between selected environmental and fish variables in estuaries. An asterisk denotes those variables that are often influenced by anthropogenic activities
(modified from Whitfield & Elliott 2002).
Evidence from estuaries such as the Mersey and Thames (UK) suggest that we have had such situations arise in the past due to gross mismanagement, particularly in terms of sewage, industrial and agricultural waste inputs, but that recovery of estuarine fish populations is possible when the degradation process is reversed (Jones 2006, Elliott & Hemingway 2008). Indeed, the Thames lost virtually its entire fish community through gross sewage pollution up to the 1960s but has since recovered to now carry more than 120 species.
Estuaries have recently been characterised as being exposed to a ‘triple whammy’ of threats – of increasing industrialisation and urbanisation, of increasing use or loss of resources (such as space, energy and biological materials such as fisheries) and of the increased threats of climate change including sea‐level rise, acidification and invasive species (Elliott et al. 2019). Whilst this has led to the declining levels of natural estuarine ecosystem functioning which is a characteristic of many estuaries around the world, there are encouraging signs of rehabilitation and restoration being practised on an increasingly larger scale in several countries (Elliott et al. 2016). This will ultimately have a positive effect on the recovery of estuary‐associated fish populations in the affected systems (Andrews & Rickard 1980, Able et al. 2008).
Figure 1.3 Biotic and abiotic factors influencing fish assemblage composition in southern African estuaries. The scale on the left‐hand side illustrates the trend from predominantly abiotic variables in the top of the diagram to mainly biotic variables at the bottom (after Whitfield 2019).
Whilst the above improvements at the habitat and pollution levels are most encouraging, there also needs to be a major focus on reducing fishing pressure on targeted angling and commercial fishery species. In many parts of the world, previously abundant estuary‐associated taxa have been reduced to less than 5% of the original spawner‐stock biomass due to over‐exploitation (Griffiths 1997). Any reduction in fishing effort is often difficult to enforce because of growing human populations and an increasing dependence on coastal fish populations to provide the protein requirements and employment opportunities for people residing within the coastal zone (Blaber et al. 2000, Wolanski et al. 2019).
There is also a poorly based perception in the public domain that aquaculture or mariculture in estuaries may be an alternative route to the production of natural resources for human consumption within these systems. Increasingly, we are becoming aware of the negative consequences of intensive, artificial raising of fish stocks in coastal waters (Silvert 1992). Apart from the transformation and loss of natural habitat and ecology as a result of this type of aquatic ‘farming’, the pollution, disease and genetic contamination consequences are serious and deserve adequate consideration before embarking on such exploits. Indeed, the first prize in terms of estuarine and coastal fish protein production are healthy and optimally exploited wild stocks that will continue to provide quality food for the long‐term future.
Primary constraints to achieving the above goal in some estuaries are deforestation and degradation of river catchments, dams without protection/maintenance of environmental flows built into their design, port developments and extensive dredging that alters productive littoral habitats, unplanned and poorly serviced urban settlements near estuaries and industrial estates that ignore or deliberately abuse environmental pollution legislation. In some large systems, nutrient over‐enrichment and resulting eutrophication, hypoxia and habitat loss are a major threat, e.g. in Chesapeake Bay nutrient overload is viewed as the most important constraint in maintaining or restoring estuary health (Cooper & Brush 1993).
Figure 1.4 Diagram showing some of the potential environmental modifications to fish habitats and life‐history stages brought about by estuarine and coastal marine global change (after Whitfield 1996).
Plastic debris, heavy metals and persistent organic pollutants (POPs) in particular are remaining an increasing problem in estuaries and pose a severe threat to the health of fish within these systems (Ferreira et al. 2005, Oliva et al. 2012). Bioaccumulation and biomagnification of metals and POPs are a major concern, as well as the trophic transfer of microplastics (Vendel et al. 2017), which are having a negative impact on the integrity of estuarine systems and their associated ichthyofauna. This is an extremely short‐sighted sequence of events since humans are major end users of fish associated with estuaries. Overall, there is a sequential model that starts with impacts on habitats in both estuarine and marine environments and ends up negatively influencing all life stages of estuary‐associated fish species (Figure 1.4).
The above threats are what may be termed endogenic pressures in that they occur mainly within estuaries and thus their management has to tackle both the causes and consequences of change (Figure 1.5); however, we also need to consider those exogenic pressures which emanate from outside the estuary (e.g. climate change) and where management has to deal with the consequences (Elliott 2011). Therefore, if one adds the impact of human‐induced accelerated climate change and invasive species introductions to the environmental and biotic pressures on fish in estuaries, then the conclusion has to be reached that urgent management attention is required to assist the continued viability of estuaries from a fish and fisheries perspective. This applies particularly to the ichthyofauna located in proximity to major coastal towns and cities where pollution pressure is often greatest (Nie et al. 2005). The breakdown in connectivity between freshwater, estuarine and marine systems is also an area of major concern, particularly the impact of structures such as dams, weirs and barrages on diadromous species (Hall et al. 2012).
Figure 1.5 Diagrammatic representation of the hierarchical responses of fishes along a time‐response scale to environmental stressors, with all levels of fish response needing to be taken into account by estuarine managers
(modified from Adams 2005).
Over‐exploitation of coastal fish stocks is a global phenomenon, and it has been argued that estuaries in particular should be protected from this pressure, mainly because these habitats are the nursery grounds for a large number of fish species. Indeed, many countries have Marine Protected Areas (MPAs) and similar designations such as Special Areas of Conservation (SACs) and No‐take Zones, where fish within these areas are protected from exploitation. However, the creation of similar Estuarine Protected Areas (EPAs) are less well developed and, where they do occur, recreational and sometimes even subsistence fishing is often permitted. If one compares the fishing and wider anthropogenic impacts on estuaries compared to the sea, especially given the severity of pressures on a unit area basis, then estuaries should be prioritized before the marine environment in terms of the creation of new coastal protected areas.
1.3 Estuary definition and types
In this book, we have defined an estuary as follows: “An estuary is a semi‐enclosed