Department of Water 28 Science supporting estuary management Issue Issue1, February 28, July 2000 2009 Fish diets and food webs in the Swan–Canning estuary Untangling food How do we know CONTENTS Who eats what in the webs helps us what fish eat? Swan estuary ...............2 The construction of a robust food web for an estuary 1. Detritivores ..............2 manage the estuary requires good quantitative data on the dietary 2. Herbivores ...............3 Sound information on the feeding relationships compositions of the various fauna found in that 3. Benthophages .........4 within an estuary is crucial for understanding the environment. These dietary data can be in the form 4. Hyperbenthophages 7 ways in which that estuary functions. Ideally, such of numbers, weights or volumes of the various items food webs describe not only which animal feeds in the gut, which are then typically expressed as a 5. Planktivores .............8 on which food source(s), but how, from a dietary percentage of the corresponding totals for those 6. Piscivores ................8 point of view, each of the main plant and animal items. Most of the information on trophic levels, 7. Opportunists ............9 groups are interrelated. These data, together with feeding groups and food items that is presented in this A generalised food web biochemical studies, then enable the ways in which article has been derived from volumetric dietary data for the Swan estuary ..10 energy is transferred within and among different obtained over the last 30 years by research students trophic levels* to be determined. and staff at the Murdoch University Centre for Fish Summary ...................10 and Fisheries Research. Some of the data is derived The construction of food webs needs to take into Further reading .......... 11 directly from the Swan–Canning estuary and some account the fact that the food source of a species Glossary ....................12 from other south-western Australian estuaries to help often changes as that species increases in body create a more complete food web. Acknowledgments .....12 size. It should also recognise that, while some fish For more information .12 species are relatively specific in the types of food The components of food webs in estuaries can they consume, many of those found in estuaries are be broadly allocated to one of the following four highly opportunistic in their feeding behaviour. Thus, trophic levels. the diets of fish can vary substantially within and 1. Primary producers. Comprise mainly more particularly among estuaries, according to the phytoplankton (microalgae found in the water availability of the different potential food sources. column), benthic micro- and macroalgae, seagrass Reliable food webs provide the types of data that and other aquatic plants. For convenience, detritus, are required for refining environmental management which consists mainly of decomposing plans for conserving the living resources of estuaries. plant material, is also included in this For example, they enable environmental managers category. and scientists to predict the likely impact of the reduction or loss of certain predators or prey on the ecosystem, which may occur through the harvesting of those species or by changes in their habitats. River Science * Technical terms highlighted in blue are defined in the glossary July 2009 at the end of the document. Page 1 2. Primary consumers. Organisms that feed predominantly on primary producers. Examples Who eats what in include: (a) meiofauna, i.e. very small invertebrates, such the Swan estuary as nematodes (small worm-like animals) and Each of the fish species found in the Swan–Canning certain copepods, that live on and in the (estuary estuary and other estuaries in south-western Australia floor) substrate can be allocated to one of the six functional feeding groups listed in Table 1. Note that, in terms of the (b) small benthic macroinvertebrates, i.e. small type of food ingested, some species straddle two invertebrates, such as polychaete worms, groups. For example, a number of large fish species gastropod and bivalve molluscs and amphipod that feed on mobile benthic macroinvertebrates also crustaceans, that live on or in the substrate ingest substantial amounts of fish. (c) epibenthic macroinvertebrates, i.e. larger inver- We now describe the different sources of food for tebrates, such as prawns and crabs, that move more freely over the substrate surface than the each of the above groups and provide selected previous group examples of the organisms, and particularly the fish, which ingest those different sources of energy. (d) fish (e) zooplankton, i.e. very small invertebrates, such as calanoid copepods and the larval stages of 1. Detritivores (feed on other invertebrates, that are found floating in compost) the water column Detritivores feed on decomposing plant material, (f) surficial fauna,i.e. insects that fall on the water which constitutes most of the detritus in estuaries. The surface, such as flying ants detritus is derived mainly from either neighbouring 3. Secondary consumers. Animals that feed mainly land (typically fringing vegetation), tributary on the primary consumers. These comprise mainly rivers or the estuary itself (aquatic macrophytes, small fish species, such as hardyheads and gobies, micro- and macroalgae). The fine detrital particles and larger fish species such as Yelloweye Mullet. found in and on the substrate constitute the main 4. Tertiary consumers. Animals that feed to a food for two fish species that are abundant in the large extent on secondary consumers. This group Swan–Canning estuary, namely the Perth Herring includes not only some of the larger fish species, Nematalosa vlaminghi and the Sea Mullet Mugil such as Mulloway, Yellowtail Flathead and Tailor, cephalus. Detritivores also ingest the considerable but also birds, such as pelicans and cormorants, bacterial load that is associated with detritus and and humans. which can act as an extra energy source. Table 1 Functional feeding groups of fish and other aquatic animals. Feeding group Food category Consumer Detritivores Predominantly detritus Adult Sea Mullet, Perth Herring, King and River prawns Herbivores Mainly macrophytes and filamentous algae Eastern Striped Grunter, River Garfish, and the Black Swan Benthophages Predominantly meiofauna and the more Many fish species, including all whiting sedentary benthic macroinvertebrates species, Estuary Cobbler, Blowfish, gobies, Common Silverbiddy, Western Gobbleguts, Elongate and Ogilby’s hardyhead, Yellowtail Grunter and Blue Swimmer Crab Hyperbenthophages Mobile, larger invertebrates living over the Flounders, flatheads, Australian Herring, Little substrate Black and Little Pied cormorants Planktivores Mainly zooplankton, but occasionally also Fish larvae, jellyfish, Australian Anchovy, phytoplankton. Also flying insects that land on Sandy Sprat and Mullet Hardyhead the water surface Piscivores Substantial amounts of fish Flounders, flatheads, Australian Herring, Tailor, Mulloway, dolphins, cormorants and pelicans Opportunists Diverse range of food ingested (i.e. three Black Bream and Yelloweye Mullet River Science or more of the above food categories) and July 2009 dietary composition sometimes differs markedly between estuaries Page 2 Figure 1 Different stages in the life cycle of Yelloweye Mullet (photograph by F. Valesini, Murdoch University). Figure 2 Juvenile Yellowtail Grunter (photograph by D. Morgan, Figure 3 Tarwhine (photograph by C. Wakefield, Murdoch Murdoch University). University). Coarse detritus is also sometimes ingested by those montagneana and Chaetomorpha linum and the red fish species that consume benthic or epibenthic prey, alga Gracilaria verrucosa. such as the Yelloweye Mullet Aldrichetta forsteri A seasonal study of the dietary composition of (Fig. 1), Yellowtail Grunter Amniataba caudavittata the Yellowtail Grunter demonstrated that, in the (Fig. 2) and the gobies Favonigobius lateralis and upper Swan estuary, the juveniles of this species Pseudogobius olorum. ingested large volumes of algae, and particularly G. verrucosa. However, the extent of this herbivory varied greatly with the time of year, with the 2. Herbivores (vegetarians volumetric contribution of plant material being as high as 50 per cent in summer of the estuary) but only about two per cent in Herbivores feed on plants. The most common spring. macroscopic plants found in south-western Australian estuaries are the seagrasses, e.g. the paddleweed Halophila ovalis, the eelgrass Zostera River Science marina and the river reed Ruppia megacarpa, July 2009 and macroalgae, e.g. the green algae Cladophora Page 3 Figure 4 King George Whiting (photograph by F. Valesini, Murdoch University). Figure 5 Wallace’s Hardyhead (photograph by Figure 6 Southwestern Goby (photograph by D. Morgan, Murdoch University). M. Allen, Murdoch University). Tarwhine Rhabdosargus sarba in the lower Swan However, on the basis of studies elsewhere, the estuary have been found to feed on increasingly River Garfish Hyporhamphus regularis, which was greater amounts of seagrass as they increase in body previously relatively abundant in the Swan estuary, size (Fig. 3). Recent studies have demonstrated that probably feeds largely on macroalgae. sparids, such as the Tarwhine, have the digestive enzymes required to break down this plant material and thus the ability to incorporate (assimilate) the 3. Benthophages (feed off released carbon. the estuary floor) The Black Swan Cygnus atratus, the faunal