Factors influencing the timing and frequency of spawning and fecundity of the goldlined seabream (Rhabdosargus sarba) (Sparidae) in the lower reaches of an estuary

We have studied the reproductive biology of the goldlined seabream (Rhabdosargus sarba) in the lower Swan River Estuary in Western Australia, focusing particularly on elucidating the factors influencing the duration, timing, and frequency of spawning and on determining potential annual fecundity. Our results demonstrate that 1) Rhabdosargus sarba has indeterminate fecundity, 2) oocyte hydration commences soon after dusk (ca. 18:30 h) and is complete by ca. 01:30-04:30 h and 3) fish with ovaries containing migratory nucleus oocytes, hydrated oocytes, or postovulatory follicles were caught between July and November. However, in July and August, their prevalence was low, whereas that of fish with ovaries containing substantial numbers of atretic yolk granule oocytes was high. Thus, spawning activity did not start to peak until September (early spring), when salinities were rising markedly from their winter minima. The prevalence of spawning was positively correlated with tidal height and was greatest on days when the tide changed from flood to ebb at ca. 06:00 h, i.e., just after spawning had ceased. Because our estimate of the average daily prevalence of spawning by females during the spawning season (July to November) was 36.5%, individual females were estimated to spawn, on average, at intervals of about 2.7 days and thus about 45 times during that period. Therefore, because female R. sarba with total lengths of 180, 220, and 260 mm were estimated to have batch fecundities of about 4500, 7700, and 12,400 eggs, respectively, they had potential annual fecundities of about 204,300, 346,100 and 557,500 eggs, respectively. Because spawning occurs just prior to strong ebb tides, the eggs of R. sarba are likely to be transported out of the estuary into coastal waters where salinities remain at ca. 35‰. Such down-stream transport would account for the fact that, although R. sarba exhibits substantial spawning activity in the lower Swan River Estuary, few of its early juveniles are recruited into the nearshore shallow waters of this estuary

A new proportionality-based back-calculation approach, which employs traditional forms of growth equations, improves estimates of length at age

The performance of a new proportionality-based back-calculation approach, describing the relationship among length, otolith size, and age using traditional growth curves and assuming a bivariate distribution of deviations from those curves, was evaluated. Cross-validation was used for six teleost species to compare predictions of expected lengths or otolith sizes at age, given otolith size or length, respectively, with those of other proportionality-based approaches that incorporate age. For four species, and particularly Acanthopagrus butcheri when using a biological intercept, better estimates were produced using the new model than were produced using the regression equations in the other back-calculation approaches. Back-calculated lengths for A. butcheri estimated using this model were more consistent with observed lengths, particularly when employing a biological intercept, than those obtained using other proportionality-based approaches and also a constraint-based approach known to produce reliable estimates. By selecting somatic and otolith growth curves from a suite of alternatives to better describe the relationships among length, otolith size, and age, the new approach is likely to produce more reliable estimates of back-calculated length for other species

Differential changes in production measures for an estuarine-resident sparid in deep and shallow waters following increases in hypoxia

This study determined how productivity measures for a fish species in different water depths of an estuary changed in response to the increase in hypoxia in deep waters, which had previously been shown to occur between 1993–95 and 2007–11. Annual data on length and age compositions, body mass, growth, abundance, biomass, production and production to biomass ratio (P/B) were thus determined for the estuarine-resident Acanthopagrus butcheri in nearshore shallow (<2 m) and offshore deep waters (2–6 m) of the upper Swan River Estuary in those two periods. Length and age compositions imply that the increase in hypoxia was accompanied by the distribution of the majority of the older and larger A. butcheri changing from deep to shallow waters, where the small fish typically reside. Annual densities, biomass and production in shallow waters of <0.02 fish m−2, 2–4 g m−2 and ∼2 g m−2 y−1 in the earlier period were far lower than the 0.1–0.2 fish m−2, 8–15 g m−2 and 5–10 g m−2 y−1 in the later period, whereas the reverse trend occurred in deep waters, with values of 6–9 fish net−1, 2000–3900 g net−1, 900–1700 g net−1 y−1 in the earlier period vs < 1.5 fish net−1, ∼110 g net−1 and 27–45 g net−1 y−1 in the later period. Within the later period, and in contrast to the trends with annual abundance and biomass, the production in shallow waters was least during 2008/09, rather than greatest, reflecting the slow growth in that particularly cool year. The presence of substantial aggregations of both small and large fish in shallow waters accounts for the abundance, biomass and production in those waters increasing between those periods and thus, through a density-dependent effect, provide a basis for the overall reduction in growth. In marked contrast to the trends with the other three production measures, annual production to biomass ratios (P/B) in shallow waters in the two years in the earlier period, and in three of the four years of the later period, fell within the same range, i.e. 0.6–0.9 y−1, but was only 0.2 y−1 in 2008/09, reflecting the poor growth in that year. This emphasises the need to obtain data on P/B for a number of years when considering the implications of the typical P/B for a species in an estuary, in which environmental conditions and the growth of a species can fluctuate markedly between years

Marked deleterious changes in the condition, growth and maturity schedules of Acanthopagrus butcheri (Sparidae) in an estuary reflect environmental degradation

As Acanthopagrus butcheri typically completes its life within its natal estuary and possesses plastic biological characteristics, it provides an excellent model for exploring the ways and extent to which a fish species can respond to environmental changes over time. The environment of the Swan River Estuary in south-western Australia has deteriorated markedly during the last two decades, reflecting the effects of increasing eutrophication and hypoxia in the upper regions, where A. butcheri spends most of the year and spawns. In this study, the biological characteristics of A. butcheri in 2007-11 were determined and compared with those in 1993-95. Between these two periods, the condition factor for females and males of A. butcheri across their length ranges declined by 6 and 5%, respectively, and the parameters k and L∞ in the von Bertalanffy growth curves of both sexes underwent marked reductions. The predicted lengths of females and males at all ages ≥1 year were less in 2007-11 than in 1993-95 and by over 30% less at ages 3 and 6. The ogives relating maturity to length and age typically differed between 1993-94 and 2007-10. The L50s of 156 mm for females and 155 mm for males in 2007-10 were less than the corresponding values of 174 and 172 mm in 1993-94, whereas the A50s of 2.5 years for both females and males in 2007-10 were greater than the corresponding values of 1.9 and 2.0 years in 1993-94. The above trends in condition, growth and maturity parameters between periods are consistent with hypotheses regarding the effects of increasing hypoxia on A. butcheri in offshore, deeper waters. However, as the density of A. butcheri declined in offshore, deeper waters and increased markedly in nearshore, shallow waters, density-dependent effects in the latter waters, although better oxygenated, also probably contributed to the overall reductions in growth and thus to the changes in the lengths and ages at maturity

Comparisons between the biology of two co-occurring species of whiting (Sillaginidae) in a large marine embayment

We compare the biology of the tropical species Sillago analis and the temperate species Sillago schomburgkii in Shark Bay, a large subtropical marine embayment on the west coast of Australia. This environment constitutes approximately the southernmost and northernmost limits of the distributions of these two species, respectively. The annuli visible in sectioned otoliths of S. analis and S. schomburgkii form annually. Their numbers were thus used to age the individuals of these two species, which are morphologically very similar and live in the same habitats. Although the growth rates of S. analis and S. schomburgkii are very similar until maturity is attained, they subsequently diverge, with S. schomburgkii investing relatively more energy into somatic growth. The maximum total lengths and ages of both the females (320 mm, 6 years) and males (283 mm, 8 years) of S. analis were not as great as those of the females (383 mm, 9 years) and males (302 mm, 9 years) of S. schomburgkii. In Shark Bay, S. schomburgkii spawns earlier and longer than S. analis, i.e. August-December vs. January-March, which would result in the juveniles of these two species recruiting into nursery areas at different times. In addition, S. schomburgkii spawns earlier and for longer in Shark Bay than in temperate marine waters 800 km further south, presumably reflecting the fact that, in that subtropical embayment, water temperatures over which this species typically spawns are attained earlier and last for longer. However, although environmental conditions in Shark Bay and those temperate marine waters differ markedly, the growth of the corresponding sexes of S. schomburgkii in these two water bodies is similar

Biological parameters required for managing Western Blue Groper, Blue Morwong and Yellowtail Flathead

This study provides the sound quantitative data that are required by managers for developing plans for conserving the stocks of the Western Blue Groper Achoerodus gouldii, the Blue Morwong (previously Queen Snapper) Nemadactylus valenciennesi and the Yellowtail Flathead (previously Bar-tailed Flathead) Platycephalus endrachtensis in south-western Australian waters. The first two species are commercially and recreationally important in coastal waters and the third is one of the most important angling species in the Swan River Estuary. All three species have been identified by managers as requiring detailed studies of their biology, and Blue Morwong and Yellowtail Flathead are among a small suite of species selected as indicator species for the status of fish populations in marine and estuarine waters, respectively, in south-western Australia. As juveniles, Western Blue Groper typically occupy reef areas in protected inshore waters along the coast and around neighbouring islands. As the individuals of this species increase in size, they move offshore to deeper and more exposed waters over reefs. Spawning occurs in the latter environment, between early winter and mid-spring. The maximum length and age we recorded for Western Blue Groper were 1162 mm and 70 years, respectively, the latter age being the greatest by far yet recorded for any species of wrasse. However, most of the growth of this species occurs in the first 20 years of life. The Western Blue Groper is shown to be a monandric protogynous hermaphrodite, namely all of its individuals begin life as females and, after maturing, many subsequently change sex to males. Females typically first become mature at about 650 mm and 15-20 years and typically change to males at lengths of about 800-850 mm and ages of about 35- 39 years. As sex change takes place over a narrower range in lengths (650 to 900 mm) than in ages (15 to 49 years), that change is apparently related more to size than age. The fact that sex change is typically accompanied by a change in body colour from green to blue can be used to determine the approximate size at which females change to males, without having to cut open the fish to determine whether it possesses ovaries or testes. Growth curves fitted to the lengths at age of individuals of each sex of this hermaphroditic species using a novel technique demonstrated that, with increasing age, the lengths of males became increasingly greater than those of females. Thus, at ages 15, 30 and 60 years, the “average” lengths of females were approximately 600, 670 and 680 mm, respectively, those of males were approximately 695, 895 and 975 mm, respectively. As the Western Blue Groper is very long-lived and maturity and particularly sex change occur late, it is potentially very susceptible to overfishing. Thus, because the mortality estimates and per recruit analyses indicate that, at present, this species is close to or fully exploited, fisheries managers will need to take a precautionary and watchful approach to managing and thus conserving the stocks of this species. As with Western Blue Groper, the Blue Morwong moves to deeper, offshore waters as it increases in size and then matures and spawns in those waters. Although Blue Morwong has a maximum length of close to 1 m and thus, like Western Blue Groper, is a moderately large fish species, it has a far shorter life span, namely 21 years compared with 70 years. While female Blue Morwong do not grow to as large a size as their males (max. lengths = 846 and 984 mm, respectively), the maximum age of both sexes was 21 years. From the growth curves, the average lengths attained by ages 3, 6 and 10 years were 435, 587 and 662 mm, respectively, for females, compared with 446, 633 and 752 mm, respectively, for males. Both sexes exhibited little growth after 10 years of age. Juveniles of Blue Morwong less than 400 mm in total length were found exclusively in shallow, coastal waters on the south coast, whereas their adults were abundant in offshore waters of both the south and lower west coasts. The lengths and ages at which females and males typically mature in offshore waters of the south coast were about 600-800 mm and about 7-9 years. In contrast, the vast majority of females caught in offshore waters of the lower west coast (where they were of a similar length and age range to those in offshore waters on the south coast) became mature at lengths of 400-600 mm and 3-4 years of age. The attainment of maturity by Blue Morwong at far lesser lengths and ages on the lower west coast than south coast suggests that the former coast provides better environmental conditions for gonadal maturation and spawning. Furthermore, the contrast between the almost total absence of the juveniles of Blue Morwong in nearshore waters on the lower west coast and their substantial numbers in comparable waters on the south coast indicates that the larvae of this species produced on the lower west coast are transported southwards to the south coast, where they become juveniles. As spawning occurs between mid-summer and late autumn, the larvae, which spend a protracted period in the plankton, would be exposed, on the lower west coast, to the influence of the southwards-flowing Leeuwin Current at the time when that current is strongest. Although Blue Morwong is caught by recreational line fishing and commercial gillnet fishing when they are as young as 3-4 years, they do not become fully vulnerable to these fisheries until they are about 9 years old. Consequently, the individuals of this species can potentially breed over a number of years before they become particularly prone to capture by fishers. Mortality estimates and per recruit analyses suggest that the Blue Morwong in south-western Australia is currently not overfished. A greater resilience to fishing by Blue Morwong than Western Blue Groper reflects, in part, its shorter lifespan, gonochorism (namely, it is not hermaphroditic) and early maturity. The Yellowtail Flathead spawns in the Swan River Estuary between late spring and early autumn and completes the whole of its life cycle in this system. Although its females attain a far larger length (615 mm) than its males (374 mm), this species, unlike some of its relatives, is not a protandrous hermaphrodite, namely, it does not change from male to female with increasing body size. As the maximum age of both sexes is eight years, the far greater length attained by females is largely related to the far faster growth of that sex. Females outnumbered males in each age class in which the sample size exceeded 25, with the overall sex ratio being 2.7 females: 1 male. As the minimum legal length for retention of Yellowtail Flathead is 300 mm, and relatively few males exceed this length, the recreational fishery which targets this species is largely based on its females. The estimates of mortality and results of the per recruit analyses provided no evidence that the Yellowtail Flathead is currently overfished. From a management point of view, it is advantageous that the current size limit for Yellowtail Flathead exceeds the average length at which its females (259 mm) attain maturity. Furthermore, this species appears to be resilient to capture and release. The biological data provided in this study will be very useful for the ongoing development of management policies for three important commercial and/or recreational species in south-western Australian waters and will alert managers to the need to monitor closely the status of Western Blue Groper

Determination of the biological parameters required for managing the fisheries of four tuskfish species and western yellowfin bream

The levels of commercial and recreational exploitation of the fish stocks of certain species in the ecologically very important environment of Shark Bay, Western Australia, are high and continuing to increase. However, the amount of biological data that can be used for management purposes for most of those species is minimal. The ability to sustain stocks of such species, in the face of increasing fishing pressure, by setting in place appropriate management plans is dependent on acquiring strategic information on the biology of those species

Biology, stock status and management summaries for selected fish species in south-western Australia

Many people are interested in fishes, but often it can be difficult to access reliable information on the biology, status and management of a particular fish species. Although there is a long history of world class fisheries research and management in Australia, the full details are generally only available in a fragmented manner in various scientific journals, books and reports. In some cases, these sources of information can be difficult to find and, even for fisheries researchers and managers, who usually have access to scientific journals, certain types of information such as unpublished reports or student theses, can be hard to acquire. The first objective in developing this guide was thus to collate a range of important details relating to current biological understanding, stock status and management for 30 of Western Australia’s most important and/or well known, temperate marine fish species. The second objective was to provide a comprehensive list of publications relevant to each species, to enable easier access to more detailed information on those species