icolls - Sustainable Tourism CRC

ECOLOGY, THREATS AND MANAGEMENT OPTIONS FOR SMALL ESTUARIES AND ICOLLS
environments (Kitting, Fry & Morgan 1984; Miller, Geider & MacIntyre 1996; Middelburg, Barranguet,
Boschker, Herman, Moens & Heip 2000).
In addition to the influence of sewage discharges on community composition, it is likely that hydrology
(entrance status) may have also influenced the sampled diversity of Tallows Creek in this study. For example,
the presence of some taxa with strong freshwater affinities (e.g. Corixids) in our samples reflects the closed
entrance status of Tallows Creek at the beginning of our sampling. As Griffiths (1999) and Dye and Barros
(2005a) observed, time since connection with the ocean is likely to be a key driving force in determining the
structure of communities in intermittently open estuaries. Our collections, although restricted in space and time,
appear to support these conclusions, although more detailed studies that follow a series of opening and closing
events are required to independently quantify the influence of entrance status on community structure and
composition.
Patterns of Fish Movement Inferred from Nitrogen Isotope Signatures
Many researchers have used stable isotopes to assess movements of biota within and between aquatic ecosystems
(Hobson 1999; Rubenstein & Hobson 2004; Herzka 2005). Whilst most studies use δ 13 C signatures to examine
patterns of movement of consumers between two or more habitats with different source δ 13 C signatures (Cunjak,
Roussel, Dietrich, Cartwright, Munkittrick & Jardine 2005), Hansson et al. (1997) showed that enriched δ 15 N
signatures from sewage effluent can also provide useful information relating to residency and movement of fish
between sites. On the basis of our findings, we propose that the greatly enriched nitrogen isotope signatures of
biota in Tallows Creek can provide useful information on the movement of commercially important fish species
into and out of this system during the intermittent periods of connectance with the ocean (i.e. when the entrance
is artificially or naturally opened).
Schlacher et al. (2005) proposed the use of fish δ 15 N signatures as an indicator in estuarine environments
receiving wastewater. They provided evidence of high site fidelity for a range of estuarine fish species and claim
this residency facilitates their use as an indicator of nutrient inputs. In our study, we confirmed that resident taxa
have elevated δ 15 N signatures in Tallows Creek, a system receiving substantial sewage inputs. However, the
presence of comparatively unenriched individuals in Tallows Creek following an entrance opening event during
our sampling period revealed that recent recruits into the system could be discriminated on the basis of their
comparatively lower δ 15 N signatures. This discrimination between residents and recent immigrants provides
evidence for movements of fish species into Tallows Creek and can provide useful insights into increasing our
understanding of movements between habitats with vastly different nitrogen isotope baseline signatures.
Knowledge of patterns of movement into and out of estuaries is scant for most species. For Mugil cephalus,
there is conflicting evidence for the site fidelity proposed by Schlacher et al. (2005). While Field (1987) reported
that juvenile mullet (including Mugil cephalus) show a high level of site fidelity and tend not to mix with
neighbouring groups or move between neighbouring estuaries, Pusey, Kennard and Arthington (2004) report that
at various points in their life cycle, substantial movements within and between estuaries is a common trait of this
species (particularly on the east coast of Australia). Specifically, individuals have been reported to re-enter the
marine environment (from estuaries) during periods of high rainfall (due to washing out of their detrital food
resources). Pusey, Kennard and Arthington (2004) note that when such movements occur, M. cephalus
individuals tend not to feed and this has important implications for the use of elevated nitrogen isotope
signatures (like those observed in resident biota in Tallows Creek) as potential indicators of movements of M.
cephalus individuals caught offshore. We propose that these characteristics of M. cephalus movements facilitate
their use as indicators of emigration and immigration patterns into and out of neighbouring estuarine habitats,
particularly when heavily 15 N-enriched systems (like Tallows Creek) can leave such a strong and lasting mark
(especially during periods of non-feeding movements) on individual δ 15 N signatures. Pusey, Kennard and
Arthington (2004) indicated that following these ‘wash out’ runs, M. cephalus individuals tend to re-enter the
original estuary or move to a neighbouring estuary (typically to the north). Monitoring stable isotope signatures
of immigrants in neighbouring estuaries (like Belongil Creek in this study) might therefore aid in assessing
movement patterns in this species and the contribution of sewage effluent across broader spatial scales.
For the other species for which we identified new recruits into Tallows Creek (Ambassis marianus), there is
very little information on movement patterns into and between estuaries. However, Pusey, Kennard and
Arthington (2004) indicate that substantial upstream and downstream migrations occur for many freshwater
ambassid species and Broadfoot, Berghuis and Heidenreich (2000) observed upstream migrations of Ambassis
spp (thought to be mostly A. marianus) through a tidal barrage fishway in the Kolan River (in south-east
Queensland). Given that A. marianus inhabits brackish waters along the Queensland and New South Wales
coasts (Grant 2002), it is not surprising that movements within and between estuaries would occur. We propose
that the enriched isotopic signature of Tallows Creek and its resident biota may be able to provide ideal
34