Coulson, Peter Graham (2008) The biology of three teleost species with divergent life cycle characteristics and their implications for fisheries management. PhD thesis, Murdoch University.
The overall aim of this thesis was to determine the size and age compositions, growth and reproductive biology of Western Blue Groper (Achoerodus gouldii), Blue Morwong (Nemadactylus valenciennesi) and Yellowtail Flathead (Platycepahlus endrachtensis) in south-western Australian waters, in which these three species have divergent life cycle characteristics. As A. gouldii and N. valenciennesi are commercially and recreationally important in coastal waters and P. endrachtensis is one of the most recreationally important species in the Swan River Estuary, these biological data were then used to produce estimates of mortality and spawning stock biomass per recruit for each of these species. The biological data and stock assessment parameters were finally employed comparatively to ascertain whether any of the three species possessed characteristics that would make them particularly susceptible to the effects of fishing and whether there was evidence that any of the species is fully or even overexploited.
Achoerodus gouldii typically uses reefs in protected inshore waters along the coast and around neighbouring islands as a nursery habitat and then, as it increases in size, moves to deeper, offshore reefs, where it spawns between early winter and mid-spring. The maximum total length and age of A. gouldii were 1162 mm and 70 years, the latter being the greatest age by far yet recorded for any species of labrid. However, most growth occurs in the first 20 years of life. Histological and demographic analyses demonstrated that all individuals begin life as females and, after attaining maturity, many become males, i.e. A. gouldii is a monandric protogynous hermaphrodite. The L50 at maturity and sex change were 653 and 821 mm, respectively, which correspond to ages of c. 17 and 37 years, respectively. As sex change took 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. Since sex change is typically accompanied by a change from green to blue, body colour can be used as a proxy for determining the length (L50) at which females change to males. von Bertalanffy 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 estimated lengths at age of females were c. 600, 670 and 680 mm, respectively, whereas those of males were c. 695, 895 and 975 mm, respectively.
As A. gouldii is very long-lived and sexual maturity, and even more particularly sex change, occur late, this labrid is potentially very susceptible to overfishing. Thus, because the mortality estimates and per recruit analyses indicated 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 A. gouldii, N. valenciennesi moves to deeper, offshore waters as it increases in size and then matures and spawns in those waters. Although N. valenciennesi has a maximum length of nearly 1 m and thus, like A. gouldii, is moderately large, it has a far shorter life span, i.e. 19 vs 70 years. While female N. valenciennesi does not grow to as large a size as its males (max. lengths = 846 and 984 mm, respectively), the maximum age of both sexes was 19 years. From the growth curves, the females by ages 3, 6 and 10 years havd attained, on average, lengths of 435, 587 and 662 mm, respectively, compared with 446, 633 and 752 mm, respectively, for males. Both sexes grew little after 10 years of age.
Juvenile N. valenciennesi < 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 females and males typically mature in offshore waters of the south coast at lengths of c. 600-800 mm and ages of c. 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 N. valenciennesi at far lesser lengths and ages on the lower west coast than south coast suggests that the former coast provides better environmental conditions for the gonadal maturation and spawning of this species. Furthermore, the contrast between the almost total absence of the juveniles of N. valenciennesi 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 at its strongest.
Although N. valenciennesi is caught by recreational line fishing and commercial gillnet fishing when they are as young as 3-4 years old, 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 suggested that N. valenciennesi in south-western Australia is not currently overfished. A greater resilience to fishing by N. valenciennesi than A. gouldii presumably reflects, in part, its far shorter lifespan, earlier maturity and possession of gonochorism rather than hermaphroditism.
Platycephalus endrachtensis spawns in the Swan River Estuary between late spring and early autumn and completes the whole of its life cycle in this system. Although females attain a far larger length (615 mm) than males (374 mm), females and males were present in each age class. These data, together with a detailed examination of histological sections of a wide size and age range of individuals, demonstrated that this species, unlike some of its relatives, is not a protandrous hermaphrodite, i.e. it does not change from male to female with increasing body size. The combination of the presence of females and males in all age classes and the observation that all of the large number of individuals between 374 and 615 mm were females shows that the far greater length attained by that sex is largely related to its faster growth rate. The fact that females outnumbered males in each age class of P. endrachtensis in which the sample size was substantial, i.e. > 25, with the overall sex ratio being 2.7 females: 1 male, indicate that there has been strong selection for egg production in this species. As the minimum legal length for retention of P. endrachtensis is 300 mm, and relatively few males exceeded this length, the recreational fishery which targets this species is based largely on its females.
The estimates of mortality and results of per recruit analyses provided no evidence that P. endrachtensis is currently overfished. From a management point of view, it is advantageous that the current size limit for this species exceeds the average length at which its females (259 mm) attain maturity. Furthermore, this species appears to be resilient to capture and release.
The data presented in this thesis demonstrate that A. gouldii possesses biological characteristics which make it potentially more prone to the effects of fishing than is the case with either N. valenciennesi or P. endrachtensis. This presumably accounts, at least in part, for the indications that A. gouldii is the only one of these three species that is likely to be close to or at full exploitation.