High Fish Densities Research Articles

Benthic community structure and ecosystem functions in above- and below-waterfall pools in Borneo

Waterfalls are geomorphic features that often partition streams into discrete zones. Our study examined aquatic communities, litter decomposition and periphyton growth rates for above- and below-waterfall pools in Ulu Temburong National Park, Brunei. We observed higher fish densities in below-waterfall pools (0.24 fish m−2 vs. 0.02 fish m−2 in above-waterfall pools) and higher shrimp abundance in above-waterfall pools (eight shrimp/pool vs. less than one shrimp/pool in below-waterfall pools). However, macroinvertebrate densities (excluding shrimp) were similar among both pool types. Ambient periphyton was higher in below-waterfall pools in 2013 (4.3 vs. 2.8 g m−2 in above-waterfall pools) and 2014 (4.8 vs. 3.4 g m−2 in above-waterfall pools), while periphyton growth rates varied from 0.05 to 0.26 g m−2 days−1 and were significantly higher in below-waterfall pools in 2014. Leaf litter decomposition rates (0.001 to 0.024 days−1) did not differ between pool types, suggesting that neither shrimp nor fish densities had consistent impacts on this ecosystem function. Regardless, this research demonstrates the varied effects of biotic and abiotic factors on community structure and ecosystem function. Our results have highlighted the importance of discontinuities, such as waterfalls, in tropical streams.

Mortality of Cryptocaryon irritans in sludge from a digester of a marine recirculating aquaculture system

Cryptocaryon irritans, a marine protozoan fish parasite with a life cycle that includes cyst formation, constitutes a major concern in intensive recirculating aquaculture systems (RASs), due to its potential propagation in a system with high fish density and minimal water change. In this study, the survival of C. irritans in sludge collected from an up-flow anaerobic sludge blanket (UASB) reactor of a RAS system was studied using a PCR assay. The detection sensitivity of the different life stages in sludge was determined to be 50 tomonts, 5 theronts and 5 trophonts. Dead C. irritans DNA was not detectable after 24h of inoculation. The survival of the different C. irritans life stages in anaerobic conditions, as present in the UASB reactor, was evaluated. Theronts died within 3h post-exposure to anaerobic conditions. Protomonts encysted within a day post-exposure, but unlike in aerobic conditions, the resulting tomonts did not hatch. Excystment of theronts from tomonts was delayed in anaerobic conditions, and occurred only after transfer to an aerobic environment. However, the tomonts did not die after 96h of such exposure and the release rate of theronts did not decrease, compared to the control. The results suggest that infective theronts cannot originate from the UASB reactor and protomonts or theronts that reach the UASB do not survive. However, tomonts appear to be able to survive the reactor's conditions, and if they are released from it with the water outflow, they could hatch in aerobic parts of the RAS.

Effects of small-sized crucian carp (Carassius carassius) on the growth of submerged macrophytes: Implications for shallow lake restoration

Reestablishment of submerged macrophytes is considered important when restoring shallow eutrophic lakes. To improve water clarity and consequently the growth conditions of macrophytes, removal of plankti-benthivorous fish has been used. In sub-tropical shallow lakes, however, rapid recruitment of small fish, especially benthivores during restoration, may hamper early reestablishment of submerged macrophytes. Crucian carp (Carassius carassius) and Vallisneria natans are common species dominated in sub-tropical shallow lakes. To investigate the effect of small benthivorous fish on the growth of Vallisneria natans, a 28-day outdoor controlled experiment was undertaken in 12 mesocosms with three densities of Carassius carassius − low (10gm−2), high (40gm−2) and no fish (all in four replicates). The results showed that the fish significantly increased chlorophyll-a concentrations and periphyton biomass in both fish treatments, most significantly at high density for Chl a. This concurs with an increase in nutrient concentrations, likely mediated by fish sediment disturbance and excretion, and a reduction of zooplankton biomass (less algal grazing). Increased concentrations of inorganic suspended solids with increasing fish density further enhanced turbidity, causing shading of the macrophytes. Accordingly, the relative growth rate, ramet number and root/leaf ratio of V. natans decreased significantly at low and high fish density compared with the controls, but the effects did not depend on fish density. However, mean leaf length rose with increasing fish density, likely to allow the plants to obtain more light. Overall, our results show that not only large-bodied carp, as demonstrated frequently, but also small-sized crucian carp posed a constraint on submerged macrophyte reestablishment, and we conclude that crucian carp may hamper restoration efforts in sub-tropical shallow lakes. Restoration by biomanipulation should therefore target also small-sized crucian carp.

Protein markers and mechanisms of Bacterial Cold Water Disease in Rainbow Trout (Oncorynchus mykiss) revealed using mass spectrometry proteomic analysis

Aquaculture is a rapidly expanding agricultural field, especially at the international level. One of the largest issues which arise from raising high densities of fish in a confined area is infection and rapid transmission of disease within the population. There are currently few federally approved pharmaceutical treatments for fish, making rapid and accurate diagnosis essential for maintaining healthy populations. The current study focuses on the infection of rainbow trout (Oncorynchus mykiss) by the gram‐negative bacteria Flavobacterium psychrophilum which manifests as Bacterial Cold Water Disease (BCWD). The approach involves examining the physiological effects of infection in these fish at the protein level. Using LC‐MS techniques, the majority of proteins in the kidney, spleen, plasma, and muscles at site of infection are analyzed for differential abundance between infected and control populations. Artificially infected cohorts are compared against fish taken from an active hatchery to determine whether lab results are reflected in a real‐world context. Sets of proteins which change in abundance as a response to BCWD infection make ideal candidates as bio‐markers which can be used in the development of diagnostic tools. This method also reveals the underlying mechanisms of the infection and response through gene ontology and pathway analysis. Plasma was found to have the greatest regulation in terms of significantly different protein abundance. For this reason and the relative ease of sampling plasma without fish sacrifice, plasma appears to the best tissue for diagnostic purposes. The greatest protein regulation in the plasma was seen in enzymes involved in glycolysis, actin cytoskeletal regulation, immune response, and oxidation‐reduction regulation.Support or Funding InformationSupported by NSF Grant IOB‐1355098

In‐stream structure alters density‐dependent fish effects in stream ecosystems

AbstractThe independent effects of in‐stream structure (ISS) and fish foraging on stream properties have been well documented, but few studies have explored the interactive effects of ISS and fishes on streams. Herein, we tested the independent and interactive effects of ISS and a generalist fish (Blacktail shiner, Cyprinella venusta) on suspended organic matter (SOM), benthic algae, invertebrate density and fish growth using experimental mesocosms. We found that Blacktail shiner foraging affected all of the ecosystem properties; however, in some cases, the fish effects differed between mesocosms with and without ISS. Specifically, mesocosms with ISS provided greater surface area for invertebrate colonisation and enhanced food resources for Blacktail shiner. As a result, benthic foraging by Blacktail shiner was reduced in these mesocosms. The reduced benthic foraging in turn enhanced benthic algae and benthic invertebrates via a bottom‐up, nutrient excretion pathway. The ISS‐dependent effects of fish on these stream properties, however, were only evident at low and intermediate fish densities (1 and 2 fish·m−2 respectively). This was likely because at the highest fish density (4 fish·m−2) intense fish foraging overrode any mediating effects of ISS. Furthermore, fish growth decreased with fish density because of intraspecific competition, but this negative effect on growth was reduced in mesocosms with ISS because of the increased forage base. However, the positive effect on fish growth was weak and only marginally significant. Our data suggest that fish‐mediated effects on streams are context dependent, changing with microhabitat availability (e.g. ISS) and density of the fish population.

Fine-tuning for the tropics: application of eDNA technology for invasive fish detection in tropical freshwater ecosystems.

Invasive species pose a major threat to aquatic ecosystems. Their impact can be particularly severe in tropical regions, like those in northern Australia, where >20 invasive fish species are recorded. In temperate regions, environmental DNA (eDNA) technology is gaining momentum as a tool to detect aquatic pests, but the technology's effectiveness has not been fully explored in tropical systems with their unique climatic challenges (i.e. high turbidity, temperatures and ultraviolet light). In this study, we modified conventional eDNA protocols for use in tropical environments using the invasive fish, Mozambique tilapia (Oreochromis mossambicus) as a detection model. We evaluated the effects of high water temperatures and fish density on the detection of tilapia eDNA, using filters with larger pores to facilitate filtration. Large-pore filters (20μm) were effective in filtering turbid waters and retaining sufficient eDNA, whilst achieving filtration times of 2-3min per 2-L sample. High water temperatures, often experienced in the tropics (23, 29, 35°C), did not affect eDNA degradation rates, although high temperatures (35°C) did significantly increase fish eDNA shedding rates. We established a minimum detection limit for tilapia (1 fish/0.4megalitres/after 4days) and found that low water flow (3.17L/s) into ponds with high fish density (>16fish/0.4megalitres) did not affect eDNA detection. These results demonstrate that eDNA technology can be effectively used in tropical ecosystems to detect invasive fish species.

Is there a feeding selectivity of Sarpa salpa over Caulerpa spp vs. seagrasses in Mediterranean meadows?

Event Abstract Back to Event Is there a feeding selectivity of Sarpa salpa over Caulerpa spp vs. seagrasses in Mediterranean meadows? Candela Marco-Méndez1*, Luis Ferrero-Vicente1, Patricia Prado2 and Jose Luis Sáchez-Lizaso1 1 University of Alicante, Marine Science and Applied Biology, Spain 2 IRTA, Aquatic ecosystems, Spain Mediterranean seagrass ecosystems are endangered by increased colonization of Caulerpa species, which may replace them, affecting key ecosystem processes such as herbivore preferences and the local herbivory pressure. The fish Sarpa salpa (L.) is one of the main macroherbivores in the western Mediterranean seagrass meadows which is also known to feed on a wide range of macroalgae such as Caulerpa species providing a certain resistance to invasion of native assemblages. In this study we investigate if S. salpa feed selectively on Caulerpa spp vs. seagrasses and we quantify consumption rates in order to elucidate the potential role of this herbivore on the spread of invasive species in Mediterranean seagrass meadows. For this aim, during summer-autumn 2012 we compared S. salpa herbivory on C. prolifera and C. cylindracea to that on Posidonia oceanica and Cymodocea nodosa in a mixed meadow. A combination of field experiments and dietary analyses were used to investigate consumption rates, dietary contributions, and feeding preferences for the different macrophytes, including the role of epiphytes and nutrient contents in mediating fish herbivory. In summer, C. nodosa was the most consumed macrophyte, probably influenced by higher fish densities, higher nutritional quality of leaves and epiphytes, and by differences in epiphyte community structure. Feeding observations suggest that fish may have a variable diet, although with a consistent selection of mixed patches with C. nodosa and C. prolifera. Indeed, food choice experiments suggest that when seagrass leaves are not epiphytized, fish prefer feeding on C. prolifera. Gut content and stable isotopic analyses supported the dietary importance of epiphytes and C. prolifera but also suggested that C. cylindracea could occasionally be an important food item for S. salpa. Our results highlight the role of epiphytes in S. salpa feeding decisions but also suggest that C. nodosa and C. prolifera may have an important contribution to fish diet. This variability in S. salpa feeding decisions confirms the need to carry out multiple dietary approaches for understanding of the potential effects of herbivory in complex systems with multiple macrophytes species. Acknowledgements This research was funded by the Department of Marine Science and Applied Biology (University of Alicante). We thank Pablo Sánchez-Jérez and especially David Izquierdo-Gomez and Pablo Arechavala-López whose support has been essential in providing S. salpa individuals and teaching us dissection and field measurement techniques. Keywords: food choice, Cymodocea nodosa, Posidonia oceanica, Caulerpa prolifera, Caulerpa cylindracea. Conference: XIX Iberian Symposium on Marine Biology Studies, Porto, Portugal, 5 Sep - 9 Sep, 2016. Presentation Type: Oral Presentation Topic: 1. ECOLOGY, BIODIVERSITY AND VULNERABLE ECOSYSTEMS Citation: Marco-Méndez C, Ferrero-Vicente L, Prado P and Sáchez-Lizaso J (2016). Is there a feeding selectivity of Sarpa salpa over Caulerpa spp vs. seagrasses in Mediterranean meadows?. Front. Mar. Sci. Conference Abstract: XIX Iberian Symposium on Marine Biology Studies. doi: 10.3389/conf.FMARS.2016.05.00056 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 27 Apr 2016; Published Online: 02 Sep 2016. * Correspondence: PhD. Candela Marco-Méndez, University of Alicante, Marine Science and Applied Biology, Alicante, 03690, Spain, candela.marco@ua.es Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Candela Marco-Méndez Luis Ferrero-Vicente Patricia Prado Jose Luis Sáchez-Lizaso Google Candela Marco-Méndez Luis Ferrero-Vicente Patricia Prado Jose Luis Sáchez-Lizaso Google Scholar Candela Marco-Méndez Luis Ferrero-Vicente Patricia Prado Jose Luis Sáchez-Lizaso PubMed Candela Marco-Méndez Luis Ferrero-Vicente Patricia Prado Jose Luis Sáchez-Lizaso Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Influence of riparian forests on fish assemblages in temperate lowland streams

The characteristics of riparian vegetation along streams vary with natural and anthropogenic factors. Deforestation for agricultural purposes has consequences for the physical in-stream structure and function, such as the predominance of autotrophic or heterotrophic stream metabolism. Open canopy lowland streams are often dominated by macrophytes, with potential direct and indirect effects on the fish community. We tested for possible differences in the structure (relative abundance of species, mean body size, and density) and composition (species richness, species identity, and different trophic groups) of fish assemblages between open canopy streams (OCS) and riparian forest streams (RFS), including pool and riffle habitats, in temperate lowland Denmark. OCS reaches exhibited higher alpha and beta diversity and frequently hosted rare species. Almost 50 % of the recorded species appeared only in OCS. OCS also had smaller mean body size of fish and tended to have higher fish densities. The relative abundance of the different trophic groups did not differ between the two streams types, but the RFS had a higher abundance and occurrence frequency of intolerant salmonids. Our results suggest that modification of riparian habitats can affect richness patterns and that strong functional changes may occur as a consequence of forest clearance through changes in the relative importance of a keystone species, trout (Salmo trutta).

Effect of fine wood on juvenile brown trout behaviour in experimental stream channels

AbstractIn‐stream wood can increase shelter availability and prey abundance for stream‐living fish such as brown trout, Salmo trutta, but the input of wood to streams has decreased in recent years due to harvesting of riparian vegetation. During the last decades, fine wood (FW) has been increasingly used for biofuel, and the input of FW to streams may therefore decrease. Although effects of in‐stream FW have not been studied as extensively as those of large wood (LW), it is probably important as shelter for small‐sized trout. In a laboratory stream experiment, we tested the behavioural response of young‐of‐the‐year wild brown trout to three densities of FW, with trout tested alone and in groups of four. Video recordings were used to measure the proportion of time allocated to sheltering, cruising and foraging, as well as the number of aggressive interactions and prey attacks. Cruising activity increased with decreasing FW density and was higher in the four‐fish groups than when fish were alone. Foraging decreased and time spent sheltering in FW increased with increasing FW density. Our study shows that juvenile trout activity is higher in higher fish densities and that trout response to FW is related to FW density and differs from the response to LW as reported by others.

Effects of hydrographic conditions of ponds on juvenile fish assemblages in the Kakum mangrove system, Ghana

The importance of mangrove ecosystems as nursery grounds for fishes is well established and documented. This paper reports on the possible role of hydrographic dynamics in the selection and utilization of tropical mangrove ponds as nursery habitats for juvenile fishes of ecological and commercial importance. The study was conducted in the Kakum River Estuary mangrove ecosystem near Elmina in the Central Region of Ghana during the peak of the monsoon season. Fish assemblages were sampled using a cast net and a pole seine. Aspects of hydrographic conditions of the ponds were studied. A total of 265 fish specimens, belonging to 18 species and 12 families were sampled. The commonest fish was the blackchinned tilapia Sarotherodon melanotheron, a typical brackish water fish in West Africa, which accounted for 66.4% of the total fish sampled. Ten of the 18 species belonging to 7 families, namely Elops lacerta, Eucinostomus melanopterus, Porogobius schlegelii, Gobionellus occidentalis, Mugil bananensis, Liza falcipinnis, Epinephelus sp., Serranus accraensis, Penaeus notialis and Callinectes amnicola were of marine origin. Spatial and temporal variations in the prevailing hydrographic conditions appeared to influence the fish species composition, density and size classes to varying degrees. Changes in salinity, dissolved oxygen, pH, conductivity and pond size correlated significantly with abundance of juvenile fish. Major fluctuations in fish species composition occurred on a weekly basis with changing pond depth and volume, indicating the dynamic nature of mangroves ponds that may serve the ecological needs of different species over time. Smaller juvenile fishes seemed better adapted to high variations in hydrographic conditions compared to larger juveniles. Higher fish densities and lower species diversity were encountered when the ponds were shallow compared to deep ponds. The utilization of mangrove tidal ponds as nurseries by juvenile fish may therefore be influenced primarily by the salinity and pond size.

Freshwater treatment of amoebic gill disease and sea-lice in seawater salmon production: Considerations of water chemistry and fish welfare in Norway

Abstract Amoebic gill disease (AGD) and sea lice are two of the most significant disease issues that the Norwegian Atlantic salmon aquaculture industry is facing. Although both diseases respond to various extents, to freshwater treatment, the chemistry, interactions, and efficacy of treatment can be variable. These variations can have significant impacts upon the success and failure of treatment and costs to the production cycle. Although it is known that soft freshwater is most effective in bathing of Atlantic salmon with AGD and that most of the freshwaters in Norway fall into the soft category, the low alkalinity and buffering capacity of such waters may impact on the pH and metal toxicity of the water source in use. Similarly dissolved organic carbon can be beneficial in treatment, although sequestration of metal ions can be reversed as the water pH drops due to high densities of fish and accumulations of carbon dioxide. Alternative treatments such as the use of oxidative disinfectants such as hydrogen peroxide used for AGD and sea lice control may have potential although the interactions in seawater with organic loads and dissolved organic carbon are unclear. Similarly the use of oxidative disinfectants in freshwater will depend upon the water chemistry and interactions with treatment chemicals, fish, and water organic content. The logistics of treating large biomasses of Atlantic salmon on marine farms are challenging. The use of well boats offers potential although maintaining water quality during treatments is essential for both AGD and sea lice treatments to optimize fish welfare and treatment efficacy.

Patchwork of oil and gas facilities in Saudi waters of the Arabian Gulf has the potential to enhance local fisheries production

AbstractBecause of the increasing oil industry development in the Arabian Gulf, hundreds of oil and gas facilities have been installed in both offshore and inshore areas during the last few decades. However, no studies have been conducted till now on the influence of these platforms on the structure and composition of marine faunal assemblages. The present work addresses this issue to propose environmental management measures connected to the utilization of fishery resources. Offshore and inshore surveys were carried out along the Saudi Gulf waters using trawl and beach-seine nets, respectively. Data relative to only fish (offshore) and fish and invertebrates (inshore) were collected concurrently with several factors: density of oil and gas facilities (offshore), distance to the nearest coastal platform (inshore), oceanographic variables, and habitat characteristics. Results of offshore surveys indicated higher fish density—both total and of fishery resources—in locations with a higher number of oil and gas facilities within a 5 km radius, whereas biomass density was not significantly different. Hence, oil and gas facilities seem to serve as nursery areas for small fish. For inshore communities, more species and diversity were found in stations closer to coastal oil and gas facilities. In addition, among the five coastal embayments sampled, those with more oil and gas facilities had more species. The findings of the present work support the hypothesis of a positive net ecological role of oil and gas platforms of the Saudi Arabian Gulf, with the implication that this effect could be extended to improve the sustainability of important fishery resources.

Capture and Brief Invasive Procedures Using Electronarcosis Does Not Appear to Affect Postrelease Habits in Male Atlantic Sturgeon During the Spawning Season

AbstractWith advances in technology and demand for life history information, researchers are increasingly conducting invasive procedures on fish that require an anesthetic. This study examined the effectiveness of electronarcosis as a field anesthetic on Atlantic Sturgeon Acipenser oxyrinchus oxyrinchus during the spawning season in the James River, Virginia. Concerns about sampling Atlantic Sturgeon during spawning runs prompted our examining whether movements after capture, narcosis, and tagging were noticeably modified. An electronarcosis system, which consisted of a power supply, fiberglass tank, and hardware cloth, were set up both on land and on a sampling boat. During the spawning season Atlantic Sturgeon were caught and implanted with Vemco V16 telemetry tags using electronarcosis as an anesthetic. Anesthesia induction, surgery, and recovery averaged 5 min. Telemetry data from the tagging year was compared with returning fish tagged in previous years. This showed movements during the spawning season and spawning exit dates were similar between the two groups, suggesting that electronarcosis was effective and time‐efficient for conducting invasive procedures. Capturing and implanting transmitters in Atlantic Sturgeon during spawning runs does not appear to modify spawning movements. Managers may benefit from targeting adult Atlantic Sturgeon and other iteroparous anadromous fishes in rivers during spawning periods, which is typically very efficient due to high densities of fish during that period.Received February 21, 2014; accepted December 31, 2014

Dynamics of intracohort cannibalism and size heterogeneity in juvenile barramundi (Lates calcarifer) at different stocking densities and feeding frequencies

Abstract Intracohort cannibalism is a major concern for the aquaculture of piscivorous fish species. This study investigates the effects of stocking density and feeding frequency on dynamics of cannibalism, growth and size distribution in juvenile barramundi ( Lates calcarifer ) over time. Cannibalism was significantly reduced or delayed by the use of high feeding frequency and low fish density. In the fish fed once a day, food restriction stimulated cannibalism through heterogeneous growth and size dispersal due to hunger and food competition. The number of potential cannibals gradually emerged from the conspecifics and provoked cannibalism over time. The increase of feeding frequency from one to three times a day enabled most fish to gain access to the formulated diet and achieve homogeneous growth rates through the reduction of food competition. Consequently, the emergence of potential cannibals and the dynamics of size dispersal, skewness and cannibalism at a high feeding frequency were postponed in comparison with the feeding once a day. The increase in fish density intensified cannibalism regardless of feeding frequency. The present study indicates that the increase of feeding frequency and decrease of fish density can significantly reduce size heterogeneity and consequently alleviate or postpone the cannibalism in barramundi.

Environmental and socioeconomic assessment of a poorly known coastal section in the southern Mexican Caribbean

The Uvero-Punta Piedras section, a low-developed coastal strip located in the southern area of Quintana Roo, Mexico, is experiencing habitat degradation driven by unplanned population sprawl, unregulated tourism expansion and overfishing. The main objective of this study is to provide an environmental (coastal vegetation and coral reef condition) and socioeconomic (human population condition) baseline data of a poorly documented and weakly managed zone by the use of a set of rapid assessment methodologies, looking to assist the regional efforts to manage these coastal resources. To achieve this goal we used a series of surveying methods including remote sensing, geographic information systems and in situ land/underwater surveys, to allow a broad characterization of the resources' condition in the study zone. Results showed that reef habitat was dominated by macro-algae (61.2 ± 7.6%), followed by soft coral (gorgonians, 12.4 ± 4.1%), hard coral (8.2 ± 5.1%) and sponges (2.5 ± 1.3%). Zooanthids and tunicates represented less than 1% of the total; coral and macro algae estimates suggest a decline from records of 15 years ago. Highest fish densities were (144 ± 124 individuals/100 m2), while lowest were (83 ± 25 individuals/100 m2). The total average fish density recorded could also be reflecting a decrease trend in the reef community structure. The main economic activities in the area are fishing and land clearing. From December 2003, 22% (corresponding to 44 ha) of the total original vegetation cover in the studied area was deforested. By February 2007, the deforested area increased 4.4 ha more. Should this tendency continue, by 2025 it would only maintain 24% of its total coastal vegetation cover representing a huge habitat loss. The future of economic activities in these areas lie directly on the establishment of appropriate management strategies for the protection and conservation of the renowned biodiversity that this area comprises.

Seasonal depletion of resources intensifies trophic interactions in subarctic freshwater fish communities

Summary Climate change in recent decades has resulted in an increase in both the density and diversity of consumers in subarctic freshwater ecosystems. Despite this, harsh winter conditions in the region limit productivity and may serve as a bottleneck driving trophic interactions within the fish community, potentially determining the outcome of climate change for resident biota. However, due to the difficulties in sampling during winter months, few studies have assessed seasonal variation in the ecological interactions between native and range‐expanding species in subarctic regions. We examined seasonal variation in activity, diet (stomach content) and resource use (δ13C and δ15N stable isotope ratios) of two fishes: the resident cold‐water‐adapted European whitefish (Coregonus lavaretus L.) and the range‐expanding cool‐water‐adapted ruffe (Gymnocephalus cernua L.) in lakes with low and high consumer density. Results were contrasted with seasonal availability of benthic and pelagic resources and indirect measures of individual fitness, that is condition, growth and estimated lipid content (muscle elemental C : N ratio) of both species. The effects of fish density were apparent in both the diet and resource use of whitefish, which altered their diet and displayed reduced stomach fullness during winter in high‐density lakes. This was associated with an overall reduction in whitefish growth, condition and estimated lipid content in high‐density lakes, which was especially pronounced during winter. Ruffe utilised a greater proportion of profundal resources than whitefish in both summer and winter, potentially exploiting a vacant niche. Ruffe maintained condition and lipid reserves throughout the winter, highlighting the potential for further northward range expansion of the species. Winter acts as a seasonal bottleneck in subarctic lakes, but assumptions that this bottleneck primarily selects against warmer‐water‐adapted invasive species such as ruffe may be too restrictive. The effects of seasonal resource depletion on fish condition and invertebrate density were most severe in lakes with a high density of fish, and in these conditions, the cold‐water‐adapted resident whitefish was most adversely affected.

Influência de probióticos na prevalência parasitária e níveis de glicose e cortisol em tilápia do Nilo

A adição de probióticos em rações visa atuar como um bioestimulante da flora intestinal das tilápias. A fase de alevinagem em tanques-rede utiliza grande densidade de peixes, o que pode potencializar os efeitos de estresse. Os probióticos surgem como uma alternativa para a mitigação ou prevenção de danos. O objetivo deste trabalho foi avaliar a influência dos probióticos Bacillus cereus var. Toyoi e Bacillus subtilis C-3201 na prevalência parasitária e verificar o estado de saúde por meio do perfil bioquímico de glicose e cortisol e do fator de condição de Fulton (Kf) para atuar como indicador quantitativo do bem-estar de tilápia do Nilo da linhagem GIFT cultivada em tanques-rede. Foram utilizados 1800 juvenis de tilápia do Nilo distribuídos em 20 tanques-rede de 0,5 m x 0,5 m x 0,7 m instalados em tanques com baixa renovação de água. O grupo de peixes sem a adição de probióticos (T1) recebeu apenas dieta basal. Os probióticos liofilizados foram incluídos na ração utilizando 2 % de óleo vegetal na mesma proporção utilizada em todos os grupos experimentais. Na ração basal foram adicionados níveis de 0,5 % B. cereus (T2), 0,5 % de B. subtilis (T3), 0,25 % de B. cereus + 0,25 % B. subtilis (T4). As dietas com adição de B. cereus e/ou B. subtilis não foram eficientes no controle da infestação ectoparasitária por monogenóides e trichodinídeos. Houve maior (p

Community assemblages of commercially important coral reef fishes inside and outside marine protected areas in the Philippines

Establishment of marine protected areas (MPA) is among the most commonly implemented initiatives for coral reef conservation and fisheries management in the Philippines. However, there are concerns that the MPAs in the country may not work because of their generally small sizes and high fishing pressures from the burgeoning highly resource-dependent population. In this study, we assessed the assemblages of seven commercially important coral reef fish families (Acanthuridae (excluding genus Zebrasoma), Labridae (subfamily Scarinae only), Lutjanidae, Serranidae (subfamily Epinephelinae only), Mullidae, Haemulidae and Lethrinidae) inside and outside MPAs in 37 coastal municipalities in the Philippines. A total of 12,354 individuals belonging to 114 species (33 species of Acanthuridae, 27 species of Scarinae, 17 species of Lutjanidae, 16 species of Epinephelinae, 9 species of Mullidae, 6 species of Haemulidae, and 6 species of Lethrinidae) were recorded. Overall, reef fishes inside MPAs were more diverse than outside MPAs based on Shannon–Wiener index of diversity. Reefs inside MPAs had an average of four more fish species than outside MPAs. Both inside and outside MPAs had comparable equitability values which are characterized by fish communities that are largely dominated by few species only. Higher fish densities, especially fishes with ⩾25cm (total length), were also recorded inside MPAs. However, we also found some patterns suggesting that more efforts must be made in order to effectively protect many of the commercially important coral reef fishes from the impacts of fishing. Our study provides valuable science-based insights that can be used to improve coral reef conservation and fisheries management initiatives in the country. Moreover, it can also serve as crucial information that can be used for monitoring and evaluation of MPA effectiveness, particularly on commercially important coral reef fishes in the country.

Nonlinear and density-dependent fish habitat selection across physiochemical gradients in an invasive macrophyte habitat

Invasive macrophytes drive substantial changes to freshwater fish habitats. Such changes can influence how fish select for habitats, but fish–habitat relationships in many invasive macrophytes are often poorly understood at micro-scales. Fish habitat use is influenced by dissolved oxygen (DO) and habitat complexity, but this response can be nonlinear and dependent upon fish density. This study tested whether microhabitat use of sunfish (Lepomis spp.) in invasive macrophyte beds was density-dependent. Fish were sampled with underwater video point counts in six 0.405 ha experimental ponds with surface-matted hydrilla Hydrilla verticillata and stocked with varying densities of sunfishes. Regression models were used to evaluate the key drivers for fish habitat selection across DO, complexity and fish densities. Both fish occurrence and fish counts were positively influenced by DO and negatively influenced by habitat complexity, but the fish counts-DO relationship was dome-shaped and both fish occurrence and counts depended upon fish densities. For example, at high fish densities, fish used low and high DO and high macrophyte complexity; at low fish densities, fish avoided such areas. Fish counts peaked at intermediate DO and low macrophyte complexity. Density-dependent fish habitat selection appeared to mitigate detrimental effects of invasive macrophytes on fish habitats, indicating that fish can and do use ‘inhospitable’ habitats with potential positive population growth. Understanding density-dependent habitat selection is needed when evaluating the quality of habitats, as apparently inhospitable habitats can be utilized when fish density is high.

Dynamic hypoxic zones in Lake Erie compress fish habitat, altering vulnerability to fishing gears

Seasonal degradation of aquatic habitats from hypoxia occurs in numerous freshwater and coastal marine systems and can result in direct mortality or displacement of fish. Yet, fishery landings from these systems are frequently unresponsive to changes in the severity and extent of hypoxia, and population-scale effects have been difficult to measure except in extreme hypoxic conditions with hypoxia-sensitive species. We investigated fine-scale temporal and spatial variability in dissolved oxygen in Lake Erie as it related to fish distribution and catch efficiencies of both active (bottom trawls) and passive (trap nets) fishing gears. Temperature and dissolved oxygen loggers placed near the edge of the hypolimnion exhibited much higher than expected variability. Hypoxic episodes of variable durations were frequently punctuated by periods of normoxia, consistent with high-frequency internal waves. High-resolution interpolations of water quality and hydroacoustic surveys suggest that fish habitat is compressed during hypoxic episodes, resulting in higher fish densities near the edges of hypoxia. At fixed locations with passive commercial fishing gear, catches with the highest values occurred when bottom waters were hypoxic for intermediate proportions of time. Proximity to hypoxia explained significant variation in bottom trawl catches, with higher catch rates near the edge of hypoxia. These results emphasize how hypoxia may elevate catch rates in various types of fishing gears, leading to a lack of association between indices of hypoxia and fishery landings. Increased catch rates of fish at the edges of hypoxia have important implications for stock assessment models that assume catchability is spatially homogeneous.