Do spatial differences in the abundance of two serranid fishes in estuaries of tropical Australia reflect long-term salinity patterns?

Abstract

Epinephelus coioides and E. malabaricus are closely related species of serranid fishes. They are similar in appearance, occupy the same habitats and are represented in estuaries of northeastern tropical Australia almost entirely by juveniles. Fish traps were used to sample these 2 species from 3 estuaries on the northeast coast of tropical Australia over a 2 yr period Patterns of distribution. abundance and size structure were compared. Despite the sirmlarities between the 2 species, they showed quite different patterns of distribution and abundance, both within and between estuaries. In seaward parts of the estuaries the numbers of E. coioides were similar to or greater than those of E. malabaricus. However, in upstream areas E. malabaricus became much more abundant than E. coioides. The reasons for this change in dominance are unknown. However, upstream areas of the estuaries exhibited extreme levels of salinity (either hypersaline or hyposahne) and catches of the 2 species correlated with the maximum deviation in salinity. This suggests that i f salinity, or some correlated variable, does influence the abundance of the 2 species, it is through long-term effects rather than shortterm changes. Regardless of the reason, these spatial differences in abundance suggest that E. malabaricus is better equipped to deal with the challenges of estuary life than is E. coioides.

Ecological drivers of spatial variability among fish fauna of 21 tropical Australian estuaries

“…Both wet and dry tropics experience extensive seasonal changes in salinity, with the real difference being the extent to which the wet and dry phases dominate. In this case, it might not be surprising that fish fauna adapted to meet the challenges of living in a rapidly fluctuating envi-h r r r S i n u o s i t y T i d a l r a n g e I n t e r t i d a l a r e a S u b t i d a l a r e a M a n g r o v e a r e a S a l t m a r s h a r e a E n t r a n c e w i d t h C l o s e s t e s t u a r y S e d i m e n t i n d e x E s t u a r y c l a s s i f i c a t i o n E n t r a n c e s t r u c t u r e I n f l u e n c e a t m o u t h ronment, with periodic saline and freshwater phases (Sheaves 1996), is not greatly influenced by salinity regimes. The lack of response to proximity is more difficult to explain but indicates local-level responses of a widespread fauna at the scale of estuaries rather than regions.…”

“…Site-level sinuosity involved categorising each site as comprising a straight section of channel, a medium angled bend, or a sharp angled bend. Although salinity and turbidity only represented snapshots of conditions at each site, and were thus likely to be poor indicators of important trends (Sheaves 1996), they were included as the best available indicators of the physical conditions prevailing at the sites at the time of sampling. To characterise the similarities and differences between estuaries, the relationship between site-scale physical variables (averaged over reaches) and spatial Table 2.…”

Temporal dynamics of fish assemblages of natural and artificial tropical estuaries

“…In contrast, there have been few attempts to systematically study the temporal dynamics of these assemblages, and those which have been made have usually been limited in temporal resolution, and have provided only coarse-scale temporal information (Sheaves 2006), or fine detail confined to a single estuary system (Robertson & Duke 1990a). Other studies that have included a temporal component have been primarily concerned with spatial factors (Blaber et al 1989, Sheaves 1998, physical factors (Cyrus & Blaber 1992, Sheaves 1996b, attributes of estuaries (Ley 2005), or fisheries management status (Ley et al 2002). Robertson & Duke (1990a) made a detailed temporal assessment of the fish assemblage of Alligator Creek in tropical eastern Australia.…”

Scale-dependent variation in composition of fish fauna among sandy tropical estuarine embayments

“…For instance, even within Australia's north-eastern tropics, some areas are characterised by substantial rainfall over most of the year, while areas only 100 km away may display marked seasonality, with heavy rainfall confined to a brief wet season each year. Consequently, estuaries in these areas range from wet-tropic habitats, with reduced salinities over much of the year, to dry-tropic habitats that have only a brief period of depressed salinity and which can progress rapidly to hypersaline conditions during the 'dry' season (Sheaves 1996). Additionally, tropical estuaries provide a diversity of habitats, including mangrove forests, intertidal and subtidal seagrass beds, sand and mud banks, deep channels, steep 'snaggy' banks, rocky outcrops and open-water environments.…”