Characteristics of the ichthyofauna of a temperate microtidal estuary with a reverse salinity gradient, including inter‐decadal comparisons

Abstract

Data on the fish fauna of the Leschenault Estuary on the lower west coast of Australia were collected and used as a model to elucidate the characteristics of permanently open estuaries with a reverse salinity gradient, which undergo seasonal changes similar to many other estuaries with Mediterranean climate. Focus was placed on determining (1) the relationships of the number of species, density, life cycle category and species composition of fishes with region (within estuary), season and year and salinity, (2) whether species are partitioned along the lengths of such systems and (3) the extent and significance of any inter‐decadal changes in species composition. The analyses and interpretation involved using multi‐factorial permutational multivariate analysis of variance (PERMANOVA) and analysis of similarity (ANOSIM) designs, and three new or recently published visualization tools, i.e. modified non‐metric multidimensional scaling (nMDS) plots, coherent species curves and segmented bubble plots. The base, lower, upper and apex regions of the Leschenault Estuary, along which the salinity increased in each season except in winter when most rainfall occurs, were sampled seasonally for the 2 years between winter 2008 and autumn 2010. Estuarine residents contributed twice as many individuals, but less than half the number of species as marine taxa. While the numbers of marine species and estuarine residents declined between the base or lower and apex regions, the individuals of marine species dominated the catches in the base region and estuarine residents in the other three regions. Ichthyofaunal composition in each region underwent conspicuous annual cyclical changes, due to time‐staggered differences in recruitment among species, and changed sequentially along the estuary, both paralleling salinity trends. Different groups of species characterized the fauna in the different regions and seasons, thereby partitioning resources among species. The ichthyofauna of the apex region, in which salinities reached 54 and temperatures 36° C, recorded the highest maximum density and, in terms of abundance, was dominated (90%) by three atherinid species, emphasizing the ability of this family to tolerate extreme conditions. Comparisons between the data for 2008–2010 and 1994 demonstrate that the spotted hardyhead Craterocephalus mugiloides and the common hardyhead Atherinomorus vaigiensis had colonized and become abundant in the Leschenault Estuary in the intervening period. This represents a southwards extension of the distribution of these essentially tropical species during a period of increasing coastal water temperatures as a result of climate change. The abundance of weed‐associated species, e.g. the western gobbleguts Ostorhinchus rueppellii and the soldier Gymnapistes marmoratus, increased, whereas that of the longfinned goby Favonigobius lateralis decreased, probably reflecting increases in eutrophication and siltation, respectively.

Baseline survey of the fish fauna of a highly eutrophic estuary and evidence for its colonisation by Goldfish (Carassius auratus)

“…The ability to osmoregulate in a broad range of salinities is advantageous in the Vasse and Wonnerup estuaries as salinities can change from[90 in April to\1 in June (Lane et al 2011). The total number of six fish species recorded during the present study is far lower than that recorded in the shallow, nearshore waters of the nearby permanently open Swan-Canning (61), Peel-Harvey (53) and Leschenault (43) estuaries (Valesini et al 2014;Veale et al 2014;Potter et al 2016). It is also less than the number of species found in estuaries on the south coast of Western Australia (i.e.…”

“…The abundances of the various fish species recorded were dispersion-weighted to down-weight the effects of those species whose numbers exhibited erratic differences among replicate samples due to schooling (Clarke et al 2006), followed by a square-root transformation to down-weight the contributions of species with consistently high values (across replicates within a group) in relation to those with consistently low values (Veale et al 2014). The two replicate samples from each site were averaged and the resultant pre-treated data used to construct a Bray-Curtis resemblance matrix which was, in turn, subjected to a one-way analysis of similarities (ANOSIM) test (Clarke and Green 1988) in PRIMER v7.…”

“…The prevalence of these species in estuaries, particularly those with a seasonal connection to the ocean, has been reported elsewhere in south-western Australia Valesini et al 2009). The large abundances of these species in these systems, many of which undergo marked changes in salinity ), reflect the fact that these species are highly euryhaline (Hoeksema et al 2006;Veale et al 2014). The ability to osmoregulate in a broad range of salinities is advantageous in the Vasse and Wonnerup estuaries as salinities can change from[90 in April to\1 in June (Lane et al 2011).…”

“…This result reflects the fact that (1) the faunas of both estuaries were dominated to a similar extent by the same three species, (2) the marked variability in the densities of fish recorded at the various sites within either estuary and (3) that the environments of both estuaries were broadly similar at the time of sampling. Although salinities were significantly higher in the Vasse (18.3) than Wonnerup (15.7) Estuary, these differences were not great enough to inhibit colonisation by the three dominant species, all of which are euryhaline (Hoeksema et al 2006;Veale et al 2014). Furthermore, although three of the six species recorded were unique to a particular estuary, none of these species occurred in appreciable densities; for example, F. lateralis was only caught at a single site in the Wonnerup Estuary and constituted only five individuals.…”

Dolphin Prey Availability and Calorific Value in an Estuarine and Coastal Environment

“…documented since the 1970s using small seine nets (Potter et al, 2000;Veale et al, 2014). The major changes in the Inlet include increases in species associated with warmer water and macroalgae, which correspond to increasing sea temperatures and macroalgal cover.…”