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.

Ichthyofauna of a eutrophic microtidal estuary following engineering intervention  Species composition changed after opening of an artificial subsidiary entrance channel  Increased tidal exchange and persistent high salinities influenced species composition  Species composition also reflects extent of macrophyte abundance  Great value of long-term data for predicting changes in estuarine ichthyofaunas

This study of Pelates octolineatus is the first to use individually aged fish to describe the life cycle of a terapontid, a speciose and abundant Indo-West Pacific family. On the lower west Australian coast, this species uses dense seagrass as a nursery area and, after ~1 year of life when approaching 100 mm in total length (TL), moves into deeper waters over sparser seagrass where it matures at the end of its second year at ~140-170 mm. The maximum TL and age were 256 mm and 10 years. A modified von Bertalanffy curve, allowing for a linear increase in the growth coefficient with age, improved the fit to the lengths at age of older P. octolineatus. Growth was even better described by extending this model to allow for seasonality through incorporating a sine-based curve. This model described well the seasonality exhibited by modal progressions in monthly length-frequency distributions. Instantaneous growth rates, particularly of the youngest age classes, peaked in the warm, summer months and the amplitude of seasonal change in these rates declined with increasing age. Gonadal recrudescence occurred in early spring as temperature and day length increased and spawning peaked in late-spring to mid-summer when temperatures were approaching their maxima.

The biological characteristics of a marine and macrophyte-associated species (Pelates octolineatus) in a large microtidal, eutrophic estuary in 2008-10 were determined. Comparisons are made with those of individuals remaining in coastal waters and during two earlier periods in the estuary when plant biomass differed markedly. P. octolineatus start entering the Peel-Harvey Estuary in mid-summer, soon after metamorphosis, with many remaining there until autumn when they are ~15 months old.These individuals, and older fish that re-entered the estuary in summer, then return to the sea where they spawn from late spring to early summer. Most P. octolineatus in the estuary were less than or equal to the length at maturity and all were <4 years old, whereas individuals up to 10 years old were caught in coastal embayments, emphasising that the estuary acts mainly as a nursery for this terapontid. Growth in the estuary was seasonal and peaked earlier and was greater than in marine waters. Abundance of P. octolineatus in the estuary was greater in 2008-10 and 1980-81 than in 1996-97, when macrophytes were less abundant. The results demonstrate how a marine estuarineopportunist can benefit from using both estuaries and coastal waters as a nursery area and capitalise on variations in environmental conditions.