coastal marine waters as alternative nursery areas
In contrast, those fish taxa with euryoecious tolerances (i.e. wide tolerances to several environmental variables), especially euryhaline characteristics, are best adapted to an estuarine existence (Whitfield 2019). For example, both estuarine (ES) and marine estuarine‐opportunist/dependent (MEO/MED) species are euryoecious and have the ability to tolerate the spatially and temporally widely varying conditions found within estuaries. This feature was shown by the multivariate analysis of salinity tolerances amongst estuarine fishes carried out by Bulger et al. (1993), where estuarine salinity regions were defined according to their biological characteristics. In that analysis, many estuarine species were shown to have a wide salinity tolerance such that salinity regions in estuaries were overlapping in distribution. One family with species able to persist in extremely high salinities are the atherinids, with Atherinosoma elongatum and its congener Atherinosoma microstoma being recorded in salinities up to 150 in estuaries in southern Australia (Tweedley et al. 2019). Moreover, another atherinid, Menidia beryllina, was observed in salinities of 120 in the Laguna Tamaulipas, Texas, USA (Copeland 1967).
Although most MEO taxa use estuaries opportunistically, some marine species are dependent on estuaries during their juvenile stages and are therefore not using estuaries opportunistically. Ray (2005) emphasises the need to determine which fishes ‘must’ use estuaries (i.e. obligate dependents), and which therefore will be at risk if estuarine habitats are lost, from those which ‘may’ use estuaries (i.e. facultative dependents). Whilst Elliott et al. (2007) did not distinguish between the above two guilds and treated them together as a marine migrant category, Potter et al. (2015a) divided them into marine estuarine‐opportunist and marine estuarine‐dependent (Table 2.2, Figure 2.10).
From the above, it is apparent that marine estuarine‐opportunists are able to use alternative marine nursery areas, whereas marine estuarine‐dependents do not have alternative suitable nursery habitats nearby. For example, the 0+ juveniles of Mugil cephalus in Western Australia are almost exclusively found in estuaries in south‐western Australia, where there are numerous rivers (Lenanton & Potter 1987, Chuwen et al. 2009). However, similar sized juvenile M. cephalus are also abundant in nearshore waters of regions further north where there are no estuaries. Hence, this species does not have to rely on estuaries as a nursery area, but uses them opportunistically when they are present. Similarly, juvenile Pleuronectes platessa in the North Sea use estuaries opportunistically as nursery grounds, but also employ other suitable shallow, sandbank habitats (Elliott & Hemingway 2002). In contrast, 0+ juveniles of Rhabdosargus holubi are abundant in South African estuaries but seldom recorded in adjacent marine waters (Wallace et al. 1984). The terms opportunist and dependent therefore illustrate fundamental differences in the importance of estuaries to particular species (Blaber et al. 1989).
Estuaries provide essential routes for the migration of diadromous species (Elliott & Hemingway 2002, Able 2005). The term diadromy is taken here, and by others such as McDowall (1988), not to imply a tolerance to stable, variable or low salinities, but rather an ability of a fish to change its physiology while moving between water bodies of different and stable salinities. Hence, the classical diadromous species such as anguillid eels moving from freshwater to seawater to breed (catadromy) and salmonids and lampreys moving in the opposite direction (anadromy) undergo a major physiological adjustment
