Johann D. Bell

Abundance of macrofauna in dense seagrass is due to habitat preference, not predation

SummaryTwo main hypotheses compete to explain why prey abundance decreases when seagrass density is reduced. One proposes that predators are more successful amongst seagrass of lower density; the other invokes habitat choice by prey. We reduced the density of seagrass in the presence, and in the absence, of predators in a field experiment to discriminate between these hypotheses. When seagrass was manipulated abundances of all six prey species decreased simultaneously both in the presence and in the absence of predators. We conclude that correlations of prey abundance and shoot density within a seagrass bed are proximately due to habitat preference of dense seagrass by prey. We report another experiment which supports this conclusion and shows that habitat preference is exercised at the earliest opportunity. However, the habitat preferences may have been selected by predation pressure.

Variation in seagrass height and density over a wide spatial scale: Effects on common fish and decapods

Abstract This study examines whether relationships between abundances of fishes and decapods and the leaf height and density of seagrass determined from small scale field experiments within a seagrass bed were also evident at a wider spatial scale, i.e. among beds within a bay. Separate comparisons were made for common species of fishes and decapods associated with the seagrasses Zostera capricorni Aschers, and Posidonia australis Hook f. Of the 23 significant responses by species to leaf height or density in the small scale experimental study, 15 were not repeated at the wider scale. Abundances of species with inconsistent responses at the two scales were not correlated meaningfully with other major environmental variables such as sediment size, area of seagrass bed or distance from the ocean. We reconcile the high incidence of effects of leaf height and density within beds, and their scarcity among beds, with a model of how postlarvae settle into, and move within, seagrass beds. The model holds that (1) larvae of fishes and decapods are distributed patchily when ready to settle, (2) larvae do not discriminate among beds when they settle, and (3) individuals do not leave a bed soon after settling but do redistribute within the bed to select microsites favouring survival.

Importance of local changes in leaf height and density to fish and decapods associated with seagrasses

Abstract The height and density of seagrass leaves were manipulated to about one-third normal levels to test whether associated fish and decapods were affected. The experiments tested for effects of height, density and their interaction, and were run separately in single beds of Zostera capricorni Aschers, and Posidoniaaustralis Hook f. to determine if effects were consistent for each seagrass. Abundances of individual species were often affected significantly by the manipulations. In Zostera, abundances of six species decreased when leaves were shortened, whereas abundances of three species increased, and seven species decreased, when leaves were thinned. In Posidonia, abundances of four species decreased when leaves were shortened, while abundances of two species increased and five decreased when leaves were thinned. Significant height × density interactions did not occur for any of the 13 species analysed from Zostera, but did occur for two of the 12 species from Posidonia. Several species, in both seagrasses, did not respond to the manipulations. Of the six species that were analysed in both seagrasses, four responded in the same way to height and density, and a fifth species responded consistently to height. There were significant effects on the number of individuals in feeding guilds. However, effects on guilds masked (opposite) effects, and the lack of effects, for component species, and showed little consistency between Zostera and Posidonia. This was also true for effects on the total number of individuals. The data do not support an existing model predicting responses of species richness and abundance to changes in physical complexity of seagrasses.

Location of seagrass beds in estuaries: effects on associated fish and decapods

Abstract This study examined whether there were differences in abundance, species composition, and species richness of juvenile fish and decapods associated with Zostera capricorni Aschers, beds at different distances (zones) along an estuary. It also determined whether such variation was consistent between two arms of an estuary and if it occurred in different seasons. Total abundance of fish did not differ among zones or arms but abundance of decapods was often greatest near the mouth of each arm. More species of fish and decapods were caught in the lower and middle reaches of the estuary than in the upper reaches, although this depended on the arm. There were significant differences in abundances among zones for 13 of the 16 species studied separately in May, and for 17 of the 20 species examined in November/December. These differences were usually expressed as zone × arm interactions, and often occurred because individuals were absent or scare in one arm. Patterns of mean abundance among zones and arms were different for most species. Each combination of zone and arm had a distinct assemblage of fish and decapods. Artificial seagrass units (ASUs) were used to control for variation in physical complexity of Zostera among zones. There were striking similarities in patterns of abundance among zones and arms for the majority of species abundant enough for analysis in both Zostera and ASUs. We conclude that 1. (1) the location of a seagrass bed within an estuary has a significant effect on abundances of juveniles of many species of fish and decapods, 2. (2) for most species, these effects are due to the position of the bed, not to its size, shape, leaf height or leaf density, and 3. (3) the most likely cause of these effects is variation in distribution and availability of competent larvae.

Fish larvae settling in seagrass: do they discriminate between beds of different leaf density?

A field experiment was designed to test the hypothesis that settling fishes do not discriminate between artificial seagrass units (ASUs) with dense and sparse leaves, but do occur in greater numbers in any type of ASU than on bare sandy substratum. The experiment also tested for the effects of excluding predators on the outcome of settlement patterns. A total of 2360 fishes, from 29 species, settled on the experimental units. Only one species, Achoerodus viridis (Richardson) (Labridae), settled in great numbers. A. viridis did not discriminate between dense and sparse ASUs or between shelter in the form of seagrass and shelter in the form of a cage alone, but did discriminate between shelter (ASUs, cages, or both together) and no shelter (bare sand). Postsettlement predation was insignificant. Patterns of settlement for all other species combined were similar to those for A. viridis. We conclude that abundances of juvenile fishes in isolated ASUs were not due to settlement preferences based on physical complexity of seagrass, or postsettlement predation. Rather, they were due to availability of larvae prepared to settle indiscriminately into any shelter.

Artificial seagrass: How useful is it for field experiments on fish and macroinvertebrates?

Twenty-four artificial seagrass units (ASUs), each of 7 m2, were submerged near Zostera capricorni Aschers, beds for 6 wk. Fish, decapods and cephalopods associated with ASUs and with comparable areas within adjacent Zostera beds were then collected. ASUs yielded significantly fewer species than Zostera areas but there was no significant difference in number of individuals. Relative abundances of species were similar in samples from ASUs and Zostera, and there were few significant differences in abundances of the seven major faunal groups. Our interpretation of these data is that artificial seagrass attracts vague macrofauna typical of real seagrass, that ASUs had fewer species than Zostera only because they had less time to accumulate species, and that ASUs should prove a valuable tool in field experiments on macrofaunal assemblages.