but that anadromous species dominated in terms of relative fish abundance. In the Amazon Estuary (Brazil), there was an increasing representation of marine guilds from the Guamá River mouth into the more marine Marajó Bay, but the trophic guilds underwent very little change along the same environmental gradient (Mourão et al. 2014).
Long‐term trends (2003–2010) in the fish assemblage of the Mondego Estuary (Portugal) were analysed from both a life‐history and trophic perspective (Nyitrai et al. 2012). In dry years, estuarine residents were the dominant group but in wet years the MEO species increased in abundance, possibly due to increased volumes of olfactory cues being carried from estuaries into the marine environment as may occur for Anguilla rostrata (Sullivan et al. 2006). When analysed by trophic groupings, the abundances of planktivorous and invertebrate consumers were higher in dry years, whilst the opposite trend occurred with omnivorous fishes. Although the balance between fish trophic groupings in South Africa changed according to zoogeographic region and estuarine typology, all estuaries were dominated by detritivores (Harrison & Whitfield 2012). More recently, Whitfield & Harrison (2020) have used the same comprehensive fish dataset to highlight the lack of redundancy in the various trophic groupings in temperate South African estuaries.
Functional fish guilds have also been used to document estuarine degradation due to anthropogenic pressures. For example, Fonseca et al. (2013) found that, in estuaries subject to chemical pollution, the abundance of marine estuarine‐opportunist species (in terms of life‐history categories) and numbers of piscivorous fishes (from a trophic perspective) were reduced. Indeed, a number of indices have been developed that are based on the functional attributes of fish guilds (e.g. fish community indices used for UK, European, South African and Australian estuaries) and this trend is likely to expand globally (e.g. Harrison & Whitfield 2004, Hallett et al. 2012).
A final example of the value of using a fish functional group approach to comparing fish assemblages in estuarine systems across different spatial and temporal scales is provided by the review of Gess & Whitfield (2020). These authors conducted a detailed analysis of the life‐history styles of fishes from a fossil southern African Devonian estuarine lake habitat and a modern estuarine lake on the subcontinent, and came to the surprising conclusion that assumed advanced forms of fish reproduction, such as ovovipary and vivipary, were more prevalent in estuaries in the Devonian than is the case in estuaries today. Ovipary is the overwhelmingly dominant breeding behaviour of modern estuary‐associated teleosts but the above fossil record suggests a high representation of probable ovoviparous fishes (e.g. coelacanths and placoderms) which are absent from Holocene estuaries. The Actinopterygii, which are dominant in modern estuaries around the world, were poorly represented in Devonian estuary fossils beds but they, together with other extinct fish taxa, also used these systems mainly as nursery areas. Trophic guilds within fish assemblages from the two periods were also similar, with zoobenthivores and piscivores well represented in estuaries across the above geological time scale (Gess & Whitfield 2020).
2.7 Acknowledgements
We thank estuarine ichthyologists from around the world for access to their work and without which this review would not have been possible. We are also grateful for the graphics assistance of Susan Abraham who worked with us on the preparation of figures in this chapter.
2.8 References
1 Able, K.W. 2005. A re‐examination of fish estuarine dependence: evidence for connectivity between estuarine and ocean habitats. Estuarine, Coastal and Shelf Science 64, 5–17.
2 Able, K.W. & Fahay, M.P. 2010. Ecology of Estuarine Fishes: Temperate Waters of the Western North Atlantic. John Hopkins University Press, Baltimore, USA.
3 Able, K.W., Fahay, M.P., Witting, D.A., et al. 2006. Fish settlement in the ocean vs. estuary: comparison of pelagic larval and settled juvenile composition and abundance from southern New Jersey, U.S.A. Estuarine, Coastal and Shelf Science 66, 280–290.
4 Able, K.W., Hagan, S.M., Kovitvongsa, K., et al. 2007. Piscivory by the mummichog (Fundulus heteroclitus): evidence from the laboratory and salt marshes. Journal of Experimental Marine Biology and Ecology 345, 26–37.
5 Able, K.W., Morson, J.M. & Fox, D.A. 2017. Food habits of large nektonic fishes: trophic linkages in Delaware Bay and the adjacent ocean. Estuaries and Coasts 41, 866–883.
6 Aguilar‐Medrano, R., Durand, J.R., Cruz‐Escalona, V.H., et al. 2019. Fish functional groups in the San Francisco Estuary: understanding new fish assemblages in a highly altered estuarine ecosystem. Estuarine, Coastal and Shelf Science 227, 106331.
7 Akin, S., Buham, E., Winemuller, K.O., et al. 2005. Fish assemblage structure of Koycegiz Lagoon‐Estuary, Turkey: spatial and temporal distribution patterns in relation to environmental variation. Estuarine, Coastal and Shelf Science 64, 671–684.
8 Albaret, J.‐J. 1999. Les peuplements des estuaires et des lagunes. In: Les Poissons des eaux Continentales Africaines (ed., Lévèque, C. & Paugy, D. ), pp. 325–349. IRD, Paris.
9 Albaret, J.‐J., Simier, M., Darboe, F.S., et al. 2004. Fish diversity and distribution in the Gambia Estuary, West Africa, in relation to environmental variables. Aquatic Living Resources 17, 35–46.
10 Augspurger, J.M., Warburton, M. & Closs, G.P. 2017. Life‐history plasticity in amphidromous and catadromous fishes: a continuum of strategies. Reviews in Fish Biology and Fisheries 27, 177–192.
11 Ayvazian, S.G., Deegan, L.A. & Finn, J.T. 1992. Comparison of habitat use by estuarine fish assemblages in the Acadian and Virginian zoogeographic provinces. Estuaries 15, 368–383.
12 Ayvazian, S.G., Johnson, M.S. & McGlashan, D.J. 1994. High levels of genetic subdivision of marine and estuarine populations of the estuarine catfish Cnidoglanis macrocephalus (Plotosidae) in southwestern Australia. Marine Biology 118, 25–31.
13 Baker, R. & Sheaves, M. 2005. Redefining the piscivore assemblage of shallow estuarine nursery habitats. Marine Ecology Progress Series 291, 197–213.
14 Balon, E.K. 1975. Reproductive guilds of fishes: a proposal and definitions. Journal of the Fisheries Research Board of Canada 32, 821–864.
15 Barletta, M., Barletta‐Bergan, A., Saint‐Paul, U., et al. 2005. The role of salinity in structuring the fish assemblages in a tropical estuary. Journal of Fish Biology 66, 45–72.
16 Barletta, M. & Blaber, S.J.M. 2007. Comparison of fish assemblages and guilds in tropical habitats of the Embley (Indo‐West Pacific) and Caeté (Western Atlantic) estuaries. Bulletin of Marine Science 80, 647–680.
17 Beckley, L.E. 1985. Tidal exchange of ichthyoplankton in the Swartkops estuary mouth, South Africa. South African Journal of Zoology 20, 15–20.
18 Begg, G.W. 1984a. The Estuaries of Natal. Part 2. Natal Town and Regional Planning Report 55, 631 pp.
19 Begg, G.W. 1984b. The comparative ecology of Natal's smaller estuaries. Natal Town and Regional Planning Report 62, 1–182.
20 Bennett, B.A. 1989. A comparison of the fish communities in nearby permanently open, seasonally open and normally closed estuaries in the south‐western Cape, South Africa. South African Journal of Marine Science 8, 43–55.
21 Blaber, S.J.M. 1974. Field studies of the diet of Rhabdosargus holubi (Pisces: Teleostei: Sparidae). Journal of Zoology, London 173, 407–417.
22 Blaber, S.J.M. 1976. The food and feeding ecology of Mugilidae in the St Lucia lake system. Biological Journal of the Linnean Society 8, 267–277.
23 Blaber,
