J. P. Salini

Species composition and biomasses of fishes in different habitats of a tropical Northern Australian estuary: Their occurrence in the adjoining sea and estuarine dependence

Abstract The fish communities of the five habitats comprising the Embley estuary in tropical north-east Australia were studied for two and a half years. The fish faunas of each habitat were significantly different in both biomass and species composition. Mean biomasses were estimated as 7·1 g m−2 to 16·1 g m−2 for open water channels, 5·0 g m−2 for sandy mud beaches, 0·5 to 1·8 g m−2 for seagrass areas, 8·2 g m−2 for small mangrove creeks and inlets, and 70·6 g m−2 for intertidal mudflats adjacent to mangroves. The species composition and biomass of the fish population in the estuary were compared with those of offshore waters in adjoining Albatross Bay. Of the 197 species recorded in the estuary, 91 were also recorded in the bay. They fell into six species categories: (1) juveniles found only in the estuary, (2) juveniles found only offshore, (3) juveniles that live both in the estuary and offshore, (4) adults found only in the estuary, (5) adults that live only offshore, and (6) adults that occur in both areas. Of the 106 species caught only inside the estuary, 59 also occur in shallow marine areas, which could not be sampled by trawling. This estuarine/shallow marine component formed at least one-third of the biomass in all estuarine habitats. The juveniles of 17 species of this group were found only in the estuary. Thirty species from the Embley (17 of which were Gobiidae) were considered truly estuarine. The number of species recorded, the biomasses in the various habitats and the differences between the fish faunas of the habitats are compared with published data from other tropical estuaries. The relatively high number of species from the Embley and the variations in biomasses and communities emphasize the importance of adequate sampling of all estuarine habitats. The results are also discussed in relation to the concepts of ‘estuarine dependence’ and ‘estuarine opportunism’. We concluded that ‘estuarine dependence’ is a valid concept and that at least one-third of the species from the Embley estuary are ‘estuarine-dependent’: they make up at least half the fish biomass in all estuarine habitats.

Fish communities and the nursery role of the shallow inshore waters of a tropical bay in the gulf of Carpentaria, Australia

The species composition of the fishes of the inshore zone between a tropical estuary and offshore trawling grounds were studied. The objectives were to investigate the affinities of the fishes with those of adjacent areas, to examine the influence of abiotic factors on species composition, and to assess the role and importance of the zone as a nursery area. The shallow (

Diets of piscivorous fishes in a tropical Australian estuary, with special reference to predation on penaeid prawns

The diets of fish from the tropical Embley Estuary in the eastern Gulf of Carpentaria, Australia, were analysed with particular reference to piscivory and predation on juveniles of commercially important penaeid prawns from October 1986 until July 1988. Of the 77 species caught, 52 were piscivorous, and of these 37 ate penaeid prawns. The most numerous piscivores wereScomberoides commersonianus, Arius proximus, Lates calcarifer, Polydactylus sheridani andRhizoprionodon acutus, the first four of which accounted for over 90% of all prawns eaten. Twenty species are commercially important species of prawns. The proportions of penaeids in the diets varied seasonally, according to the density of penaeids in the estuary. The proportion was highest during the pre-wet period (November) and lowest during the dry period (July–August). Predation on prawns was highest in the lower and middle reaches of the river. Estimates of the rates of predation on the two most common juvenile commercial prawns,Penaeus merguiensis andP. semisulcatus, are presented and discussed.

The extent of population genetic subdivision differs among four co-distributed shark species in the Indo-Australian archipelago

BackgroundThe territorial fishing zones of Australia and Indonesia are contiguous to the north of Australia in the Timor and Arafura Seas and in the Indian Ocean to the north of Christmas Island. The area surrounding the shared boundary consists of a variety of bio-diverse marine habitats including shallow continental shelf waters, oceanic trenches and numerous offshore islands. Both countries exploit a variety of fisheries species, including whaler (Carcharhinus spp.) and hammerhead sharks (Sphyrna spp.). Despite their differences in social and financial arrangements, the two countries are motivated to develop complementary co-management practices to achieve resource sustainability. An essential starting point is knowledge of the degree of population subdivision, and hence fisheries stock status, in exploited species.ResultsPopulations of four commercially harvested shark species (Carcharhinus obscurus, Carcharhinus sorrah, Prionace glauca, Sphyrna lewini) were sampled from northern Australia and central Indonesia. Neutral genetic markers (mitochondrial DNA control region sequence and allelic variation at co-dominant microsatellite loci) revealed genetic subdivision between Australian and Indonesian populations of C. sorrah. Further research is needed to address the possibility of genetic subdivision among C. obscurus populations. There was no evidence of genetic subdivision for P. glauca and S. lewini populations, but the sampling represented a relatively small part of their distributional range. For these species, more detailed analyses of population genetic structure is recommended in the future.ConclusionCooperative management between Australia and Indonesia is the best option at present for P. glauca and S. lewini, while C. sorrah and C. obscurus should be managed independently. On-going research on these and other exploited shark and ray species is strongly recommended. Biological and ecological similarity between species may not be a predictor of population genetic structure, so species-specific studies are recommended to provide new data to assist with sustainable fisheries management.

Bycatch diversity and variation in a tropical Australian penaeid fishery; the implications for monitoring

Abstract The composition of teleost and elasmobranch bycatch in two tropical Australian penaeid prawn fisheries was examined using scientific surveys. These covered the major fishing grounds in February and October 1997. On average 82% of the bycatch was teleosts and elasmobranchs, resulting in a high proportion of the bycatch which is unlikely to survive capture. The bycatch is highly diverse (over 350 species of teleosts and elasmobranchs) and dominated by species which occur rarely (75% of species occurred in Overall the general catch characteristics (e.g. total bycatch weight, catch rate of teleosts and elasmobranchs), bycatch composition and individual species abundances showed significant variation among the regions and between February and October. The variation due to regional differences was much greater than between the two times of year. The inclusion of covariates (depth, acoustic measures of seabed roughness and hardness, commercial prawn catch and commercial fishing effort) explained some of the observed variation but there was still significant regional variation present. The primary implication of the results is the clear need for future monitoring strategies to be stratified with respect to region. Monitoring from a single region or time is unlikely to be representative of the fishery.

Biomasses, catch rates and abundances of demersal fishes, particularly predators of prawns, in a tropical bay in the Gulf of Carpentaria, Australia

The demersal fish fauna of Albatross Bay, in the eastern Gulf of Carpentaria, northern Australia, was sampled on seven cruises from August 1986 to November 1988, using a random stratified trawl survey. Four depth zones between 7 and 45 m were sampled during both day and night. The mean biomass of fish from all seven cruises was 297 kg ha−1 for days trawls and 128 kg ha−1 for night trawls. The overall mean catch rates were 922 kg h−1 for day trawls and 412 kg h−1 for night trawls. There were marked differences between cruises in both the biomass and catch rate. Approx 890 000 fish of 237 species were collected. Of these, 25 species comprised 82% of the total biomass and 74% of the overall catch rate. The dominant families were Leiognathidae, Haemulidae and Clupeidae, with Sciaenidae and Dasyatidae important at night.Leiognathus bindus was the most abundant species. Twenty-five species occurred in more than 50% of trawls, withCaranx bucculentus the most frequently caught (96% of all trawls). Thirty four species were predators on prawns; their absolute mean biomass was 50 kg ha−1 during the day and 39 kg ha−1 at night. The corresponding catch rates were 171 and 125 kg h−1. Multiple-regression analyses were used to discriminate the effects of diel, seasonal, depth and cruise patterns. Of the 31 most abundant species, 15 showed diel patterns of abundance; 11 species showed seasonal patterns of abundance; 23 species had differential depth distribution; and 13 species showed significant cruise-to-cruise variation in abundance. Cruise variations in abundance were tested against salinity, temperature, tidal exchange, plankton biomass and prawn abundances as well as periods (and lags) of total rainfall prior to sampling. Only total rainfall showed any significant correlation. Total rainfall over a period of 6 wk immediately prior to sampling showed significant positive correlations with the abundances of five species, with overall daytime catch rates, and with the suite of 34 prawn predators. Rainfall and river runoff into Albatross Bay were significantly correlated. In Albatross Bay, the complex of factors affecting fish abundances and the magnitude of between-cruise differences indicate that such tropical communities may be unpredictable and are not seasonally constant. The high catch rates in Albatross Bay relative to similar tropical areas elsewhere are discussed and attributed to the light exploitation of the Albatross Bay stocks. Other than a prawn fishery, there is no commercial trawling in Albatross Bay. Hence, the only fishing mortality is a result of by-catch from prawn trawling. The annual total of such fish by-catch is probably less than 10% of the estimated standing stock of 93 000 tonnes.

Pronounced genetic population structure in a potentially vagile fish species (Pristipomoides multidens, Teleostei; Perciformes; Lutjanidae) from the East Indies triangle

The East Indies triangle, bordered by the Phillipines, Malay Peninsula and New Guinea, has a high level of tropical marine species biodiversity. Pristipomoides multidens is a large, long‐lived, fecund snapper species that is distributed throughout the East Indies and Indo‐Pacific. Samples were analysed from central and eastern Indonesia and northern Australia to test for genetic discontinuities in population structure. Fish (n = 377) were collected from the Indonesian islands of Bali, Sumbawa, Flores, West Timor, Tanimbar and Tual along with 131 fish from two northern Australian locations (Arafura and Timor Seas) from a previous study. Genetic variation in the control region of the mitochondrial genome was assayed using restriction fragment length polymorphism and direct sequencing. Haplotype diversity was high (0.67–0.82), as was intraspecific sequence divergence (range 0–5.8%). FST between pairs of populations ranged from 0 to 0.2753. Genetic subdivision was apparent on a small spatial scale; FST was 0.16 over 191 km (Bali/Sumbawa) and 0.17 over 491 km (Bali/Flores). Constraints to dispersal that contribute to, and maintain, the observed degree of genetic subdivision are experienced presumably by all life history stages of this tropical marine finfish. The constraints may include (1) little or no movement of eggs or larvae, (2) little or no home range or migratory movement of adults and (3) loss of larval cohorts due to transport of larvae away from suitable habitat by prevailing currents.

Allozyme and morphological variation throughout the geographic range of the tropical shad, hilsa Tenualosa ilisha

Abstract The population structure of the shad, hilsa Tenualosa ilisha, was investigated with both allozymes and morphometric analysis. Fish samples were collected from nine sites within Bangladesh and compared with samples from four other countries that covered the entire species range. Five polymorphic loci, Idh-l, Idh-m, Mdh-l, Mdh-m and Pgm, were interpretable in liver and muscle with starch–gel electrophoresis, although levels of variation were low. No significant differences in allele frequencies were detected within Bangladesh or within the Bay of Bengal (SE India and Myanmar) samples. Significant differences in allele frequencies occurred between Kuwait, Bangladesh and Indonesia. Nine morphological measurements and two meristic counts were compared among the same fish that were collected for genetic analysis. Discriminant function analyses (DFA) of transformed measurement ratios varied widely between sites, even those separated by short distances (

Species composition and biomasses of fishes in tropical seagrasses at Groote Eylandt, northern Australia

The species composition and biomasses of fishes in the tropical seagrasses of Groote Eylandt, northern Australia, were studied in 1989 and 1990. A total of 156 species was recorded. Tall dense seagrass, short seagrass and control (no seagrass) sites in different depths were compared. Shallow ( H ) and evenness ( E ) were higher in non-vegetated areas. In slightly deeper water ( −2 , which is low relative to other inshore tropical areas. The possible causes—the characteristics of adjacent habitats (coral reefs and mangroves) and the role of seagrasses in the life cycle of fishes are discussed. It is suggested that habitat structure is a major determinant of the species composition of fish in tropical seagrass areas, primarily because it affects food availability, both for small residents and juveniles, and for visiting predators.

Towards better management of Australia’s shark fishery: genetic analyses reveal unexpected ratios of cryptic blacktip species Carcharhinus tilstoni and C. limbatus

The common blacktip shark (Carcharhinus limbatus) and the Australian blacktip shark (C. tilstoni) are morphologically similar species that co-occur in subtropical and tropical Australia. In striking contrast to what has been previously reported, we demonstrate that the common blacktip shark is not rare in northern Australia but occurs in approximately equal frequencies with the Australian blacktip shark. Management of shark resources in northern Australia needs to take account of this new information. Species identification was performed using nucleotide sequences of the control, NADH dehydrogenase subunit 4 (ND4) and cytochrome oxidase I (COI) regions in the mitochondrial genome. The proportion of overall genetic variation (FST) between the two species was small (0.042, P < 0.01) based on allele frequencies at five microsatellite loci. We confirm that a third blacktip species (C. amblyrhynchoides, graceful shark) is closely related to C. tilstoni and C. limbatus and can be distinguished from them on the basis of mtDNA sequences from two gene regions. The Australian blacktip shark (C. tilstoni) was not encountered among 20 samples from central Indonesia that were later confirmed to be common blacktip and graceful sharks. Fisheries regulators urgently need new information on life history, population structure and morphological characters for species identification of blacktip shark species in Australia.