Title

Landscape structure influences the abundance, size structure, movements and trophic linkages of the western rock lobster, Panulirus cygnus, in shallow coastal waters

Date of Award

2009

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Natural Science

Faculty

Faculty of Computing, Health and Science

First Advisor

Glenn Hyndes

Second Advisor

Russ Babcock

Third Advisor

Mat Vanderklift

Abstract

Shallow marine landscapes can be viewed as mosaics of habitat patches. The spatial arrangement of habitat patches influences community structure within patches as well as processes linking patches, such as consumer-prey dynamics and movements, by altering the context within which species interact with each other and with their environment. The shallow (<20 >m) coastal waters along the lower west coast of Australia comprise a mosaic of limestone patch reefs and other benthic habitats (such as seagrass meadows, pavement reef and bare sand) and the resulting spatial patterns are known to influence several ecological processes. This study tested whether the spatial arrangement of these habitats influences the density, size structure and trophic linkages of a major benthic consumer, the western rock lobster (Panulirus cygnus). This study also examined the spatial extent of their movements linking patches within this landscape (foraging movements) and from this landscape to more offshore waters (migratory movements).

To examine the influence of patch reef characteristics on lobster density and size structure, visual census was carried out at 14 patch reefs during six periods (three per year) in Jurien Bay, on the mid-west coast of Australia. A range of physical and biological characteristics of the reefs on a landscape scale (i.e. depth and distance from shore and surrounding habitat) and on a local scale (reef biota and ledge availability), were measured. Distance-based redundancy analyses revealed that the depth and distance from shore (i.e. its physical location) were the strongest predictors of the density and size structure of lobsters inhabiting patch reefs. Densities were greatest, and the proportion of _3 year old lobsters was highest, on reefs/or survival of lobsters on these nearshore reefs followed by a movement of larger 4-5 year old lobsters to deeper reefs further offshore. Time of year and management zoning were significant predictors of legal-sized lobster density, with lobsters more abundant in November, before the start of the fishing season and migratory period, and in zones where lobster fishing was banned.

The influence of patch reef characteristics on the diet of lobsters was examined through gut content and stable isotope analyses of lobsters collected from eight patch reefs with three differing surrounding benthic habitats. Mixing model analysis using stable isotopes revealed that the habitat surrounding a reef influenced the long-term diet and nutrition of P. cygnus. Lobsters from reefs surrounded by seagrass (Amphibolis spp. and Posidonia spp.) consumed more mobile invertebrates but less articulated coralline red algae than those from patch reefs surrounded by macroalgae-dominated pavement and sand. Acoustic telemetry confirmed that lobsters use all of these habitats to forage, albeit within close proximity to reef edge (90% of foraging movements were within 60 m). The scale of movements was found to be similar among habitats, but dietary differences suggest that the availability of animal prey and growth rates may differ.

Past research has suggested that a large proportion of pale-shelled 4-5 year old P. cygnus in shallow waters, termed “whites”, undergo offshore migratory movements during November- January. Acoustic tracking of 22 white lobsters using an array of receivers in this study indicated that at least 50% of these whites remained on a shallow inshore reef within a no-take zone over the migration season, with direct evidence that only one individual migrated offshore. This suggests that, in the absence of fishing, many legal-sized whites can remain within shallow coastal landscapes, at least throughout the migration season. Indeed, the proportion of largescale movements made by whites was no greater than that made by dark-shelled ‘reds’, which are commonly believed to display more resident behaviour.

The spatial arrangement of patch reefs within shallow water landscapes clearly plays a role in the ecology of western rock lobsters. The arrangement of habitat patches was a strong predictor of long-term diet and nutrition, highlighting the trophic linkage between patch reefs and surrounding habitat, although the extent of movement into these habitats is limited. In comparison, the influence of surrounding habitat on density and size structure was not as great as that of the physical position of patch reefs. Life-cycle processes that vary over a depth and cross-shelf gradient appear to be more important. The observed patterns suggest that settlement/survival of puerulus larvae and movements by large juveniles are likely to be key processes that influence density and size structure of lobsters. These results suggest that spatially explicit models of the distribution, size structure, trophic-linkages, and perhaps growth of P. cygnus, should incorporate landscape measures, including depth and distance from shore, and configuration of benthic habitats. Results here provide evidence that the densities of legalsized P. cygnus in shallow coastal waters are likely to have been altered through fishing.

LCSH Subject Headings

Edith Cowan University. Faculty of Computing, Health and Science -- Dissertations

Western rock lobster -- Habitat -- Western Australia -- Jurien Bay

Reef ecology -- Western Australia -- Jurien Bay

Dissertations

Access Note

Access to this thesis - the full text is restricted to current ECU staff and students by author's request. Email request to library@ecu.edu.au

Access to this thesis is restricted. Please see the Access Note below for access details.

Share

Paper Location

 
COinS