S. J. M. Blaber

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.

Turbidity and salinity in a tropical Northern Australian estuary and their influence on fish distribution

Turbidity and salinity and their influences on fish distribution were studied for two and a half years in the Embley Estuary in tropical northern Australia. Both turbidity and salinity varied significantly during the year but three clearly distinguishable seasonal patterns existed. These are referred to as the Wet, Early Dry and Late Dry Seasons. During each of these seasons distinct gradients of turbidity and salinity were present. The turbidity and salinity gradients were continuous with those in the adjacent marine environment of Albatross Bay. The levels and ranges of both factors were largely determined by the seasonal rainfall patterns in the catchment of the Embley River. The distribution and abundance of the 45 most common species was analysed in relation to turbidity, salinity and temperature patterns in the estuary. These data showed that fish densities within the estuary were related to turbidity and salinity but not temperature. There was a strong inverse relationship between turbidity and salinity. The Catch per Unit Effort (CPUE) of each species was determined in each of three broad ranges of turbidity and salinity. From this, patterns related to these two factors were found for 30 of the 45 species of fish.

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 (

Species composition, community structure and zoogeography of fishes of mangrove estuaries in the Solomon Islands

Mangrove estuaries in the Solomon Islands are well developed, but are small and isolated from each other by extensive fringing coral reef lagoons. A total of 136 species of fish were recorded from 13 estuaries (6 estuaries in Kolombangara, 3 in New Georgia, 3 in Rendova and 1 in the Florida Group); none contained more than 50 species. Sampling took place during five 3 wk expeditions from 1986 to 1988. The mean biomass of 11.60 g m−2 is comparable with that of similar estuaries in northern Australia. Cluster-analysis revealed two patterns of fish species composition. The first group, in which Gobiidae were the most numerous taxon, inhabited soft, muddy-bottom estuaries. The second group, dominated by Pomacentridae, lived in hard-bottom, log-strewn estuaries. The role of the estuaries as nursery grounds for coral reef species was assessed and found to be insignificant, but they are used as feeding grounds by mobile piscivorous species. The species composition of Solomon Island estuaries was compared with that of other Indo-Pacific estuaries. No endemic species were found and the fauna is typical of such mangrove systems throughout the region. However, several taxa that are common in Australia or New Guinea were not found, notably Ariidae, Centropomidae,Pomadasys, and Sciaenidae. These absentees, and the fish fauna as a whole, are discussed in relation to the position of the Solomon Islands at the western edge of the Pacific Plate, the effects of deep-ocean trenches, the recent geological origin of the islands, and possible methods of colonisation from nearby mangroves in Australia and New Guinea. The importance of larval durations and dispersal to colonisation are discussed in relation to oceanic circulation patterns in the Solomon Sea.

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.

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.

Diets of lagoon fishes of the Solomon Islands: Predators of tuna baitfish and trophic effects of baitfishing on the subsistence fishery

Abstract Analyses of the stomach contents of 122 species of predators from fringing reef lagoons of the Solomon Islands showed that 45 species ate baitfish species caught commercially. Baitfish (mainly Stolephorus and Spratelloides species) comprised nearly 25% by weight of their combined diets, but in only 28 of these predators were baitfish a major proportion of the diet. To simulate the subsistence fishery, fishing competitions (using drop-lining, spear-fishingand trolling) were held. A total of 183 species of 31 families was recorded. Drop-lining, the predominant technique, contributed most to the overall catch, although trolling was important in one area. Catches from traditional poisoning methods were also monitored. We concluded that most of the fish caught by the subsistence fishery do not eat baitfish. Some pelagic species, mainly Scombridae, are baitfish predators, but are caught only by trolling. Hence, unless there is a marked increase in trolling among subsistence fishermen, there is little evidence that the commercial baitfishery at present directly affects the food chain on which the subsistence reef fishery depends.

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.

Predation on penaeid prawns by fishes in Albatross Bay, Gulf of Carpentaria

Fishes were trawled from Albatross Bay, on the west coast of Cape York, north Queensland (12°45′S′; 141°30′E) during 4 yr, from August 1986 to April 1989. Penaeids were the first or second most important prey item by dry weight in 14 of the 34 penaeid-eating fish species, and in 12 of the species by frequency of occurrence. Eighteen species of Penaeidae were identified in fish stomachs. The five commercially important species comprised over 70% by dry weight of all the penaeids eaten by all the fishes;Metapenaeus ensis, Penaeus semisulcatus andP. merguiensis comprised 22, 28 and 11%, respectively. Commercially unimportant penaeids comprised 85% by numbers of all penaeids eaten. Larger fishes ate larger penaeids, mainly commercially important species, while smaller fishes ate smaller penaeids, mainly commercially unimportant species. All penaeid-eating fishes also ate some teleost prey and many were primarily piscivorous. Most penaeid-eating fish species took more benthic prey than bentho-pelagic and pelagic prey combined. The fishes with the strongest predation impact on commercially important penaeids wereCaranx bucculentus and four species of elasmobranchs. The highest impact on commercially unimportant penaeids was made by several species of smaller but abundant fishes. An overall annual estimate of 2950 t yr−1 of commercially important penaeids is eaten by all fishes, a much higher figure than the average 870 t yr−1 taken by the fishery. This study highlights the need for accurate measurement of the abundance of penaeid predators as well as analyses of their diets when assessing the impact of predators on prawn stocks.

Variable Success in Breeding of the Roseate Tern Sterna dougallii on the Northern Great Barrier Reef

Breeding of Roseate Terns Sterna dougallii on the northern Great Barrier Reef (11°-15°S) was monitored between 1976 and 1994, and they were recorded breeding on the inshore rocky islands in most years, usually starting April-May. Between 50-5000 pairs were recorded breeding each year and indicate that the northern Great Barrier Reef is an important breeding area for Roseate Terns during peak years when large colonies of 1000-5000 pairs formed on low sand cays in the far northern Great Barrier Reef (11°s). On at least three occasions, breeding success was poor in these large colonies, with high mortality of the almost-fledged chicks. Growth in weight and wing length of Roseate Tern chicks was measured at Eagle Island (14°42′S) between 1983 and 1986 in order to assess the influence of food on growth rates and survival. Chicks took up to 35 days to fledge, significantly longer than the ecologically similar, more numerous Black-naped Tern Sterna sumatrana at the same site at the same time and also almost 10 days longer than Roseate Tern chicks from temperate North American colonies. These data suggest that conditions in northern Australia are rarely favourable for the rapid growth and breeding success of Roseate Terns. Pelagic fish populations around inshore islands in tropical Australia may only be able to support limited numbers of Roseate Terns during the breeding season. This makes northeastern Australian populations of this globally threatened species extremely vulnerable to changes in the availability of their prey during the breeding season.