the assumption that most freshwater and marine fish species have relatively free access to the abundant and diverse food resources and habitats provided by estuaries is a fallacy that is not supported by ichthyological studies that traverse these ecosystem boundaries or transition zones (Barletta et al. 2008).
All fish species have a set of well‐defined tolerances to environmental characteristics, and those tolerances may remain constant or change during their lifecycles, for example the ability by some taxa to live for part of their lives in the sea, estuary or catchment. Most fish species, especially those which have life cycles closely tied to either rivers or the sea, are generally confined to those particular aquatic environments and seldom stray into estuaries (Whitfield et al. 2017). However, those taxa that are able, because of their tolerances, to traverse the ecological divisions have access to highly productive waters that provide ideal nurseries and adult feeding grounds for a variety of fish trophic groups (Elliott & Hemingway 2008).
Table 1.1 Geographic divisions, salinity ranges and zones (Venice System) and the approximate distribution of different categories of ichthyofauna found in a ‘generalized’ estuary
(modified from an estuarine invertebrate concept published by Carriker 1967).
| Estuary divisions | Salinity ranges | Salinity zones | Fishes and approximate distribution range within estuaries | ||||
|---|---|---|---|---|---|---|---|
| River reaches | <0.5 | Limnetic |
Anadromous migrants |
Freshwater taxa |
  |
 |
 |
| Estuary head | 0.5–5 | Oligohaline | |||||
| Upper reaches | 5–18 | Mesohaline | |||||
| Middle reaches | 18–25 | Polyhaline | |||||
| Lower reaches | 25–30 | Polyhaline | |||||
| Estuary mouth | 30–40 | Euhaline |
There is little doubt that the highly variable and sometimes widely fluctuating physico‐chemical conditions in estuaries are a challenge to many marine or freshwater fish species that would otherwise make extensive use of these systems (Haedrich 1983, Cowan et al. 2013). However, those taxa that are euryoecious (i.e. a wide ability for various variables, including euryhaline for salinity and eurythermal for temperature) and eurytopic have been able to exploit estuaries and have the benefit of occupying one of the most productive natural aquatic ecosystems on the planet (Day et al. 2013). This gave rise to the idea of the stress‐subsidy phenomenon in which a highly variable environment is stressful for those species not able to tolerate the conditions but a subsidy for those able to tolerate the varying conditions, and thus thrive (Elliott & Quintino 2019).
Some marine species have become so closely associated with estuarine systems that they are termed ‘estuary dependent’, especially in terms of nursery area use by the juveniles (Miller et al. 1984). This term has been widely and often loosely used to refer to a wide range of fish species that are found in estuaries (Able 2005). In reality, there is a cline in the association by fish taxa found in estuaries (Figure 1.1), ranging from those species that are completely dependent on estuaries for at least part or all of their life cycle, to those fish that are most abundant in either marine or freshwater environments and only use estuaries opportunistically to varying degrees (Elliott et al. 2007, Potter et al. 2015, Whitfield 2020).
The occupation of an estuary by a particular fish species depends on the availability of a suitable niche for that species. As an initial model, Figure 1.2 summarises the way in which a niche is produced and then occupied by fishes within estuaries, but it also illustrates that most of the driving variables are prone to anthropogenic impacts from a variety of sources. The nature and extent of these impacts feature in a number of chapters within this book (see Chapters 6–11 in particular) and highlight the major role that humans play in determining the future of estuarine fish assemblages going forward. In reality, the full range of abiotic and biotic drivers of fish assemblage structure is very complex and diverse, with a simplified depiction of known factors influencing southern African fish communities being shown in Figure 1.3. Despite having worked on some of these drivers for several decades, it would be true to say that, in many instances, our current understanding of the detailed influences on estuarine fish communities is still in its infancy.
Figure 1.1 Diagrammatic representation of the gradual change in the level of estuary association by fish guilds not dependent on estuaries (lower left) to those completely dependent on estuaries (upper right) (after Whitfield 2020).
1.2 Reasons why this synthesis is important
Anthropogenic impacts on estuarine morphology and hydrodynamics are significant and widespread. Human modifications of riverine flow (e.g. major dams) and estuarine morphology (e.g. canalization) have the ability to substantially modify or completely disrupt natural ecoclines between catchments and the sea (de Groot 2002). These aspects then give rise to important and fundamental questions (Box 1.1).
To what extent are human developments and activities transforming estuaries into systems that even eurytopic fish species will find difficult to colonise?
Have certain systems already reached that tipping point and been switched into habitats that are not able to support viable estuarine fish assemblages?