'Fish in hot water": the challenges facing fish and fisheries research in tropical estuaries

Tropical estuarine areas comprise small systems of a few km, larger estuaries, coastal lakes of hundreds of km 2 and vast shallow coastal waters that are contiguous with estuaries and have similar reduced salinities. Many of the world's great estuaries are in the tropics, e.g. the Amazon, Orinoco, Congo, Zambezi, Niger, Ganges and Mekong. The distribution of tropical and subtropical estuaries approximately follows that of mangroves. Like estuaries everywhere, they are a focus of human activity and are among the most exploited of ecosystems. In few other places do the activities of fishers, industrialists, shippers, farmers, conservationists, sports enthusiasts and biologists overlap to such an extent. Quite apart from the possible eects of all these activities, the fishes of subtropical and tropical estuaries already face one of the most rigorous of aquatic environments; but despite this, species diversity and productivity are high. Only in tropical estuaries are animals from such a wide range of taxa so closely associated, annelid worms, prawns, crocodiles, birds, ' hippos ', dolphins and of course fishes, all may form part of the overall community, often with functional ecological links. Unfortunately, the diculties of working in these often inhospitable environments, has meant that biologists have favoured projects in more appealing areas, such as coral reefs. While it is still true that most estuarine research is conducted in industrialized developed countries, nearly all of which are in cold or temperate regions, there has been a recent upsurge in tropical estuarine fish research. This is being driven by two imperatives, food security and the conservation and maintenance of biodiversity. Both these problems require knowledge of the ecology of tropical estuarine fishes, particularly their relationships with the environment and the extent to which they are dependent on estuaries or adjacent habitats for survival.     

Changes in tropical fish assemblages associated with small-scale fisheries: a case study in the Pacific off central Mexico

Information from artisanal marine fisheries inColima, Mexico is analyzed to find evidence ofchange in the composition of fish assemblagesresulting from fishing pressure. Catchcomposition from up to 20 boats per day, during2 to 6 days a month, were recorded from 1983 to1998. Stability of the species composition wasanalyzed using rank correlation methods, andmeasures of diversity (specific richness,Shannon-Wiener index, evenness) were estimatedfrom data. The resulting time-series wereanalyzed to detect trends. No significantdifferences were found in the number of speciescaught by month nor were significant trends indiversity detected. The composition of the fishassemblages is dynamic. The presence ofpersistent and resilient species that definedthe values obtained of diversity and evennesswas detected, but did not determine thestructure of the community, which is apparentlydefined by other factors. The changes incomposition of the catch probably are aconsequence of environmental fluctuations andrandom events, and not a direct consequence offishing pressures on the fish community.     

Convergence of temperate and tropical stream fish assemblages

The hypothesis of convergence takes the deterministic view that community (or assemblage) structure can be predicted from the environment, and that the environment is expected to drive evolution in a predictable direction. Here we present results of a comparative study of freshwater fish assemblages from headwater streams in four continents (Europe, North America, Africa and South America), with the general objective of testing whether these assemblages display convergent structures under comparable environmental conditions (i.e. assemblage position in the stream longitudinal continuum). We tested this hypothesis by comparing species richness and trophic guilds of those stream fish assemblages represented in available data from multiple sites on each continent. Independent of phylogenetic and historical constraints, fish assemblage richness and trophic structure in the four continents converged along the stream continua to a substantial degree. For the four continents, assemblage richness increased, the proportion of invertivorous species decreased, and the proportion of omnivorous species increased from upstream to downstream, supporting theoretical predictions of the river continuum concept. However, the herbivore/detritivore and piscivore guilds were virtually absent from our small European and North American stream sites, unlike our African and South American stream sites. This divergence can be linked to differences in energy availability between temperate and tropical systems.     

Habitat specialization in tropical continental shelf demersal fish assemblages

The implications of shallow water impacts such as fishing and climate change on fish assemblages are generally considered in isolation from the distribution and abundance of these fish assemblages in adjacent deeper waters. We investigate the abundance and length of demersal fish assemblages across a section of tropical continental shelf at Ningaloo Reef, Western Australia, to identify fish and fish habitat relationships across steep gradients in depth and in different benthic habitat types. The assemblage composition of demersal fish were assessed from baited remote underwater stereo-video samples (n = 304) collected from 16 depth and habitat combinations. Samples were collected across a depth range poorly represented in the literature from the fringing reef lagoon (1-10 m depth), down the fore reef slope to the reef base (10-30 m depth) then across the adjacent continental shelf (30-110 m depth). Multivariate analyses showed that there were distinctive fish assemblages and different sized fish were associated with each habitat/depth category. Species richness, MaxN and diversity declined with depth, while average length and trophic level increased. The assemblage structure, diversity, size and trophic structure of demersal fishes changes from shallow inshore habitats to deeper water habitats. More habitat specialists (unique species per habitat/depth category) were associated with the reef slope and reef base than other habitats, but offshore sponge-dominated habitats and inshore coral-dominated reef also supported unique species. This suggests that marine protected areas in shallow coral-dominated reef habitats may not adequately protect those species whose depth distribution extends beyond shallow habitats, or other significant elements of demersal fish biodiversity. The ontogenetic habitat partitioning which is characteristic of many species, suggests that to maintain entire species life histories it is necessary to protect corridors of connected habitats through which fish can migrate.     

Ecological structure of tropical fish assemblages in wet-zone streams of Sri Lanka

The introduction of four fish species into a depauperate stream system in central Sri Lanka provided a 'natural experiment' that enabled us to determine the ecological structure of wet-zone stream fish assemblages. All the species indigenous to this 'introduction' stream system also co-occur naturally with the introduced species in nearby streams. Analyses of habitat use and dietary requirements revealed that most species in the assemblages were segregated on the basis of macrohabitat, microhabitat and food, regardless of origin of the fishes. Macrohabitat and microhabitat utilizations by species were similar in each stream of the 'introduction' system despite differences in stream conditions. Thus species distributions, relative to each other in ecological space, were consistent among streams, and with the combined data. High overlaps along the resource axes of velocity, depth, substratum and food were few. A major proportion of the high overlaps was due to associations with introduced species; especially Barbus nigrofasciatus and Barbus cumingii. The other two introduced species, Barbus titteya and Rasbora vaterifloris , were more specialized, and interacted relatively less with indigenous species. The indigenous species, however, exhibited pronounced complementarity along three resource axes representing depth, velocity and food. Rasbora vaterifloris and B. nigrofasciatus grew to larger sizes in one stream which had fewer indigenous species, suggesting competitive release. In this stream, the most common indigenous species was a dietary specialist that fed on diatoms, and B. nigrofasciatus , which fed heavily on diatoms in other streams, switched to feeding more on macrophytes. Overall, the data suggest that these assemblages are predictable, co-evolved systems with competition serving as an important structuring force that reinforces species segregation.     

Objective selection of surrogate families to describe reef fish assemblages in a subtropical marine park

Effective representation of biodiversity in a marine park can be limited by lack of sampling at a suitable scale due to various methodological, logistical and taxonomic constraints. Surrogates that describe key components of biodiversity can benefit management planning and assist evaluation of zoning arrangements by improving efficiency and effectiveness of sampling. Reef fish are considered an important component of biodiversity in the Solitary Islands Marine Park (SIMP), New South Wales, Australia. Fish assemblages were surveyed using 30-min timed counts at 68 sites spread across the extent of shallow reef in the SIMP. The overall assemblage was compared with various subsets of taxa using the RELATE procedure in PRIMER to determine useful surrogates. Two families, Labridae and Pomacentridae, showed a high concordance with overall patterns and the highest correlation in estimating species richness by site. These families were the two most speciose (43, 32 species, respectively) comprising 30% of the species richness out of 66 families and 254 species. Surveying a subset of species that includes these two families has utility for marine park management in the SIMP, including evaluating the influence of ‘no take’ zones on assemblage patterns and systematic planning for biodiversity representation.     

Morphological correlates of river velocity and reproductive development in an ornamented stream fish

Adaptive phenotypic divergence can arise when environments vary in ways favoring alternative phenotypic optima. In aquatic habitats, the costs of locomotion are expected to increase with water velocity, generally favoring a more streamlined body and the reduction of traits that produce drag. However, because streamlining in fish may come at the cost of maneuverability, the net benefits of drag reduction can differ not only among habitats, but also among individuals (or classes of individuals) that rely on locomotion for different uses (e.g., males vs. females or adults vs. juveniles). We tested these predictions by exploring relationships among river velocity, body streamlining, ornamental fin size, and male reproductive condition in the steelcolor shiner (Cyprinella whipplei), a small-bodied North American cyprinid. Overall, males in peak reproductive condition (defined by the development of sexually dimorphic tubercles) had less streamlined bodies and larger ornamental fins than males in lower reproductive condition or individuals lacking these secondary sexual characters (females and immature males). There was a relationship between river velocity and body streamlining only for males in peak reproductive condition, but it was in the opposite direction of our predictions: these males were less streamlined in faster rivers. We found only weak support for the prediction that ornamental fin size would be negatively associated with river velocity. Overall, these results suggest either that drag is not an important selective pressure in these habitats, or that the sexual selection advantages of a deep body and large fin compensate any natural selection costs for C. whipplei males. This study highlights the often overlooked diversity of selective pressures acting on streamlining in fishes, and can offer novel insights and predictions allowing a more nuanced understanding of fish ecomorphology.     

Morphological adaptations of the digestive tract of tropical cyprinids and cichlids to diet

Eight cyprinids with four different feeding habits fall into two distinct groups according to morphological adaptations of the intestine: (i) carnivorous and omnivorous cyprinids with large gut diameter and large mucosal surface which decreases from the foregut to the hindgut; (ii) benthivorous and phytoplanktivorous cyprinids with small gut diameter and small mucosal surface which is more or less uniformly built along the intestine, although relative gut length may vary considerably (1.45–6.10). Although the intestine of phytoplanktivorous cyprinids is extremely elongated, it appears less adapted for processing a refractory diet than that of cichlids with similar feeding habits.     

Spatial structure of fish assemblages in a tropical estuarine lagoon: combining multivariate and geostatistical techniques

The spatial structure and seasonal changes of estuarine fish assemblages in the Ciénaga Grande de Santa Marta (CGSM) were analysed based on four seasonal comprehensive surveys conducted in 1993-1994 and 1997. Geostatistical and multivariate techniques were used to: (a) determine seasonal changes in spatial distribution of the species richness, and (b) identify assemblages of estuarine fish and their relation to abiotic factors. Potential biotic interactions affecting the assemblage structure were also explored. A total of 11075 individuals representing 39 species were collected, with Eugerres plumieri, Diapterus rhombeus, Micropogonias furnieri, Mugil incilis, Cathorops spixii, Elops saurus and Anchovia clupeoides as dominant species between seasons. Spatial distribution of fish richness differed between rainy and dry seasons in each year, whereas species mapping showed spatial patchiness in 1993-1994 and gradients during 1997. Strong evidence of species saturation was found in all seasons, suggesting biotic interactions limiting species richness at a threshold density of ca. 50 ind./5000 m². Marine, marine-estuarine and freshwater species were classified in each season according to their capability to cope with salinity fluctuations. Associations defined by functional feeding guilds were also identified. Empirical and statistic evidence showed that fish assemblages differed between seasons within each year, and each assemblage was always dominated by a small number of species, notably E. plumieri in both years 1993-1994 and 1997. Between-season differences in fish assemblage structure in the CGSM seem to be driven by abiotic factors; however, evidence of species saturation could suggest the existence of density-dependent factors operating together.     

Ichthyofaunal assemblages in estuaries: A South African case study

This review places the life-history styles of fishes associated with South African estuaries in a global context and presents a classification system incorporating all the major life-history categories for estuary-associated fish species around the world. In addition, it documents the early life histories of the major fish groups in South African estuaries, with particular emphasis on the differing modes of estuarine utilization by marine, estuarine and freshwater taxa.This review details factors influencing the ichthyofaunal community structure in South African estuaries. The availability of fish for recruitment into an estuary depends primarily upon the distributional range of euryhaline marine and estuarine species, with tropical and temperate taxa showing marked abundance trends. Within a particular biogeographic region, however, estuarine type and prevailing salinity regime have a major influence on the detailed ichthyofaunal structure that develops. There is an increasing preponderance of marine fish taxa when moving from a freshwater-dominated towards a seawater-dominated type of system, and a decline in species diversity between subtropical estuaries in the north-east and cool temperate systems in the south-west. Similar declines in fish species diversity between tropical and temperate estuaries in other parts of the world are highlighted.Fish assemblages in estuaries adjust constantly in response to changing seasons, salinities, turbidities, etc. Despite persistent fluctuations in both the biotic and abiotic environment, the basic ichthyofaunal structure appears to have an underlying stability and to be predictable in terms of the response of individual species to specific conditions. This stability seems to be governed by factors such as the dominance of eurytopic taxa within estuarine assemblages and the robust nature of food webs within these systems. The predictability arises from factors such as the seasonality associated with estuarine spawning cycles and juvenile fish recruitment patterns. These patterns, together with a well-documented resilience to a wide range of physico-chemical and biotic perturbations, appear to be an underlying feature of fish assemblages in estuaries around the world.In contrast to marine fish species, estuary-associated taxa have received little conservation attention. Apart from the designation of protected areas, the main direct means of conserving estuary-associated fish stocks include habitat conservation and controls over fishing methods, effort, efficiency and seasonality. Of these, the conservation of fish habitats, the most important, because healthy aquatic environments invariably support healthy fish populations. The use of estuarine sanctuaries for fish conservation is briefly reviewed, as well as the legislation governing the USA National Estuarine Research Reserve System (NERRS) and the Australian Marine and Estuarine Protected Area (MEPA) system. It is concluded that South Africa requires an expansion of the existing Estuarine Protected Area (EPA) network, as well as the upgrading of selected 'estuarine reserves' where fishing is permitted, into 'estuarine sanctuaries' where no exploitation of biological resources is allowed.     

The structure of shallow water fish assemblages in sandy beaches of a tropical bay in the southwestern Atlantic

Ichthyological Research - The spatial and seasonal dynamics of shallow water fish assemblages and their relationship with physical-chemical variables were investigated on Guanabara Bay, one of the...     

Functional diversity and trait–environment relationships of stream fish assemblages in a large tropical catchment

1. The species composition of stream fish assemblages changes across the longitudinal fluvial gradient of large river basins. These changes may reflect both zonation in species distributions and environmental filtering of fish traits as stream environments change from the uplands to the lowlands of large catchments. Previous research has shown that taxonomic diversity generally increases in larger, lowland streams, and the River Continuum Concept, the River Habitat Template and other frameworks have provided expectations for what functional groups of fishes should predominate in certain stream types. However, studies addressing the functional trait composition of fish assemblages across large regions are lacking, particularly in tropical river basins. 2. We examined functional trait–environment relationships and functional diversity of stream fish assemblages in the Río Grijalva Basin in southern Mexico. Traits linked to feeding, locomotion and life history strategy were measured in fishes from streams throughout the catchment, from highland headwaters to broad, lowland streams. Relationships between functional traits and environmental variables at local and landscape scales were examined using multivariate ordination, and the convex hull volume of trait space occupied by fish assemblages was calculated as a measure of functional diversity. 3. Although there were a few exceptions, functional diversity of assemblages increased with species richness along the gradient from uplands to lowlands within the Grijalva Basin. Traits related to swimming, habitat preference and food resource use were associated with both local (e.g. substratum type, pool availability) and landscape‐scale (e.g. forest cover) environmental variables. 4. Along with taxonomic structure and diversity, the functional composition of fish assemblages changed across the longitudinal fluvial gradient of the basin. Trait–environment relationships documented in this study partially confirmed theoretical expectations and revealed patterns that may help in developing a better understanding of general functional responses of fish assemblages to environmental change.     

Ecological integrity assessment of the fish assemblages of the Matigulu/Nyoni and Umvoti estuaries,KwaZulu-Natal,South Africa

The estuarine fish community index (EFCI) and an estuarine fish guild assessment (EFGA) were used in 2004 and 2008 to determine the ecological state and effects of selected ecosystem variables on the Umvoti and Matigulu/Nyoni estuaries. The EFCI indicated that the Matigulu/Nyoni Estuary was in a moderate state and that the Umvoti Estuary was in a poor to very poor state. The EFGA revealed a clear spatial differentiation, based on the estuaries’ fish community structures, that was being driven primarily by the freshwater-dominated state of the Umvoti Estuary and the poor water quality of the Umvoti Estuary. The analysis of estuarine fish assemblage characteristics was shown to contribute towards the management and conservation of these systems.     

Crustacean zooplankton assemblages in freshwaters of tropical Australia

Throughout tropical Australia eulimnetic zooplankton is composed of 7 calanoid copepods, 5 cyclopoid copepods and 12 cladocerans. A further 10 cyclopoids and 9 cladocerans occur as littoral strays, making 43 species in all. Dominants include Diaptomus lumholtzi, Mesocyclops notius, Thermocyclops decipiens, Diaphanosoma excisum, Ceriodaphnia cornuta, and Moina micrura. Momentary species composition averages 1.0 calanoids, 1.3 cyclopoids and 2.0 cladocerans. These assemblages are not only less complex than in temperate Australia, but also are not as rich as in most other tropical regions. The main contributors to these differences are calanoids and cladocerans respectively. Nevertheless, as elsewhere, most zooplankters in tropical Australia are eurytropic, widely distributed and small.     

Review of the life history characteristics,ecology and fisheries for deep-water tropical demersal fish in the Indo-Pacific region

Deep-water tropical fishes support locally significant commercial fisheries, high value recreational fisheries, and culturally and economically important artisanal and subsistence fisheries throughout the Indo-Pacific region. The main species captured by these fisheries are deep-water snappers (Lutjanidae), groupers (Epinephelidae), and emperors (Lethrinidae). Quantitative assessments of deep-water tropical fisheries have been limited by a lack of adequate biological and fisheries data. We review the biology and ecology of deep-water tropical fishes, discuss the implications for assessment and management of tropical deep-water fisheries, and provide perspectives on future research priorities. We found that biological and fisheries information is lacking for the majority of deep-water tropical fishes. Furthermore, many studies were constrained by low samples sizes and the use of methods that have not been validated or verified. Most species for which reliable information was available were reported to have extended longevities (>20 years), low rates of natural mortality (M < 0.15), and slow to modest growth rates (K = 0.12–0.25). These life history traits indicate a low production potential for many deep-water tropical fishes, and suggest that sustainable exploitation rates and potential yields may be low. There is a need for more representative and adequate studies of deep-water tropical fishes and for improved fisheries data collection and the use of consistent methods in addition to information sharing to facilitate the development of robust data-poor assessment techniques for these species.