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

P.G. Coulson; I.C. Potter; S.A. Hesp; N.G. Hall

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.

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

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