Tests of artificial substrata as nursery habitat for young fish

Recruitment areas for freshwater fish are often negatively affected by eutrophication and physical disturbances. Vegetated areas, which are important nursery habitats, are reduced and water turbidity increased. As a method of compensation, we tested artificial substrata for young-of-the-year fish. The structures were made of spruce bundles, with and without surrounding nets, and placed in a hyper-eutrophic very turbid environment and in an undisturbed area with clear water. Both habitats were devoid of submerged vegetation. Young fish abundance in treated areas was compared with adjacent reference sites. In the clear water area, the abundance of all investigated species – perch, pike, bream, silver-bream and roach – was higher in areas with artificial refuges. A similar response was evident for cyprinids in the turbid environment. High abundance of pikeperch and ruffe appeared in the hyper-eutrophic test area. Neither of these species, nor perch, was attracted to the artificial refuges. The lack of response in perch and pikeperch suggests that the importance of structural refuge decreases in very turbid water for these species. Of the two methods tested, spruce bundles with surrounding nets generally attracted most young fish, implying that the nets further increased the refuge capacity by reducing predation risk. The conclusion is that artificial habitats could improve recruitment habitats and that protective devices can increase refuge capacity.     

Osmoregulatory ability and juvenile habitat preference in some penaeid prawns

An investigation was made of the osmoregulation, over a salinity (S) range 3–50‰, of early juvenile and adult penaeid prawns, whose nursery-ground preferences range from rivers of widely fluctuating salinity (banana prawn Penaeus merguiensis de Man; greasyback prawn Metapenaeus bennettae Racek & Dall) through open estuaries with very occasional low salinities (tiger prawn Penaeus esculentus Haswell) to open bays and lower estuaries with fairly stable high salinities (king prawn, P. plebejus Hesse). Adult banana prawns osmoregulated well between S 15–40‰ with isosmotic point ≈ S 27‰, but could not adapt below S 7 ‰, whereas adult greasyback prawns osmoregulated extensively from S 3–50 ‰ with isosmotic point ≈ S 23 ‰. Adult tiger and king prawns had limited osmoregulatory abilities with isosmotic points ≈ S 30 ‰ and lower lethal limits of ≈ S 10 and 7‰, respectively. Early juveniles of all four species were, however, all highly efficient osmoregulators and the curves of the three Penaeus spp. were almost identical below their isosmotic points, which were appreciably lower than those of the adults. Lower lethal limits were S 3‰ or below. It is concluded that nursery-ground selection by the species is unlikely to be determined by osmoregulatory ability. Possible factors determining nursery-ground selection are discussed.     

Saltmarsh as habitat for fish and nektonic crustaceans: Challenges in sampling designs and methods

This is a review of research into the ecological role of saltmarsh as habitat used directly by fish and nektonic crustaceans such as shrimp (prawns) and portunid crabs. The quality of information about direct use of saltmarshes by nekton is poor, with even the most influential works suffering from obvious limitations. Attempts to generalize about nekton use of saltmarshes are hampered by sampling difficulties, poor sampling design, and inconsistent reporting of flooding regime and landscape structure. The difficulty in sampling nekton from shallow, vegetated marsh flats while inundated contributes to the fragmentary results of nekton work. A range of sampling methods have been described that vary in portability, size, amount of above-ground structure, escape rates of nekton, and expense; none has yet become standard. Poor experimental design in studies of saltmarsh nekton includes lack of replication, limited spatial scale, and lack of baseline data before management changes are made. Attempts to determine effects on commercial fisheries of losses in area or quality of saltmarsh by correlations between catch data and marsh loss should use active adaptive management. A second type of study, aimed at understanding how the saltmarsh is important to fisheries, can be tackled usefully at a smaller scale. To assist in the comparison of results from different studies, flooding regime should be reported as a proportion of time the marsh is submerged, and landscape structure as proportions of the marsh that are covered by intertidal flats, pools and drainage creeks. Flooding regime and landscape structure at the sites and at the time of sampling should then be put in the context of the typical pattern for the marsh under study.     

Rates at which post-larval prawns are digested by a small predatory fish and the implications for predation studies

Juvenile Monocanthus chinensis digested most of the flesh of post-larval Penaeus monodon within 60 min, and most exoskeletal components within 180 min. Rapid cooling of predator samples should improve estimates of consumption of young prawns.     

Horizontal gradient in fish assemblage structures in and around a seagrass habitat: some implications for seagrass habitat conservation

Horizontal gradients in fish assemblage structures in and around a seagrass habitat were evaluated by visual observations. The assemblage structures clearly differed between open microhabitats lacking seagrass, i.e., nearby sand and the area adjoining the outer margin of the seagrass bed (referred to as “outer gap”), and microhabitats with seagrass within the overall seagrass bed, although not differing among the latter microhabitats, including both edge and core portions. Such open microhabitats were found to be not always inferior, but nearly equal to or even sometimes greatly superior in fish species’ diversity and/or abundance to the microhabitats with seagrass. In particular, the outer gap was always ranked first in total species’ number and had outstanding abundance in spring. Similar open microhabitats adjoining seagrass walls facing the sand patch within the seagrass bed (referred to as “inner gap”) in spring were also characterized by higher fish species and individual numbers. The 11 most abundant fishes showed four discrete distribution patterns (three recognized herein and one implied by precedent studies), such contributing to a horizontal gradient in the fish assemblage structure. While no fishes showed a preference for the edge or core of the seagrass bed, group-forming juveniles of several species favored gap microhabitats, suggesting that, in addition to the traditionally recognized edge and core microhabitats, the concept of gap microhabitats should be included in seagrass conservation ecology.     

'Cells' and 'organisms' as a habitat for DNA.

Although the bulk of the hereditary information in bacteria is organized as a single chromosome, it has been known for some years now that bacteria may also carry pieces of self-replicating extrachromosomal DNA. These units are known as plasmids. Sometimes such plasmids carry the information necessary to give rise to mature bacterial viruses under appropriate conditions, but in other cases they specify the production of enzymes and other proteins which alter the bacterial phenotype. Plasmids are often inessential for survival of bacteria, although they may widen the range of environmental conditions under which they flourish. Thus plasmids may be thought of as adventitious additions to the genetic content of bacterial cells. Recently it has become clear that furthur organizational units of DNA are to be found in bacterial cells. These units are called insertion sequences and transposons. Unlike plasmids and the chromosome, however, these DNA units do not carry enough genetic information to specify their own independent replication: they must rely on plasmids or the chromosome for that purpose. Nevertheless they behave in many respects as independent functional units. Although it is possible to think of the chromosome, plasmids and transposons/insertion sequences as three distinct hierarchies of bacterial DNA, genes may move from one hierarchy to another; and such transitions have important implications for the evolution of bacterial populations. Moreover, their study in bacteria may throw much light on the type of DNA interactions occurring in higher cells.     

Active habitat selection for sand by juvenile western king prawns, Melicertus latisulcatus (Kishinouye)

This paper presents the results of a series of habitat selection experiments aimed at determining if juvenile Melicertus latisulcatus generally occur on intertidal sand- and mud-flats as a result of active selection of unvegetated areas, or due to extrinsic factors (e.g. differential predation). In the laboratory, juvenile M. latisulcatus showed a clear preference for habitats containing sand irrespective of the presence or absence of predators. If sand was not available, artificial seagrass was chosen as a secondary preference but was avoided when sand alone was also present. Importantly, the combinations of habitats chosen for testing allowed us to determine that artificial seagrass provided a good surrogate for real seagrass, and that the presence of potential food (epiphytes) did not appear to influence habitat selection. There was also no difference in the habitat selected between day and night, and only minor differences with prawn size. Thus, juvenile M. latisulcatus appear to have a hierarchy of mechanisms for avoiding predators, with burying in sand being the preferred option. If burying is not possible, then seagrass is used for shelter. Active habitat selection to avoid predation appears likely to play a substantial role in determining the distribution of these animals on unvegetated sand- and mud-flats.     

Principles for linking fish habitat to fisheries management and conservation

A set of eight simple ecological and social principles is proposed that could enhance the understanding of what constitutes fish 'habitat' and, if implemented, could contribute to improved management and conservation strategies. The habitat principles are a small, interrelated sub‐set that may be coupled with additional ones to formulate comprehensive guidelines for management and conservation strategies. It is proposed that: 1) habitat can be created by keystone species and interactions among species; 2) the productivity of aquatic and riparian habitat is interlinked by reciprocal exchanges of material; 3) the riparian zone is fish habitat; 4) fishless headwater streams are inseparable from fish‐bearing rivers downstream; 5) habitats can be coupled – in rivers, lakes, estuaries and oceans, and in time; 6) habitats change over hours to centuries; 7) fish production is dynamic due to biocomplexity, in species and in habitats; 8) management and conservation strategies must evolve in response to present conditions, but especially to the anticipated future. It is contended that the long‐term resilience of native fish communities in catchments shared by humans depends on incorporating these principles into management and conservation strategies. Further, traditional strategies poorly reflect the dynamic nature of habitat, the true extent of habitat, or the intrinsic complexity in societal perspectives. Forward‐thinking fish management and conservation plans view habitat as more than water. They are multilayered, ranging from pools to catchments to ecoregions, and from hours to seasons to centuries. They embrace, as a fundamental premise, that habitat evolves through both natural and anthropogenic processes, and that patterns of change may be as important as other habitat attributes.     

Exploitation and habitat degradation as agents of change within coral reef fish communities

Over‐exploitation and habitat degradation are the two major drivers of global environmental change and are responsible for local extinctions and declining ecosystem services. Here we compare the top‐down effect of exploitation by fishing with the bottom‐up influence of habitat loss on fish communities in the most diverse of ecological systems, coral reefs. Using a combination of multivariate techniques and path analyses, we illustrate that the relative importance of coral cover and fishing in controlling fish abundance on remote Fijian reefs varies between species and functional groups. A decline in branching Acropora coral is strongly associated with a decline in abundance of coral‐feeding species, and a decrease in coral‐associated habitat complexity, which has indirectly contributed to reduced abundance of small‐bodied damselfish. In contrast, reduced fishing pressure, brought about by declining human populations and a shift to alternate livelihoods, is associated with increased abundance of some piscivores and fisheries target species. However, availability of prey is controlled by coral‐associated habitat complexity and appears to be a more important driver of total piscivore abundance compared with fishing pressure. Effects of both fishing and coral loss are stronger on individual species than functional groups, as variation in the relative importance of fishing or coral loss among species within the same functional group attenuated the impact of either of these potential drivers at the functional level. Overall, fishing continues to have an influence on Fijian fish communities; however, habitat loss is currently the overriding agent of change. The importance of coral loss mediated by climate change is expected to have an increasing contribution to fish community dynamics, particularly in remote locations or where the influence of fishing is waning.     

Diversity and geneflow in a migratory frugivorous fish: implications for Amazonian habitat connectivity

Colossoma macropomum is an ecologically and economically important fish distributed throughout the major tributaries of the Amazon River. C. macropomum require a suite of habitat types for different life stages making them potentially susceptible to the impacts of habitat fragmentation and alteration. As a means of better understanding the potential impacts of development, baseline data on connectivity and patterns of gene flow in species from relatively undisturbed habitat will be of value to monitor potential ecosystem impacts of anthropogenic habitat alteration on native fish communities. We used 13 single sequence repeat markers to determine if fine-scale structuring could be detected at the landscape scale at the Pacaya Samiria National Reserve, Perú. We also applied a model testing approach to evaluate the strength of different migration models, including panmixia, stepping stone and isolation models. Bayesian clustering detected a single genetic grouping across 131 fish. However, a comparison of marginal likelihoods for alternative migration models across PSNR supported a stepping stone model, rather than panmixia (Probability ~1.0). These results demonstrate that even in highly migratory fish with limited genetic structure, the effects of anthropogenic aquatic habitat alterations can be explored using genetic data.     

Simple predictions of instream habitat model outputs for fish habitat guilds in large streams

1. In the context of a generalised modification of hydraulic conditions in medium to large streams, modelling the impacts of stream regulation on fish communities in multiple streams is an important challenge for basic and applied freshwater ecology. Conventional instream habitat models such as PHABSIM link a hydraulic model with preference curves for various species to estimate habitat value changes with discharge in stream reaches. Despite world‐wide applications, they have been scarcely used in multiple sites with multiple species. 2. We assigned 21 size classes of European fish species to four habitat guilds (cluster analysis grouping size classes with comparable microhabitat preference curves). Then, we ran a conventional instream habitat model on 28 French stream reaches belonging to the `barbel zone', to estimate habitat values versus discharge curves for the 21 size classes. We summarised the outputs as mean habitat values for guilds, and tested if they were predictable from average characteristics of reaches (discharge, depth, width, particle size). 3. As was obtained elsewhere for populations, habitat values for guilds were strongly related to average, dimensionless characteristics of reaches. The Reynolds number of reaches, equivalent to a discharge per width unit, reflected most of the discharge‐dependent changes in habitat values (within reaches). In particular, habitat values of species preferring bank (respectively midstream) microhabitats decreased (respectively increased) with increasing Reynolds number. The Froude number at median discharge was the major predictor of reach‐dependent but discharge‐independent variations in habitat values. Habitat values of species preferring riffle versus pool or bank microhabitats were higher in reaches with high Froude numbers. These relationships were consistent with existing knowledge on the different species. 4. Such results suggest that the input variables required to estimate habitat values for fish communities can be greatly simplified, as illustrated by a general estimation of the sensitivity of species preferring midstream habitats to discharge changes in any reach. Cost‐efficient alternatives to conventional instream habitat models should facilitate their validation in multiple sites, a point that remains critical in instream habitat modelling of fish communities.     

Interaction between Water and Wind as a Driver of Passive Dispersal in Mangroves

Although knowledge on dispersal patterns is essential for predicting long-term population dynamics, critical information on the modalities of passive dispersal and potential interactions between vectors is often missing. Here, we use mangrove propagules with a wide variety of morphologies to investigate the interaction between water and wind as a driver of passive dispersal. We imposed 16 combinations of wind and hydrodynamic conditions in a flume tank, using propagules of six important mangrove species (and genera), resulting in a set of dispersal morphologies that covers most variation present in mangrove propagules worldwide. Additionally, we discussed the broader implications of the outcome of this flume study on the potential of long distance dispersal for mangrove propagules in nature, applying a conceptual model to a natural mangrove system in Gazi Bay (Kenya). Overall, the effect of wind on dispersal depended on propagule density (g l^-1). The low-density Heritiera littoralis propagules were most affected by wind, while the high-density vertically floating propagules of Ceriops tagal and Bruguiera gymnorrhiza were least affected. Avicennia marina, and horizontally floating Rhizophora mucronata and C. tagal propagules behaved similarly. Morphological propagule traits, such as the dorsal sail of H. littoralis, explained another part of the interspecific differences. Within species, differences in dispersal velocities can be explained by differences in density and for H. littoralis also by variations in the shape of the dorsal sail. Our conceptual model illustrates that different propagule types have a different likelihood of reaching the open ocean depending on prevailing water and wind currents. Results suggest that in open water, propagule traits (density, morphology, and floating orientation) appear to determine the effect of water and wind currents on dispersal dynamics. This has important implications for inter- and intraspecific variation in dispersal patterns and the likelihood of reaching suitable habitat patches within a propagule''s viable period.     

Fish habitat modelling as a tool for river management

This paper presents an integrated modelling approach to simulate and assess ecological effects of physical habitat changes in rivers. An ecohydraulic simulation tool was created by combining a 1D hydraulic model based on HEC-RAS software and the fish habitat module of CASiMiR, a fuzzy logic-based ecohydraulic modelling system. This tool was applied on a river stretch commonly occurring in Belgium and elsewhere in Europe. In particular the effect of weir removal on habitat suitability for bullhead (Cottus gobio L.) was simulated. Physical conditions of the studied stretch after weir removal were simulated with a hydraulic model. CASiMiR linked these conditions to ecological expert knowledge to calculate habitat suitability for three life stages of bullhead at four different flow rates based on fuzzy logic. Results indicated that after weir removal, habitat suitability increased significantly for all life stages and all flow rates. The presented approach is promising regarding fish community assessment and ecological river engineering.     

Structure-forming corals and sponges and their use as fish habitat in Bering Sea submarine canyons

Continental margins are dynamic, heterogeneous settings that can include canyons, seamounts, and banks. Two of the largest canyons in the world, Zhemchug and Pribilof, cut into the edge of the continental shelf in the southeastern Bering Sea. Here currents and upwelling interact to produce a highly productive area, termed the Green Belt, that supports an abundance of fishes and squids as well as birds and marine mammals. We show that in some areas the floor of these canyons harbors high densities of gorgonian and pennatulacean corals and sponges, likely due to enhanced surface productivity, benthic currents and seafloor topography. Rockfishes, including the commercially important Pacific ocean perch, Sebastes alutus, were associated with corals and sponges as well as with isolated boulders. Sculpins, poachers and pleuronectid flounders were also associated with corals in Pribilof Canyon, where corals were most abundant. Fishes likely use corals and sponges as sources of vertical relief, which may harbor prey as well as provide shelter from predators. Boulders may be equivalent habitat in this regard, but are sparse in the canyons, strongly suggesting that biogenic structure is important fish habitat. Evidence of disturbance to the benthos from fishing activities was observed in these remote canyons. Bottom trawling and other benthic fishing gear has been shown to damage corals and sponges that may be very slow to recover from such disturbance. Regulation of these destructive practices is key to conservation of benthic habitats in these canyons and the ecosystem services they provide.     

Chwaka Bay (Zanzibar,East Africa) as a nursery ground for penaeid prawns

The penaeid prawns are recruited in the sheltered sandy beaches and mangrove areas of Chwaka Bay (Zanzibar) at post-larval stage (7 mm). Year round incursions with a maximum during the warmer months of December to March were observed. February to March is considered as the peak recruitment period. Out of six species of penaeids represented in the area, Penaeus latisulcatus (75%) and Penaeus indicus (15%) were dominant. The recruitment pattern indicated greatest incursions of post-larvae with the flood spring tides of the night when the tidal flow is strong.The juvenile population of P. latisulcatus is distributed in those intertidal sandflats with a rich growth of seagrass and P. indicus in the muddy areas of mangrove forests. P. indicus showed affinity for euryhaline conditions, whereas P. latisulcatus showed no preference for lower salinity. Provision of food and shelter are considered as important factors for their nursery dependence. P. latisulcatus attained a size of 60–70 mm in five to six months and P. indicus 110–120 mm in six to eight months during their nursery phase. These juvenile penaeids were found to be omnivorous, feeding on animal products, plant material and detritus.The maturing P. latisulcatus emigrate back to the sea when they are about 65 mm, whereas P. indicus move out at about 120 mm. A positive correlation between post-larval recruitment and juvenile abundance was observed.     

Influence of habitat degradation on fish replenishment

Temperature-induced coral bleaching is a major threat to the biodiversity of coral reef ecosystems. While reductions in species diversity and abundance of fish communities have been documented following coral bleaching, the mechanisms that underlie these changes are poorly understood. The present study examined the impacts of coral bleaching on the early life-history processes of coral reef fishes. Daily monitoring of fish settlement patterns found that ten times as many fish settled to healthy coral than sub-lethally bleached coral. Species diversity of settling fishes was least on bleached coral and greatest on dead coral, with healthy coral having intermediate levels of diversity. Laboratory experiments using light-trap caught juveniles showed that different damselfish species chose among healthy, bleached and dead coral habitats using different combinations of visual and olfactory cues. The live coral specialist, Pomacentrus moluccensis, preferred live coral and avoided bleached and dead coral, using mostly visual cues to inform their habitat choice. The habitat generalist, Pomacentrus amboinensis, also preferred live coral and avoided bleached and dead coral but selected these habitats using both visual and olfactory cues. Trials with another habitat generalist, Dischistodus sp., suggested that vision played a significant role. A 20 days field experiment that manipulated densities of P. moluccensis on healthy and bleached coral heads found an influence of fish density on juvenile weight and growth, but no significant influence of habitat quality. These results suggests that coral bleaching will affect settlement patterns and species distributions by influencing the visual and olfactory cues that reef fish larvae use to make settlement choices. Furthermore, increased fish density within the remaining healthy coral habitats could play an important role in influencing population dynamics.