D.T. Brewer

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.

Sustainability of fishery bycatch: a process for assessing highly diverse and numerous bycatch

In tropical prawn (shrimp) trawl fisheries it is daunting to assess the sustainability of bycatch species because they are diverse and there is little historical and biological information for quantitative stock assessments. We developed a process to examine the likely impact of prawn trawling on the sustainability of bycatch species and applied this to fish bycatch in the Australian Northern Prawn Fishery. The 411 fish bycatch species were ranked with respect to biological and ecological criteria that contributed to two overriding characteristics, namely first, their susceptibility to capture and mortality due to prawn trawling, and second the populations capacity to recover after depletion. The rank of each species on these two characteristics determined its relative capacity to sustain trawling, and therefore its priority for research and management. Species that were the least likely to be sustainable came from the families Apogonidae, Ariidae, Bathysauridae, Callionymidae, Congridae, Diodontidae, Labridae, Opisthognathidae, Plotosidae, Synodontidae and Tetraodontidae. These species are highly susceptible to capture by trawls, they are benthic or demersal, their primary habitat is soft sediments, and their diet may include prawns. The recovery capacity of these species is also low, with the estimated removal rate by trawling high. The species that were the most likely to be sustainable came from the families Carangidae, Clupeidae, Ephippidae, Scombridae, Sphyraenidae and Terapontidae. They are less susceptible to capture by trawls, they are generally pelagic, their primary habitat is not in trawl grounds, and they have a broad depth distribution and range in the fishery. These species also have a greater capacity to recover, as most individuals have bred before capture, and a low estimated removal rate by trawling. The final ranking of the species must be used with caution because of the assumptions made in the process. However, the process is a valuable first step towards ensuring the sustainability of the bycatch species. Because of the simplicity of the process, it can be readily used in fisheries, particularly those with diverse bycatch, to manage the sustainability of their bycatch.

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 (

An assessment of Bycatch Reduction Devices in a tropical Australian prawn trawl fishery

Our study assessed 16 different Bycatch Reduction Devices (BRDs) for use in Australias Northern Prawn Fishery (NPF), during three cruises in the Gulf of Carpentaria. The study was a response to pressure from conservation and community groups to make harvesting of the sea more selective and less damaging to the non-target communities. The approach was to incrementally improve the performance of BRDs after each stage: underwater observations in a flume tank and engineering trials at sea; rigorous and controlled scientific trials on the RV Southern Surveyor; testing under normal commercial fishing conditions. Bycatch Reduction Devices were improved throughout these procedures; only the best-performing devices were tested in the next stage. All four inclined grids (Super Shooter, Nordmore grid, AusTED and NAFTED) were extremely effective at excluding large sharks, stingrays and sea turtles. They were also effective at excluding some of the unwanted small-fish catch, especially when used in combination with other BRDs, such as a fisheye or square-mesh window. Fish exclusion ranged between 0 and 39%. Exclusion of sea snakes was substantially increased by using BRDs, such as the AusTED or square-mesh windows. The ability of BRDs to maintain the catch of the target species was part of the assessment. The Super Shooter had the best prawn retention rate, losing only between 2 and 12% of prawns in scientific trials and gaining 3.4% during the commercial trials. The Super Shooter also performs well in areas where the other inclined grids tend to clog with sponges or other large objects. The NAFTED, which was used only during the commercial trials, lost only 3.3% of the prawn catch of a standard trawl. There is evidence that the fisheye may increase prawn catches. Other devices lost substantial numbers of prawns, but modifications could greatly improve their performance. Some of these devices could be used successfully in their present form. The performance of most BRDs were also strongly affected by the weather and fishing procedures, such as delay in haulback of the trawl. This study has shown that there are BRDs that can be of major benefit to prawn trawl fishers in the NPF, by excluding much of the unwanted bycatch while maintaining catches of commercially valuable prawns. The main benefits to the NPF fishery of using BRDs are: • fewer or no sea turtles caught or killed; • fewer small fish to be sorted from catches; • fewer large animals (stingrays, sharks and sea turtles) that can damage valuable prawns, resulting in a higher catch value; and • decreased fishing impacts on bycatch species, which helps to maintain ecological biodiversity and resilience of these demersal communities. Failure of the NPF and other Australian prawn trawl fisheries to voluntarily adopt BRDs may result in an involuntary adoption brought on by pressures from community, conservation and trade bodies.

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.

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.

The effectiveness of fish and shrimp trawls for sampling fish communities in tropical Australia

Catches of fish in a Frank and Bryce fish trawl during the day and in a Florida Flyer shrimp trawl at night were compared over 2 years (1992, 1993). The effectiveness of a Frank and Bryce demersal trawl as a sampling tool for demersal fish communities was assessed from 122 stations in the far northern section of the Great Barrier Reef. Day-time and night-time catches of the Frank and Bryce net were compared to identify diurnal and nocturnal species. Of a total of 259 species caught, 138 were caught both during the day and at night. An analysis of variance was used to identify species that were caught exclusively or in greater biomass during the day or night; 17 species (mainly small apogonids and mullids) were significantly nocturnal and one (a leiognathid) was significantly diurnal. The overall catch rate of the fish trawl during the day was 395.8 kg h−1 (s.e. 141.3) and that of the shrimp trawl at night was 28.0 kg h−1 (s.e. 1.7). A total of 340 species was caught by both types of net—the fish trawler caught 236 species, the shrimp trawler 243, and 141 were common to both trawl types. An analysis of variance was used to help identify species that were caught in greater biomass by either trawl. Fifty-eight fish species each had a greater biomass in the fish trawl than in the shrimp trawls, for 60 species it was the reverse and for 32 species there was no difference. The fish caught only by the fish trawl were mainly pelagic species (mackerels, carangids) or specimens of large species (lutjanids, sharks and rays), while the fish caught only by the shrimp trawl were small benthic species (apogonids, platycephalids, scorpaenids and flatfish). Over 80% of the fish caught by both nets were small (< 30 cm SL). The length-frequency distributions of only seven species differed significantly between fish and shrimp trawl catches, with the shrimp trawl generally catching the smaller component. No large commercially or recreationally important species of reef fish were caught by the shrimp trawl. The results of this study indicate that each type of trawl net will catch different species and, where they catch the same species, they catch different numbers, sizes and weights. Neither gear type can be used on its own for an adequate description of the fish community.

Diet and dentition in tropical ariid catfishes from Australia

SynopsisThe diets of 13 species of ariid catfishes from the tropical waters of the Gulf of Carpentaria are described and compared. Fishes were collected from two estuaries and inshore and offshore marine areas. Up to 10 species have been recorded from a single estuary. Although all are carnivorous and consume a variety of prey, diet analyses and statistical ordination reveal three feeding guilds - piscivores, polychaete-eaters and molluscivores. The diets of most species are similar between sites. There are strong relationships between dietary guild and the size and arrangement of the palatine teeth. The piscivorous group of catfish (guild 1) have large mouths with relatively large multiple palatine tooth plates, either in a band or in a triangular pattern and armed with sharp recurved teeth. The primarily polychaete-feeding group (guild 2) have a variable mouth size but it is usually smaller than that of guild 1 fish; their palatine teeth plates are fewer and smaller, and they have small, sharp recurved teeth. Guild 3 eat mainly molluscs, and have a small mouth and large posteriorly situated palatine plates with globular, truncated teeth. Overlaps in diet between species are probably reduced by differential distribution patterns within estuaries and different habitat preferences. The mouth-width and tooth-plate arrangements of ariids in tropical Australia are suitable for dealing with broad classes of prey rather than specific items, conferring dietary flexibility. This probably optimizes the trade-off for most species between occupation of broad feeding niches and the ability to shift diet easily.