Biogeography of tuna and billfish communities

Aim The aims of this study were: (1) to identify global communities of tuna and billfish species through quantitative statistical analyses of global fisheries data; (2) to describe the spatial distribution, main environmental drivers and species composition of each community detected; and (3) to determine whether the spatial distribution of each community could be linked to the environmental conditions that affect lower trophic levels by comparing the partitions identified in this study with Longhurst’s biogeochemical provinces. Location The global ocean from 60° S to 65° N. Methods We implemented a new numerical procedure based on a hierarchical clustering method and a nonparametric probabilistic test to divide the oceanic biosphere into biomes and ecoregions. This procedure was applied to a database that comprised standardized data on commercial longline catches for 15 different species of tuna and billfish over a period of more than 50 years (i.e. 1953–2007). For each ecoregion identified (i.e. characteristic tuna and billfish community), we analysed the relationships between species composition and environmental factors. Finally, we compared the biogeochemical provinces of Longhurst with the ecoregions that we identified. Results Tuna and billfish species form nine well‐defined communities across the global ocean. Each community occurs in regions with specific environmental conditions and shows a distinctive species composition. High similarity (68.8% homogeneity) between the spatial distribution of the communities of tuna and billfish and the biogeochemical provinces suggests a strong relationship between these species and the physical and chemical characteristics of the global ocean. Main conclusions Despite their high tolerance for a wide range of environmental conditions, these highly migratory species are partitioned into clear geographical communities in the ocean at a global scale. The similarity between biogeochemical and biotic divisions in the ocean suggests that the global ocean is a mosaic of large biogeographical ecosystems, each characterized by specific environmental conditions that have a strong effect on the composition of the trophic web.     

Putting all the pieces together: integrating current knowledge of the biology,ecology, fisheries status,stock structure and management of yellowfin tuna (<Emphasis Type="Italic">Thunnus albacares</Emphasis>)

Yellowfin tuna (Thunnus albacares; YFT) is an apex marine predator inhabiting tropical and sub-tropical pelagic waters. It supports the second largest tuna fishery in the world. Here, we review the available literature on YFT to provide a detailed overview of the current knowledge of its biology, ecology, fisheries status, stock structure and management, at global scale. YFT are characterized by several peculiar anatomical and physiological traits that allow them to survive in the oligotrophic waters of the pelagic realm. They are opportunistic feeders, which allows fast growth and high reproductive outputs. Globally, YFT fisheries have expanded over the last century, progressively moving from coastal areas into the majority of sub-tropical and tropical waters. This expansion has led to a rapid increase in global commercial landings, which are predominantly harvested by industrial longline and purse seine fleets. For management purposes, YFT is divided into four stocks, each of which is currently managed by a separate tuna Regional Fisheries Management Organization. Our current understanding of YFT stock structure is, however, still uncertain, with conflicting evidence arising from genetic and tagging studies. There is, moreover, little information about their complex life-history traits or the interactions of YFT populations with spatio-temporally variable oceanographic conditions currently considered in stock assessments. What information is available, is often conflicting at the global scale. Finally, we suggest future research directions to manage this valuable resource with more biological realism and more sustainable procedures.     

Modelling the Spatial Behaviour of a Tropical Tuna Purse Seine Fleet

Industrial tuna fisheries operate in the Indian, Atlantic and Pacific Oceans, but concerns over sustainability and environmental impacts of these fisheries have resulted in increased scrutiny of how they are managed. An important but often overlooked factor in the success or failure of tuna fisheries management is the behaviour of fishers and fishing fleets. Uncertainty in how a fishing fleet will respond to management or other influences can be reduced by anticipating fleet behaviour, although to date there has been little research directed at understanding and anticipating the human dimension of tuna fisheries. The aim of this study was to address gaps in knowledge of the behaviour of tuna fleets, using the Indian Ocean tropical tuna purse seine fishery as a case study. We use statistical modelling to examine the factors that influence the spatial behaviour of the purse seine fleet at broad spatiotemporal scales. This analysis reveals very high consistency between years in the use of seasonal fishing grounds by the fleet, as well as a forcing influence of biophysical ocean conditions on the distribution of fishing effort. These findings suggest strong inertia in the spatial behaviour of the fleet, which has important implications for predicting the response of the fleet to natural events or management measures (e.g., spatial closures).     

What do warming waters mean for fish physiology and fisheries?

Environmental signals act primarily on physiological systems, which then influence higher-level functions such as movement patterns and population dynamics. Increases in average temperature and temperature variability associated with global climate change are likely to have strong effects on fish physiology and thereby on populations and fisheries. Here we review the principal mechanisms that transduce temperature signals and the physiological responses to those signals in fish. Temperature has a direct, thermodynamic effect on biochemical reaction rates. Nonetheless, plastic responses to longer-term thermal signals mean that fishes can modulate their acute thermal responses to compensate at least partially for thermodynamic effects. Energetics are particularly relevant for growth and movement, and therefore for fisheries, and temperature can have pronounced effects on energy metabolism. All energy (ATP) production is ultimately linked to mitochondria, and temperature has pronounced effects on mitochondrial efficiency and maximal capacities. Mitochondria are dependent on oxygen as the ultimate electron acceptor so that cardiovascular function and oxygen delivery link environmental inputs with energy metabolism. Growth efficiency, that is the conversion of food into tissue, changes with temperature, and there are indications that warmer water leads to decreased conversion efficiencies. Moreover, movement and migration of fish relies on muscle function, which is partially dependent on ATP production but also on intracellular calcium cycling within the myocyte. Neuroendocrine processes link environmental signals to regulated responses at the level of different tissues, including muscle. These physiological processes within individuals can scale up to population responses to climate change. A mechanistic understanding of thermal responses is essential to predict the vulnerability of species and populations to climate change.     

Spatial Scale,Means and Gradients of Hydrographic Variables Define Pelagic Seascapes of Bluefin and Bullet Tuna Spawning Distribution

Seascape ecology is an emerging discipline focused on understanding how features of the marine habitat influence the spatial distribution of marine species. However, there is still a gap in the development of concepts and techniques for its application in the marine pelagic realm, where there are no clear boundaries delimitating habitats. Here we demonstrate that pelagic seascape metrics defined as a combination of hydrographic variables and their spatial gradients calculated at an appropriate spatial scale, improve our ability to model pelagic fish distribution. We apply the analysis to study the spawning locations of two tuna species: Atlantic bluefin and bullet tuna. These two species represent a gradient in life history strategies. Bluefin tuna has a large body size and is a long-distant migrant, while bullet tuna has a small body size and lives year-round in coastal waters within the Mediterranean Sea. The results show that the models performance incorporating the proposed seascape metrics increases significantly when compared with models that do not consider these metrics. This improvement is more important for Atlantic bluefin, whose spawning ecology is dependent on the local oceanographic scenario, than it is for bullet tuna, which is less influenced by the hydrographic conditions. Our study advances our understanding of how species perceive their habitat and confirms that the spatial scale at which the seascape metrics provide information is related to the spawning ecology and life history strategy of each species.     

Maximum entropy model: Estimating the relative suitability of cetacean habitat in the northern Savu Sea,Indonesia

An understanding of cetacean distribution is necessary to gain insights into crucial ecological processes for species conservation management. However, cetacean habitat preference and distribution in the northern Savu Sea, Indonesia, are still poorly understood. We use maximum entropy modeling with five environmental predictors to describe habitat preference and distribution of seven cetacean species in the northern Savu Sea. Our study confirms that static predictors are the most important variables in explaining habitat preferences of seven cetacean species in the northern Savu Sea. Seasonally, each cetacean species has a different environmental preference. Globicephala macrorhynchus prefers the open nearshore areas adjacent to deep waters. Stenella attenuata is associated with nearshore habitats and higher productivity areas. Stenella longirostris prefers nearshore habitat during coastal upwelling events. Tursiops truncatus prefers nearshore sheltered habitat. Grampus griseus habitats are highly related to depth and steep gradients around the isobaths of 200–1,000 m. Feresa attenuata are associated with deep waters and cooler sea temperature. Pseudorca crassidens prefers protective bays with productive waters. This study provides important information for reviewing marine spatial plans of the northern Savu Sea, specifically on managing species fisheries interaction, optimizing surveillance, and regulating marine traffic.     

The challenge of novel abiotic conditions for species undergoing climate-induced range shifts

Coincident with recent global warming, species have shifted their geographic distributions to cooler environments, generally by moving along thermal axes to higher latitudes, higher elevations or deeper waters. While these shifts allow organisms to track their thermal niche, these three thermal axes also covary with non-climatic abiotic factors that could pose challenges to range-shifting plants and animals. Such novel abiotic conditions also present an unappreciated pitfall for researchers – from both empirical and predictive viewpoints – who study the redistribution of species under global climate change. Climate, particularly temperature, is often assumed to be the primary abiotic factor in limiting species distributions, and decades of thermal biology research have made the correlative and mechanistic understanding of temperature the most accessible and commonly used response to any abiotic factor. Receiving far less attention, however, is that global gradients in oxygen, light, pressure, pH and water availability also covary with latitude, elevation, and/or ocean depth, and species show strong physiological and behavioral adaptations to these abiotic variables within their historic ranges. Here, we discuss how non-climatic abiotic factors may disrupt climate-driven range shifts, as well as the variety of adaptations species use to overcome abiotic conditions, emphasizing which taxa may be most limited in this capacity. We highlight the need for scientists to extend their research to incorporate non-climatic, abiotic factors to create a more ecologically relevant understanding of how plants and animals interact with the environment, particularly in the face of global climate change. We demonstrate how additional abiotic gradients can be integrated into global climate change biology to better inform expectations and provide recommendations for addressing the challenge of predicting future species distributions in novel environments.     

Modelling the niche for a marine vertebrate: a case study incorporating behavioural plasticity,proximate threats and climate change

The integration of satellite telemetry, remotely sensed environmental data, and habitat/environmental modelling has provided for a growing understanding of spatial and temporal ecology of species of conservation concern. The Republic of Cape Verde comprises the only substantial rookery for the loggerhead turtle Caretta caretta in the eastern Atlantic. A size related dichotomy in adult foraging patterns has previously been revealed for adult sea turtles from this population with a proportion of adults foraging neritically, whilst the majority forage oceanically. Here we describe observed habitat use and employ ecological niche modelling to identify suitable foraging habitats for animals utilising these two distinct behavioural strategies. We also investigate how these predicted habitat niches may alter under the influence of climate change induced oceanic temperature rises. We further contextualise our niche models with fisheries catch data and knowledge of fisheries ‘hotspots’ to infer threat from fisheries interaction to this population, for animals employing both strategies. Our analysis revealed repeated use of coincident oceanic habitat, over multiple seasons, by all smaller loggerhead turtles, whilst larger neritic foraging turtles occupied continental shelf waters. Modelled habitat niches were spatially distinct, and under the influence of predicted sea surface temperature rises, there was further spatial divergence of suitable habitats. Analysis of fisheries catch data highlighted that the observed and modelled habitats for oceanic and neritic loggerhead turtles could extensively interact with intensive fisheries activity within oceanic and continental shelf waters of northwest Africa. We suggest that the development and enforcement of sustainable management strategies, specifically multi‐national fisheries policy, may begin to address some of these issues; however, these must be flexible and adaptive to accommodate potential range shift for this species.     

The key role of the Northern Mozambique Channel for Indian Ocean tropical tuna fisheries

The Northern Mozambique Channel (NMC) is a tropical area of ~?1 million km² where pelagic fisheries supply proteins to more than 9 million people living in Comoros, Mayotte, and along the coasts of Mozambique, Tanzania and Madagascar. Although uncertain, statistics suggest that about 20,000 mt of tropical tuna and other pelagic fish are annually caught by artisanal fisheries in the area. The NMC is also a major seasonal fishing ground for high-seas fleets that export an annual average catch of more than 20,000 mt to tuna can and sashimi markets of high-income countries for a value estimated to be more than 100 million USD. The fisheries productivity of the NMC appears to be highly variable in relation to strong annual and seasonal variability in oceanographic conditions. Our review shows that the NMC is a key feeding area for tropical tunas and a major spawning area for skipjack tuna thanks to warm waters and strong mesoscale activity that results in the enrichment of surface waters and efficient energy transfers enabled by short food chains. Projections of climate models under future warming scenarios predict some strong changes in the oceanographic conditions of the NMC which has already experienced substantial warming over the last decades. Changes in the pelagic ecosystem of the NMC could have dramatic consequences on the coastal populations that are expected to increase towards 100 million people by 2100. Improving monitoring systems and collecting information on the socio-economics of coastal fisheries is crucial to assess the dependence of NMC populations on tuna resources and empower the countries to more involvement in the management of tuna stocks.

     

Projected migrations of southern Indian Ocean albatrosses as a response to climate change

Anthropogenic climate change is altering the geographical distribution and regular movements of species. Highly-mobile pelagic seabirds, such as albatrosses, are particularly threatened by human activities, such as fisheries bycatch. Predicting the impact of climate change on how these animals roam the ocean is an important step towards making informed conservation decisions. In this study, we used a mechanistic model of migratory movements to predict how the migration of albatross species that breed in the southern Indian Ocean may change between now and the end of the century. The model is able to generate non-breeding movement patterns of albatrosses that correspond to empirical patterns from tracking data, thus providing confidence in the ability of the model to make future predictions. We projected the model using environmental conditions for 2100 based on a scenario assuming high emissions (IPCC RCP 8.5). Overall, we found very little projected change in the non-breeding distribution of albatrosses compared to the present. Some change, however, is predicted for large albatrosses, which, due to their size, are more affected by wind, and are projected to migrate further eastwards in the future scenario. These results contrast with previous analyses focusing on the breeding distribution that used statistical modelling, such as habitat and species distributions models, and predicted poleward shifts in geographical distributions of various seabird species including albatrosses. Therefore, it highlights the need for formal comparison of predicted changes in distribution during different phases of the annual cycle of the albatrosses and/or integration of the different approaches. Our analysis also predicts that the overlap of albatrosses with Regional Fisheries Management Organisations (RFMOs) during the non-breeding season will remain similar in 2100 compared to today. This implies that large-scale by-catch mitigation measures implemented through fisheries management organisations will remain important over the next hundred years of climate change.     

The Effects of Sub-Regional Climate Velocity on the Distribution and Spatial Extent of Marine Species Assemblages

Many studies illustrate variable patterns in individual species distribution shifts in response to changing temperature. However, an assemblage, a group of species that shares a common environmental niche, will likely exhibit similar responses to climate changes, and these community-level responses may have significant implications for ecosystem function. Therefore, we examine the relationship between observed shifts of species in assemblages and regional climate velocity (i.e., the rate and direction of change of temperature isotherms). The assemblages are defined in two sub-regions of the U.S. Northeast Shelf that have heterogeneous oceanography and bathymetry using four decades of bottom trawl survey data and we explore temporal changes in distribution, spatial range extent, thermal habitat area, and biomass, within assemblages. These sub-regional analyses allow the dissection of the relative roles of regional climate velocity and local physiography in shaping observed distribution shifts. We find that assemblages of species associated with shallower, warmer waters tend to shift west-southwest and to shallower waters over time, possibly towards cooler temperatures in the semi-enclosed Gulf of Maine, while species assemblages associated with relatively cooler and deeper waters shift deeper, but with little latitudinal change. Conversely, species assemblages associated with warmer and shallower water on the broad, shallow continental shelf from the Mid-Atlantic Bight to Georges Bank shift strongly northeast along latitudinal gradients with little change in depth. Shifts in depth among the southern species associated with deeper and cooler waters are more variable, although predominantly shifts are toward deeper waters. In addition, spatial expansion and contraction of species assemblages in each region corresponds to the area of suitable thermal habitat, but is inversely related to assemblage biomass. This suggests that assemblage distribution shifts in conjunction with expansion or contraction of thermal habitat acts to compress or stretch marine species assemblages, which may respectively amplify or dilute species interactions to an extent that is rarely considered. Overall, regional differences in climate change effects on the movement and extent of species assemblages hold important implications for management, mitigation, and adaptation on the U.S. Northeast Shelf.     

Large‐scale distribution of tuna species in a warming ocean

Tuna are globally distributed species of major commercial importance and some tuna species are a major source of protein in many countries. Tuna are characterized by dynamic distribution patterns that respond to climate variability and long‐term change. Here, we investigated the effect of environmental conditions on the worldwide distribution and relative abundance of six tuna species between 1958 and 2004 and estimated the expected end‐of‐the‐century changes based on a high‐greenhouse gas concentration scenario (RCP8.5). We created species distribution models using a long‐term Japanese longline fishery dataset and two‐step generalized additive models. Over the historical period, suitable habitats shifted poleward for 20 out of 22 tuna stocks, based on their gravity centre (GC) and/or one of their distribution limits. On average, tuna habitat distribution limits have shifted poleward 6.5 km per decade in the northern hemisphere and 5.5 km per decade in the southern hemisphere. Larger tuna distribution shifts and changes in abundance are expected in the future, especially by the end‐of‐the‐century (2080–2099). Temperate tunas (albacore, Atlantic bluefin, and southern bluefin) and the tropical bigeye tuna are expected to decline in the tropics and shift poleward. In contrast, skipjack and yellowfin tunas are projected to become more abundant in tropical areas as well as in most coastal countries' exclusive economic zones (EEZ). These results provide global information on the potential effects of climate change in tuna populations and can assist countries seeking to minimize these effects via adaptive management.     

Habitat and behaviour of yellowfin tuna Thunnus albacares in the Gulf of Mexico determined using pop-up satellite archival tags

This study presents the first data on movement, habitat use and behaviour for yellowfin tuna Thunnus albacares in the Atlantic Basin. Six individuals were tracked in the Gulf of Mexico using pop-up satellite archival tags. Records up to 80 days in length were obtained, providing information on depth and temperature preferences as well as horizontal movements. Thunnus albacares in the Gulf of Mexico showed a strong preference for the mixed layer and thermocline, consistent with findings for this species in other ocean basins. Fish showed a diel pattern in depth distribution, remaining in surface and mixed layer waters at night and diving to deeper waters during the day. The vertical extent of T. albacares habitat appeared to be temperature limited, with fish generally avoiding waters that were >6° C cooler than surface waters. The vertical and thermal habitat usage of T. albacares differs from that of bigeye Thunnus obesus and bluefin Thunnus thynnus , Thunnus orientalis and Thunnus maccoyii tunas. These results are consistent with the results of earlier studies conducted on T. albacares in other oceans.     

Global variation in freshwater physico-chemistry and its influence on chemical toxicity in aquatic wildlife

Chemical pollution is one of the major threats to global freshwater biodiversity and will be exacerbated through changes in temperature and rainfall patterns, acid–base chemistry, and reduced freshwater availability due to climate change. In this review we show how physico-chemical features of natural fresh waters, including pH, temperature, oxygen, carbon dioxide, divalent cations, anions, carbonate alkalinity, salinity and dissolved organic matter, can affect the environmental risk to aquatic wildlife of pollutant chemicals. We evidence how these features of freshwater physico-chemistry directly and/or indirectly affect the solubility, speciation, bioavailability and uptake of chemicals [including via alterations in the trans-epithelial electric potential (TEP) across the gills or skin] as well as the internal physiology/biochemistry of the organisms, and hence ultimately toxicity. We also show how toxicity can vary with species and ontogeny. We use a new database of global freshwater chemistry (GLORICH) to demonstrate the huge variability (often >1000-fold) for these physico-chemical variables in natural fresh waters, and hence their importance to ecotoxicology. We emphasise that a better understanding of chemical toxicity and more accurate environmental risk assessment requires greater consideration of the natural water physico-chemistry in which the organisms we seek to protect live.     

Patterns of cetacean sighting distribution in the Pacific exclusive economic zone of Costa Rica based on data collected from 1979-2001

Nineteen species of cetaceans (families Balaenopteridae, Kogiidae, Physeteridae, Ziphiidae and Delphinidae) occur in the Costa Rican Pacific Exclusive Economic Zone (EEZ). Based on data recorded from the EEZ by the Southwest Fisheries Service Center, Cascadia Research Collective, and CIMAR between 1979-2001, we mapped the distribution of 18 cetacean species. Our results suggest that the majority of the cetacean species use primarily oceanic waters, particularly those species within the families Balaenopteridae, Kogiidae. Physeteridae and Ziphiidae. Members of the family Delphinidae showed a wide variety of distribution patterns: seven species are widespread throughout the EEZ, four appear to be exclusively pelagic, and two are primarily coastal. Overall, three cetacean species appear to have populations concentrated in coastal waters: Stenella attenuata graffmani. Tursiops truncatus, and Megaptera novaeangliae. These three may be more susceptible to human activities due to the overlap of their ranges with fishery areas (tuna and artisanal fisheries), and an uncontrolled increase of touristic whale watching activities in several parts of their range. The distribution maps represent the first comprehensive representation of cetacean species that inhabit Costa Rican Pacific waters. They provide essential base-line information that may be used to initiate conservation and management efforts of the habitats where these animals reproduce and forage.     

Indications of strong adaptive population genetic structure in albacore tuna (Thunnus alalunga) in the southwest and central Pacific Ocean

Albacore tuna (Thunnus alalunga) has a distinctly complex life history in which juveniles and adults separate geographically but at times inhabit the same spaces sequentially. The species also migrates long distances and presumably experiences varied regimes of physical stress over a lifetime. There are, therefore, many opportunities for population structure to arise based on stochastic differences or environmental factors that promote local adaptation. However, with the extent of mobility consistently demonstrated by tagged individuals, there is also a strong argument for panmixia within an ocean basin. It is important to confirm such assumptions from a population genetics standpoint for this species in particular because albacore is one of the principal market tuna species that sustains massive global fisheries and yet is also a slow‐growing temperate tuna. Consequently, we used 1,837 neutral SNP loci and 89 loci under potential selection to analyze population genetic structure among five sample groups collected from the western and central South Pacific. We found no evidence to challenge panmixia at neutral loci, but strong indications of structuring at adaptive loci. One population sample, from French Polynesia in 2004, was particularly differentiated. Unfortunately, the current study cannot infer whether the divergence is geographic or temporal, or possibly caused by sample distribution. We encourage future studies to include potentially adaptive loci and to continue fine‐scale observations within an ocean basin, and not to assume genome‐wide panmixia.     

Comparative feeding strategies of yellowfin tuna around St Helena and adjacent seamounts of the South Atlantic Ocean

Yellowfin tuna are the mainstay of the traditional tuna fisheries in St Helena waters, but there is limited knowledge of their ecology and feeding behaviour in the area. In this study yellowfin tuna stomach contents were used to assess spatio-temporal changes in feeding strategy and consider the role of tuna in the local ecosystem. Comparisons of the feeding spectra of yellowfin tuna between inshore regions of St Helena and oceanic seamounts demonstrated that in both areas the species was largely piscivorous. In inshore waters yellowfin consumed more neritic fauna, including significant numbers of crab megalopa, whereas around seamounts the diet included a greater diversity of epi- and mesopelagic fish and squids. The most important fish prey species in inshore waters was the St Helena butterflyfish Chaetodon sanctahelenae, and around seamounts was the pufferfish Lagocephalus lagocephalus. Results indicate that the diet spectrum of yellowfin tuna in St Helena waters is relatively similar to those of conspecifics living in waters with relatively low productivity, with strategies indicative of food-poor ecosystems. The availability of coastal fauna may make areas around islands and seamounts more attractive for feeding aggregations of yellowfin tuna, compared to the open ocean. The relatively unselective feeding of yellowfin tuna means that stomachs can provide valuable data on the species diversity, particularly in remote areas with limited opportunities for dedicated research expeditions.     

Local,regional and global markets: what drives the tuna fisheries?

Tuna products are amongst the most popular seafoods in the world and widely traded across the globe. Their global trade developed at a very early stage in the growth and development of tuna fisheries. In this article, recent evolutions of tuna markets in terms of products (for both sashimi and cannery-grade tuna products), market structures, and trade are introduced followed by a comprehensive study of global integration through price linkages. Most studies show a high degree of market integration and competition through prices at the world-wide level. Finally, we introduce some original results about the relationship between catches and prices (estimated coefficients of demand elasticity and flexibility), and provide answers to a few key questions for tuna fisheries and markets, including: How do consumers respond to price changes? Are fish price changes fully transmitted to consumers? Is there any economic incentive for fishers to comply with reduced catch quotas? Do fishers target particular tuna species according to the relative price of tuna species?     

Papyrus swamps and the respiratory ecology of Barbus neumayeri

Synopsis In seasonally flooding fresh waters, dissolved oxygen and fluctating water levels combine to create strong seasonal changes in habitat availability and dispersal pathways for fishes. In this study we demonstrate how respiratory mode can affect the use of and dispersal through hypoxic papyrus swamps for a small cyprinid fish, Barbus neumayeri, a species which uses aquatic surface respiration (ASR) in response to severe hypoxia. Monthly records of the distribution and relative abundance of B. neumayeri across 28 stations in a papyrus swamp in Uganda were used to examine seasonal patterns of habitat use and movement. The distribution of B. neumayeri was very restricted during the drier months. Most fish were captured in open pools and channels with large areas of open water surface and higher oxygen. Movement of B. neumayeri was limited to short periods during the wet season when peak water levels produced high levels of oxygen and pathways for movement. Three lines of evidence suggest that these patterns relate to its respiratory mode. These include: the positive :relationship between dissolved oxygen and the number of stations used per month, the positive relationship between fish density at dry season stations and dissolved oxygen, and the absence of fish from stations with. little open water surface area during the dry season. Significant differences in the gill morphology between B. neumayeri from the papyrus swamp and those from the main river suggest that papyrus swamps may contribute to the diversification of B. neumayeri by limiting movement and demanding specialization for extremely hypoxic waters.     

Contemporary perspectives on the niche that can improve models of species range shifts under climate change

Pioneering efforts to predict shifts in species distribution under climate change used simple models based on the correlation between contemporary environmental factors and distributions. These models make predictions at coarse spatial scales and assume the constancy of present correlations between environment and distribution. Adaptive management of climate change impacts requires models that can make more robust predictions at finer spatio-temporal scales by accounting for processes that actually affect species distribution on heterogeneous landscapes. Mechanistic models of the distribution of both species and vegetation types have begun to emerge to meet these needs. We review these developments and highlight how recent advances in our understanding of relationships among the niche concept, species diversity and community assembly point the way towards more effective models for the impacts of global change on species distribution and community diversity.