Influence of sectioning otoliths on marginal increment trends and age and growth estimates for the flathead Platycephalus speculator

Comparisons are made between estimates of ages and growth of the flathead from a temperate Western Australian estuary, using data obtained from whole and sectioned otoliths. Use of whole otoliths frequently underestimated age by one year in 2+ to 4+ fish and two years in 5+ to 10+ fish, and by as much as five or six years in the oldest fish (11+ and 12+). The respective 95% confidence limits for the parameters L∞, K, and To in the von Bertalanffy growth equations for males, calculated using data from sectioned otoliths, overlapped those calculated from data for whole otoliths, and the same was true for K with females. This similarity in growth curves in particularly the first four years of life can be attributed to the fact that c74 and 65% of the growth of males and females, respectively, occurred in the first three years, when under-estimates of age were limite

Habitat, reproductive biology and size composition of Parequula melbournensis, a gerreid with a temperate distribution

Trawling was carried out over sandy substrates in the shallow (5 15 m) and deeper (20 35 m) waters of four regions along 250 km of the lower west coast of Australia, during seven consecutive seasons. This yielded 32 752 individuals of the gerreid Parequula melbournensis, which constituted c. 42% of the total number of fish. Densities of P. melbournensis were greatest in the most southern region, reaching a seasonal maximum of 835 fish ha-1 at one site in that region. Since P. melbournensis is restricted largely to the southern coastline of Australia, it hits a temperate rather than subtropical or tropical distribution and thus is not a typical gerreid. Furthermore, unlike most other gerreids, it does not spend part of its life cycle in either estuaries or nearshore marine waters. The maximum total length of P. melbournensis was 175 mm, with the length at maturity (L50) being 115 mm in females and 121 mm in males. No clear monthly trends were exhibited by gonadosomatic indices, the prevalence of mature ovaries and the oocyte size frequency distributions of female P. melbournensis, and no clear and consistent modes were observed in length frequency data for this species. These strong indications that spawning occurs throughout the year were substantiated by the occurrence of post-ovulatory follicles in the ovaries of large fish in all months but August, and by the presence in that month of advanced yolk granule oocytes in some ovaries, which implies that spawning was imminent. The spawning of P. melbournensis throughout the year contrasts with the far more restricted spawning periods recorded for other teleosts in the same temperate Australian waters. In this respect, P melbournensis exhibits the characteristics of the essentially tropical family to which it belongs. Annuli, which could be detected on the otoliths of c. 40% of fish, suggest that the majority of P. melbournensis were <3 years old

Patterns in the abundance and size-distribution of syngnathid fishes among habitats in a seagrass-dominated marine environment

Syngnathid fishes were sampled using a 1 m wide beam trawl during the day and night in each season from summer 1996/1997 to summer 1997/1998 in five habitat types in an approximately 90 km2 area located on the lower west coast of Australia. The seagrasses Amphibolis griffithii, Posidonia sinuosa and Posidonia coriacea and shallow unvegetated sand were all in depths of 4-9 m, while deep habitat, which comprised mainly bare sand with isolated patches of the seagrasses Heterozostera tasmanica and Halophila ovalis, occurred at depths of 12-16 m. While A. griffithii and P. sinuosa each formed dense monospecific meadows, P. coriacea occurred in sparse clumps surrounded by areas of bare sand and patches of H. tasmanica. While catches of spotted pipefish Stigmatopora argus occurred mainly in P. sinuosa and P. coriacea, individuals of this species that exceeded ca. 55 mm in snout-vent length were far more abundant in the former habitat whereas smaller fish occurred mostly in the latter. Densities of S. argus were similar in P. sinuosa and P. coriacea, but differed between seasons, and a season/habitat interaction was present. In contrast, wide-bodied pipefish Stigmatopora nigra were collected mainly in P. coriacea and deep habitat and, although the densities were greater in P. coriacea than in deep habitat, the size-distributions of this species in these habitats were similar. Notably, S. nigra was never collected in P. sinuosa. Although less abundant than the above pipefish, the long-snouted pipefish Vanacampus poecilolaemus was collected almost exclusively in P. sinuosa. Few syngnathids were collected from shallow unvegetated sand or from A. griffithii, which differed markedly in plant structure from both Posidonia species. It is suggested that these syngnathid species occupy habitats that best enable them to remain inconspicuous to predators. Both S. argus and S. nigra have green or brown colouration and mimic strap-like seagrass leaves which they grasp with prehensile tails, whereas V. poecilolaemus, which is dark coloured and lacks a prehensile tail, most likely lies among the rich detrital material that accumulates beneath the dense P. sinuosa canopy. The movement of larger-sized S. argus from the narrow-leaved P. coriacea to the relatively broad-leaved P. sinuosa may enable the adults of this species to be better camouflaged than if they had remained in the former habitat. As they grow to only approximately half the size of S. argus, S. nigra of all sizes could remain concealed in narrow-leaved seagrasses like P. coriacea and H. tasmanica. The presence of more S. nigra in P. coriacea compared with deep habitat probably reflected the greater expanse of seagrass canopy available in the former habitat, while the inability of either Stigmatopora species to mimic the short leaves of A. griffithii could help to explain their scarcity in this seagrass. The almost total absence of S. nigra and small S. argus from P. sinuosa could be due to their inability to grasp the relatively broad leaves of this seagrass or the presence in this habitat of predators that selectively predate these smaller pipefish. That very few small-sized S. argus were present in P. coriacea during winter 1997, despite the fact that males of both Stigmatopora species carried broods of young in all seasons, suggests that the reproductive output of this species and/or the survival of recruits varied during the study period

Comparisons between the age structures, growth and reproductive biology of two co-occurring sillaginids, Sillago robusta and S. bassensis, in temperate coastal waters of Australia

Sillago robusta and S. bassensis occupy the open sandy areas of the deeper waters (20–35 m) of the inner continental shelf of the lower west coast of Australia. Comparisons were made of their age and size compositions, growth rates, ages and sizes at first maturity, and spawning times. Most S. robusta were less than 3 years old (maximum 7 years), while S. bassensis often reached 7 years of age (maximum 10 years). The maximum length of S. robusta (200 mm) was far shorter than S. bassensis (328 mm), whereas the reverse pertained for the von Bertalanffy growth coeffcient (K), i.e. c. 1.0 v. c. 0.3. Differences in K reflect the fact that c. 80% of the asymptotic length is achieved by S. robusta after 2 years, but not until 6 years of age by S. bassensis. Sexual maturity was reached by up to 50% of S. robusta at the end of their first year of life, and by almost all fish at the end of their second year of life. Most S. bassensis did not reach maturity until the end of their third year of life. In both species, those individuals that reached maturity early, were significantly longer than those that did not reach maturity at the same age. The gonadosomatic indices and proportions of mature gonads, and the numbers of vitellogenic and hydrated oocytes and post-ovulatory follicles, were relatively high in ovaries of both species between December and March. Although both species spawn in these 4 months, some S. bassensis also spawn between September and November and in March and April. In the middle of the spawning period, the ovaries of S. robusta and S. bassensis frequently contained oocytes that ranged widely in size and development, together with post-ovulatory follicles, suggesting that both species are multiple spawners. Juveniles of S. robusta remain in the deeper waters of the inner continental shelf, whereas those of S. bassensis migrate inshore to their nursery areas in surf zones. As S. robusta reaches sexual maturity at an earlier age and smaller size than S. bassensis, it is probably more advantageous for the juveniles of this species to remain in deeper water, and thereby conserve energy for gonadal maturation, rather than migrating into shallow waters for only a very few months

Characteristics of the ichthyofaunas of southwestern Australian estuaries, including comparisons with holarctic estuaries and estuaries elsewhere in temperate Australia: A review

Data on the species compositions and the ages, sizes, reproductive biology, habitats and diets of the main species in the ichthyofaunas of seven estuaries in temperate southwestern Australia have been collated. Twenty-two species spawn in these estuaries, of which 21 complete their lifecycles in the estuary. The latter group, which includes several species of atherinids and gobies with short lifecycles, make far greater contributions to the total numbers of fish in the shallows of these estuaries than in those of holarctic estuaries, such as the Severn Estuary in the United Kingdom. This is presumably related in part to far less extreme tidal water movements and the maintenance of relatively high salinities during the dry summers, and thus to more favourable conditions for spawning and larval development. However, since estuaries in southwestern Australia have tended to become closed for periods, there would presumably also have been selection pressures in favour of any members of marine species that were able to spawn in an estuary when that estuary became landlocked. Furthermore, the deep saline waters, under the marked haloclines that form in certain regions during heavy freshwater discharge in winter, act as refugia for certain estuarine species. The contributions of estuarine-spawning species to total fish numbers in the shallows varied markedly from 33 or 34% in two permanently open estuaries to ≥ 95% in an intermittently open estuary, a seasonally closed estuary and a permanently open estuary on the south coast, in which recruitment of the 0 + age class of marine species was poor. The larger estuarine species can live for several years and reach total lengths of ~ 700 mm and some estuarine species move out into deeper waters as they increase in size. Several marine species use southwestern Australian estuaries as nursery areas for protracted periods. However, sudden, marked increases in freshwater discharge in winter and resultant precipitous declines in salinity in the shallows, and in other regions where haloclines are not formed, are frequently accompanied by rapid and pronounced changes in ichthyofaunal composition, partly due to the emigration of certain marine species. In contrast, the ichthyofaunal compositions of macrotidal holarctic estuaries undergo annual, cyclical changes, due largely to the sequential entry of the juveniles of different marine species for short periods. The ichthyofaunal compositions of the narrow entrance channels, wide basins and saline riverine reaches of large, permanently open southwestern Australian estuaries vary, reflecting the marked tendency for some species to be restricted mainly to one or two of these regions. Comparative data indicate that the characteristics determined for ichthyofaunas in southwestern Australian estuaries apply in general to estuaries elsewhere in temperate Australia

Composition of the fish fauna of a permanently open estuary on the southern coast of Australia, and comparisons with a nearby seasonally closed estuary

The fish faunas of the outer basin (Nornalup Inlet), inner basin (Walpole Inlet) and saline region of the main tributary (Frankland River) of the permanently open Nornalup-Walpole Estuary on the southern coast of Western Australia, were sampled bimonthly for a year using seine and gill nets, and also during a further two months by the former method. Although the Nornalup-Walpole Estuary is permanently open, the catches of fish in its shallows were dominated (98.4%) by estuarine-spawning species, thereby paralleling the situation in the nearby and seasonally closed Wilson Inlet. In contrast, larger representatives of several marine species were present in appreciable numbers in the offshore, deeper waters of both of these estuaries. The delayed recruitment of marine species into these estuaries apparently reflects the distance that the juveniles of these species have to travel from the areas where they are believed predominantly to spawn. The larger representatives of marine species made a greater contribution to the fish faunas of the offshore, deeper waters in the Nornalup-Walpole Estuary than in Wilson Inlet (64.5 vs 36.9%) and, unlike the situation in the latter estuary, they included five species of elasmobranchs, two of which (Mustelus antarcticus and Myliobatis australis) were relatively abundant. Classification and ordination of the combined data for both estuaries demonstrated that the composition of the fish fauna in the offshore, deeper waters of the outer basin of the Nornalup-Walpole Estuary was particularly distinct, with some marine species being restricted to these waters. This is presumably related both to the presence of a permanently open entrance channel and the relatively deep waters found in Nornalup Inlet, which allow the ready exchange of water between the sea and estuary and the maintenance of high salinities in the deeper regions of the outer basin for much of the year. The fish faunas in Walpole Inlet and the tributaries of both the Nornalup-Walpole Estuary and Wilson Inlet were more similar to each other than they were to those in the more seawards end of either estuary. This similarity reflects the apparent preference of certain teleosts, such as the estuarine species Acanthopagrus butcheri and the marine species Mugil cephalus and Aldrichetta forsteri for reduced salinities and/or features associated with riverine environments

Age, growth and reproduction of Sillago schomburgkii in south-western Australian, nearshore waters and comparisons of life history styles of a suite of Sillago species

Samples from sheltered nearshore waters in south-western Australia, in which Sillago schomburgkii spends its entire life cycle, have been used to determine the age structure, growth rate, age and length at first sexual maturity, and spawning period of this whiting species. Several S. schomburgkii reached four to seven years in age and one 12+ fish was caught. The respective maximum and asymptotic lengths (L(∞)) were 350 and 333 mm for females and 348 and 325 mm for males, while the growth coefficients (K) for females and males were 0.53 and 0.49, respectively. Sexual maturity was attained by both sexes of S. schomburgkii at ca. 200 mm, a length reached at the end of the second year of life. Monthly trends exhibited by gonadosomatic indices, the proportions of mature gonads and the prevalence of advanced oocytes and post-ovulatory follicles demonstrate that S. schomburgkii spawns predominantly from December to February. The presence of yolk vesicle and yolk granule oocytes and post-ovulatory follicles in the same ovaries during the spawning period, indicate that S. schomburgkii is a multiple spawner. The patterns of growth of the five Sillago species, that occur in south-western Australian marine waters, fall into two categories. The first, which consists of S. burrus and S. robusta, has a small L(∞), i.e. 300 mm, and has a low K, i.e. ≤ 0.5. The lengths and ages at maturity of S. schomburgkii, S. bassensis, S. burrus and S. robusta, as well as of S. analis and S. flindersi found elsewhere in Australia, are linearly related to their asymptotic lengths and maximum ages, respectively. The two smallest species, S. burrus and S. robusta, attain maturity at ca. 130 mm. However, the former species, whose juveniles occupy productive nearshore waters, grows rapidly and reaches this length by the end of the first year of life, whereas the latter species, which is restricted to deeper waters, grows more slowly and thus does not attain this length until a year later. Sillago flindersi, which is slightly larger than S. burrus and S. robusta, migrates out into deeper waters and attains maturity at ca. 170 mm and two years of age. Although S. schomburgkii, S. analis and S. bassensis attain maturity at ca. 200 mm and reach similar lengths, the first two of these species, which remain in nearshore waters and display more rapid growth, reach maturity one year earlier than the last species, which migrates out into deeper and presumably less productive waters. While S. vittata reaches a similar size and likewise migrates out into deep waters, it reaches maturity earlier, i.e. at the end of its first year of life

Seasonal, annual and regional variations in ichthyofaunal composition in the inner Severn Estuary and inner Bristol Channel

Monthly samples of fish from the intake screens of power stations at Oldbury and Berkeley in the inner Severn Estuary and Hinkley Point in the inner Bristol Channel, were used to analyse the community structures of the ichthyofauna in these regions. Marine species that use the estuary as a nursery area (marine estuarine-opportunists) were very abundant in the shallow inshore waters at Oldbury. Diadromous species were more abundant in the offshore and deeper waters at Berkeley than at Oldbury. Only one of the two species that complete their life cycles in the estuary was even moderately abundant in the inner estuary and the 15 freshwater species were relatively rare. Bass and particularly the sand goby complex were more numerous in the protected, inshore waters than the more offshore waters of the estuary. With the yellow and silver stages of the European eel, the reverse situation pertained. Seasonal changes in faunal composition were more pronounced in the inshore shallow than in more offshore deeper waters of the estuary. This largely reflected the sequential immigration of large numbers of the juveniles of marine estuarine-opportunist species into the former area for relatively short periods. Although the ichthyofaunal composition in the shallows at Oldbury underwent the same pattern of cyclical variation in each of five consecutive years, the degree of intra-annual variability differed, reflecting interannual differences in the recruitment strengths of the 0+ age classes of the different marine estuarine-opportunists. These cyclical changes were not correlated strongly with either salinity or water temperature. The faunal composition of the protected inshore, more marine waters of the inner Bristol Channel differed from those in both inshore and offshore regions of the inner estuary. The species which typified the fauna of the Channel were bib, poor cod, five-bearded rockling, sole and conger eel. Although the first four of these species were relatively more abundant in these waters than in the estuary, their juveniles often made extensive use of the shallows at Oldbury. This study emphasizes that, for some marine species, the protected inshore, and more marine, waters in the Bristol Channel can act as alternative nursery areas to those provided by the inshore shallows of the Severn Estuary

Variations in the dietary compositions of morphologically diverse syngnathid fishes

We examined the diets of 12 morphologically diverse syngnathid species in shallow seagrass-dominated marine waters of south-western Australia to determine whether they differed among species that varied in body form, size and snout morphology, and in particular whether species with long snouts ingested more mobile prey. Although all species consume mainly small crustaceans, the dietary compositions of these species often vary markedly. We suggest that these differences are related to factors that influence both their foraging capabilities and/or locations. Those species with long snouts (e.g. the common seadragon Phyllopteryx taeniolatus and long-snouted pipefish Vanacampus poecilolaemus) consume far more relatively mobile prey than species with short snouts. Species with short snouts (e.g. the pug-nosed pipefish Pugnaso curtirostris and Macleay's crested pipefish Histiogamphelus cristatus) mainly consume slow moving prey. Spotted pipefish, Stigmatopora argus, and wide-bodied pipefish, Stigmatopora nigra, restrict their diets to planktonic copepods, probably because their small gape size limits their ability to feed on alternative larger prey. Both the short-snouted seahorse, Hippocampus breviceps, and West Australian seahorse, Hippocampus subelongatus, ingest mainly slow-moving prey, even though the latter species possesses a moderately long snout. This may reflect the fact that seahorses are weak swimmers that anchor themselves to vegetation or the substrate with a strongly prehensile tail and rarely venture into open water to pursue mobile prey. In contrast, the relatively large P. taeniolatus, which resides above, rather than within, the macrophyte canopy, consumes mysids, which aggregate in open water above the seabed. Those pipefishes with characters that imply relatively enhanced mobility, such as well developed caudal fins and non-prehensile tails, are trophically diverse, suggesting that they are able to feed either on the sediment or phytal surfaces or in the water column