Date of Award

2002

Degree Type

Thesis

Degree Name

Bachelor of Science Honours

Faculty

Faculty of Communications, Health and Science

First Advisor

Dr Glenn Hyndes

Abstract

Three distinct seagrass habitats were sampled to determine whether fish assemblages differed between meadows comprising of different seagrass species with different morphological characteristics and whether plant morphology influences species assemblages. Three seagrass habitats consisting of Posidonia sinuosa, Posidonia coriacea and meadows of a mixture of P. coriacea and Heterozostera tasmanica in the Success Bank region, off the coast of Fremantle, Western Australia were selected. For each habitat, sampling was carried out using a 1m wide beam trawl over a distance of 50m at six replicated locations, on three occasions between June and September 2002. Seagrass samples were collected at each location. On each sampling, occasion a 0.025m2 quadrat was used to collect data on leaf area index (LAI), seagrass density and biomass, leaf width and length, and epiphytic algal biomass. MDS ordinations and ANOSIM showed that P. sinuosa habitats contain a significantly different composition of fish species to P. coriacea habitats (with or without H. tasmanica). Species composition was similar in P. coriacea on its own or mixed with H. tasmanica. SIMPER showed that the; differences between P. sinuosa and the P. coriacea habitats reflected the greater abundances of Stigmatopora argus, Siphonognathus radiatus and Scobinichthys granulatus, whereas Stigmatopora nigra was restricted to the P. coriacea habitats only. ANOVA demonstrated that total densities and biomass of fish and species richness were greatest in P. simwsa, while little difference occurred for those variables between the twn P. coriacea habitats. ANOVA indicated that densities of S. argus and Siphamia cephalotes did not differ between habitats, while densities of S. radiatus and S. nigra differed between P. sinuosa and the P. coriacea habitats. Densities of S. granulatus differed only between P. sinuosa and P. coriacea with H. tasmanica. Seagrass leaf density, leaf area index (LAI), leaf width, dry seagrass biomass and dry epiphytic biomass differed significantly among the three habitats, where as leaf length did not differ between these habitats. Regression analysis indicated that leaf area index influenced species richness, fish abundance and biomass, while leaf width influenced the abundance of S. argus and S. radiatus, and leaf density influenced the abundances of S. nigra and S. granulatus. BIOENV revealed that leaf width, leaf density and LAI influenced the fish composition in the seagrass meadows. Specific plant features appear to influence the fish assemblages associated with these habitats. Plant morphology also separated size-Classes of an abundant seagrass species supporting the "nursery habitat" theory. Artificial seagrass and live animals were used in laboratory experiments to evaluate habitat preference of the most abundant seagrass-associated fish species in the absence of predators and food, and to determine whether juvenile and/or adult-sized fish exhibit a preference for a particular seagrass morphology, corresponding to those of P. sinuosa, P. coriacea and H. tasmanica. Habitat preference experiments were conducted separately for each size class in three experimental aquaria containing artificial seagrass to simulate three different seagrass habitats. Each aquarium contained two different seagrass habitats and ten fish of the same size class. Habitat preference observations were made at hourly intervals over a 1 0-hour period. Three replicates were conducted for each experiment. The laboratory experiments showed that both juvenile and adult-sized S. argus had a strong preference towards the narrow leaves of P. coriacea and particularly H. tasmanica. However, the preference was more pronounced for the juvenile fish. Thus, seagrass morphology, specifically leaf width, appears to play a significant role in the habitat selection of S. argus. However, the ability to avoid predation is equally dependant on their body shape, size and ability to mimic their surroundings. The findings described in this study have shown that plant morphology appears to play a significant role in influencing fish faunal assemblages associated with seagrass meadows. The results of this study have clear implications for the environmental management of coastal marine ecosystems, highlighting the need to conserve seagrass meadows of different plant morphology to maintain the biodiversity of the fish assemblages in those regions.

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