Trophic interactions among the macrofauna of an eelgrass community

Robertson, Alistar Ian.

Trophic interactions among the macrofauna of an eelgrass community

Authors

Robertson, Alistar Ian.

Date

1978

Publisher

University of Melbourne

Affiliation

Science Collected Works

Full item page

Document Type

PhD thesis

Access Status

Only available to University of Melbourne staff and students, login required

URI

https://hdl.handle.net/11343/341010

Description

Typescript (photocopy)

Dept. of Zoology

Thesis (PhD) -- University of Melbourne, Faculty of Science, 1979

Includes bibliographical references.

Abstract

This project investigated the dynamics of the trophic interactions between the macrobenthos and the predatory fish of an intertidal eelgrass community. The aim was to provide information on the ecology of many macrofaunal species inhabiting eelgrass flats in Westerport Bay; illustrate the dependence of fish species on these areas; and indicate the factors which are important in maintaining the structure of eelgrass communities. Investigations of the population dynamics of the 15 most abundant fish species showed that four species were permanent residents of intertidal flats, nine were tidal transients (i.e. move over the flats with incoming tides) and two were seasonal residents. Ten species breed over or near the eelgrass flats; while the remainder are represented only by juveniles. These areas are therefore important as nursery areas for immature fish of many species. The number and biomass of fish exhibited definite diel and seasonal changes. Of the 15 major species, nine were diurnal and five nocturnal inhabitants of eelgrass flats. Only one species was present in equal numbers during day and night. The total number and biomass of fish was lowest during the winter when most species moved away from the intertidal flats, presumably as a result of decreased food availability. Numbers and biomass increased during spring and summer (although at different rates) due to return of large fish and recruitment of 0+ aged fish from spring breeding periods. Maximum numbers and biomass were encountered during autumn, when there was the greatest density of large fish as well as 0+ aged recruits. Analysis of feeding relations showed that many fish depend on eelgrass flats for large proportions of their diets. Most, fish were carnivores and crustaceans were the major dietary group. Three species were omnivores consuming algae and detritus in addition to animal foods. Only one species, which was present in low numbers, consumed live eelgrass to a significant degree. Apart from permanent ontogenetic dietary shifts in the juveniles of some species, the diets of most fish exhibited seasonal changes which generally came about through fluctuations in the proportions of a few major prey types, rather than through prey switching. Most fish fed in distinct habitat patches within the eelgrass system, and exhibited marked patterns of food resource partitioning. The study of production by the major infaunal and epifaunal macrobenthic taxa indicated that the animals which are epifaunal on or among the eelgrass fronds are more productive than the infauna. Among the infauna, the active crustaceans are more productive than the sedentary infaunal polychaetes, and this was probably a result of the higher levels of predation on active crustaceans. Total secondary production in heavily-grassed areas of the eelgrass flats was found to be almost twice that in bare mud and lightly-grassed areas. Measurement of the rate of food consumption by the major fish species indicated that most of the food for the fish fauna came from bare mud and lightly-grassed areas of eelgrass flats, and predation pressure (defined as the amount of secondary production removed by predatory fish) was much higher in these areas than in heavily-grassed areas. It was concluded that although physical stresses and the differing above-ground biomass of eelgrass in regions of the intertidal flats set limits on the number and type of species that can inhabit different habitat patches within the eelgrass community, predation by fish was probably the major factor influencing the structure of the macroinvertebrate guild of bare mud and lightly-grassed areas of eelgrass flats. Competition for space and food has probably played a major role in structuring the invertebrate guild of heavily-grassed areas. The distinct pattern of resource allocation among the species of the fish guild, as well as the morphological specializations for feeding exhibited by most species were explained as responses to interspecific competition for food. The habits of only one fish species appeared to be controlled primarily by predation.

Keywords

Marine ecology -- Australia -- Victoria -- Western Port, Fishes -- Australia -- Victoria -- Western Port.

Collections

Science Collected Works - Theses