Shrimp Density Research Articles

Length at sex transition in northern shrimp (Pandalus borealis) off West Greenland in relation to changes in temperature and stock size

Abstract Length at sex transition of northern shrimp (Pandalus borealis) off West Greenland decreased in the years 1991–2002. A pronounced increase in bottom temperature occurred in the middle of this period, and stock size increased substantially during the past years. Length at sex transition differed within the area and the possible effects of changing temperature and abundance were studied for five different regions on the West Greenland shelf. On average, length at sex transition declined by 1.7 mm carapace length (CL) and mean bottom temperature increased by 1.9 °C. The change in length at sex transition was significantly correlated with bottom temperature in three out of the five regions and for all regions combined. No clear density-dependence was detected despite of a substantial increase in northern shrimp density in parts of the area and no evidence was found that northern shrimp decrease size at sex change during periods of low female abundance to compensate for a decreased reproductive potential. The observed change in length at sex transition was apparently independent from the fishery. Earlier maturation due to higher temperature was identified as the principal cause for the decrease in the length at sex transition. Because maximum female length is regarded to be proportional to length at sex transition and female fecundity increases with size, total egg production of a given year class over its lifespan would be lower when sex transition occurs at a smaller size if not compensated otherwise, e.g. through an increase of the proportion of females that spawn in each instead of every second year.

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.

Contribution of the natural biota associated with substrates to the nutritional requirements of the post-larval shrimp, Penaeus esculentus (Haswell), inhigh-density rearing systems

The contribution of epiphytes associated with physical substrates to the nutritional requirements of post-larval shrimp, Penaeus esculentus Haswell, was determined in high-density rearing systems (3000, 6000 and 11 000 m−3). Stable isotope signatures of epiphytes on polyethylene mesh substrate, AquaMats™ and tank walls were compared with shrimp signatures. Two methods were used: the determination of carbon and nitrogen natural abundance ratios; and 15N-nitrogen enrichment ratios after the addition of 15N-ammonium to tanks. Using the natural abundance technique and a simple mixing model, epiphytes were found to contribute substantially to the carbon requirements of post-larval shrimp (39–53%). This was despite the addition of formulated feed at satiation levels. There was no indication of a reduced contribution of carbon from epiphytes to shrimp nutrition at higher shrimp densities. The lack of a difference in the 15N/14N ratios of the two food sources meant that mixing models could not be used to calculate the contribution of nitrogen from epiphytes vs. artificial feed to shrimp nutrition. Using the 15N-nitrogenenrichment method, the amount of nitrogen contributed by epiphytes to shrimp nutrition over 24 h could be determined. This method showed that nitrogen from epiphytes was assimilated by shrimp. 15N-enrichment methods provided a more accurate alternative to natural abundance techniques, particularly when the stable isotope signals ofthe food sources are similar. This experiment hasshown the benefits in providing substrates for P.esculentus in high-density rearing systems to provide an additional food source for shrimp.

The impact of bait-pumping on populations of the ghost shrimp Trypaea australiensis Dana (Decapoda: Callianassidae) and the sediment environment

This study investigated the relative effects of bait-pumping and the associated disturbance of mudflats on populations of the ghost shrimp Trypaea australiensis and various sediment properties at Coronet Bay, Western Port, southern Australia. The experiment followed a BACI design, with 3 months of sampling before and after the simulated impact. Control plots were left undisturbed while two disturbance treatments (impact and procedural control) were bait-pumped. Ghost shrimp were removed from the impact plots, whereas ghost shrimp were allowed to re-burrow in the procedural control plots. The purpose of the procedural control was to test for the effect of the disturbance associated with bait-pumping. Initial destruction of burrows and compaction of sediment due to bait-pumping and trampling of the mudflat, reduced sediment porosity and created more reducing conditions to depths of 20 cm. The proportion of small particles (<250 μm) and abundance of benthic microalgae increased in surface sediments, whereas organic carbon content decreased. Ghost shrimp density decreased considerably and was slow to recover over the experimental period. Both disturbance treatments responded similarly indicating that the mudflat disturbance associated with bait-pumping was the major cause of the effects observed.

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.

Effects of extreme low flows on freshwater shrimps in a perennial tropical stream

Summary Long‐term data on rainfall suggests that perennial rainforest streams rarely are subject to drying of riffles or pools in the wet, non‐seasonal Caribbean climate of Puerto Rico. Unusually low rainfall in 1994 caused some headwater riffles to dry out completely, resulting in isolated pools, reduced pool volumes and loss of access to microhabitats by benthic invertebrates. From 1992 to 1998, shrimp populations were sampled bimonthly using baited traps in six pools along 1200 m (from 305 to 480 m in altitude) of Quebrada Prieta, a second‐order headwater stream in the Luquillo Experimental Forest (Caribbean National Forest). Following contraction of the smaller and shallower pools in the most upstream section of the stream, mean densities of the dominant shrimp (Atya lanipes) increased from 22 to 75 shrimp m−2 of pool area during the 1994 drought year. A second common species (Xiphocaris elongata) increased from 5 to 14 shrimp m−2. A smaller percentage of adults of both species was gravid during the drought. Following the 1994 drought (1995–1998), densities of both shrimp species and reproductive activity of Atya returned to predrought (1990–1993) levels. However, the reproductive activity of Xiphocaris remained lower than in the predrought period. It is suggested that prolonged droughts, even in tropical rainforest biomes, may significantly alter aquatic communities through localised crowding effects resulting from habitat contraction, and lead to prolonged decreases in reproductive output. Consequently, major alterations in aquatic populations and communities would be predicted by current climate change scenarios of decreased total rainfall and increased variability.

Stable isotope indicators of movement and residency for brown shrimp (Farfantepenaeus aztecus) in coastal Louisiana marshscapes

Brown shrimp (Farfantepenaeus aztecus) are an important commercial aquatic species experiencing loss of inshore marsh nursery habitat in coastal Louisiana. To study inshore brown shrimp movements and identify aspects of essential habitat important for sustaining brown shrimp populations, we collected juvenile brown shrimp in April and May 2000, the time of annual maximum brown shrimp abundance, in a small 1-km2 marsh area on the central Louisiana coast. Drop sampling showed average shrimp densities of 1.6–2.4 m−2 in shallow marsh ponds and seining indicated lower densities of 0.5–0.9 m−2 in nearby shallow channel and open bay sites. Smaller shrimp (< 50 mm) fed disproportionately on benthic diatoms and small harpacticoid copepods, while large shrimp fed more frequently on larger-bodied amphipods and tanaids. We developed novel chemical approaches to estimate patterns of shrimp residency and movement using carbon and nitrogen stable isotopic determinations. Resident shrimp had isotopic values similar to average foods and showed consistent isotopic spacings between fast and slow turnover tissues. Residency was highest (47–55%) in ponds and shallow channel habitats and much less in open bays and deep channels (4–27%). There was sparse evidence for dietary specialization among individull shrimp. The results support the view that small 10–20 mm postlarval and juvenile brown shrimp arriving in estuaries from offshore waters continue movement through sub-optimal habitats (deep channels and open bays), but exhibit much less movement once an optimal habitat (marsh ponds or shallow channel margins) is reached. This study also indicated that combining estimates of shrimp densities, residency, growth rate, and mortality allows evaluation of the importance of different habitat types for shrimp production. Shallow ponds that in many ways resemble fertile aquaculture ponds appear to be hot spots for brown shrimp production, and coastal preservation and restoration efforts should focus on these areas as important for sustaining shrimp fisheries.

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.

Comparative effects of the large bioturbators, Trypaea australiensis and Heloecius cordiformis, on intertidal sediments of Western Port, Victoria, Australia

The effect of bioturbation by the ghost shrimp Trypaea australiensis and semaphore crab Heloecious cordiformis was compared in sediment-filled tanks in the laboratory. Effect of bioturbator density was also investigated with high- and low-density treatments. It was hypothesised that the two species would influence the sediment profile in different ways owing to their contrasting burrowing and feeding habits. Both species increased porosity of surface sediments relative to control tanks. Crab activity did not alter redox potential, but low densities of shrimp created more oxidising conditions and high densities of shrimp created more reducing conditions than controls. Burial of tracer particles by crabs was restricted to the top 5 cm, whereas shrimp mixed particles to depths of 25 cm. Bioturbator density had little effect on the extent of particle mixing. The presence of both shrimp and crabs increased benthic microalgae in the sediments relative to the controls. Again, crabs had the greatest effect at the sediment surface, whereas shrimp also enhanced concentrations at 25 cm depth. High densities of shrimp had the greatest effect. Overall, shrimp bioturbation influenced deeper sediments than crab bioturbation, but there was no clear density-dependent effect.

Water quality control using Spirulina platensis in shrimp culture tanks

A cyanobacterium (Spirulina platensis) was co-cultured with black tiger shrimp (Penaeus monodon) for water quality control. We evaluated the effects of: (1) three S. platensis trial conditions on inorganic nitrogen concentrations at one shrimp density (S. platensis trial conditions included: absent, nonharvested and semicontinuous harvesting) and (2) two shrimp densities on inorganic nitrogen concentrations, with and without S. platensis. Semicontinuous harvesting of S. platensis at one shrimp density resulted in significantly reduced (P<0.05) inorganic nitrogen concentrations (NH4, NO2 and NO3). With S. platensis absent, ammonium and nitrite concentrations ranged from 0.5 to 0.6 mg l−1, while nitrate concentrations ranged from 16 to 18 mg l−1 by day 44. With nonharvested S. platensis, considerable variability occurred with nitrogen concentrations. Semicontinuous harvest of S. platensis reduced nitrate to 4 mg l−1, while ammonium and nitrite ranged from 0.0 to 0.15 mg l−1, respectively. The factorial evaluation of shrimp density versus presence and absence of S. platensis resulted in greatly reduced nitrogenous compounds with S. platensis present regardless of shrimp density, and only moderately increased nitrogen with greater shrimp density. Without S. platensis, all nitrogen compounds were substantially elevated and shrimp survived was significantly reduced at high shrimp density.

Water quality and plankton densities in mixed shrimp-mangrove forestry farming systems in Vietnam

Water quality and plankton densities were monitored in shrimp ponds at 12 mixed shrimp-mangrove forestry farms in Ca Mau province, southern Vietnam, to detail basic water chemistry and assess whether conditions are suitable for shrimp culture. In general, water quality was not optimal for shrimp culture. In particular, ponds were shallow (mean ± 1SE, 50.5 ± 2.8 cm), acidic (pH < 6.5), had high suspended solids (0.3 ± 0.03 g l-1), low chlorophyll a/phytoplankton concentrations (0.2 ± 0.05 μg l-1 and 8600 ± 800 cells l-1 respectively) and low dissolved oxygen (DO) levels (3.7 ± 0.15 mg l-1). Eight out of the 12 farms sampled had potentially acid sulphate soils (pH < 4.2). Salinity, DO and pH were highly variable over short time-periods (hours); DO in particular was reduced to potentially lethal levels (1-2 mg l-1). Seasonal variations in water chemistry and plankton communities (i.e. salinity, DO, phosphate, temperature, phytoplankton and zooplankton densities) appear to be driven by differences in rainfall patterns. The presence or absence of mangroves on internal pond levees ('mixed' versus 'separate' farms) and the source of pond water (rivers versus canals) were of lesser importance in determining water quality patterns and plankton biomass. Zooplankton and macrobenthos densities were sufficient to support the current (low) stocking densities of shrimp. However, natural food sources are not adequate to support increases in production by stocking hatchery reared post larvae. Increasing productivity by fertilization and/or supplemental feeding has the potential for adverse water quality and would require improvements to water management practices. Some practical strategies for improving water quality and plankton densities are outlined.

Maintenance of androdioecy in the freshwater shrimp Eulimnadia texana: sexual encounter rates and outcrossing success

The clam shrimp Eulimnadia texana has a rare mating system known as androdioecy, in which males and hermaphrodites cooccur but there are no pure females. In this species, reproduction takes place by outcrossing between males and hermaphrodites, or by selfing within a hermaphrodite; this system provides a unique opportunity to examine the adaptive significance of outcrossing and selfing in animals. Our study examined mating behavior in hermaphrodites and males from two populations to understand the propensity of these shrimp to mate and to estimate a parameter of a model developed by Otto et al. (American Naturalist 141:329–337), which predicts the conditions for stability of the mixed mating system in E. texana. Here we present evidence that mating frequency is environmentally sensitive, with greater numbers of encounters and matings per male when males are rare and in younger males. However, the effects of shrimp density, relative male frequency, and shrimp age interact in a complex way to determine male mating success. Overall, mating frequency was determined by a combination of encounter rates between the sexes and the proportion of encounters resulting in mating. The mating rates were then used to estimate one of four parameters of the Otto et al. model, and these estimates were combined with previous estimates of the other three parameters to examine the fit of the predicted to the observed sex ratios in the two populations. Key words: androdioecy, branchiopod crustaceans, evolution of breeding systems, inbreeding, mating success, mating tactics, mixed mating system, selffertilization. [Behav Ecol 13:561–570 (2002)]

Level and percentage recovery of resistance to oxytetracycline and oxolinic acid of bacteria from shrimp ponds

The bacterial level of the water, sediment and cultured shrimp ( Penaeus monodon) from different ponds were determined using a general medium, a presumptive Vibrio medium and a presumptive Pseudomonas– Aeromonas medium. Samples were taken from ponds that had not used any antimicrobial, ponds that had previously used and also ponds that were currently using oxolinic acid (OXA). The bacterial level in the sediment was higher than in the water using all three media. More bacteria existed in the pond system than in the receiving environment. Shrimp hepatopancreas harbored more bacteria than the lymphoid organ. The Vibrio density of the pond and bacterial levels in the shrimp were correlated with the use of the antimicrobial. The Vibrio level of the pond samples and microbial density of shrimps were higher from ponds that had not used any antimicrobials. The percentage recoveries of resistance to oxytetracycline (OTC) and OXA in bacteria from shrimp ponds and cultured shrimps were also determined using Zobell's marine agar, Pseudomonas– Aeromonas selective agar and thiosulfate citrate bile sucrose agar (TCBS) with the addition of either 25 μg/ml OTC or 25 μg/ml OXA. Presumptive Vibrio bacteria and other bacterial taxa recovered from the pond/receiving water/sediment from all three sites showed some degree of resistance to OTC and OXA. However, a higher percentage recovery of strains resistant to OTC than to OXA was observed among the presumptive vibrios and other bacterial taxa. Pseudomonas and Aeromonas bacteria were more resistant to OXA compared with the vibrios. All bacterial taxa resistant to OTC were more readily recovered from the water samples than from the sediment samples. In general, between the samples from the pond and from the receiving environment, a higher percentage of resistant strains was observed in the latter. Higher percentage recovery of bacteria resistant to OXA was observed in shrimp from ponds currently using OXA than those from ponds that had not used any or those that had previously used them. The results of the present study showed that the percentage recovery of resistance reflected the pattern of antimicrobial use.

Model of white spot syndrome virus (WSSV) epidemics in Litopenaeus vannamei.

White spot syndrome virus (WSSV) is devastating shrimp aquaculture throughout the world, but despite its economic importance no work has been done on modeling epidemics of this pathogen. Therefore we developed a Reed-Frost epidemic model for WSSV in Litopenaeus vannamei. The model includes uninfected susceptible, latently infected, acutely infected, and dead infected shrimp. The source of new infections during an outbreak is considered to be dead infected shrimp. The transmission coefficient, patency coefficient, virulence coefficient, and removal coefficient (disappearance of dead infected shrimp) control the dynamics of the model. In addition, an explicit area parameter is included to help to clarify the distinction between density and absolute shrimp population size. An analysis of the model finds that as number of shrimp, initial dose, transmission coefficient, patency coefficient, virulence coefficient, or removal coefficient changes, the speed of the epidemic changes. The model predicts that a threshold density of susceptible shrimp exists below which an outbreak of WSSV will not occur. Only initial dose, transmission coefficient, removal coefficient, and area coefficient affect the predicted threshold density. Increases in the transmission coefficient reduce the threshold value, whereas increases in the other factors cause the threshold value to increase. Epidemic models may prove useful to the shrimp aquaculture industry by suggesting testable hypotheses, some of which may contribute to the eventual control of WSSV outbreaks.

Evaluation of a sweep net electrofishing method for the collection of small fish and shrimp in lotic freshwater environments

A major limitation of methods for collecting small fish and shrimp is that no single method is equally effective for all species or all life history stages. We compared the efficacy of the sweep net electrofishing (SNE) method with two other commonly used sampling methods for the collection of small fish and shrimp, the standard sweep net (SW) and the point abundance electrofishing (PAE) method developed by Copp & Penaz (1988). The effectiveness of the three methods was compared under three different current speeds in an Australian lowland river. The SNE method collected an order of magnitude more shrimp and a greater size range than the other two methods, irrespective of the current speed. The SNE method captured a similar number of fish to the SW method, but significantly more individuals than the PAE method. However, the two electrofishing methods caught a greater size range of both fish and shrimp than did the SW method. The effectiveness of the SNE method appears to be due to its ability to immediately capture shocked individuals, thereby lessening the likelihood of escape. Whilst the SNE method is not quantitative, its high level of effectiveness is likely to be particularly useful in systems with low densities of fish and shrimp.

Marsh terracing as a wetland restoration tool for creating fishery habitat

Terracing is a relatively new wetland-restoration technique used to convert shallow subtidal bottom to marsh. This method uses existing bottom sediments to form terraces or ridges at marsh elevation. A terrace field is constructed by arranging these ridges in some pattern that maximizes intertidal edge and minimizes fetch between ridges; the intertidal area is planted with marsh vegetation. We examined the habitat value of terracing for fishery species at Sabine National Wildlife Refuge, Louisiana (USA) in spring and fall 1999 by quantifying and comparing nekton densities in a 9-yr-old terrace field and nearby reference area using a 1-m2 drop sampler. Decapod crustaceans were more abundant than fishes, composing 62% and 95% of all organisms we collected in spring and fall, respectively. White shrimp Litopenaeus setiferus, daggerblade grass shrimp Palaemonetes pugio, blue crab Callinectes sapidus, and brown shrimp Farfantepenaeus aztecus accounted for 94% of all crustaceans, whereas 60% of all fishes were gulf menhaden Brevoortia patronus. Mean densities of white shrimp (fall), daggerblade grass shrimp, blue crab, and brown shrimp (spring) were significantly greater in terrace marsh than on non-vegetated bottom in the reference pond. Densities of most nekton on non-vegetated bottom were similar in the terrace field and the reference pond, but gulf menhaden and white shrimp had higher densities at terrace pond sites and brown shrimp (spring) were more abundant at reference pond sites. The pattern for biomass was similar to that for density in that the mean biomass of most species was significantly greater at terrace marsh sites than reference pond sites and similar at terrace and reference pond sites. Terrace marsh, however, was not functionally equivalent to natural marsh, as mean densities of daggerblade grass shrimp (fall), brown shrimp (spring), and blue crab and mean biomass of white shrimp (fall), striped mullet Mugil cephalus (spring), and spotted seatrout Cynoscion nebulosus (fall) were greater at reference marsh sites than terrace marsh sites. Using these density and biomass patterns and the percentage of marsh and pond area in the terrace field, we concluded that terrace fields support higher standing crops of most fishery species compared with shallow marsh ponds of similar size. Future restoration projects could include design changes to increase the proportion of marsh in a terrace field and enhance the habitat value of marsh terraces for fishery species.

Effects of two commercially available low-protein diets (21% and 31%) on water and sediment quality, and on the production of Litopenaeus vannamei in an outdoor tank system with limited water discharge

Effects of feeding Litopenaeus vannamei with commercial low-protein diets on shrimp performance and on water and sediment quality were studied under high shrimp density with limited water discharge. Twelve tanks (each 10.5 m 2) were stocked with juvenile shrimp (average weight, 1.69 g) at a density of 40/m 2. Shrimp in six tanks were fed a 21% protein diet, and shrimp in the other six tanks were fed a 31% protein diet. Selected water quality parameters were monitored over a 94-day period. Shrimp growth, survival, feed conversion ratio (FCR) and selected sediment quality parameters were evaluated at harvest. Shrimp survival was significantly higher ( P=0.041) in the 31% treatment (96.2%) than in the 21% treatment (90.6%). Mean final weight of the shrimp in the 31% treatment (14.04 g) also was significantly higher ( P<0.001) than in the 21% treatment (12.17 g). The FCR of the shrimp in the 31% treatment (1.75) was significantly lower ( P<0.001) than in the 21% treatment (2.15). The gross production of the 31% treatment (540 g/m 2) was 22% higher than that of the 21% protein diet (441 g/m 2). No significant differences were found between the 21 and 31% treatments in the daily-measured water quality parameters (dissolved oxygen, temperature, pH, Secchi readings, and salinity) and the weekly-measured parameters (total ammonia-N, nitrite-N, nitrate-N, reactive and total phosphorus, 5-day carbonaceous biochemical oxygen demand (cBOD 5), chemical oxygen demand (COD), total suspended solids and volatile suspended solids (VSS)). Differences in sediment quality parameters (volume of sediment, COD, cBOD 5 and VSS) between treatments were not statistically significant. This work shows that the 31% protein diet provided better production results than the 21% protein diet, yet it did not significantly deteriorate the water or sediment quality.

Density and Water Exchange‐Dependent Growth and Survival of Litopenaeus setiferus Postlarvae

AbstractThis present study was designed to investigate the effects of stocking density and water exchange on the growth rate, survival and performance index of L. setiferus postlarvae under controlled laboratory conditions. The experiment was done with postlarvae (PL10 to PL40) at densities of 50, 150, 250 and 350 shrimp/m2 and various different water exchanges rate per day (0, 6, 12 and 18%). The maximum growth rate was obtained for shrimp with 12% water exchange per day at all densities. A reduction of the maximum growth rate was observed in relation to density with the highest values in shrimp stocked in a density of 50 and 150 shrimp/m2 (mean value of 0.53 mg/d) and the lowest in shrimp stocked in a density of 350 shrimp/m2 (0.24 mg/d). The multiple regression equation obtained to relate performance index (growth rate* survival : PI), shrimp density (X1) and water exchange (X2) was: PI = 0.31 + (0.001) X1+ 0.039 X2+ 2.28 × 10−6 X12+ (−0.0017) X22+ (0.000026)X1X2,R2= 0.78; P &gt; 0.03. According to this equation the optimum shrimp density‐water exchange comhination was between 5 to 12% of water exchange at stocking density of between 50 and 150 shrimp/m2. Salinity, ammonia‐N and nitrite‐N increased according to the time spent in tanks without water exchange. With no (0%) water exchange, water quality parameters measured were outside the optimum for L. setiferus postlarvae. The use of optimum density and water exchange in a nursery system for L. setiferus with optimum variables established is proposed.

Shrimp predation on 0-group plaice: contrasts between field data and predictions of an individual-based model

An individual-based model has been used to predict the effects of size-dependent predation by shrimp Crangon crangon on populations of newly settled 0-group plaice Pleuronectes platessa. The model predicted changes in population size and thus mortality rate of plaice during settlement and the following period until the plaice reached 30 mm in length and were invulnerable to further shrimp predation. Population parameters derived from seven years' catch data for 0-group plaice and shrimp from the west coast of Scotland were supplied to the model for direct comparison of predicted and observed mortality rates for each of the seven years. The contrast of predicted and observed mortality was used to test the hypothesis that predation by shrimp is a major factor regulating populations of juvenile plaice. There was no evidence for control of plaice populations by shrimp predation at the study site, since a negative relationship between plaice mortality and shrimp density was observed. Alternative explanations for the divergence of prediction and observations included (i) a general failure of the model to adequately describe predator and prey populations, (ii) limited effects of consumption of plaice by shrimp, and (iii) responses of both species to a third factor such as temperature producing an apparent association of survival and recruitment in the respective 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.