Distribution Of Shrimp Research Articles

Vertical distribution and behaviour of shrimp Pandalus borealis larval stages in thermally stratified water columns: laboratory experiment and field observations

AbstractBy combining field data and laboratory observations of larvae in a simulated thermal gradient, we described the ontogenetic changes in vertical distribution and behaviour of early stages of shrimp Pandalus borealis in thermally stratified water columns. Both in the laboratory and at stations in the north‐western Gulf of St Lawrence, the first two larval stages appear to actively select and maintain a position in the upper layer of warmer temperatures, within the thermocline and above the cold (<1°C) intermediate layer. Stage III larvae were distributed deeper in the water column and in colder waters than the previous two stages. Stage IV and V larvae showed the highest degree of swimming activity in the laboratory and a much wider range (from surface to ∼200 m) in vertical distribution in the field. The shift to deeper waters and settlement to the bottom habitat appears to happen after the fifth moult, at stage VI. We propose that the pattern of vertical distribution in the field reflects the adjustment of the different developmental stages to the distribution of preferred prey. The description of the ontogenetic change in the vertical distributions and movements of early stages of P. borealis should be valuable information for future attempts to model larval transport and dispersion, and for detecting settlement/recruitment areas using 3D ocean circulation models. The identification of the thermal habitat of the different larval stages and the timing for settlement at the bottom also provides important information for the development of temperature‐dependent growth models up to the first juvenile stages.

Field distribution and osmoregulatory capacity of shrimps in a temperate European estuary (SW Spain)

The spatial distribution of the six most common crustacean decapods in the benthos and hyperbenthos of the Guadalquivir estuary (SW Spain) has been analysed in relation to their osmoregulatory capacities (at 20 °C). Field densities along the estuarine salinity gradient revealed that, although the species studied showed salinity tolerance ranges in the field that do overlap to some extent, there was a considerable spatial and/or salinity-related segregation between several of them. Concerning their isosmotic points and their osmoregulatory salinity ranges, two main groups of species were distinguished: species with higher isosmotic points and tight ranges in osmoregulation, represented by marine species that entered the estuary from open sea and remained there for only part of the year ( Crangon crangon, Melicertus kerathurus and Palaemon serratus); and those with slightly lower isosmotic points and wide ranges in osmoregulation, represented by estuarine species which completed their life cycle in brackish water ( Palaemon longirostris, Palaemon macrodactylus and Palaemonetes varians). For all the species studied, their field distributions were clearly biased towards the lower end of the salinity ranges within which they osmoregulate. Nevertheless, individuals of the less euryhaline species ( M. kerathurus and P. serratus) were mainly found in less saline water when the estuarine gradient was displaced downstream and low salinities occurred close to the river mouth.

Monitoring the Prestige oil spill impacts on some key species of the Northern Iberian shelf

Selected key components of the continental shelf benthic and demersal communities were monitored for the two years following the Prestige oil spill (POS) in order to identify the possible ecological effects of the oil. This work includes the first results regarding changes in abundance, distribution and food habits of hake ( Merluccius merluccius), four-spot megrim ( Lepidorhombus boscii), Norway lobster ( Nephrops norvegicus) and Pandalid shrimp ( Plesionika heterocarpus) populations of Galician and Cantabrian Sea shelves following the POS. Significant reductions in the abundance of Norway lobster, Plesionika heterocarpus and four-spot megrim were detected in the POS maximum impact area, located over the Galician shelf. Noteworthy recoveries were observed in the 2004 abundance indices of four-spot megrim and Plesionika. On the other hand, no significant effects were detected in the abundance or distribution of hake juveniles even though the tar aggregates were bound by the same oceanographic drift events as the hake recruits were during the winter of 2003 ( Navidad current) in different water column layers of the Cantabrian Sea. Feeding patterns of the four species analysed did not present apparent modifications that can be related to the POS.

Use of replanted mangroves as nursery grounds by shrimp communities in Gazi Bay, Kenya

This study quantitatively assessed the distribution of postlarval and juvenile shrimps in natural, degraded and replanted stands of Sonneratia alba mangroves in Gazi Bay, Kenya in 2002–2003. Two plantations (matrix and integrated) differing in historical status and planting strategy were studied. Sampling was conducted using stake nets (2 mm mesh), each net enclosing 9 m 2 of intertidal microhabitat. A total of 615 shrimps from 19 species/taxa were caught, including several penaeid species of major commercial importance. Penaeids dominated the catch (66%) followed by Macrobrachium spp. (16%) and Acetes sp. (6%). Shrimp abundance ranged from 0.42 to 10.0 ind. per net (9 m 2) for individual sites across spring tides and significant differences were detected between sites and over time ( p<0.001). Results showed no significant difference in diversity of species/taxa between sites. However, multivariate analysis revealed significant differences in community assemblages between sites, except for the natural stand and integrated plantation. These two sites harbored higher abundances of the majority of all taxa caught. The observed distribution patterns are discussed with regard to measured environmental parameters such as elevation, structural complexity and sediment characteristics.

Macrofaunal distributions and habitat change following winter–spring releases of freshwater into the Breton Sound estuary, Louisiana (USA)

We examined the effect of freshwater inflows on the aquatic environment and macrofauna in the intermediate and brackish zones of the Breton Sound estuary. Following water releases from the Caernarvon Freshwater Diversion Structure in winter 2000 and spring 2001, we compared environmental conditions and the abundance and distribution of nekton in May 2001 between the inflow area, which receives freshwater directly from the structure, and a nearby reference area. We used these data and stable isotope analyses for C, N, and S in brown shrimp Farfantepenaeus aztecus and two species of grass shrimp ( Palaemonetes paludosus and Palaemonetes intermedius) to test four null hypotheses: (1) water quality and SAV (submerged aquatic vegetation) coverage were similar between the inflow and reference areas, (2) macrofaunal abundance and biomass were similar between the two areas, (3) stable isotopic values of brown shrimp and grass shrimp were similar between areas, habitat types, and species, and (4) brown shrimp distributions were unaffected by river inputs. Freshwater from the structure clearly influenced the estuarine environment within the inflow area. Releases from the Caernarvon structure freshened the inflow area as intended and increased SAV and daytime dissolved oxygen concentrations. The response by macrofauna to these increased freshwater flows and habitat changes involved mostly changes in density and biomass rather than shifts in species composition. Although we detected no strong effect of the freshwater diversion on brown shrimp abundance or size in the inflow area, results of the sulfur stable isotope analysis indicated that brown shrimp collected in the inflow area had been growing in higher salinity waters, possibly following downstream displacement by the diversion. Species that would benefit most from continued freshwater diversions are likely to be those species that both use SAV as nursery habitat and thrive in a low-salinity environment. Nutrients carried by water from the structure were incorporated into the estuarine food web, and these nutrient inputs, together with an increase in SAV habitat, may enhance overall secondary productivity in the inflow area.

Spatial distribution of brown shrimp (Farfantepenaeus aztecus) on the northwestern Gulf of Mexico shelf: effects of abundance and hypoxia

We used fishery-independent hydrographic and bottom trawl surveys on the northwestern Gulf of Mexico shelf from 1983–2000 to test for density dependence and effects of hypoxia (dissolved oxygen ≤ 2.0 mg·L–1) on the spatial distribution of brown shrimp (Farfantepenaeus aztecus). Spatial distribution of shrimp was positively related to abundance on the Texas shelf but negatively related to abundance on the Louisiana shelf. Density dependence was weak, however, and may have been due to factors other than density-dependent habitat selection. Males were distributed over a broader area and further offshore than were females, though differences in spatial distribution between sexes were not large (~10%–15%). Large-scale hypoxia (up to ~20 000 km2) on the Louisiana shelf occurs in regions of typically high shrimp density and results in substantial habitat loss (up to ~25% of the Louisiana shelf), with shifts in distribution and associated high densities both inshore and offshore of the hypoxic region. We discuss these results in terms of the generality of density-dependent spatial distributions in marine populations and potential consequences of habitat loss and associated shifts in distribution due to low dissolved oxygen.

Day–night changes in the spatial distribution and habitat preferences of freshwater shrimps, Gammarus pulex, in a stony stream

Summary1. As many invertebrates are nocturnal, their spatial distribution and habitat preferences may change from day to night. Both aspects are examined for Gammarus pulex by testing the hypotheses: (i) a power function was a suitable model for the spatial distribution of the shrimps in both day and night; (ii) diurnal and nocturnal spatial distributions were significantly different; (iii) diurnal and nocturnal habitat preferences were significantly different. Five different life‐stages were treated separately. To ensure that the conclusions were consistent, large samples were taken near midday and midnight in April, June and November over 4 years at two sites about 3 km apart in a stony stream: downstream (n = 30) and upstream (n = 50).2. The first and second hypotheses were supported at both sites. A power function, relating spatial variance (s2) to mean (m), was an excellent fit in all analyses (P &lt; 0.001, r2 &gt; 0.91), i.e. the spatial variance was density‐dependent. All five life‐stages were aggregated in the day. At night, the degree of aggregation increased for juveniles at higher densities but decreased for juveniles at lower densities, increased for immature females and males, but decreased slightly for mature females and especially mature males, the latter being close to a random distribution. There were no significant differences between sites, in spite of the lower numbers at the downstream site.3. The third hypothesis was tested at only the upstream site and supported by comparisons between shrimp densities and 13 physical variables (distance from bank, water depth, water velocity, ten particle size‐classes), and three non‐physical variables (dry weights of bryophytes, leaf material, organic detritus). During the day, densities were strongly related to particle sizes with the following preferences: 0.5–8 mm for juveniles, 8–256 mm for the other life‐stages with a weaker relationship for males. There were no significant positive relationships with the other variables, apart from bryophytes for immature shrimps and adults. At night, densities were unrelated to particle size; juveniles and immature shrimps preferred low water velocities near the banks, often where leaf material and organic detritus accumulated, females often preferred medium water velocities slightly away from the banks, and males showed no habitat preferences.4. Day samples do not provide a complete picture of habitat preferences and probably identify refuge habitats. Day–night changes in spatial distribution and habitat preferences are an essential part of the behavioural dynamics of the shrimps and should be investigated in other species.

How to increase juvenile shrimps in mangrove waters?

The distribution, density and species composition of juvenile shrimps were studied in three mangrove zones of Pichavaram, situated on the southeast coast of India. Nine species of shrimps dominated with Metapenaeus monoceros, M. brevicornis and Penaeus indicus were recorded. This shrimp resource was higher in mangrove-lined waters than that in neritic waters. This field observation was experimentally proved using litter bags. The average catch around the litter bags was 0.80 individuals day−1 for Rhizophora leaves, 1.6 ind. day−1 for Avicennia leaves and 0.16 ind. day−1 in control. In general, abundance of juvenile shrimps was the highest around the leaves that were decomposed for 40 days and colonized with high counts of nitrogen-fixing azotobacters. Finally, a ‘vegetation trap’ technique using mangrove leaves has been proposed for efficient schooling of juvenile shrimps. There was about 3-fold increase in number of shrimp juveniles around the trap than at control waters.

Spatial distribution of symbiotic shrimps (Periclimenes holthuisi, P. brevicarpalis, Thor amboinensis) on the sea anemone Stichodactyla haddoni

This study investigated the spatial distribution patterns of three shrimp species, Periclimenes holthuisi, P. brevicarpalis, and Thor amboinensis on the sea anemone Stichodactyla haddoni in the laboratory. Anemones were partitioned into five zones (mouth, inner tentacle, outer tentacle, upper column, and lower column), and shrimp distribution on these zones was determined. Regardless of species, significantly higher numbers of shrimps chose outer tentacles (>40%) over other zones during daytime. Such distribution might be attributed to their feeding practices as these crustaceans clipped and ate parts of the outer tentacles. Periclimenes holthuisi also showed varying temporal distribution patterns on their hosts. At night when anemones contracted their tentacles, shrimp moved in significant numbers from the outer tentacle region either to the column or off the anemones. Shrimps returned to the tentacles during daytime when anemones expanded their tentacles. Thus, spatial and temporal distribution of shrimps depend upon their feeding activities and degree of anemone expansion.

Aquatic Insect Assemblages in Shrimp-dominated Tropical Streams, Puerto Rico1

We examined the effects of shrimp on insects in Puerto Rico by assessing insect assemblages in streams relative to varying shrimp densities and also by manipulating shrimp presence. Insect assemblages were typical of tropical island streams and insects were found in all stream habitats. Shrimp impacted insects directly via predation and competition and indirectly through removing particles from the water column. Variation in shrimp distribution is likely to be an important factor determining insect-assemblage structure in tropical streams.

Distribution and life history of caridean shrimps in regulated lowland rivers in southern Australia

Caridean shrimps are an integral component of lowland river ecosystems in south-eastern Australia, but their distributions may be affected by flow alteration. Monthly shrimp samples were collected from slackwaters in three hydrologically distinct sections of the heavily regulated Campaspe River and the less regulated Broken River for three consecutive years. The distributions of Paratya australiensis, Caridina mccullochi and Macrobrachium australiense, along with their life history in river sections with different hydrology are outlined. Paratya australiensis and M. australiense occurred in all sections, but C. mccullochi was absent from sections of the Campaspe River that received irrigation flows during summer/autumn. Shrimp larvae were most abundant in summer (December–February) and juvenile recruitment continued through to mid autumn (April). Breeding and recruitment of P. australiensis occurred for longer than other shrimps. Apart from large adult and berried M. australiense, all life stages of shrimps commonly occurred in slackwaters, particularly the larval and juvenile stages. Irrigation flows in summer/autumn probably adversely affect the size, extent and arrangement of slackwaters, at a time when they may be critical habitats for C. mccullochi larval development and recruitment. Dams and weirs in the Campaspe River may have influenced shrimp abundance and the timing of breeding.

Spatial distribution and structure along ecological gradients: penaeid shrimps in a tropical estuarine habitat of Mexico

The spatial structure of salinity, submerged aquatic vegetation (SAV) and 4 shrimp (Far- fantepenaeus spp.) populations along an estuarine habitat was evaluated seasonally, based on data gathered every 1 km along a 1-dimensional transect in Celestun Lagoon, Mexico. Geostatistical tech- niques were used to evaluate autocorrelation among data points along the transect, and to predict their spatial distribution by kriging. The estuarine habitat was characterized by persistent salinity gra- dients, decreasing from the seaward (1 km) to the inner (18 km) zone of the lagoon. SAV biomass was almost always spatially structured and best described by spherical variograms. Spatial partitioning be- tween SAV species was evident: the seagrasses Halodule wrightii and Ruppia maritima dominated the seaward and central zones of the lagoon, respectively, whereas the green alga Chara fibrosa was con- strained to the inner zone. Shrimp abundance also presented a marked spatial structure best described by spherical variograms. Spatial structure of shrimps, discriminated by species (F. aztecus, F. brasilien- sis, F. duorarum and F. notialis) and population components (recruits, juveniles, subadults) was most evident in the Nortes season (i.e. the season of strong northerly winds) and poorly defined in the Dry season. Spatial partitioning among species and population components was not detectable. Total shrimps and recruits were generally most abundant close to the mouth of the lagoon, although juvenile and subadult abundance peaked farther from the mouth than recruits. Distance from the mouth was the most important factor determining the spatial distribution of shrimps along the estuarine habitat. At local scales, abundance and type of SAV beds were crucial factors determining the abundance and spatial distribution of shrimps. Analysis at different spatial scales allowed us to identify the importance of spatial structure in biotic and abiotic variables of this estuarine habitat.

A comparison of two methods for sampling the Gulf of California mud shrimp, Neotrypaea uncinata (Crustacea: Thalassinidea)

Methods for collecting mud shrimp are highly variable, and the efficiency of these techniques is poorly known. In October 1999, on a mud flat near Puerto Peñasco, Sonora, México, we compared the 'kiwi' method for extracting mud shrimp from their burrows, with a core sampling method designed to accomplish the same goal. The coring method involved pressing a 10 cm × 90 cm PVC pipe into the substratum, creating suction in the pipe, and withdrawing a core of mud and the shrimp within. The kiwi method required 8-10 researchers to march in a circle on the mud flat to gradually liquefy the mud, thereby forcing shrimp to the surface as the circle closed inward. Two transects of 30 cores conducted at high and at low tide yielded similar numbers of adult animals of both sexes. Previous results show that mud shrimp tend to aggregate within burrows when breeding. In this collection, the distribution of mud shrimp in cores did not differ from a Poisson distribution, suggesting that these shrimp were non-reproductive. The kiwi method provided no such information on mud shrimp distributions. Moreover, this method generated significantly fewer intact mud shrimp than were obtained in cores. Our results indicate that compared to the kiwi method, the coring method generates more animals, is less destructive of mud shrimp habitat, and provides more detailed information on mud shrimp populations.

Salinity tolerance of brown shrimpFarfantepenaeus aztecus as it relates to postlarval and juvenile survival, distribution, and growth in estuaries

The brown shrimp,Farfantepenaeus aztecus, is the major component of the Gulf of Mexico shrimp fishery, and it is critical that we understand its environmental requirements. Brown shrimp spend a large portion of their post-larval (PL) and juvenile life within estuaries distributed along salinity gradients and yet our understanding of the salinity tolerance of various age groups is limited. A series of 48-hr bioassays were conducted in which various ages ofF. aztecus (PL-10, PL-13, PL-15, PL-17, PL-20, and PL-23) were acclimated from a salinity of 26‰ to 1‰, 2‰, 4‰, 8‰, 12‰, and 26‰ in order to determine their tolerance to these salinities. Finally, PL-80.F. aztecus were transferred directly from 25‰ to 2‰, 4‰, and 8‰ waters to study the effects of rapid salinity reductions on juvenile survival. Survival of 10-and 13-day-old PLs was significantly, different from the control (26‰) for all salinities tested. Survival of PL-15 shrimp and older was significantly lower than survival of the controls at 1‰ and 2‰ but similar to the control at all other salinities tested. A 4-wk growth trial was conducted with juvenile shrimp at 2‰, 4‰, 8‰, and 12‰. There was no significant difference in survival among treatments, although shrimp maintained at 8‰ and 12‰ grew significantlymore than shrimp maintained at 2‰ and 4‰. There was no growth difference between shrimp at the two low salinities or between shrimp at the two high salinities. Survival of juveniles transferred directly from 25‰ to various salinities were 100% at 25‰, 94.2% at 8‰, 67.3% at 4‰, and 63.5% at 2‰. These results suggest that PL-13 and younger brown shrimp would have a better chance of survival by delaying entry into estuaries susceptible to rapid salinity declines. The brown shrimp juveniles would, be more densely distributed in areas with salinities greater than 4‰ than in salinities less than 4‰. Although food availability, and bottom type also affect shrimp distribution survival and growth, salinity may also greatly affect the shrimp and its fishery.

Implications of a fluctuating fish predator guild on behavior, distribution, and abundance of a shared prey species: the grass shrimp Palaemonetes pugio

In some systems, the identity of a prey species' dominant predator(s) may not be constant over time. In cases in which a prey species exhibits different responses to various predator species, such changes in predator identity may have population-wide consequences. Our goals were to determine (1) whether mortality of and refuge use by the grass shrimp, Palaemonetes pugio, were predator-specific, and (2) how effects of prey size and habitat interacted with predator type. Striped bass ( Morone saxatilis) exerted twice as much predation pressure as mummichog ( Fundulus heteroclitus), although not equally as great on large (female) and small (male) shrimp. Mummichog, which fed preferentially on large shrimp, forced a partitioning of habitat between the two shrimp size classes. In contrast, large and small shrimp occupied similar habitats when subjected to striped bass, which fed on both size classes equally. Refuge use of grass shrimp depended on predator type. In the presence of mummichog, which occupied shallower depths in the water column than striped bass, shrimp stayed deep and close to structural habitat. Striped bass, which were deeper, caused shrimp to move high in the water column away from structural habitat. When both predators were present, shrimp distribution was similar to that when only striped bass were present, striped bass predation rate was enhanced, and overall mortality was higher than with either predator alone. Results suggest that at times when mummichogs are the dominant predators, large (female) shrimp experience higher predation than small (male) shrimp and are physically separated from their potential mates. When striped bass are more abundant, male and female shrimp may share a similar, shallow, less structure-oriented distribution and be subjected to higher mortality. When both predators are present, mortality rates may be higher still. This predator-, size-, and habitat-specificity of grass shrimp behavior suggests significant population and distribution consequences of fluctuating predator guilds and fluctuating cover of structural habitats in the field.

Density and distribution of water boatmen and brine shrimp at a major shorebird wintering area in Puerto Rico

The Cabo Rojo salt flats are an important wintering area for migratory shorebirds. Their quality is intimately related to prey availability, as prey are needed to meet energetic requirements. Understanding prey dynamics is, therefore, a key element of shorebird conservation plans. To this end, we monitored the density and distribution of water-boatmen (Trichocorixa spp.) and brine shrimp (Artemia spp.) in relation to water salinity from September to November of 1994 and 1995. Salinity ranged from 4 to 292 ppt, and gradients were related to hydrological alterations (e.g., salt extraction) and connection to the ocean. Brine shrimp were restricted to areas of highest salinity (≥ 106 ppt), whereas water-boatmen to areas of lowest salinity ( 100 ppt. Lowering water salinity did not result in osmolal related mortality. Results underscored the sensitivity of water boatmen to high salinity, particularly when the difference in salinity between the ‘source’ and ‘destination’ localities widened. Water boatmen density increased in one lagoon as salinity decreased from 65 to 47 ppt. On the basis of our experiments, local adult survivorship improved and immigration and subsequent survival of adults, if any, was not hindered. The density of nymphs also suggested that hatching occurred concurrently. The foraging value of the salt flats can be enhanced by maintaining salinity at < 65 ppt in selected management units and minimizing differences in salinity concentrations among them.

Abundance and distribution of juvenile white shrimp Litopenaeus setiferus within a tidal marsh landscape

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 232:213-223 (2002) - doi:10.3354/meps232213 Abundance and distribution of juvenile white shrimp Litopenaeus setiferus within a tidal marsh landscape Stacey R. Webb*, R. T. Kneib University of Georgia Marine Institute, Sapelo Island, Georgia 31327, USA *Present address: Florida Department of Environmental Protection, 2600 Blair Stone Road, M.S. 3555, Tallahassee, Florida 32309, USA. E-mail: stacey.webb@dep.state.fl.us ABSTRACT: We measured variation in low-tide abundance of juvenile white shrimp Litopenaeus setiferus in subtidal creek channels relative to the amount of intertidal drainage area and creek edge in an adjacent tidal marsh. Samples were collected over a 2 yr period (1998 and 1999) and included 201384 individual shrimp taken by cast net during July to November from 4 different tidal creek sub-systems in the Sapelo Island National Estuarine Research Reserve on the coast of Georgia (USA). Total drainage areas of the sub-systems ranged from 0.59 to 1.19 km2. Variation in intertidal and subtidal areas, and creek edge (interface between the vegetated marsh and open water) within the upper, middle and lower reaches of each sub-system was measured from high-resolution scaled (1:16 000) aerial photographs. Shrimp were most abundant and smaller in the upper reaches of tidal channels, where the adjacent intertidal drainage area and vegetated marsh edge were greatest. Regression analysis of shrimp abundance, measured as mean catch per unit effort, showed a strong positive relationship (p < 0.001; R2 = 0.86) with the amount of intertidal marsh edge in associated reaches (upper, middle or lower) of the creek sub-systems. Distribution and abundance of shrimp within tidal creeks probably reflected a link between estuarine landscape structure and accessibility of resources in the vegetated marshes that support shrimp production. Among the implications for management of shrimp resources in marsh habitats, we recommend a precautionary approach in making land-use decisions that would reduce structural complexity of marsh landscapes, particularly in the upper reaches of tidal creeks where juvenile white shrimp are concentrated and complex networks of creek channels enhance access to intertidal resources within the estuarine nursery. KEY WORDS: crustacean · Edge · Essential fish habitat · Estuarine nursery · Intertidal · Penaeid Shrimp · Sapelo Island Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 232. Online publication date: May 03, 2002 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2002 Inter-Research.

Distribution of migratory fishes and shrimps along multivariate gradients in tropical island streams

Among the 16 species of fishes and shrimps studied at 51 sites along several small streams at Basse Terre, Guadeloupe, Lesser Antilles, more than 60% showed a spatial distribution significantly influenced by environmental conditions. These included altitude, basin size, terrestrial vegetation and land use. However, the range of the species habitat was generally high, except for three species limited to the downstream stretches (two fishes and one shrimp). The habitat characteristics of fish and shrimp species tended to differ, but the difference was not significant (P<0·05). On the contrary, the habitat characteristics of amphidromous and catadromous species clearly differed, with a lower occurrence at the most elevated sites of catadromous species than amphidromous species.

Distribution of migratory fishes and shrimps along multivariate gradients in tropical island streams

Among the 16 species of fishes and shrimps studied at 51 sites along several small streams at Basse Terre, Guadeloupe, Lesser Antilles, more than 60% showed a spatial distribution significantly influenced by environmental conditions. These included altitude, basin size, terrestrial vegetation and land use. However, the range of the species habitat was generally high, except for three species limited to the downstream stretches (two fishes and one shrimp). The habitat characteristics of fish and shrimp species tended to differ, but the difference was not significant (P&lt;0·05). On the contrary, the habitat characteristics of amphidromous and catadromous species clearly differed, with a lower occurrence at the most elevated sites of catadromous species than amphidromous species.

Population Variability of Four Sympatric Penaeid Shrimps (Farfantepenaeus spp.) in a Tropical Coastal Lagoon of Mexico

Spatial and temporal variations in the relative abundance of the species and population components ofFarfantepenaeus shrimps were investigated in Celestun Lagoon, Mexico. The influence of salinity, water temperature and aquatic vegetation biomass on the distribution and abundance of shrimps was evaluated. The paper documents for the first time an annual analysis of the population variability of Farfantepenaeus notialis in its new range extension for the north-western Atlantic Ocean. A total of 11335 shrimps belonging to four Farfantepenaeus species (F. aztecus, F. brasiliensis, F. duorarum and F. notialis) were collected monthly during 1997 at three hydrological zones (seaward, middle, inner). The species represented 9·3, 25·1, 31·5 and 22·4% respectively of the total collection, the remaining 11·7% corresponding to small unidentified shrimps classified as recruits. The four penaeid species occurred throughout the year. Recruits, juveniles and subadults had a clear bimodal pattern, with a gap of 1–4 months between peaks of each population component, indicating a strong linkage between life stages. Two major peaks of recruits occurred during December–February and May–June: the first might be related to F. duorarum and F. notialis, while the second one could be attributed to F. brasiliensis and F. aztecus, suggesting differential recruitment timing. An ontogenetic spatial partitioning is also suggested to explain the observed patterns. The climatic seasons generated strong differences in temperature, salinity and aquatic vegetation biomass, which in turn affected the spatial patterns of shrimp density. The significant preference of all species, recruits and juveniles for the seaward zone highlights its strong nursery role for Farfantepenaeus species.