Causas e consequências da onivoria de peixes em ecossistemas aquáticos

Omnivory is a predominant feeding strategy among tropical fishes, but knowledge about its causes and consequences of this pattern is scarce. In this study we hypothesized that tropical fish feed lower in food web as a way to compensate a higher energetic demand, which increases with increasing water temperature and body size. Information about 8172 freshwater and marine fish species from whole world, from tropical and temperate ecosystems, showed that the trophic position of non-carnivore fish decreases with increasing body size in tropical but not in temperate ecosystems. This result indicates that the higher energetic demand of large-bodied tropical fish should exert a selective force in favor of omnivory. As a consequence, trophic dynamics in tropical freshwater ecosystems should have different patterns comparing to temperate ones, with major implications for water management and restoration of eutrophic aquatic ecosystems. Another hypothesis of this work was that effects of tropical omnivorous planktivorous fish on planktonic communities depend of primary producers stoichiometric composition, which depends of light availability relative to nutrients ratios. A mesocosm experiment, manipulating light availability and planktivorous fish presence, confirmed our hypothesis indicating that resource stoichiometric composition (consequently nutritional quality), determine trophic structure of pelagic food webs in tropical lakes. Finally another mesocosm experiment indicated that the removal of omnivorous benthivorous fish should be more efficient than removal of omnivorous planktivorus fish, as a way to improve water quality in tropical lakes and reservoirs. This last experiment showed that omnivorous planktivorous fish increase phytoplankton biomass due to trophic cascade interactions, without increasing nutrient concentrations in the water column. On the other hand, omnivorous benthivorous fish, feeding on detritus and other benthonic food sources and excreting nutrients in the water column, are responsible for translocate nutrient from sediments to the water column, increasing phosphorus pool and phytoplankton biomass. Thereby, internal phosphorus supply should be reduced and water quality of eutrophicated lakes could be improved by removing omnivorous benthivorous fish.

Causas e consequências da onivoria de peixes em ecossistemas aquáticos

Omnivory is a predominant feeding strategy among tropical fishes, but knowledge about its causes and consequences of this pattern is scarce. In this study we hypothesized that tropical fish feed lower in food web as a way to compensate a higher energetic demand, which increases with increasing water temperature and body size. Information about 8172 freshwater and marine fish species from whole world, from tropical and temperate ecosystems, showed that the trophic position of non-carnivore fish decreases with increasing body size in tropical but not in temperate ecosystems. This result indicates that the higher energetic demand of large-bodied tropical fish should exert a selective force in favor of omnivory. As a consequence, trophic dynamics in tropical freshwater ecosystems should have different patterns comparing to temperate ones, with major implications for water management and restoration of eutrophic aquatic ecosystems. Another hypothesis of this work was that effects of tropical omnivorous planktivorous fish on planktonic communities depend of primary producers stoichiometric composition, which depends of light availability relative to nutrients ratios. A mesocosm experiment, manipulating light availability and planktivorous fish presence, confirmed our hypothesis indicating that resource stoichiometric composition (consequently nutritional quality), determine trophic structure of pelagic food webs in tropical lakes. Finally another mesocosm experiment indicated that the removal of omnivorous benthivorous fish should be more efficient than removal of omnivorous planktivorus fish, as a way to improve water quality in tropical lakes and reservoirs. This last experiment showed that omnivorous planktivorous fish increase phytoplankton biomass due to trophic cascade interactions, without increasing nutrient concentrations in the water column. On the other hand, omnivorous benthivorous fish, feeding on detritus and other benthonic food sources and excreting nutrients in the water column, are responsible for translocate nutrient from sediments to the water column, increasing phosphorus pool and phytoplankton biomass. Thereby, internal phosphorus supply should be reduced and water quality of eutrophicated lakes could be improved by removing omnivorous benthivorous fish.

Leukocytes respiratory burst and lysozyme level in pacu (Piaractus mesopotamicus Holmberg, 1887)

Respostas imunológicas inatas são úteis para determinar o estado de saúde de peixes e avaliar o efeito de substâncias imunomoduladoras no cultivo destes animais. A atividade respiratória de leucócitos foi medida em pacu (Piaractus mesopotamicus) através de ensaio de quimioluminescência e ensaio de redução do nitroblue tetrazolium (NBT). O ensaio de redução do nitroblue tetrazolium pareceu mais adequado que o ensaio de quimioluminescência para determinação da atividade respiratória de leucócitos, uma vez que foi difícil isolar com êxito os leucócitos do sangue para o ensaio de quimioluminescência. Lisozima sérica e plasmática foram medidas por meio de ensaio turbidimétrico. Com o objetivo de inativar as proteínas do sistema complemento, as amostras de soro e plasma foram aquecidas (56 ºC por 30 minutos). Porém, este procedimento provocou a turvação das amostras de plasma e interferiu nos resultados. A atividade de lisozima no soro foi maior que no plasma, sugerindo que amostras de soro são mais apropriadas para esta análise. Este estudo estabeleceu protocolos que podem ser utilizados como ferramentas no estudo de mecanismos imunológicos do peixe tropical pacu.

Tropical prawn trawl bycatch of fish and seasnakes reduced by Yarrow Fisheye Bycatch Reduction Device

Reducing unwanted trawl bycatch is actively encouraged in Australia, particularly in prawn trawl fisheries. We tested the performance of a Bycatch Reduction Device, the Yarrow Fisheye, during two periods of commercial fishing operations in Australia's Northern Prawn Fishery, by comparing the catches of paired treatment and control nets. We compared the catch weights of the small fish and invertebrate bycatch, and the commercially important tiger prawns, from 42 trawls in 2002. The Yarrow Fisheye reduced the weight of small bycatch by a mean of 22.7%, with no loss of tiger prawn. We also compared the numbers of seasnakes caught in 41 and 72 trawls during the spring trawling seasons of 2004 and 2005, respectively. The Yarrow Fisheye reduced the catches by a mean of 43.3%. Flume-tank tests of the Yarrow Fisheye showed that this device created a slow water-flow region extending over 2 m downstream from its position in the net, and close to where the catch accumulates. Finfish and seasnakes may be exploiting this slow water-flow region to escape via the eye, Although the reductions in fish and seasnake bycatch were excellent, we think they could be further improved by relating differences in fisheye position and localised water displacements, to design and rigging changes.

Quantitative assessment of total and Gram-positive aerobic bacteria in fresh and ambient-temperature-stored sub-tropical marine fish

The current study was undertaken to enumerate Gram-positive bacteria in fresh sub-tropical marine fish and determine the effect of ambient storage (25°C) on the Gram-positive bacterial count. Total and Gram-positive bacteria were enumerated in the muscles, gills and gut of fresh and stored Pseudocaranx dentex, Pagrus auratus and Mugil cephalus on tryptone soya agar (TSA) and TSA with 0.25% phenylethyl alcohol (PEA), respectively. Initial studies indicated that PEA significantly reduced total aerobic bacterial count (TABC) whereas control Gram-positive bacteria were not affected by 0.25% PEA. TABC significantly increased in all fish body parts, whereas Gram-positive aerobic bacterial count (GABC) significantly increased only in the muscles and gills during ambient storage for 15 h. The TABC of the fish species increased from 4.00, 6.13 and 4.58 log cfu g-1, respectively in the muscles, gills, and gut to 6.31, 7.31 and 7.23 log cfu g-1 by the end of storage. GABC increased from 2.00, 3.52 and 2.20 log cfu g-1 to 4.70, 5.85 and 3.36 log cfu g-1. Within each species, TABC were significantly higher in the gills compared to that of muscles and gut; however, no significant differences were found in GABC between muscles and gills. This study demonstrated the potential importance of Gram-positive bacteria in sub-tropical marine fish and their spoilage.

Variable growth rates of the tropical estuarine fish barramundi Lates calcarifer (Bloch) under different freshwater flow conditions

Relationships between freshwater flows and growth rates of the opportunistic predatory finfish barramundi Lates calcarifer in a dry tropical estuary were examined using data from a long-term tag-recapture programme. Lagged effects were not investigated. After accounting for length at release, time at liberty and seasonal variation (e.g. winter, spring, summer and autumn), growth rates were significantly and positively related to fresh water flowing to the estuary. Effects were present at relatively low levels of freshwater flow (i.e. 2.15 m3 s-1, the 5th percentile of the mean flow rate experienced by fish in the study during time at liberty). The analysis, although correlative, provides quantitative evidence to support the hypothesis that freshwater flows are important in driving the productivity of estuaries and can improve growth of species high in the trophic chain.

Effect of changes in hook pattern and size on catch rate, hooking location, injury and bleeding for a number of tropical reef fish species

The Queensland Great Barrier Reef line fishery in Australia is regulated via a range of input and output controls including minimum size limits, daily catch limits and commercial catch quotas. As a result of these measures a substantial proportion of the catch is released or discarded. The fate of these released fish is uncertain, but hook-related mortality can potentially be decreased by using hooks that reduce the rates of injury, bleeding and deep hooking. There is also the potential to reduce the capture of non-target species though gear selectivity. A total of 1053 individual fish representing five target species and three non-target species were caught using six hook types including three hook patterns (non-offset circle, J and offset circle), each in two sizes (small 4/0 or 5/0 and large 8/0). Catch rates for each of the hook patterns and sizes varied between species with no consistent results for target or non-target species. When data for all of the fish species were aggregated there was a trend for larger hooks, J hooks and offset circle hooks to cause a greater number of injuries. Using larger hooks was more likely to result in bleeding, although this trend was not statistically significant. Larger hooks were also more likely to foul-hook fish or hook fish in the eye. There was a reduction in the rates of injuries and bleeding for both target and non-target species when using the smaller hook sizes. For a number of species included in our study the incidence of deep hooking decreased when using non-offset circle hooks, however, these results were not consistent for all species. Our results highlight the variability in hook performance across a range of tropical demersal finfish species. The most obvious conservation benefits for both target and non-target species arise from using smaller sized hooks and non-offset circle hooks. Fishers should be encouraged to use these hook configurations to reduce the potential for post-release mortality of released fish.

Speciation of Gram-positive bacteria in fresh and ambient-stored sub-tropical marine fish.

This study identified Gram-positive bacteria in three sub-tropical marine fish species: Pseudocaranx dentex (silver trevally), Pagrus auratus (snapper) and Mugil cephalus (sea mullet). It further elucidated the role played by fish habitat, fish body part and ambient storage on the composition of the Gram-positive bacteria. A total of 266 isolates of Gram-positive bacteria were identified by conventional biochemical methods, VITEK, PCR using genus- and species-specific primers and/or 16S rRNA gene sequencing. The isolates were found to fall into 13 genera and 30 species. In fresh fish, Staphylococcus epidermidis and Micrococcus luteus were the most frequent isolates. After ambient storage, S. epidermidis, S. xylosus and Bacillus megaterium were no longer present whereas S. warned, B. sphaericus, Brevibacillus borstelensis, Enterococcus faecium and Streptococcus uberis increased in frequency. Micrococcus luteus and S. warned were the most prevalent isolates from P. dentex, while E. faecium and Strep. uberis were the most frequent isolates from P. auratus and M. cephalus. With respect to different parts of the fish body. E. faecium, Strep. uberis and B. sphaericus were the most frequent isolates from the muscles, E. faecium, Strep. uberis from the gills and M. luteus from the gut. This study showed a diversity of Gram-positive bacteria in sub-tropical marine fish; however, their abundance was affected by fish habitat, fish body part and ambient storage.

Assessing the effects of line capture and barotrauma relief procedures on post-release survival of key tropical reef fish species in Australia using recreational tagging clubs.

Common coral trout, Plectropomus leopardus Lacepede, crimson snapper, Lutjanus erythropterus Bloch, saddletail snapper, Lutjanus malabaricus (Bloch & Schneider), red emperor, Lutjanus sebae (Cuvier), redthroat emperor, Lethrinus miniatus (Schneider) and grass emperor, Lethrinus laticaudis Alleyne & Macleay, were tagged to determine the effects of barotrauma relief procedures (weighted shot-line release and venting using a hollow needle) and other factors on survival. Release condition was the most significant factor affecting the subsequent recapture rate of all species. Capture depth was significant in all species apart from L. malabaricus and L. miniatus, the general trend being reduced recapture probability with increasing capture depth. Recapture rates of fish hooked in either the lip or mouth were generally significantly higher than for those hooked in the throat or gut. Statistically significant benefit from treating fish for barotrauma was found in only L. malabaricus, but the lack of any negative effects of treating fish indicated that the practices of venting and shot-lining should not be discouraged by fisheries managers for these species.

Management of 'Tough' fish syndrome (TFS) in tropical Saddletail Snapper to re-instill market confidence

Successful identification of these factors influence upon TFS will empower stakeholders to make informed decisions as to how to best utilise the resource, boost consumer confidence thus ensuring the improved profitability of the fishery into the future.

Fish mediate high food web connectivity in the lower reaches of a tropical floodplain river

High levels of hydrological connectivity during seasonal flooding provide significant opportunities for movements of fish between rivers and their floodplains, estuaries and the sea, possibly mediating food web subsidies among habitats. To determine the degree of utilisation of food sources from different habitats in a tropical river with a short floodplain inundation duration (similar to 2 months), stable isotope ratios in fishes and their available food were measured from three habitats (inundated floodplain, dry season freshwater, coastal marine) in the lower reaches of the Mitchell River, Queensland (Australia). Floodplain food sources constituted the majority of the diet of large-bodied fishes (barramundi Lates calcarifer, catfish Neoarius graeffei) captured on the floodplain in the wet season and for gonadal tissues of a common herbivorous fish (gizzard shad Nematalosa come), the latter suggesting that critical reproductive phases are fuelled by floodplain production. Floodplain food sources also subsidised barramundi from the recreational fishery in adjacent coastal and estuarine areas, and the broader fish community from a freshwater lagoon. These findings highlight the importance of the floodplain in supporting the production of large fishes in spite of the episodic nature and relatively short duration of inundation compared to large river floodplains of humid tropical regions. They also illustrate the high degree of food web connectivity mediated by mobile fish in this system in the absence of human modification, and point to the potential consequences of water resource development that may reduce or eliminate hydrological connectivity between the river and its floodplain.

Improving Profitability to Industry Through the Identification and Management of 'tough' Fish Syndrome in Tropical Saddletail Snapper (Lutjanus Malabaricus)

OBJECTIVES: 1. To determine whether incomplete rigor mortis resolution and 'cold shock' play a role in development of tough fish syndrome (TFS) in tropical Saddletail snapper. 2. To identify links between TFS and specific physiological factors in tropical Saddletail snapper. 3. Communicate findings and recommendations to stakeholders and assist with implementation of any changes to fishing or handling practices required.

Fish bioecology in relation to sediments in the Mekong and in tropical rivers

Sediments are an essential component of rivers and of their biological functioning. In addition to their influence on river geomorphology (maintenance of river forms and habitats such as pools and sand bars), sediments also include nutrients, detritus and organic debris of various sizes which interact with the river’s different life forms, including fish. The interaction between sediments and aquatic organisms, directly or indirectly through the effects of sediments on physical habitats, unquestionably influences the biodiversity and productivity of a river. The current report reviews the interactions between sediments and fish in tropical rivers and in the Mekong, and focuses more specifically on a reduction of sediment loads following dam const

New information from fish diets on the importance of glassy flying squid (Hyaloteuthis pelagica) (Teuthoidea: Ommastrephidae) in the epipelagic cephalopod community of the tropical Atlantic Ocean

Squids of the family Ommastrephidae are a vital part of marine food webs and support major fisheries around the world. They are widely distributed in the open ocean, where they are among the most abundant in number and biomass of nektonic epipelagic organisms. In turn, seven of the 11 genera of this family (Dosidicus, Illex, Martialia, Nototodarus, Ommastrephes, Sthenoteuthis, and Todarodes) are heavily preyed upon by top marine predators, i.e., birds, mammals, and fish, and currently support fisheries in both neritic and oceanic waters (Roper and Sweeney, 1984; Rodhouse, 1997). Their commercial importance has made the large ommastrephids the target of many scientific investigations and their biology is consequently reasonably well-known (Nigmatullin et al., 2001; Zuyev et al., 2002; Bower and Ichii, 2005). In contrast, much less information is available on the biology and ecological role of the smaller, unexploited species of ommastrephids (e.g., Eucleoteuthis, Hyaloteuthis, Ornithoteuthis, and Todaropsis).