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The significance of macroalgae to the diets of juvenile fish and ecosystem function in a tropical coral reef lagoon

Desfosses, Cameron (2016) The significance of macroalgae to the diets of juvenile fish and ecosystem function in a tropical coral reef lagoon. Honours thesis, Murdoch University.

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Little information is available on the contribution of macroalgae to the food web of the Ningaloo lagoon and its importance in the diets of fish associated with it. This information is important for understanding potential trophic flows from macroalgae to juvenile fish and provides the fundamental data for constructing ecosystem models.

In my Honours research, I have examined: the significance of macroalgae, its associated epibionts and infauna in the diets of juvenile and subadult fish in the Ningaloo lagoon, focussing on:
1. the significance of macroalgae in the diets of juvenile fish and how this varies between summer (February) and winter (July) (Chapter 3); and
2. the development and use of an Ecopath with Ecosim ecosystem model to assess trophic flows of macroalgae to functional fish groups (Chapter 4).

Fish were sampled in macroalgal beds by a variety of techniques (herding into fence nets by SCUBA, hand spear, and rod and line fishing) in February and July, 2015. A total of 181 fish were caught representing 11 species, with six species caught in both months. Stomach contents were identified to the lowest taxonomic resolution possible and the percent volume of items recorded. Multivariate analyses were used to identify guilds (species with similar diets), and to assess differences in diets between February and July.

The results showed that: fleshy macroalgae (e.g. Sargassum spp.) were not as important as filamentous algae to the diets of the juveniles of two nominally herbivorous fish species in February, but became more important in July. Macroalgae were not an important component in the diets of juvenile Lethrinidae, Lutjanidae, or Mullidae, though the infauna associated with the macroalgal beds was important in the diets. Analysis of feeding specialisation found that the smaller size classes of fish in February had a narrower trophic width and a more specialised feeding strategy than larger bodied fish of the same species in July. At this time, fish tended to show a generalist feeding strategy and broad niche width, possibly associated with increasing gape size of the larger fish sampled at this time. These studies provided the basis for defining three distinct functional feeding groups for the Ecopath model: herbivores, zoobenthivores, and carnivores (Chapter 4).

The Coral Bay Ecopath model was constructed by modifying an Ecopath model for the Ningaloo system and applying it to an area of the macroalgal beds to examine trophic flows from macroalgae to higher trophic levels. The Coral Bay model had 29 functional groups based on the functional fish feeding groups (adults and juveniles of herbivores, zoobenthivores, carnivore, Lethrinus species and Lethrinus nebulosus - 10 groups) and broad dietary categories (13 groups) identified in Chapter 3, a competitor for algal resources (e.g. turtles - 1 group), predators to the fish groups (e.g. reef sharks and dolphins - 2 groups) and extra groups that were included due to having different functional roles (e.g. phytoplankton, squid and octopus - 3 groups). The model was balanced by adjusting biological parameters, with an emphasis on changing those with the fewest data from the region the most. Macroalgae were the dominant primary producers in the system, and comprised more than 70% of the total consumption of trophic level I groups.

Ecopath with Ecosim was used to evaluate the effects of three categories of disturbance, (17 scenarios), affecting primary production, fishing effort and simultaneous changes in both primary production and fishing effort. The results from these predicted that changes in the rate of primary production had a much larger effect on the biomass of functional groups within the Ningaloo lagoon than changes to fishing effort. Since this model was developed with a focus on trophic understanding and not fishing, and it was not possible to tune the model with data from recreational fisheries, therefore, the predictions from the scenarios involving fishing effort should be treated with caution.

The results from this study show that the macroalgal beds are important in the diets of some fish species and contribute to trophic flows in the Ningaloo lagoon. This adds to the understanding of their function as habitat for different species and highlights the value of including them in conservation planning for the Ningaloo Marine Park.

Item Type: Thesis (Honours)
Murdoch Affiliation(s): School of Veterinary and Life Sciences
Supervisor(s): Loneragan, Neil
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