Journal of Experimental Marine Biology and Ecology 255 (2000) 153±174 www.elsevier.nl/locate/jembe Evaluating the impact of predation by ®sh on the assemblage structure of ®shes associated with seagrass (Heterozostera tasmanica) (Martens ex Ascherson) den Hartog, and unvegetated sand habitats Jeremy S. Hindella,b,* , Gregory P. Jenkins c , Michael J. Keough a aDepartment of Zoology, University of Melbourne, Parkville 3010, Australia bQueenscliff Marine Station, Queenscliff 3225, Australia cMarine and Freshwater Resources Institute, Queenscliff 3225, Australia Received 22 November 1999; received in revised form 4 July 2000; accepted 1 September 2000 Abstract The role of ®sh predation in structuring assemblages of ®sh over unvegetated sand and seagrass was examined using enclosure and exclusion cages to manipulate the abundance of predatory ®sh from November 1998 to January 1999. In our exclusion experiment, piscivorous ®sh were excluded from patches of unvegetated sand and seagrass to measure how they altered abundances of small ®shes, i.e., ®sh , 10 cm in length. Habitats from which piscivorous ®sh were excluded contained more small ®sh than those with partial cages, which in turn contained more ®sh than uncaged areas. These patterns were consistent between unvegetated sand and seagrass areas, although the relative differences between predator treatments varied with habitat. Overall, small ®sh were more abundant in unvegetated sand than seagrass. Atherinids and syngnathids were the numerically dominant families of small ®sh and varied in complex ways amongst habitats and cage treatments. The abundance of atherinids varied inconsistently between cage treatments through time. Only during the ®nal two sampling times did the abundance of atherinids vary signi®cantly across cage treatments. Syngnathids were strongly associated with seagrass and were signi®cantly more abundant in caged than uncaged habitats. In our enclosure experiment, ®ve individuals of a single species of transient piscivorous ®sh, Western Australian salmon (Arripidae: Arripis truttacea Cuvier), were enclosed in cages to provide an estimate of the potential for this species to impact on small ®sh. The abundance of small ®sh varied signi®cantly between cage treatments. Small ®sh were more abundant in enclosure cages and exclusion cages than uncaged areas; however, there was no difference in the abundance of small ®sh in enclosure cages and partial cages, and no difference between exclusion cages and partial cages. These patterns were *Corresponding author. Queenscliff Marine Station, PO Box 138, Victoria 3225, Australia. Tel.: 1 61-3-5258- 3686; fax: 1 61-3-5258-3632. E-mail address: [email protected] (J.S. Hindell). 0022-0981/00/$ ± see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0022-0981(00)00289-6 154 J.S. Hindell et al. / J. Exp. Mar. Biol. Ecol. 255 (2000) 153 ±174 consistent amongst habitats. Atherinids and syngnathids were again the numerically dominant families of small ®sh; atherinids varied more with cage structure while syngnathids did not vary statistically between cages, blocks (locations within which a single replicate of each cage treatment was applied) or habitats. Dietary analysis of caged A. truttacea demonstrated the potential for this species to in¯uence the assemblage structure of small ®sh through predation ± atherinids were consumed more frequently in unvegetated sand than seagrass, and syngnathids were consumed only in seagrass, where they are most abundant. Observations of signi®cant cage or predation effects depended strongly on the time at which sampling was undertaken. In the case of the atherinids, no predation or cage effects were observed during the ®rst two sampling times, but cage effects and predation effects strongly in¯uenced abundances of ®sh during the third and fourth sampling times, respectively. Our study suggests that transient piscivorous ®sh may be important in structuring assemblages of small ®sh in seagrass and unvegetated sand, and seagrass beds may provide a refuge to ®shes. But the importance of habitat complexity and predation, in relation to the potentially confounding effects of cage structure, depends strongly on the time at which treatments are sampled, and the periodicity and multiplicity of sampling should be considered in future predation studies. 2000 Elsevier Science B.V. All rights reserved. Keywords: Arripis truttacea; Australia; Unvegetated sand; Caging experiment; Heterozostera tasmanica; Piscivory; Seagrass; Structural complexity; Temperate; Temporal variability 1. Introduction Predation can be an important process structuring post-settlement assemblages of ®sh (Choat, 1982), and one of the most abundant predators are other ®shes. Correlative studies often show that the abundance of piscivorous ®sh is negatively associated with the abundance of smaller (prey) ®sh (Hixon, 1991; Bailey, 1994; Connell and Kingsford, 1997, 1998). Dietary studies complement correlative analyses by demonstrating the importance of particular suites of small ®sh in the diets of predatory ®shes (Hall et al., 1990; Kingsford, 1992; Edgar and Shaw, 1995a; Connell and Kingsford, 1997; Connell, 1998). In concert, correlative studies and dietary analyses imply that predatory ®sh are important determinants of the structure of small ®sh assemblages. However, few experimental studies have unequivocally concluded that ®sh predation is an important contributor to variability in small ®sh assemblage structure (Hixon, 1991). Habitat structure is provided by both abiotic (consolidated and unconsolidated sediments and rock) and biotic (coral, wood, oyster reef, submerged and emergent vegetation) elements, but vegetation has received most attention due to its wide distribution and because animal abundances in vegetation are generally greater than alternative, usually unvegetated, areas nearby (Heck and Crowder, 1991). In marine environments, the positive association of animals with structurally complex, vegetated habitats is likely to be a re¯ection of interactions between habitat selection processes (Edgar and Robertson, 1992; Levin and Hay, 1996), hydrodynamics and larval supply (Jenkins and Black, 1994; Hamer and Jenkins, 1996) and survival as a function of refuge provision and habitat complexity (Choat, 1982; Orth et al., 1984; Orth, 1992). Predation J.S. Hindell et al. / J. Exp. Mar. Biol. Ecol. 255 (2000) 153 ±174 155 is an important source of mortality, and predation ef®ciency, as a function of detection, selection, pursuit and capture of prey (Mattila, 1995), decreases with increasing habitat complexity (Choat, 1982; Stoner, 1982; Gotceitas et al., 1997). Thus, patterns in the abundance of ®sh across habitats which differ markedly in structure may re¯ect differential predation pressure, i.e., the structure of the vegetation and the type of habitat complexity it generates determines the intensity and nature of predator±prey interac- tions, and thereby affects the structuring capacity of predation (Mattila, 1995). Seagrasses are a common form of biogenic habitat in marine and estuarine systems worldwide (Pollard, 1984; Bell and Pollard, 1989; Kemp, 1989), and compared with alternative, usually unvegetated habitat, generally contain higher numbers of predatory and other ®shes (Heck et al., 1989; Edgar and Shaw, 1995a,b; Jenkins et al., 1997b). The high, but temporally variable (Heck et al., 1989), association of small ®sh with seagrass habitat has led to a paradigm which promotes the importance of seagrass beds in the provision of nursery habitat for juvenile ®shes (Pollard, 1984; Bell and Pollard, 1989; Jenkins and Wheatley, 1998). Increased food availability (Bell and Pollard, 1989) and protection from environmental disturbance (Kemp, 1989; Edgar, 1990) are two popular theories why seagrass beds contain high abundances of small ®sh. But, it is also plausible that the structural complexity provided by aspects of the seagrass affects the ef®cacy or selectivity of predators (Levin et al., 1997), and provides juvenile ®sh with a refuge from predation (Orth et al., 1984; Orth, 1992). Previous studies suggest that broad-scale patterns in small ®sh assemblages may be in¯uenced by variable larval supply (Jenkins and Black, 1994; Jenkins et al., 1997a) and, micro-habitat selection, not predation, is the proximate cause for variability in the abundance of fauna within seagrass beds (Bell et al., 1987; Edgar and Robertson, 1992; Levin et al., 1997). However, many of these studies have manipulated predatory ®sh over relatively small (1 m2 ) spatial scales (Bell et al., 1987), which may re¯ect prey movements and behaviour more than predation effects (Englund and Olsson, 1996; Englund, 1997). Furthermore, while exclosure cages have been used extensively to manipulate abundances of predatory ®sh (Steele, 1998; Levin et al., 1997; Kennelly, 1991), few studies have used enclosure cages to assess the role of predatory ®sh in structuring assemblages of ®sh in structurally diverse habitats. A more thorough understanding of (a) the role of ®sh predation in structuring assemblages of small ®sh, and (b) the importance of seagrass beds in the provision of refuge from predation, will be gained by conducting carefully designed experiments which manipulate predator abundance using controlled enclosure and exclosure caging experiments over similar spatial and temporal scales in large plots of habitat (seagrass and unvegetated sand) that differ markedly in structural complexity. The primary aim of our study was to