Cole (1994) found that divers attracted some species of fish by feeding them. Ward (1990) questioned whether reef degradation is due to natural processes or damage inflicted by diving and other tourist activities. Gomez et al. (1981), were unable to demonstrate that human usage caused significantly more damage than natural processes on coral reefs in Florida. Most recently, Rouphael and Inglis (1997) found that diver damage was not a function of reef topography, but suggested diver disturbance may vary among different types of benthic assemblages.

Monterey County, California, known for its diverse subtidal forests of Giant Kelp (Macrocystis pyrifera) has long been an attraction for recreational divers. Because of accessibility and protection from swells, divers tend to concentrate in a relatively small (2.8 km) section of coast between the Coast Guard Jetty in Monterey and Lover's Point in Pacific Grove. Saunders et al. (1997) report that an estimated 65,000 diver days are spent annually in this region. The environment of heavily used dive sites like this may be impacted by SCUBA diving and these impacts may conflict with conservation goals (Davis and Tisdell, 1995). While Saunders et al (1997) suggest that recreational diving impacts are minimal, we are unaware of any studies examining diver disturbance to kelp forest communities.

Estimates of the type and magnitude diver of disturbance to these communities would help guide future studies of the ecological effects of such disturbances, may suggest changes in diver training, and would be of importance to the design and management of marine protected areas where recreational SCUBA diving is allowed. We estimated this disturbance by following recreational divers underwater in southern Monterey Bay kelp forests and recording disturbances these divers caused to the subtidal environment.

Diver disturbance was estimated during summer, 1997. Dive sites used were Breakwater Cove, Macabee Beach and Lover's Point Cove, all in the vicinity of Cannery Row, a popular dive region in Monterey Bay. Randomly choosing divers to follow was logistically difficult, so divers were selected primarily by convenience. With their permission, the first group of two or more divers making their shore entry concurrently with us were followed. Subjects were told that, as part of a research project, we were "observing environments that divers like to explore" so they would not be startled by our presence or substantially alter their normal diving behavior. Observations started when divers were completely submerged, observations were recorded for 20-30 min., and observations were standardized to 30 minutes.

Three categories of disturbance were recorded, number of contacts with the bottom, number of algal blades detached, and number of animals touched. A contact with the bottom was recorded if a diver purposely used the bottom for support with any part of their body, stirred up sediment with fins, or unintentionally contacted the bottom with any part of the body for >2 sec. Any algal or seagrass blade or stipe detached from the bottom or from a larger plant by anything associated with the diver was recorded as an algal blade detached. Every animal touched by the diver or their equipment was recorded as an animal touched. Only contacts with obviously visible animals were counted. We also noted if the diver was part of a SCUBA instruction class.

To provide a broader perspective on diver disturbance, questionnaires were sent to Diving Officers with considerable experience in training divers associated with universities in the Pacific Northwest. Diving Officers were asked to estimate how often beginner, average and advanced divers would unintentionally touch the bottom, stir up sediment, or detach algal blades during a 30 minute dive in a kelp forest or bed.

A total of 42 divers were observed for an average of 24.5 min. (0.4 SE) each. Standardized to a 30 min. time interval, the average diver contacted the bottom 43.1 (3.8 SE) times, touched 4.0 (0.6 SE) animals, and detached 2.0 (0.4 SE) algal blades. Class divers detached significantly more algal blades than certified divers, but differences between these two groups for other disturbances were insignificant (Table 1).

Four diving officers responded to the questionnaire, and their estimates of diver disturbance were highly variable (Table 2). They estimated that in 30 min. the average diver in kelp habitats would unintentionally contact the bottom 10 times, stir up sediment 83 times, and detach 2.6 algal blades. Note that unintentional contacts with the bottom and sediment disturbance were estimated separately in the questionnaire but combined in our observations. Diving officers also reported observing occasional destructive behavior such as deliberate removal of plants and breaking kelp floats, and noted that many recreational divers regularly handle subtidal animals.

The types of 'algal' blades detached were most commonly those of the red algae Chondracanthus spp. (= Gigartina) and Mazzaella spp. (= Iridaea), surfgrass (Phyllospadix spp., a flowering plant) and giant kelp, Macrocystis pyrifera (stipes broken). Of these, Mazzaella spp. appeared to most resistant to diver disturbance, perhaps due to its thick blade with a relatively short and robust basal portion. The most obvious disturbance resulted from divers entangling themselves in M. pyrifera fronds and then purposely breaking the fronds to free themselves. Giant Kelp fronds also frequently became wrapped around fins or tank valves without the divers' knowledge. Continued movement usually broke the stipe. Animals touched included a variety of different disturbances. Sea anemones were often touched forcefully until they retracted. Crabs were picked up and dropped. Some divers attempted to feed or pet sea lions and seals. Turban snails (Tegula spp.) and top snails (Calliostoma spp.) are abundant on giant kelp in the Monterey region (Lowry et. al., 1974; Watanabe 1984). While not recorded as an animal touched, these snails were frequently swept from giant kelp blades by divers fins.

In addition to the above observations made while submerged, we also observed but did not quantify other disturbances made by divers during shore entry and exit. The snail Olivella biplicata is common in the gently sloping, sandy areas that are common diver entry and exit points in the Monterey region. During entry and exit, these snails were commonly trampled and sediment resuspended. Diving officers also reported concern for rocky shore trampling during diver entry and exit.

Conservatively assuming a diver day is one 30 min. SCUBA dive/day, and assuming that 65,000 diver days/year (Saunders et al, 1997) is an accurate estimate of diving activity in the Monterey region, then during one year recreational SCUBA divers contact the bottom approximately 2,800,000 times, touch animals 260,000 times and detach 130,000 algal blades. In this region, most of this disturbance is concentrated in giant kelp forests along a 2.8 km section of coast. These estimates do not include disturbances to intertidal organisms during diver entry and exit, bottom disturbance caused by dive boat anchors, or detachment of giant kelp surface fronds and associated animals by the propellers of dive boats and by divers attempting to swim on the surface through these fronds. These disturbances are no doubt increasing as more people learn to SCUBA dive and, if concentrated and frequent, have the potential for negatively affecting kelp forests.

Sandy, protected areas facilitate entry for divers and are particularly popular with beginning divers. These are common habitats for the gastropod Olivella biplicata (Ricketts et al., 1985). These snails commonly bury themselves in the sand during the day and forage at night. They are trampled and their habitat disturbed as divers enter and exit. Trampling may lead to mortality of 0. biplicata, and resuspension of sediment in this habitat and in the subtidal may cause stress to other benthic animals (Rogers, 1988; Neil, 1990). Seagrasses are attached to the sand by rhizomes and shallow roots. Seagrass blades were commonly broken by divers. We also noted stands of this plant being trampled during diver entry and exit. Trampling and sediment resuspension could lead to a reduction in photosynthesis of this and other plants.

These results and observations clearly indicate that recreational SCUBA diving causes a variety of disturbances to kelp communities. While these may be relatively minor on an individual basis, they may become ecologically important to the structure of these communities when many divers concentrate in a small area. Given the natural spatial and temporal variability of giant kelp forests (review in Foster & Schiel, 1985), determining the ecological effects (and their generality) of diver disturbance would be difficult. However, increasing disturbance to the marine environment is inherent to the growth of SCUBA diving. Davis and Tisdell (1995) refer to SCUBA diving as an "activity which is making increasing demands on marine resources". It is reasonable to assume that these demands have adverse environmental effects. We have observed that some dive sites resemble hiking trails as a result of repeated diver disturbance. Responses from diver officers and our observations in other areas indicate that these disturbances are not unique to the Monterey region. They are, however, more likely to become an issue in areas where diving is offered as a major tourist attraction.

While ecological effects may be difficult to quantify and generalize, it is possible to reduce diving disturbance. Disturbance may be indirectly exacerbated by the overzealous promotion of recreational diving. Promoters of recreational diving do not necessarily ignore the environmental consequences of diving, but may believe the consequences are unimportant relative to other benefits. The dive industry has grown to the point where divers are depicted in commercials for products such as cars and alcohol (Castle, 1990). If we are in the middle of a highly commercialized "decade of diving" (Castle, 1990), it is important that the diving industry recognize environmental consequences as well as economic benefits. This may be difficult if, as Ballantine (1989) suggests, exploitation of environmental resources is often assumed to be a right.

Disturbances could also be indirectly reduced through diver training. Beginning SCUBA classes were often observed kneeling on the bottom for long periods while each diver practiced basic skills and safety procedures such as mask clearing and buddy breathing. During this time divers were more likely to stir up sediment or detach algal blades. This is probably why divers in classes detached significantly more algal blades. Certainly the need to practice these skills in the ocean is vital to diver safety, but instructors might make a better effort to choose practice locations that are least sensitive to diver disturbance. Instructors could also place a strong emphasis on buoyancy so contact with the bottom is reduced, on how to avoid entangling in kelp, on how the individual and their diving partner can untangle rather than break stipes if accidental entanglement occurs, on the environmental impacts of touching animals vs. observing them, etc.

As recreational SCUBA diving in kelp communities increases, our data suggest that more attention should be given to managing its environmental effects, particularly in areas such as parks and refuges that often attract and even encourage diving as an outdoor activity. Van't Hof (1985) notes that while marine parks are found worldwide, they often lack effective management, and Tabata (1989) and Rouphael and Englis (1997) point out that changes in park management are needed.

Diving related resource management depends on knowing how often divers interact with the environment and the effects of these interactions. Knowing the effects could lead not only to more environmentally aware dive promotion and training, but might also allow managers to determine how much diving disturbance is acceptable in a particular area. Simply limiting the number of divers, however, may be insufficient (Rouphael & Inglis, 1997). Managers could also consider other approaches such as designated training areas, entry and exit points, and underwater trails to confine major disturbances to particular areas. Rather than locating marine parks in pristine areas sensitive to diver disturbance, parks where considerable diving is anticipated might be located in areas most resilient to diver disturbance. Guidance may be provided from the longer experience with related issues in terrestrial parks (e.g., Liddle, 1975). Ultimately, the management of marine protected areas needs to be based on scientific evidence if they are to be effective (Riegl & Riegl, 1996). Marine reserves not based upon empirical data and allowing unmonitored levels of diving can be counterproductive to the conservation ideals they are supposedly based upon.

The research was funded in part by a contract with NOAA Monterey Bay National Marine Sanctuary. Additional support was provided by Moss Landing Marine Laboratories and Coastal Solutions Group. We thank A. DeVogelaere for his advice and encouragement, and K. Johnson, E. Landrau, C. Rigaud and R. Walder for their assistance with field surveys.

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Table 1. Observed disturbance by certified recreational divers and individuals in SCUBA classes. Data are mean number/30 min. (SE).*, significant difference (p<0.05)

Disturbance..................Certified Divers (n=31)...........Class Divers(n=11)

Contacts with bottom......41.0 (3.2)..........................49.1 (11.8)

Animals touched............4.5 (0.7)..........................2.7 (1.0)

Algal blades detached.......0.7 (0.6)..........................4.0 (1.0)*


Table 2. Diver disturbance estimated by diving officers. Data are mean number/30 min. (range). n=4

Unintentional Contacts with Bottom.......Displace Sediment on Bottom.........Break Kelp Stipes

10.0 (2-30).....................................83 (0-450)...............................2.6 (0-10)




The MBNMS Research Activities Panel (RAP) conducted a review of the above report to ensure adherence to accepted scientific techniques and assumptions. The RAP's analysis report on this study is enclosed here.

In addition, the MBNMS Advisory Council (SAC) requested that the MBNMS Superintendent write a letter to the dive community on this study and issue.

Reviewed: September 08, 2023
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