Interactions between dugongs and seagrasses in a subtropical environment

This study investigated the ecology of dugongs in Moreton Bay, a 1400 km2 embayment in sub-tropical south-east Queensland (27.5" S, 153.3" E). The dugongs' distribution, movements, home range, habitat selection, feeding, diet and food preferences were examined in relation to the seagrass and physical resources. I considered the ways in which dugongs affect the seagrasses and the significance of the dugongs' role in the seagrass system. Seagrasses were quantitatively mapped in two study areas, encompassing 133 krn2 of seagrass. Seven species of seagrass formed 15 recognised communities, which collapsed into five community-groups. Communities dominated by species of Halophila were the most widespread, covering 51 % of the total area of seagrass, but they were characterised by low biomass, and accounted for only 9% of the total standing crop of seagrass (total = 12,808 tonnes dry weight). In comparison, communities dominated by Zostera capricorni (broad-leaf morph) occupied only 38% of the area of seagrass, but contained 75% of the seagrass standing crop. The annual, above-ground production of seagrass within the study areas was estimated to be 41,728 tonnes dry weight. Zostera capricorni, the dominant species in terms of biomass (57.5% of total above-ground standing crop), accounted for 34% of the total, while the species of Halophila accounted for 36.6 % , despite representing only 20.5 % of above-ground standing crop. The distinct seasonality of Moreton Bay was highly correlated with pronounced changes in seagrass abundance. There was a distinct summer/autumn peak in the abundance of most species. Zostera capricorni alone had a winterlspring growth period. Averaged across all species, shoot density, above-ground and belowground biomass changed by factors of 1.9, 2.3 and 2.3, respectively, between seasons of minimum and maximum abundance. Based on 28 aerial surveys, I estimate a Moreton Bay population of 600 dugongs. During spring and summer the dugongs were relatively sedentary, however, during winter the dugongs undertook regular migrations between the feeding areas inside the Bay and the oceanic waters east of the Bay (15-40 km round-trip). By riding the flood and ebb tides in and out of the Bay, the dugongs could exploit the temperature differential of up to 5" C between areas. Sometimes they spent as little as 1.5 hr, during the top of the tide, feeding in the Bay. Some dugongs stayed in the warm water outside the Bay for days at a time in winter. Thirteen dugongs occupied an average range of 64 km2 during the periods they were satellite-tracked (mean = 50 days). Within their home range, some dugongs sequentially used distinct sub-ranges, in which they concentrated their activities for periods of up to 35 days. Some areas were rarely, if ever used by dugongs, while other areas were

Name dc:type.qualificationname

phd

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

James Cook University

Year dc:date.issued

1992

Author dc:creator

• Preen, A. R.

Language dc:language

en