The contributions of habitat structure and larval supply to broad-scale recruitment variability in a temperate zone, seagrass-associated fish

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Abstract

The contribution of habitat structure and larval supply to broad-scale spatial variability in recruitment of a temperate zone, seagrass associated fish, Sillaginodes punctata (Cuvier and Valenciennes), was investigated in Port Phillip Bay, Australia, from September to November, 1994. Replicate artificial seagrass beds were placed at five sites over a 50 km section of coastline, and artificial and adjacent natural seagrass were sampled approximately fortnightly for Sillaginodes punctata recruitment. Significant differences in recruitment amongst sites were apparent for both natural and artificial seagrass. A small but significant effect of habitat was detected where more recruits occurred in artificial relative to natural seagrass at sites with longer plant stems in the natural seagrass. The contribution of larval supply to spatial variability in recruitment was investigated by sampling natural seagrass, and concomitantly sampling the plankton immediately offshore for pre-settlement larvae. There was no significant correlation between larval abundances and recruitment, or between habitat structure and recruitment, over nine sites. We hypothesise that the high spatial variability in recruitment attributable to location is probably related to a combination of factors. These factors may include variation in larval supply, and also variation in the physical exposure of the location that influences mortality and movement of recruits in the early post-settlement stage.

Introduction

The relative importance of the various factors that determine the distribution and abundance of benthic animals is a contentious issue. Early studies tended to assume that there was an excess of young and that populations were limited by a resource required by adults, usually food or space. However, it has been increasingly demonstrated that varying recruitment of young is important in determining abundance and composition in these communities (Keough, 1984; Underwood and Denley, 1984; Roughgarden et al., 1988; Doherty and Fowler, 1994). Recent research has emphasised that causes of recruitment variability, sensu Keough and Downes (1982), are often pluralistic, and can occur in both the pre- and post-settlement stages (Doherty and Williams, 1988; Levin, 1994; Olafsson et al., 1994; Caley et al., 1996; Jenkins et al., 1997).

Recruitment has five major components: input of propagules into a given water body, transport of those propagules, planktonic mortality, settlement, and post-settlement growth/survival. Variability in these components will eventually translate into recruitment variability. Some progress has been made in the areas of input of propagules and post-settlement growth and survival, and settlement has been well studied for some invertebrate groups in the laboratory, though field data are limited. Larval transport and mortality is probably the least understood phase in the recruitment process. Most of our knowledge tends to have come from specific systems, such as coral reef fishes (Doherty and Williams, 1988) and rocky reef invertebrates (Roughgarden et al., 1988).

Important factors influencing spatial and temporal variation in recruitment depend greatly on the system and scale examined, with a case in point being the recruitment of demersal fishes. On coral reefs the influence of habitat selection appears relatively unimportant, except at a few restricted scales, compared to factors influencing the distribution of pre-settlement larvae (Doherty and Williams, 1988; Sale, 1991). Recruitment of fish to temperate zone seagrass shows strong responses to habitat structure at local scales such as individual beds (Orth et al., 1984; Bell and Westoby, 1986b). At larger scales, however, at least in one system studied, recruitment may be more influenced by the availability of planktonic larvae (Bell and Westoby, 1986c; Bell et al., 1988). Recruitment of temperate reef fish, however, shows correlations with habitat structure over larger scales, and a greater range of scales, than coral reef or temperate seagrass fishes (Jones, 1984; Connell and Jones, 1991; Levin, 1993; Carr, 1994). This is probably due to the dynamic and highly variable nature of algal distributions on temperate zone reefs at a variety of scales. At the largest scales, however, recruitment variation in temperate reef fish is again dominated by factors influencing the distribution of larvae (Cowen, 1985; Ebeling and Hixon, 1991). Because habitat structure can influence recruitment at a variety of scales, variation in recruitment attributable to non-habitat factors is best studied using artificial habitats where structure and complexity are controlled (Bell et al., 1988).

The influence of larval supply on recruitment variation in fishes has been mostly inferred from the temporal and spatial distribution of recruits, with few studies directly relating larval distributions to recruitment patterns in space or time. The development of light traps for sampling pre-settlement coral reef fishes (Doherty, 1987) has led to studies linking temporal variability in larval supply with recruitment of reef fish (Milicich et al., 1992; Meekan et al., 1993; Doherty et al., 1994). Little information is available relating larval supply to recruitment of temperate zone demersal fishes. Breitberg (1991), however, showed that the aggregated distribution of pre-settlement larvae of a temperate zone goby was reflected in post-settlement distributions. Our own recent research has found a significant temporal link between larval supply and recruitment of a temperate zone demersal fish associated with macrophyte beds, the King George whiting, Sillaginodes punctata (Hamer and Jenkins, 1996). In contrast, Levin (1996)presents results for cunner, Tautogolabrus adspersus, that are inconsistent with a positive relationship between larval supply and settlement in space.

The King George whiting, Sillaginodes punctata (Cuvier and Valenciennes), is an important commercial and recreational fish species in temperate zone Australia (Hutchins and Swainston, 1986). Juveniles are associated with shallow macrophyte habitats of sheltered bays and inlets (Jones, 1980). Larvae of Sillaginodes punctata enter Port Phillip Bay, Victoria (Fig. 1), at a size of approximately 15 to 20 mm, from August to November each year (Jenkins and May, 1994). The larval life of Sillaginodes punctata entering Port Phillip is long and variable, ranging from 100 to 170 days, and larvae are competent to settle when they enter the bay (Jenkins and May, 1994). Entry of larvae to Port Phillip appears to be largely governed by low frequency hydrodynamics (Jenkins and Black, 1994).

Recruitment of Sillaginodes punctata to seagrass beds in Port Phillip Bay over a spatial scale of 10's of kilometres has been found to be unrelated to seagrass biomass (Jenkins et al., 1996). However, a negative correlation between recruitment and distance from the bay entrance led to the suggestion that recruitment to the inner bay is limited by larval supply (Jenkins et al., 1996). This general pattern of decreasing recruitment with distance from the entrance was only disrupted by low recruitment at some sites close to the entrance (Jenkins et al., 1996). High frequency sampling of recruits in seagrass beds has shown that near the entrance recruitment is highly variable, characterised by short-lived pulses, further into the Bay there is an accumulation of recruits, whilst a site deep within the Bay had low but relatively constant recruitment (Jenkins et al., 1997). Two-thirds of the variation in recruitment at the sites near the entrance and mid-way into the Bay could be explained by larval input and wave exposure at the sites simulated by numerical hydrodynamic models (Jenkins et al., 1997). With regard to these results, in this paper we examine the hypothesis that, at a scale of 10's of kilometres, habitat structure has little influence on recruitment. To test this hypothesis, we placed artificial seagrass beds of constant habitat over a broad scale and compared recruitment variation with that in natural beds at the same site. Significant variation in recruitment to artificial seagrass would be attributable to site related factors other than habitat structure. We also examined one possible non-habitat factor that might influence recruitment strength on a broad scale; the role of larval supply. This hypothesis was tested by comparing abundances of pre-settlement larvae and recruits in seagrass beds over a scale of 10's of kilometres.

Section snippets

Study area

Port Phillip Bay is a large, semi-enclosed, predominantly tidal embayment linked to the ocean of Bass Strait by a narrow entrance (Fig. 1). The hydrodynamics are characterised by an entrance region where fast (3 m s−1) ebb and flood jets dominate the circulation, a large flood–tidal delta, known as the Sands region, where strong currents occur in the major channels, and an `inner' zone where tidal currents are weak (Black et al., 1993). On the western side of Port Phillip Bay, tidal currents

Influence of habitat structure on broad-scale recruitment

The pilot study revealed that post-settlement Sillaginodes punctata were significantly more abundant on artificial seagrass relative to bare mesh and sand habitats (Fig. 2). A low level of recruitment occurred at Edwards Point and recruits were only found on artificial seagrass (Fig. 2). Recruitment at Grassy Point was higher and although most recruits were on artificial seagrass, low numbers were recorded on the other habitat treatments (Fig. 2). Repeated measures analysis of variance of the

Discussion

The results of this study suggest that most of the variation in recruitment to seagrass beds at a broad (10's of kilometres) scale is not attributable to characteristics of seagrass structure. The pattern of recruitment found shows interannual consistency, with low recruitment close to the entrance of Port Phillip Bay, high recruitment near the entrance to the Geelong Arm, and declining recruitment within the Geelong Arm (Jenkins et al., 1996; Jenkins et al., 1997). This suggests that there is

Acknowledgements

We thank V. Gomelyuk, M. Holloway, M. Wheatley, T. Shirley and P. McGrath for assistance with field and laboratory work. We appreciate the comments of G. Edgar, D. Welsford and M. Wheatley on drafts of the manuscript. Financial support for the study was provided by the Australian Research Council and the Fisheries Research and Development Corporation of Australia.

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