Pember, Matthew Barrett (2006) Characteristics of fish communites in coastal waters of north-western Australia, including the biology of the threadfin species Eleutheronema tetradactylum and Polydactylus macrochir. PhD thesis, Murdoch University.
This study was aimed at determining the characteristics of the fish assemblages in nearshore coastal waters of the remote Canning region of tropical Western Australia and to obtain sound quantitative data on crucial aspects of the biology of the two threadfin species, which are commercially and recreationally important in those waters. The community studies focused on comparing the species compositions of the fish faunas found over bare sand and in mangroves and rock pools and on elucidating the factors that influence those compositions. The population studies concentrated on exploring the hermaphroditic characteristics, size and age structures, growth and stock status of the two species of Polynemidae.
The arid Canning coast of north-western Australia, which lies between Cape Leveuque (16 degees 21'S, 123 degrees 02'E) and Cape Keraudren (19 degrees 57'S, 119 degrees 46'E),does not contain rivers and thus also estuaries, which traditionally provide alternative fish nursery areas to those found in nearshore waters. It is also subjected to some of the largest tides in the world. The fish faunas at three widely-separated locations, i.e. Port Smith, Eighty Mile Beach and Cape Keraudren, were sampled using seine and gill netting in their shallow, nearshore waters and rotenoning in intertidal pools at the first and third of those locations with the aim of determining the following. 1) The extents to which the diversity, abundance and species composition of the fish faunas of the Canning coast are influenced by location, habitat and time of year. 2) Which species use bare sand, mangroves and/or intertidal pools as nursery areas and which are permanent residents in one or more of those habitats. 3) Identification of the commercial and recreational fish species found in each of the above three habitat types and their relative abundance in those habitats.
The fish catches from all three sampling locations collectively yielded 170 species representing 66 families. The most abundant species (with % contributions) in seine net samples collected over unvegetated sand were Stolephorus carpentariae (19.1), Herklotsichthys blackburni (14.7) and Atherinomorus lacunosus (11.8), whereas those in gill net samples taken over the same substrate were Arius proximus (26.6), Eleutheronema tetradactylum (18.8) and Polydactylus macrochir (18.7) and in mangroves were A. proximus (38.9), Valamugil buchanani (18.8) and Scomberoides commersonnianus (18.7). Ambassis vachellii (32.5) and Craterocephalus capreoli (14.4) dominated the catches obtained from intertidal pools.
Fifty three of the 170 species that were caught are fished commercially and recreationally along the Canning coast and a further 17 fish species are caught solely by recreational fishers. The most abundant of these species were the polynemids E. tetradactylum and P. macrochir, which are residents of nearshore waters, i.e. are found in these waters throughout the whole of their life cycle. Other species, such as Lutjanus russelli Epinephelus coioides and Sillago vittata, are present in nearshore waters only as juveniles and thus use these waters as a nursery.
The compositions of the ichthyofaunal samples collected by both seine and gill nets over bare sand differed markedly among locations. The compositions at Cape Keraudren, the most southerly location, were more similar to those at Port Smith, the most northerly and most protected location, than those that Eighty Mile Beach, the most exposed of the three locations. It is thus proposed that the compositions of the fish faunas found in nearshore, unvegetated waters along the Canning coast are influenced more by factors related to turbidity than those reflecting latitudinal position. The fish fauna at Eighty Mile Beach was distinguished by species typically associated with turbid waters, such as those of the Polynemidae and Sciaenidae and certain species of the Engraulidae, Ariidae, Mugilidae and Carcharhinidae. In contrast, many of those species were absent from catches made in the clearer waters of Port Smith, where the ichthyofauna was disitinguished by certain species of the Clupeidae and Atherinidae and other species associated with low turbidity, such as V. buchanani and Chanos chanos.
The compositions of the fish faunas sampled over bare sand by gill and seine nets underwent marked seasonal changes. Furthermore, these changes, particularly in the case of seine net catches, tended to undergo a conspicuous cyclical progression during the year as a result of time-staggered changes in the recruitment and emigration of certain species. The species responsible for seasonal differences in either the gill and/or seine net samples included nearshore residents, such as the clupeids H. blackburni and Spratelloides delicatulus, the polynemids E. tetradactylum and P. macrochir, the engraulidid Thryssa hamiltonii and the atherinids A. lacunosus and C. capreoli, as well as species such as the sillaginid S. vittata, which use nearshore waters as a nursery area. At a broader level, the compositions of the fish sampled by gill net in the wet and dry periods were also distinct, reflecting, in particular, the influx of certain species during the wet period, e.g. mature ariid catfish and the juveniles of a number of elasmobranchs, such as the endangered Green Sawfish Pristis zijsron, aggregate in nearshore, shallow waters during this period.
The fish faunas of the intertidal pools at Port Smith and Cape Keraudren, which were sampled using rotenone, differed markedly from each other and from those over nearby bare sand substrates on the coast. The ichthyofauna of intertidal pools at Port Smith was distinguished from that at Cape Keraudren by relatively greater numbers of C. capreoli at the former location, whereas the opposite situation pertained with Amniataba caudavittatus, Acanthopagrus latus and L. macrolepis at Cape Keraudren. These differences were attributable to differences in habitat characteristics of intertidal pools, with pools at the former location containing clearer water and greater amounts of rock, while those at the latter contained mangroves. Few species were caught in both intertidal pools and in the surrounding shallow, nearshore waters, demonstrating that the rock pools provide an important habitat for certain fish species.
The composition of the fish fauna of intertidal pools at Port Smith underwent an essentially cyclical progression over the course of a year, reflecting the timing and strength of recruitment of the juveniles of the various species. The species responsible for this recruitment-related change in the fish fauna included both nearshore residents, such as A. vachellii, C. capreoli and A. lacunosus, as well as transient species that use intertidal pools as a nursery area, i.e. E. coioides and L. russelli. In contrast, the seasonal progression in species composition in intertidal pools at Cape Keraudren was largely related to the influence of a tropical cyclone. The fish community in the period immediately following the cyclone was depauparate and distinguished, from that before the cyclone, in particular by a lack of A. vachellii and A. latus.
The Blue and King Threadfins E. tetradactylum and P. macrochir, which were among the most abundant species in the seine and gill net catches taken in unvegetated waters, are key species in the Kimberley Managed Gill Net and Barramundi Fishery (KMGBF) and important recreational target species. In addition, these species are an important source of food and of cultural significance for local indigenous communities. The importance of the shared nature of these resources and the marked fluctuations undergone by the catches of threadfin in recent years led to the second major component of this thesis, i.e. to obtain reliable data on the reproductive biology, age compositions, growth rates and mortality of E. tetradactylum and P. macrochir, of the type that are required by managers for developing plans to conserve the stocks of these species in north-western Australia.
Length and age compositions of male, bisexual (i.e. possessing gonads comprising both testicular and ovarian tissue) and female E. tetradactylum and P. macrochir, and histological characteristics of the gonads of the full size range of threadfin were examined and analysed. The results demonstrate that, in north-western Australian waters, each of these species is a protandrous hermaphrodite, i.e. individuals mature first as males before changing sex to female. Sexual maturity is attained by 50% of male E. tetradactylum and P. macrochir at ca 200 and 230 mm, respectively. For both species, these lengths approximate those attained towards the end of their first year of life. However, the lengths and ages at which these two species typically change sex differ markedly.
In the case of E. tetradactylum, transitional fish (i.e. those with bisexual gonads assumed to be changing sex) were most prevalent at total lengths between 300 and 400 mm. All fish greater than 450 mm total length possessed gonads that consisted exclusively of ovarian tissue. The L50 for sex change from male and transitional fish to females is ca 400 mm. Thus, the majority of E. tetradactylum change sex during their second or third years of life and males are rarely older than three years. In contrast, the range of lengths at which transitional individuals of P. macrochir were recorded was much broader than E. tetradactylum, i.e. between 310 and 1140 mm. The data on the prevalences of males and females in sequential age classes demonstrate that sex change can occur in P. macrochir as young as two years old and up to eight years old. The lengths at which E. tetradactylum changes sex in north-western Australia was relatively similar at all locations, whereas the L50 for sex change in P. macrochir varied markedly among sampling locations. For example, the L50 value for sex inversion was ca 790 mm at Derby, compared with ca 1160 mm at Anna Plains. The differences presumably reflect variations in environmental conditions at those localities.
Analysis of the ages of male, transitional and female E. tetradactylum revealed that the change from male to female by this species occurs over a period of ca 6 months. The presence of mature sperm in transitional gonads, in combination with trends exhibited by the GSIs of the testicular and ovarian portions of transitional gonads, demonstrate that, during the spawning season, transitional threadfin function as males. Analyses indicate that, once individual E. tetradactylum and P. macrochir change sex to become females, all individuals function as mature females during successive breeding seasons. Both species of threadfin have protracted spawning periods of ca 6 months. However, the monthly trends exhibited by the mean GSIs and the proportion of the various gonad stages show that the spawning of both species peaks during spring and early summer, i.e. September to December, and occurs on multiple occasions each year.
In north-western Australia, P. macrochir grows far larger and lives for longer than E. tetradactylum, a difference reflected in the maximum total lengths and ages recorded for these two species, i.e. 1393 mm and 10 years vs 793 mm and 6 years, respectively. In addition, P. macrochir grows faster than E. tetradactylum, attaining lengths of 322, 520 and 945 mm vs 245, 400 and 635 mm, by the end of years 1, 2 and 5, respectively. For both species, estimates of total, natural and fishing mortality were derived using different life history models, simulation based on the number of fish in samples above a specified age, relative abundance analyses and a Monte Carlo resampling approach.
The various biological parameters determined for E. tetradactylum and P. macrochir, including the estimates of mortality, were incorporated into yield per recruit, spawning biomass per recruit and spawning potential ratio analyses to determine the current impacts of fishing on each species in north-western Australia. The results of these analyses indicate that E. tetradactylum is fully exploited and that P. macrochir is overexploited. The effectiveness of various management options is evaluated.
The data produced during the first part of this thesis on the diversity, abundance and species compositions of fish faunas in different nearshore habitats along the Canning coast of north-western Australia, and the way these faunas are influenced by season, provides fisheries and environmental managers with information that will enable them to develop management plans for these habitats and their fish species. In addition, the biological data for E. tetradactylum and P. macrochir, and the results of the stock assessments performed on these species, will enable fisheries managers to develop plans for conserving the stocks of these two species in north-western Australia.