Store beach and Quarantine beach had 23.5 (± S.E. = 3) boats per beach, significantly more than the other two beaches inside the NHAR (1.16 ± 0.3 boats.beach^-1), which had similar numbers of anchored boats compared to the two beaches outside the reserve (2.8 ± 1 boats.beach^-1), as seen in Table 1 and Figure 2a. There was an average of 11.5 ± 1.4 sailing yachts and 9.8 ± 2.1 motor cruisers anchored at each of Quarantine and Store beach. The average number of speedboats in each of these two beaches was smaller (2.1 ± 0.4), as seen in Figure 2b.

　

　

　

　

　

A total of 227 litter items were collected from the six beaches, representing an overall mean of 0.095 litter items per m² of seafloor. Plastic items accounted for almost half of all debris collected, followed by glass, metal and paper (Fig.3). The majority of plastic items were supermarket-like plastic bags and the majority of the glass items were bottles for alcoholic drinks, such as beer, wine and sparkling wine (Fig.4).

　

Large mean numbers of debris items.m^-2 were found in waters adjacent to beaches associated with large mean numbers of anchored recreational boats (Fig.5). This positive correlation was statistically significant (Pearson's r = 0.96, 4 df, P < 0.01).

　

　

　

　

　

This study showed that two beaches inside the NHAR (Quarantine beach and Store beach) support regularly large numbers of anchored boats, confirming the status of these areas as popular anchorages for recreational boats in Sydney Harbour. This finding is important because it provides unprecedented quantitative information about anchoring patterns within NHAR. Furthermore, it sets recreational boating as a major human activity in this aquatic reserve. Possible explanations for the popularity of these beaches among the boating community are their pristine character and the difficulty of access (for non-boaters).

　

The significant correlation between the number of anchored boats and the amount of debris items found in waters adjacent to the beaches where these boats were anchored does not demonstrate, by itself, a causal relationship between boating and littering. Because experimental manipulation of anchoring areas for recreational boats is a rare option, correlative analyses remain one of the best available approaches to identify contributing sources of marine debris. Considering that access other than by boat to these beaches (particularly to Quarantine and Store beach) is restricted coupled with the low level of residential development on the foreshores of these beaches, it is reasonable to consider that anchored boats contributed substantially to the amount of submerged debris found.

　

Two other aspects seem to support this idea. Anchored motor cruisers and sailing yachts were more abundant than speedboats. Cruisers and yachts normally have internal cabins (typically with a table, a stove, a refrigerator and cabinets) and it is conceivable that they can carry more goods aboard (and therefore more potential litter) than open-deck speedboats. In addition, it is probable that plastic bags and glass bottles, which were the litter items most commonly found, are part of the normal set of items aboard recreational boats. The majority of marine debris originated from anchored boats was probably dropped into the sea unintentionally, because recreational boaters are considered to be reasonably sensitive to environmental issues (Gregory, 1991).

　

Previous research suggested that recreational boating is not publicly perceived as a significant cause of environmental disturbances to Sydney Harbour (Widmer et al., 2002). Indeed, it seems that recreational boating is not a main source of marine debris, when compared to other human activities. However, this study indicated that this leisure activity (when practiced intensely) can contribute locally to the amounts of submerged marine debris, particularly in areas where access by land is restricted. Because the methodology used here is inexpensive, further investigations of the role of recreational boating as a contributing source of marine debris can be performed in a wide range of locations where recreational boating is intense and where access to expensive technologies is limited (e.g. some Caribbean and South Pacific island-nations).

　

Of particular concern is the large number of glass bottles found on the seafloor of Store and Quarantine beaches. This material poses a potential risk for the health of the visitors of the reserve. Ecologically, marine debris has potential adverse effects that do not contribute to the conservation objective of the North Harbour Aquatic Reserve. The hypothesis that submerged marine debris provides novel habitat for biological fouling assemblages was experimentally tested and the results of such investigation will be presented in a forthcoming paper.

　

Haynes (1997), in a study about marine debris in the Great Barrier Reef Marine Park, argued that enforcement of regulations to control dumping of debris at sea is impractical. Other actions, such as clean-up, educational programme and pollution prevention initiatives should be considered (Ellis and Podlich, 1997). Among those, the deployment of public moorings could reduce the total number of anchored boats (and therefore the risk of litter being dropped overboard). It could also organize the spatial pattern of mooring, which is beneficial for boat users as it tends to reduce collisions and provides a more rational use of water space. It is not surprising that sectors of the boating community are receptive to the idea of public moorings at these two beaches (Lockwood, 2001). Other marine protected areas (e.g. Great Barrier Reef Marine Park) use public moorings to regulate recreational anchoring (GBRMPA, 1999). Such moorings, if wisely deployed, could also contribute to the reduction of the likelihood of anchor damage to Posidonia australis, a seagrass known to have a very limited capacity for recovery after physical disturbance (Meehan and West, 2000), which is present at the subtidal vicinity of these two beaches.

　

Sydney Harbour is a busy urban waterway where secluded areas are rare. Currently, the boating community has the informal privilege of access to the waters of Store and Quarantine beach. Visiting pristine areas is likely one of the attractions of recreational boating. In return for this privilege, if not for other reasons such as ethical considerations, it is reasonable to expect boaters to be caretakers for the area. Therefore, recreational boat users should commit themselves, with other stakeholders and regulatory agencies, to discuss managerial actions that, within an experimental framework, are aimed at reducing the amounts of submerged marine debris and improving the general environmental condition of this marine protected area.