Chapter 21. Fauna of Jetty Piles, Artificial Reefs and Biogenic Surfaces
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Chapter 21. Fauna of jetties and artificial reefs CHAPTER 21. FAUNA OF JETTY PILES, ARTIFICIAL REEFS AND BIOGENIC SURFACES ALAN BUTLER C.S.I.R.O. Marine Research, Hobart, Tasmania 7001. Email: [email protected] Figure 1. Piling of Edithburgh jetty showing sponges, ascidians and bryozoans. (CAS) Introduction This chapter is not a comprehensive description or natural history of the fauna of all jetties, artificial reefs and biogenic surfaces in Gulf St Vincent (GSV) and its approaches. It is about studies done on certain jetties, etc., in the Gulf, using them as experimental systems to increase our understanding of the larger ecosystem of which they are a part. I think of the fauna attached to pilings, artificial reefs and biogenic surfaces as a window on that larger system. Pilings have been convenient places to do experiments and make repeated observations. It has to be remembered, however, that the organisms we are studying on such surfaces are part of larger populations. They have dispersive larvae which may travel short to long distances with the currents; they have predators that move about; the assemblage on one jetty is thus connected to assemblages on other jetties and reefs. We can learn a great deal by observations and experiments at the small scale, but ultimately it only makes sense if we can successfully ‘scale up’—understand these habitats in the context of the system in which they are embedded. I say more, at the end, about this ‘scaling up’. Also, the jetties etc. are artificial—a type of substratum that was not present during the millions of years of evolution of these organisms—and are, in various respects, different from their ‘natural’ habitats. This does not mean we cannot usefully study them as a window on the whole system—and in any case, they are now part of the system—but it is important to remember that a jetty is not the same as a reef. Even though it may house many of the same species, their relative abundances, recruitment rates, growth rates, or behaviour may be 279 Chapter 21. Fauna of jetties and artificial reefs different, although one can find similar faunal compositions where conditions are similar—e.g. steep rock walls, caves and overhangs. I will talk mostly about assemblages on surfaces that are sufficiently deep or shaded to be dominated more by animals than by plants. I will concentrate on sessile or sedentary animals and say almost nothing about the motile organisms such as fishes associated with these structures although they are the main attraction for anglers and are the topic of many studies (e.g. Cappo 1995; Chapter 22). The animals attached to hard substrata such as jetty piles are predominantly suspension feeders or ‘filter-feeders’. They may pump water through their feeding structures (active suspension feeders, e.g., bivalves, sponges, ascidians) or largely depend on external currents to bring food to them (passive suspension feeders, e.g. gorgonian soft corals). This means that the fauna on pilings, rocky reefs etc. tends to vary according to the current regime. Some suspension feeders thrive in slow currents; some need a strong uni-directional or bi-directional current, but may still not be capable of surviving violent water movement; and others not only need plenty of water movement, but are so armoured against rough conditions that they find a niche where other species cannot survive. The relationship between sessile fauna and water movement is a fascinating and accessible topic for an observant underwater naturalist (Vogel 1983; Warner 1984; Denny 1988). There is some obvious variation between major taxonomic groups, and even within a group (e.g., ascidians) there are species adapted to different conditions. Water movement as a source of food is not everything, however, for a sessile suspension feeder. Space to attach to the substratum is paramount, and there is intense competition for it. There is disturbance during rough weather, there are predators, and larvae arrive at different times and need to find space to settle and grow. The ecology of these assemblages involves a complex, fascinating interaction between all these factors (Butler 1995). It means that you find different assemblages on different jetties, wrecks, artificial reefs and biogenic substrata, and the reasons for the differences are not obvious. Distribution Assemblages of sessile suspension-feeders are found wherever there are hard substrata, natural or artificial. My former students and I have studied their dynamics at a number of jetties around the SA Gulfs, especially at Rapid Bay, Ardrossan, Port Giles, Edithburgh and Stenhouse Bay. There is enormous taxonomic and functional diversity in these assemblages (Keough 1999). The predominant groups are usually sponges, ascidians, bryozoans, hydroids and corals but there are bivalves, polychaetes, crustaceans and so on (Fig. 1). Within a group, the species-diversity is often exceptionally high in southern Australia (Poore 1995 - e.g. ascidians, Kott 1997). Within the habitat-structure provided by these sessile organism, of course, there are many motile organisms (e.g. crabs, shrimps, seastars, and gastropods— Fig. 2). The detailed composition of the fauna on pilings varies from place to place. Kay & Butler (1983) described apparently stable differences between the fauna of Rapid Bay and Edithburgh jetties, and these sites have remained distinctive long after the end of their study. The difference is not so much in the list of species present as in their relative dominance. Edithburgh could be loosely described as ‘sponge-dominated’ whilst at Rapid Bay the most conspicuous element is the hard coral Culicia sp. Butler (1986) confirmed these differences and documented different compositions at three other sites. The composition of the fauna also varies within one site, and a particular difference is that between large and small isolated patches. For example, Butler (1991) recorded differences between the fauna on pier pilings and on the shell of the razor fish Pinna bicolor (see Chapter 18) at three sites. Although the details differ from site to site, sponges occupy less of the space on Pinna than on pilings; bryozoans tend to occupy more on Pinna; serpulid tubeworms and barnacles are more abundant on Pinna. Another naturally occurring, patchy source of hard substrata is the shell of the scallop Chlamys asperrima. Unlike Pinna, however, C. asperrima is predominantly occupied by sponges (Pitcher & Butler 1987; Chernoff 1987; Chapter 18). These simple observations draw attention to the complex, interacting processes that determine the dynamics of sessile assemblages – for it is the ‘dynamics’, not a static description, that is interesting in these systems. A visitor sees little or no change on a single dive on a calm day, but these assemblages are constantly changing. 280 Chapter 21. Fauna of jetties and artificial reefs Artificial reefs have been established in SA since the mid 1980s (Branden et al. 1994; McGlennon & Kinloch 1995; PIRSA 2005). SA has the largest number of officially endorsed artificial reefs in Australia (Pears & Williams 2005); with 19 listed by PIRSA (2005). Although artificial reefs have a wide range of (intended) uses worldwide (Pears & Williams 2005), in SA they have been intended mainly to enhance the experience of recreational fishers and divers, and so descriptions of their fauna tend to concentrate on fish, though some record information about algae and sessile invertebrates (Olsen et al. 1976; Reimers & Branden 1994). Reefs have been constructed from discarded tyres, but also by sinking derelict vessels. Data have seldom been recorded from these artificial reefs in a very systematic way, and there have been no experimental studies of their system-wide effects (Claudet & Pelletier 2004; Pears & Williams 2005). The evidence is that they do increase the number of fish available to recreational fishers, but there remains doubt as to whether they are increasing the biomass of fish in the system as a whole, or merely acting as ‘fish aggregating devices’. Despite some exceptions worldwide, it seems likely that in many cases they merely do the latter (see review of Pears & Williams 2005 with references to earlier reviews). This raises the possibility that they may actually increase pressure on fish stocks, and the construction of new artificial reefs is accordingly now discouraged by government. Our focus here, however, is on artificial reefs as habitats for sessile fauna. Figure 2. The gastropod Thais orbita, itself a substratum for bryozoans, laying eggs on a piling of Edithburgh jetty. (MJK) Anecdotally, the sessile, suspension-feeding fauna seems to be better developed on sunken vessels than on tyre reefs. Svane & Petersen (2001) reviewed studies of artificial reefs worldwide, and specifically attempted to view artificial reefs in the context of hard-substratum ecology, since most attention had previously been paid to fish. They asked whether artificial structures (artificial reefs, also jetties, etc.) actually increase the biomass of sessile assemblages in the system as a whole. Since such assemblages are generally space-limited, they may do so; and clearly a stable structure such as a jetty or a wreck does locally increase the production of sessile assemblages. Unfortunately, there are no studies of the dynamics of sessile fauna of artificial reefs in the SA Gulfs. Experimental Studies of Sessile Assemblages in the Gulf Ecology needs both observational studies (with time-series, not merely single snapshots) and experimental studies (Keough & Butler 1995) designed to test hypotheses that may explain the patterns seen in the observations. The fauna (and to a lesser extent, the flora) of jetty piles and biogenic hard substrata in the Gulf have been particularly valuable as an experimental system to understand the processes that control the ecology of hard-substratum assemblages. 281 Chapter 21. Fauna of jetties and artificial reefs Here the neutral term assemblage is preferred, rather than community, or biocoenosis, in agreement with Svane & Petersen (2001), who use ‘assemblage’, to describe co-occurring organisms and reserve the term ‘community’ for assemblages where a higher level of organisation has been demonstrated.