Evidence for secondary planktonic transport of post‐larvae of seagrass‐associated King George whiting

Stomach contents and stable isotope analyses were used to determine if secondary planktonic dispersal of King George whiting Sillaginodes punctata post‐larvae from shallow inshore habitats in a large embayment in south‐eastern Australia was initiated by wave disturbance. Benthic harpacticoid copepods, which live in and amongst seagrass shoots, were found in the stomachs of S. punctata caught offshore in the plankton only during rough weather. Stable isotope analyses showed that the base of nutritional support, estimated from values of δ¹³C, of S. punctata collected in the plankton changed significantly during rough (waves > 0·25 m) compared to calm (waves < 0·25 m) weather conditions. Values of δ¹³C collected from S. punctata in the plankton during rough weather were more consistent with S. punctata values found in the seagrass. Sillaginodes punctata collected in the plankton and seagrass during rough and calm weather failed to show differences in δ¹⁵N values. Dietary and isotope analyses support a model whereby newly arrived S. punctata larvae can be resuspended from seagrass beds and dispersed offshore by wave action during onshore winds. Secondary planktonic dispersal in S. punctata would provide a mechanism by which seagrass beds further inside Port Phillip Bay are colonized.     

Experimental trials on the feasibility of offshore seed production of the mussel <Emphasis Type="Italic">Mytilus edulis</Emphasis> in the German Bight: installation,technical requirements and environmental conditions

This study summarizes the activities and findings during a 2 year investigation on the grow-out of blue mussels (Mytilus edulis) and the technical requirements to withstand harsh weather conditions at an offshore location. The experimental sites were two different test areas, each 5 ha in size, 12–15 m in depth, in the vicinity of the offshore lighthouse “Roter Sand” located 15–17 nautical miles northwest of the city of Bremerhaven (Germany). Two versions of submerged longline systems were deployed: a conventional polypropylene longline in 2002 as well as a steel hawser longline in 2003, both featuring different versions of buoyancy modes. The spat collectors and grow-out ropes were suspended perpendicular from the horizontal longline for several months beginning in March of each respective year. The test sites were visited and sampled on a monthly basis using research vessels. Larval abundances in the surrounding water column reached numbers of up to 1,467 individuals m⁻³. Post-larval settlement success varied through the entire experimental period, ranging from 29 to 796 individuals of spat per meter of collector. Settled mussels reached a shell length of up to 28 mm 6 months after settlement. Based on the growth rates observed for the seed, it is projected that mussels would reach market size (50 mm) in 12–15 months post settlement, and at the observed densities, each meter of collector rope could yield 10.9 kg of harvestable mussels. The polypropylene line resisted storm conditions with wind waves of up to 6.4 m and current velocities of 1.52 m s⁻¹ and was retrieved in autumn of 2002. In contrast, the steel hawser-based line did not withstand the harsh weather conditions. The steel-based line consisted of six twisted strands that were untwisted by the strong currents and turbulences and consequently the individual strands were torn. Additionally, the line was accidentally cut by a yacht in July 2003. The biological study revealed that the tested location near “Roter Sand” has the potential to become an offshore seed production site as well as being exploitable as a grow-out site for mussel production to market size. In light of the technical results, recommendations for mussel culture strategies using a polypropylene longline system are given.     

Analysis of valine uptake by Commelina mesophyll cells in a biphasic active and a diffusional component

Valine uptake by isolated Commelina benghalensis L. mesophyll cells was measured over a wide concentration range (10^-6–4·10^-2 mol l^-1). The uptake data were subjected to iterative fitting. Experiments with carbonyl cyanide mchlorophenyl hydrazone (CCCP), diethylstilbestrol (DES), and p-chloromercuriphenylsulphonic acid (PCMBS) provided evidence that the biphasic uptake kinetics of valine consists of a diffusional component and a biphasic active uptake. The data from the control experiments, were also best fitted to one diffusional component and two Michaelis-Menten systems. The presence of two carrier systems in the plasmalemma, however, was considered to be virtual for the following reasons: (1) Both phases of active uptake were equally decreased by high concentrations of K⁺-ions. (2) Fusicoccin stimulated the active uptake in both phases to the same extent. (3) Inhibitors of the proton-driven uptake (CCCP, DES, PCMBS) similarly inhibited the active uptake at all concentrations. (4) The active uptake equally responded in both phases to changes in the pH. (5) Light also promoted the active uptake over the whole concentration range. These results strongly indicate that, despite its biphasic character, the active uptake is due to one proton-driven carrier system.Abbreviations CCCP carbonyl cyanide m-chlorophenyl hydrazone - DES diethylstilbestrol - FC fusicoccin - MES 2-(N-morpholino)ethanesulphonic acid monohydrate - PCMBS p-chloromercuriphenylsulphonic acid - v uptake velocity - S substrate concentration - K _m1 and K _m2 Michaelis constants of the apparent high- and low-affinity system, respectively - V _m1 and V _m2 maximal uptake velocities of the apparent high- and low-affinity system - k linear uptake constant     

Colchicine Induced Embryogenesis in Maize

The present study involves in vitro androgenesis of Zea mays L. using anther culture. We tested combinations of single factors and their influence on microspore induction. Embryogenic induction of microspores within anthers in in vitro conditions was the best when combination of cold treatment, TIBA (0.1 mg l^–1) in media and colchicine (0.02% during first 3 days of culture) was applied, but colchicine alone can be factor, which can stimulate or initiate embryogenesis in anther culture of maize.     

Distribution and breeding biology of offshore cichlids in Lake Malawi/ Niassa

Synopsis Lake Malawi/Niassa is the second largest rift valley lake in Africa, with an area of 28 800 km², and an average and maximum depth of 292 m and>700 m, respectively. The lake is well known for the great diversity of fish occurring in the inshore zone. However, the offshore fish community is poorly documented. To rectify this, regular sampling was undertaken over two years, using trawl and gillnets at six offshore locations. This paper reports on the species composition, spatial distribution and breeding biology of the dominant cichlids species from the offshore pelagic zone. Cichlids formed approximately 88% of the offshore fish biomass. Most abundant were two species of zooplanktivores in the genus Diplotaxodon that made up 71% of the offshore fish biomass. An undescribed species, given the cheironym D. bigeye, was mainly found at a depth of 220 m during the day, but moved into near surface waters at night when the moon was full. This species was absent from the shallow regions of the lake. The most abundant offshore species was D. limnothrissa, which was distributed evenly throughout the lake to depths of 220 m. A less common offshore zooplanktivore was Copadichromis quadrimaculatus that formed 5% of the biomass and was confined to the upper 100 m of the water column. The main piscivores were in the genus Rhamphochromis and formed approximately 10% of the offshore fish biomass. The two dominant taxa were R. longiceps and the large Rhamphochromis group, and both were more common in the southern half of the lake. The former occurred mainly in the upper 100 m of the water column and the latter mainly at depths of 100–150 m. The length at maturity and fecundity for the dominant offshore species were estimated and seasonal breeding cycles determined from gonad activity and gonado-somatic indices.     

Intensive rotifer cultures using chemostats

Continuous production of the rotifer Brachionus plicatilis rotundiformis (S-type) in an intensive chemostat culture system has been investigated. The production dynamics of rotifers in relation to different flow rates and feed regimes show that the growth rate and production depends on the type of algal feed and flow rate utilized in the culture system. It was possible to achieve a mean production of up to 318.84 × 10⁶ rotifers m^–3 d^–1 at a flow rate of 6 1 h^–1 in 100 1 chemostats and up to 261.21 × 10⁶ rotifers m^–3 d^–1 at a flow rate of 40 1 h –1 while using 1 m³ capacity rotifer chemostats as production units. The 3 fatty acid composition of rotifers while using Chlorella and Nannochloropsis in the culture system has been described. The results of this investigation show that the rotifer productivity in the continuous culture system is considerably higher than in any of the conventional culture systems described to date for aquacultural purposes.This research was financed by the Kuwait Foundation for the Advancement of Sciences (KFAS), Kuwait, under a contract research project code 86-04-02.     

Valine uptake by the scleractinian coral Galaxea fascicularis: characterization and effect of light and nutritional status

The characteristics of valine uptake by isolated microcolonies of Galaxea fascicularis (Linnaeus 1758) were studied under various conditions including light, dark and feeding. The results demonstrated the presence of: (1) a linear component which might represent either a diffusional transport or a low-affinity carrier-mediated transport (apparent carrier affinity >250 mol·l^–1), and (2) a high-affinity active carrier-mediated transport (apparent carrier affinity about 5 mol·l^-1). The latter is mediated by two different systems: (i) a Na⁺-dependent carrier, stimulated by light and operative in both fed and unfed polyps, and (ii) a Na⁺-independent carrier, light insensitive and present only in unfed polyps. Competition experiments with other amino acids show that the Na⁺-dependent carrier is highly specific for neutral amino acids, as indicated by the high inhibition constants of basic and acidic amino acids. Our results suggest that the energy supplied by zooxanthellae photosynthates is necessary for the process of amino acid uptake, and that the Na⁺-dependent carrier responsible for valine uptake by G. fascicularis is similar to the B^0,+ system.Abbreviations AA amino acid(s) - AC/HC ratio autotrophic/heterotrophic carbon - ASW artificial sea water - DOM dissolved organic material - HPLC high performance liquid chromatography - K ₁ apparent inhibition constant - K _m apparent affinity of the carrier - SE standard error - V _max maximal rate of absorption     

Distribution and breeding biology of offshore pelagic cyprinids and catfish in Lake Malawi/Niassa

Synopsis Lake Malawi/Niassa is a permanently stratified lake, consisting of an oxygenated layer of 230 m depth that overlies an anoxic zone that extends to the lake bottom at a maximum depth of over 700 m. Some fish are found throughout the oxygenated zone of the offshore waters, and lead a pelagic existence, although many of the species form part of the demersal community in shallower waters where the lake-bed is oxygenated. This paper reports on the distribution of cyprinids and catfish at six offshore locations, sampled with a mid-water trawl and gillnets, along the north-south axis of the lake. These two groups formed, respectively, 5% and 13% of the offshore fish biomass in the trawl catches. The offshore cyprinids comprised the small lake-spawningEngraulicypris sardella, the only species to have pelagic larvae, and the larger river-spawningOpsaridium microcephalum andO. microlepis. These cyprinids occurred mainly in the upper 100 m of the water column. The biomass of both larvae and adults ofE. sardella showed considerable fluctuations over the two years of study. The spawning site ofE. sardella remains a mystery, but is no longer believed to be in the offshore pelagic zone. The most abundant catfish was the mochokidSynodontis njassae, that exhibited diurnal movements from just above the oxic-anoxic boundary layer during the day to the surface layers at night. The clariid catfish of the genusBathyclarias were caught infrequently offshore, but owing to their large size, they could form a significant part of the biomass. Their abundance was difficult to quantify, as it was believed that the majority are active in the near-surface layers and were under-sampled by the trawl and gillnets. Information on some aspects of the breeding biology is included in the paper, although our knowledge in this area is very incomplete.     

Expression of a bioactive fusion protein of Escherichia coli heat-labile toxin B subunit to a synapsin peptide

The B subunit of Escherichia coli heat-labile toxin (LTB) may function as an efficient carrier molecule for the delivery of genetically coupled antigens across the mucosal barrier. We constructed vectors for the expression of LTB and LTBSC proteins. LTBSC is a fusion protein that comprises the amino acid sequence from the C-domain of rat synapsin fused to the C-terminal end of LTB. Both constructions have a coding sequence for a 6His-tag fused in-frame. LTBSC was expressed in E. coli as inclusion bodies. The inclusion bodies were isolated and purified by Ni²⁺-chelating affinity chromatography under denaturing condition. Purified LTBSC was diluted in several refolding buffers to gain a soluble and biologically active protein. Refolded LTBSC assembled as an active oligomer which binds to the GM1 receptor in an enzyme-linked immunosorbent assay (ELISA). Soluble LTB in the E. coli lysate was also purified by Ni²⁺-chelating affinity chromatography and the assembled pentamer was able to bind with high affinity to GM1 in vitro. LTBSC and LTB were fed to rats and the ability to induce antigen-specific tolerance was tested. LTBSC inhibited the specific delayed-type hypersensitivity (DTH) response and induced decreased antigen-specific in vivo and in vitro cell proliferation more efficiently than LTB. Thus, the novel hybrid molecule LTBSC when orally delivered was able to elicit a systemic immune response. These results suggest that LTBSC could be suitable for exploring further therapeutic treatment of autoimmune inflammatory diseases involving antigens from central nervous system.     

Plant regeneration from protoplasts of Enteromorpha intestinalis (Chlorophyta, Ulvophyceae) as seedstock for macroagal culture

A continuous micropropagation was established from protoplasts of thegreen alga Enteromorpha intestinalis. The effects of two differentcrude enzymes and the osmolarity at different concentrations of the enzymesolution on algal protoplast yields were tested. The optimal enzymecomposition for cell wall digestion and protoplast viability was 2%cellulase R 10 Onozuka and 2% Aplysie with 0.5 m mannitol. Largenumbers of Enteromorpha protoplasts were released (10.0 × 10⁶protoplasts from 1 g fresh thalli) and settled on a rangeof substrata. Regeneration of the protoplasts followed the normal patternfor this species. Conditions for pure cultures and efficient systems offloating supports with nets were determined to optimise the product qualityof plantlets of Enteromorpha. A promising storage process has beendeveloped which involves including protoplasts in beads of alginic acid gel.Plants regenerated from protoplasts may also be used as seedstock tofacilitate propagation for macroalgal culture.     

Optimalization of the peroxidase production by tissue cultures of horseradish <Emphasis Type="Italic">in vitro</Emphasis>

Tissue cultures of Armoracia rusticana L., both transformed with Agrobacterium rhizogenes and nontransformed, were screened for peroxidase activity. Most of the derived and tested strains exhibited 20 times higher activity [from 99 to 723 U g⁻¹(d.m.)] than the root of the intact plant [(30 U g⁻¹ (d.m.)]. The highest peroxidase activity was found in tumour culture growing on the medium without growth regulators. The influence of the addition of sugars and heavy metal ions in the medium on peroxidase production was tested. Increase in peroxidase activity was observed after cultivation of horseradish culture with cadmium, cobalt, nickel or lead ions.This work is supported by Grant Agency of Czech Republic Project No. 526/04/0135.     

Kelps on demand: Closed-system protocols for culturing large bull kelp sporophytes for research and restoration

Culturing kelps for commercial, conservation, and scientific purposes is becoming increasingly widespread. However, kelp aquaculture methods are typically designed for ocean-based farms, and these methods may not be applicable for smaller scale cultivation efforts common in research and restoration. Growing kelps in closed, recirculating culture systems may address many of these constraints, yet closed system approaches have remained largely undescribed. Extensive declines of the bull kelp (Nereocystis luetkeana), an ecologically important canopy species in the Northeast Pacific, have received widespread attention and prompted numerous research and conservation initiatives. Here, we detail two approaches for cultivating N. luetkeana sporophytes in closed recirculating systems. Nereocystis luetkeana were reared as attached thalli in custom seaweed growth flumes and also free-floating in tumble culture tanks. Careful control of stocking density, water motion, aeration, and nutrient levels allowed for rapid growth and normal morphogenesis of laboratory-grown kelp. Culture systems reached up to 3 kg · m⁻³, and individual thalli attained lengths of up to 6 m before the trials were terminated. Our results demonstrate the potential of recirculating, closed culture systems to overcome limitations associated with traditional culture methods. Recirculating systems enable the precise control of culture conditions, improving biosecurity and facilitating cultivar development and other research. Kelps can be grown away from the ocean or outside their native ranges, and seasonal or annual species can be produced year-round without seasonal constraints.     

Growth promotion and inhibition by antibiotic-producing fluorescent pseudomonads on citrus roots

Summary Forty-three strains of feeder root colonizing fluorescent pseudomonads from rough lemon (Citrus jambhiri Lush.) roots were examined for effects on rough lemon and sweet orange (Citrus sinensis Osbeck) seedlings. Plants inoculated with a single bacterial soil-drench had, after 10 months, a range of stimulatory (to 116%) and inhibitory effects (to 52%). Stimulatory bacteria particularly increased growth of root systems. Cultivar-specific inhibition and stimulation was evident in inoculations of rough lemon and sweet orange seedlings. Populations of fluorescent rhizobacteria on inoculated and noninoculated, as well as on stimulated and nonstimulated seedlings, did not differ significantly (10.8×10⁶ to 30.3×10⁶ CFU/g root). Population of fluorescent rhizobacteria on seedlings were higher than populations on feeder roots from grove trees (2.8 to 5.7×10⁶ CFU/g). Ninety-four and 81% of 251 fluorescent strains produced antibiotics against the fungusGeotrichum candidum and the bacteriumErwinia stewartii, respectively. Antibiotic activities of 90% of the antibiotic producing strains were repressed by Fe³⁺, indicating siderophore production. In comparison, only 9.6 and 15% of 94 randomly selected nonfluorescentPseudomonas strains were antibiotic producers. Differences between stimulatory and inhibitory or neutral bacteria were not apparent from antibiosis tests. On the basis of physiological tests,Pseudomonas putida was the most abundant (>62%) pseudomonad species on rough lemon roots. Growth stimulating strains appeared to be in bothP. putida andP. fluorescens groups. FewP. aeruginosa strains were identified on citrus roots.Florida Agricultural Experiment Stations Journal Series No.     

Statistic evaluation of the influence of species variation,culture conditions,surface wettability and fluid shear on attachment and biofilm development of marine bacteria

A budding coccoid bacterium, (CH1), a Vibrio sp. and a Pseudomonas sp. were investigated for factors governing their attachment to glass surfaces in static batch culture and laminar flow continuous culture systems. An analysis of variance showed that the three species exhibited very different responses. For CH1 attachment was dependent on cell density, incubation time and nutrient concentration. The Vibrio sp. was affected by nutrient concentration while the attachment of the Pseudomonas sp. was independent of cell density, incubation time and nutrient concentration. A comparison of attachment to hydrophilic and hydrophobic surfaces showed that attachment of the Vibrio sp. and CH1 to hydrophilic surfaces was 3 and 10 times greater respectively than to hydrophobic surfaces while Pseudomonas attached in equal numbers to both surfaces. The continuous culture system with defined flow hydrodynamics and growth conditions at steady state revealed a random sampling effect 3 times smaller than the batch culture system did. When the biofilm development of Pseudomonas sp. was followed during 46 h at different fluid shear under laminar and turbulent flow conditions, the former biofilm reached 3.3·10⁸ cells·cm^-2 and the latter 8.2·10⁷ cells·cm^-2.Non-common abbreviation NSS Nine salt solution     

Micropropagation of the endangered <Emphasis Type="Italic">Kniphofia leucocephala</Emphasis> Baijnath

Summary The species, Kniphofia leucocephala is extant at only one location, Langepan, KwaZulu-Natal in South Africa, where the population is threatened by afforestation and possibly grazing. Consequently, a continuous culture system was established as part of a program for the propagation and re-introduction of plants into the wild. The efficiency of the system in terms of shoot multiplication and, particularly, the frequency and rate of root initiation was strongly influenced by the concentration of benzyladenine in the shoot multiplication medium. The optimum shoot multiplication medium for subsequent root initiation contained 2 mgl⁻¹ (8.9 μM) benzyladenine alone. The shoots were successfully rooted and acclimatized. Approximately 200 shoots can be produced from one shoot after five 4-wk cycles. Thus, large numbers of plantlets can be propagated in this continuous culture system, serving conservation interests.     

Detection of Campylobacter jejuni in poultry samples using an enzyme-linked immunoassay coupled with an enzyme electrode

An enzyme-linked immunoassay coupled with a tyrosinase modified enzyme electrode was used for rapid detection of Campylobacter jejuni. The immunomagnetic separation (IMS) method was investigated to achieve optimal isolation of C. jejuni cells. Eight types of beads with three different sizes and function groups were coated with anti-C. jejuni to isolate C. jejuni from the sample solution. Bead size and coating methods were found to be major factors that influenced the capture efficacy. Streptavidin-labeled beads (2.8 μm) provided the greatest capture ability. Three blocking reagents were tested to minimize non-specific binding. Bovine serum albumin (BSA) showed the best blocking capability. Two IMS formats were tested. Competitive immunoassay cut the detection time to 1.5 h, but the detection limit was relatively high (10⁶ CFU/ml). This system was evaluated using C. jejuni pure culture and poultry samples inoculated with C. jejuni. This detection method for C. jejuni could be completed within 2.5 h and had a detection limit of 2.1×10⁴ CFU/ml. No significant difference was found between pure culture samples and poultry samples (P>0.01). A linear relationship was found between C. jejuni cell numbers and the peak current ratio in a range of 10²–10⁷ CFU/ml (R²=0.94).     

Specificity of siderophore-mediated transport of iron in rhizobia

The trihydroxamate siderophore, hydroxamate K, has been purified from culture filtrates of iron-deficient Rhizobium leguminosarum biovar viciae MNF710. The iron complex has a molecular weight of 828 and an absorption maximum at 443 nm (_M=1510). ⁵⁵Fe complexed to purified hydroxamate K was taken up by MNF710, its hydroxamate-negative mutant MNF7102 and Rhizobium leguminosarum biovar trifolii WU95 via an iron-regulated transport system, but Rhizobium meliloti U45 failed to take up the iron-siderophore complex under any conditions. A similar pattern of iron uptake was observed with ferrioxamine B. MNF710, MNF7102, U45 and WU95 all transported ⁵⁵Fe-ferrichrome but only the first three strains took up ⁵⁵Fe-ferrichrome A. All these ⁵⁵Fe-trihydroxamate uptake systems were ironregulated in MNF710, MNF7102 and WU95. In contrast, uptake of ⁵⁵Fe-rhodotorulate, a dihydroxamate, was essentially constitutive in all four organisms. Similarly, uptake of ⁵⁵Fe-citrate and ⁵⁵Fe-nitrilotriacetic acid was constitutive. None of the strains took up ⁵⁵Fe complexed with enterobactin or with pyoverdins from Pseudomonas aeruginosa ATCC15692 (PAO1) and Pseudomonas fluorescens ATCC17400.     

The biology of pygmy whitefish, Prosopium coulterii, in a closed sub-boreal lake: spatial distribution and diel movements

Using gillnets and trap nets, we examined the spatial distribution, diel movements, and environmental tolerances of pygmy whitefish, Prosopium coulterii, in a small boreal lake in north-central British Columbia. Most gillnets were set below the thermocline but we also fished a shore net in the littoral zone. During the ice-free season (May to November) there was a strong diel onshore–offshore movement: during the day pygmy whitefish were offshore and below the thermocline (water temperatures of 4–6°C) but at night they were inshore and above the thermocline (water temperatures of 12–18°C). This onshore–offshore movement occurred close to the bottom and, regardless of where they were caught, most fish were <4 m off the bottom. Oxygen concentrations in most of the hypolimnion dropped to <5.0 mg l⁻¹ in June and by late August to <1.0 mg l⁻¹; indicating pygmy whitefish can tolerate low oxygen conditions. The catch of pygmy whitefish in gillnets set below the thermocline was highly skewed: 53% of the nets were empty, 37% caught 18 or less fish, and 10% caught 70% of the total catch (742 fish). Trap nets produced similarly skewed results: most trap net sets caught no pygmy whitefish but one set caught over 2,000 individuals. Our catch data suggest that in Dina Lake #1 some pygmy whitefish aggregate.     

The mycorrhizal fungus Paxillus involutus transports magnesium to Norway spruce seedlings. Evidence from stable isotope labeling

Although it is well established that ectomycorrhizas improve the mineral nutrition of forest trees, there has been little evidence that they mediate uptake of divalent cations such as Mg. We grew nonmycorrhizal seedlings and seedlings mycorrhizal with Paxillus involutus Batsch in a sand culture system with two compartments separated by a 45-μm Nylon mesh. Hyphae, but not roots, can penetrate this net. Labeling the compartment only accessible to hyphae with ²⁵Mg showed that hyphae of the ectomycorrhizal fungus Paxillus involutus transported Mg to their host plant. No label was found in nonmycorrhizal control plants. Our data support the idea that ectomycorrhizas are important for the Mg nutrition of forest trees. This revised version was published online in June 2006 with corrections to the Cover Date.     

Calibration of visually estimated distances to migrating seabirds with radar measurements

ABSTRACT Censusing seabirds from coastal areas requires reliable estimates of bird numbers and the distances of the birds from the coastline. Logistical constraints make visual estimation of distances the only feasible method in many studies. We tested the accuracy of visually estimated offshore distances of six migratory seabird species in the Strait of Gibraltar using simultaneous measurements obtained by radar. Most birds (91%) were detected within 3 km of the coast and we truncated our calibration at this distance. We found a strong correlation between radar and visual estimates (R²= 0.83, P < 0.0001). The magnitude of errors in visual estimates was moderate and ranged from 0.08 to 0.20 for different distances and observers. Among the factors potentially affecting the accuracy of visual estimates of distance to seabird in our study were observer identity, bird species, bird behavior, and weather; the most parsimonious model in our study included observer identity as the only predictor, and no model with more than one predictor had a smaller Akaike's information criterion value. Radar can be used to help train observers and to reduce biases in visual estimates of distances by means of calibration. When no other methods are available to accurately measure distances to seabirds, visual estimates of distances, recorded by experienced observers and once calibrated with radar (or other ground‐truthing methods), may be acceptable for different species under a wide range of environmental conditions.