Effects of cage netting colour and density on the skin pigmentation and stress response of Australian snapper Pagrus auratus (Bloch & Schneider, 1801)

The unnaturally dark pigmentation of cultured Australian snapper Pagrus auratus can be improved through dietary astaxanthin supplementation and by holding fish in tanks with a white background. The practical application of these laboratory‐based findings was examined with two experiments to establish if the advantages of transferring fish to light coloured tanks before harvest could be achieved on‐farm using white cages and to determine the effects of fish density on skin colour. For the first experiment, snapper (mean TL=29.7 cm) were transferred from a commercial snapper sea cage to black or white netted cages and fed diets supplemented with unesterified astaxanthin (supplied as Lucantin^® Pink, BASF) at 0 or 39 mg kg^?1 for 42 days. Skin colour was measured using the CIE (black–white), (green–red), (blue–yellow) colour scale. Snapper held in white netting cages became significantly lighter (higher ) than snapper held in black cages; however, values were not as high as previous laboratory‐based studies in which snapper were held in white plastic‐lined cages. Snapper fed astaxanthin displayed significantly greater and values, and total carotenoid concentrations after 42 days. In addition, total carotenoids were higher in fish from black than white cages. The second experiment was designed to investigate whether density reduced the improvements in skin colour achieved by holding fish in white coloured cages and whether cage colour affected stress. Snapper (mean weight=435 g) were acclimated to black cages and fed 39 mg kg^?1 astaxanthin for 44 days before transferring to black or white plastic‐lined cages at 14 (low), 29 (mid) or 45 (high) kg m^?3 for 7 days after which time skin colour, plasma cortisol and plasma glucose concentrations were measured. Skin lightness () was greater in snapper transferred to white plastic‐lined cages with the lightest coloured fish obtained from the lowest density after 7 days. Density had no effect on plasma cortisol or glucose levels after 7 days, although plasma cortisol was elevated in snapper from black cages. For improved skin colouration we recommend feeding unesterified astaxanthin at 39 mg kg^?1 for approximately 6 weeks and transferring snapper to white plastic‐lined cages or similar at low densities for short periods before harvest rather than producing fish in white netting sea cages subject to biofouling.     

Effects of different manure levels on fish growth, mortality and yield in a horizontally-integrated fish-cum-poultry farming system in Nigeria

Optimum manure rate in horizontally-integrated fish ponds, using poultry droppings, was evaluated in two agricultural zones of Imo state of Nigeria between July 1994 and January 1995. It was aimed at resolving the problem of frequent cases of excessive algal blooms, oxygen depletion and stress-related mortalities in fish ponds. Results indicate that poultry droppings, applied at the rate of 5000 kg ha^?1 per year gave the best yield (23.5 M.t ha^?1 for Heterobranchus bidorsalis Geofrey St Hillaire, 1809; and 11.2 tons ha^?1 for Oreochromis niloticus Linnaeus, 1758), higher fish recovery and optimum tolerance levels of the physico-chemical parameters of water for fish growth. The estimated costs of production at this level of fertilization were 2 734 000.00 per hectare (equivalent to US$33 351.5), with the highest net revenue of 1 555 000.00 (US$18 963), and a cost: benefit ratio of 1: 2.3. This implies that for every 1.00 (US$0.01) invested in the production system at this rate of pond fertilization, the highest net profit of 1.30 (US$0.02) accrued to the farmer compared with any other manure load of the pond.     

Phenotypic and Genetic Variation in Two North American Arctic Charr,Salvelinus alpinus,Stocks Cultured in a Recirculating Aquaculture System

Arctic charr, Salvelinus alpinus, were obtained as eggs from two North American sources, an eastern (Fraser River, Canada) and a western (Bristol Bay, Alaska, USA) stock. Fish were pit tagged (eastern , western ) and stocked communally into three replicated 7‐m³ tanks at a density of 7.6 ± 1.2 kg/m³. Each tank was supplied with 2 ppt salinity water from a recirculating biological filtration system. Fish were fed a commercial diet (48%P, 20%F) from automatic feeders. Fish were harvested approximately 24 mo after hatching at a final tank density of 56.2 ± 1.3 kg/m³. Mean daily water temperature was 10.4 C with a range of 5.9–12.6 C and mean dissolved oxygen was 12.6 mg/L with a range of 9.7 (91% of saturation) to 17.8 mg/L (149% of saturation). Individual fish were evaluated for total, carcass, and fillet weight, sex, stage of sexual maturity, fillet fat, fillet color, and gonad weight. The western stock fish grew faster and were significantly larger ( ) (P < 0.0001) than the eastern stock ( ). Heritabilities were low for all traits and ranged from 0.08 ± 0.06 to 0.28 ± 0.09. Genetic correlations for body size traits were highly positive and significant (0.99–1.00). Genetic correlations of fillet color and body size traits were positive, but not significant. Genetic correlations of fillet fat with body size traits were negative, but not significant. Genetic correlation of fillet fat and fillet color was positive and significant (0.55 ± 0.27). Random genetic variation was assessed from microsatellite variability on 30 fish from each stock (eastern and western) and 32 fish from a third commercial source of unknown origin for comparison. The amount of variation detected in the western stock was higher than the eastern and commercial stocks. The eastern stock was composed of an admixture of two sources, the commercial stock was composed by three different sources, and the western stock was composed by three to four source populations. Data will be used to calculate breeding values on a separate group of captive sibling adult broodfish and a line selected for carcass weight and maturity at 3 yr will be developed.     

Sampling techniques for burbot in a western non‐wadeable river

Burbot, Lota lota (L.), populations are declining throughout much of their native distribution. Although numerous aspects of burbot ecology are well understood, less is known about effective sampling techniques for burbot in lotic systems. Occupancy models were used to estimate the probability of detection () for three gears (6.4‐ and 19‐mm bar mesh hoop nets, night electric fishing), within the context of various habitat characteristics. During the summer, night electric fishing had the highest estimated detection probability for both juvenile (, 95% C.I.; 0.35, 0.26–0.46) and adult (0.30, 0.20–0.41) burbot. However, small‐mesh hoop nets (6.4‐mm bar mesh) had similar detection probabilities to night electric fishing for both juvenile (0.26, 0.17–0.36) and adult (0.27, 0.18–0.39) burbot during the summer. In autumn, a similar overlap between detection probabilities was observed for juvenile and adult burbot. Small‐mesh hoop nets had the highest estimated probability of detection for both juvenile and adult burbot (0.46, 0.33–0.59), whereas night electric fishing had a detection probability of 0.39 (0.28–0.52) for juvenile and adult burbot. By using detection probabilities to compare gears, the most effective sampling technique can be identified, leading to increased species detections and more effective management of burbot.     

Stocking Density and its Influence on Growth of Spotted Wolffish,Anarhichas minor,in Shallow Raceways

We compared growth properties of spotted wolffish with initial mean weight (± SE) 3.65 (± 0.15) kg reared in shallow raceways at three different densities (50, 70, and 90 kg/m², corresponding to approx. 170, 240, and 310 kg/m³) in a 447‐day‐long growth trial at ambient temperature (mean (± SD), 4.2 C ± 1.9). Growth in all three groups was comparable, but there was a trend toward higher growth at the highest density. Final mean weights (kg ± SE) were 4.90 (± 0.15), 4.80 (± 0.16), and 5.05 (± 0.11), at the 50, 70, and 90 kg/m² density, respectively. Initially, growth rates were higher at the 90 kg/m² density group compared to the two other densities, but overall growth rates were similar in all three groups. Feed conversion efficiency differed and was higher at the two highest densities compared to lowest density. The acid–base balance was influenced by the density treatment as there was a general trend toward higher partial pressure of CO₂ and HCO in the 90 kg/m² group. Calculated productivity increased almost linearly with increasing stocking density and was found to be 24, 42, and 66 g/m²/day at 50, 70, and 90 kg/m², respectively. The results show that the optimum density conditions for farming large spotted wolffish, both with respect to growth rate, feed conversion, and productivity are at densities equal to or higher than 90 kg/m².     

Evolutionary assembly rules for fish life histories

We revisit the empirical equation of Gislason et al. (2010, Fish and Fisheries 11 :149–158) for predicting natural mortality (M, year^?1) of marine fish. We show it to be equivalent to , where L_∞ (cm) and K (year^?1) are the von Bertalanffy growth equation (VBGE) parameters, and L (cm) is fish length along the growth trajectory within the species. We then interpret K in terms of the VBGE in mass , and show that the previous equation is itself equivalent to a ?? power function rule between M and the mass at first reproduction (W_α); this new ?? power function emerges directly from the life history that maximizes Darwinian fitness in non‐growing populations. We merge this M, W_α power function with other power functions to produce general across‐species scaling rules for yearly reproductive allocation, reproductive effort and age at first reproduction in fish. We then suggest a new way to classify habitats (or lifestyles) as to the life histories they should contain, and we contrast our scheme with the widely used Winemiller–Rose fish lifestyle classification.     

A novel combined recirculating treatment system for intensive marine aquaculture

This study established a pilot‐scale recirculating treatment system that coupled an ecological process with a biological process to achieve adequate water quality and to minimize the water consumption for intensive marine culture. The recirculating treatment system consisted of a settling cell, a biofilter tank, a bivalve tank and gravel beds. The toxic pollutants, threatening the growth of bivalves, were reduced by the settling cell and the biofilter tank, so that the polyculture of shrimp and bivalves could be achieved. The living bivalve tank could function well as a remover of remaining small suspended solids (SS), and other pollutants. As the SS was reduced to a very low level by bivalve tank before the water flowing into the gravel beds, the risk of clogging was prevented. The studies suggested that the system maintained high removal efficiencies of SS, ammonium nitrogen () and nitrite nitrogen () and could contribute to the increase in shrimp yield.     

Modelling fish growth: Model selection,multi-model inference and model selection uncertainty

Model selection based on information theory is a relatively new paradigm in biological sciences with several advantages over the classical approaches. The aim of the present study was to apply information theory in the area of modelling fish growth and to show how model selection uncertainty may be taken into account when estimating growth parameters. The methodology was applied for length–age data of four species of fish, taken from the literature. Five-candidate models were fitted to each dataset: von Bertalanffy growth model (VBGM), generalized VBGM, Gompertz growth model, Schnute–Richards growth model, and logistic. In each case, the ‘best’ model was selected by minimizing the small-sample, bias-corrected form of the Akaike information criterion (AIC). To quantify the plausibility of each model, given the data and the set of five models, the ‘Akaike weight’ $w_i$ of each model was calculated. The average model was estimated for each case based on $w_i$. Following a multi-model inference (MMI) approach, the model-averaged asymptotic length ${\overline{L}}_{\infty }$ for each species was estimated, using all five models, by model-averaging estimations of L_∞ and weighting the prediction of each model by $w_i$. In the examples of this study, model selection uncertainty caused a magnification of the standard error of the asymptotic length of the best model (up to 3.9 times) and thus in all four cases estimating L_∞ from just the best model would have caused overestimation of precision of the asymptotic length. The VBGM, when used for inference, without being the best model, could cause biased point estimation and false evaluation of precision. Model selection uncertainty should not be ignored even if VBGM is the best model. Multi-model inference by model-averaging, based on Akaike weights, is recommended for making robust parameter estimations and for dealing with uncertainty in model selection.     

Formulation of an Alternative Culture Medium for Arthrospira maxima (Spirulina) Based on “Tequesquite,” a Traditional Mineral Resource

Culture media formulation for Arthrospira maxima (formerly designated as Spirulina maxima) production is a constraining factor. Use of synthetic media could be expensive because most of the commercial strains require salinity conditions higher than 20 g/L. In this study, a traditional, pre‐Hispanic mineral resource, known as “tequesquite,” was used as the base for the formulation of an alternative and low‐cost culture medium. Tequesquite is available from outcrop areas located in the basin of the former Lake Texcoco, where A. maxima thrived in pre‐Hispanic times. The effect on the biomass production of different tequesquite‐based formulations, varying N, P, and , was tested and compared with results obtained with Zarrouk's medium (ZM). Growth of biomass in some alternative formulations showed no significant differences compared with ZM. Production in a bubble‐column photobioreactor of the alternative medium reached a maximum of X = 2570.4 mg/L dry weight, compared with X = 2931.4 mg/L dry weight obtained with ZM. The proposed tequesquite‐medium formulation has equivalent N and P concentrations as ZM, but only 50% of NaHCO₃, thus reducing the medium's cost. In addition, it avoids the use of seawater or NaCl to provide the salinity condition required for the adequate growth of Arthrospira.     

An overview of stress physiology of fish transport: changes in water quality as a function of transport duration

This study brings an integrated analysis about the relationship between water deterioration and its physiological consequences in live fish transport. The analysis was focused on the transport water and its deterioration, and physiological challenges imposed on the fish. Usual commercial handling procedures employed to mitigate fish stress during transport were discussed. Future topics of research for the establishment of safer fish transport protocols were proposed. Transport was classified into short (≤8 h) or long transport (>8 h). The main issue in short transports should be the prevention of water pH reduction, while in long transports it is the increase in ammonia. Plasma cortisol is the most employed marker for stress and is acutely elevated upon short episodes of transport, but remains elevated even in long‐transport events. Plasma glucose is perhaps a better marker for handling stress. Plasma lactate, pH, osmolality CO₂ and ions should be more often evaluated. Plasma Na⁺ and Cl^‐ are very useful markers of acidosis, due to their respective exchange for H⁺ and , for acid–base regulation. The establishment of species‐specific transport protocols should be preceded by such combined analyses of water and physiological parameters.     

Pharmacokinetics of minocycline in crucian carp (Carassius auratus) after intravenous and oral administration

The pharmacokinetic of minocycline was studied after a single intravenous as well as oral dose (5 mg/kg body weight) in crucian carp (Carassius auratus) reared in freshwater at 10°C. Plasma samples were randomly collected from six fish at each sampling time. Plasma concentrations were determined by high‐performance liquid chromatography and further subjected to noncompartmental analysis. Initial concentration of minocycline just after intravenous administration was calculated as 7.320 μg/ml, while the other parameters after intravenous injection were determined as flows: apparent elimination rate constant (λ_z) of 0.064 per hr, apparent elimination half‐life () of 10.82 hr, total body clearance (Cl) of 142.72 ml/hr/kg, volume of distribution (Vz) of 2,227.38 ml/kg and volume of distribution at steady‐state (Vss) of 1,937.08 ml/kg. While after oral administration, the λ_z, , mean absorption time (MAT), absorption half‐life (t_1/2ka) and bioavailability were determined as 0.059 per hr, 11.74, 5.55, 3.84 hr, and 81.98%, respectively, and the peak concentration was observed as 1.474 ± 0.362 μg/ml at 8 hr. It was shown that minocycline was slowly but relatively completely absorbed, extensively distributed, and slowly eliminated in crucian carp. Based on the ratios of AUC_0–24 hr/MIC90, a minocycline dosage of 5 mg/kg body weight administered intravenously or orally would be only effective to successfully treat crucian carp infected by bacterium with MIC values ≤0.25 μg/ml.     

Chronic exposure to nitrate significantly reduces growth and affects the health status of juvenile Nile tilapia (Oreochromis niloticus L.) in recirculating aquaculture systems

Studies on chronic or acute toxicity of nitrogen species on fish in recirculating aquaculture systems (RAS) usually focused on adverse effects of total ammonia nitrogen (TAN: sum of NH₃ + NH₄⁺) and nitrite (), while underestimating the potential effects of high nitrate accumulation on growth and health status of fish. In our study, Nile tilapia (Oreochromis niloticus) were exposed to five different nitrate concentrations (0, 10, 100, 500 and 1000 mg L^?1 ‐N) over 30 days. Growth parameters (feed conversion ratio (FCR), specific growth rate (SGR), hepatosomatic index (HSI)), blood samples (concentrations of haemoglobin, methaemoglobin, plasma /) and the histology of the gills were studied to evaluate growth and health status of the fish. At the highest nitrate concentration, the fish showed significantly reduced growth and impaired health status (SGR, FCR, plasma /, haemoglobin and methaemoglobin concentration), demonstrating that too high nitrate concentrations can negatively influence tilapia production in RAS. Here, we recommend not exceeding concentrations of 500 mg L^?1 ‐N in juvenile tilapia culture to ensure an optimal health and growth status of the fish, as below that concentration no effects on the tilapia have been observed.     

Observation of three‐dimensional flow structures and effluent transport around fish cages using a towed ADCP and free‐fall multi‐parameter profiler

A field campaign was conducted around salmon cages, using a combination of a towed ADCP and a free‐fall multi‐parameter profiler, in order to investigate flow structures and the possible distribution of effluent materials. Two transect observations showed that hydrographic conditions changed dramatically within 5 days, from highly stratified open water conditions to weak stratification. Three‐dimensional observation revealed that flow was blocked behind the cages and that the blockage was reduced as the distance from the cages increased. The flow speed was positively correlated with the intensity of the backscattering signal. The R² value was high immediately behind the cages and decreased with distance from the cages. The flow time series behind the cages exhibited a k^–2 power law spectrum that was consistent with a typical internal wave spectrum. This suggests that eddies shed from the cages were highly influenced by stratification. We estimated the rate of kinetic energy dissipation, , from YODA Profiler data based on the Thorpe scale approach. We also estimated the eddy diffusivity coefficient, . Both and followed a lognormal probability density function. The mean was consistent with a one‐dimensional diffusion model assessed from the R² values of flow speed and backscatter intensity.     

Characterization of a novel marine origin aerobic nitrifying–denitrifying bacterium isolated from shrimp culture ponds

A novel marine origin Bacillus subtilis strain H1 isolated from a shrimp culture pond effectively removed NH₄⁺‐N, ‐N and ‐N, with a maximum ammonium, nitrite and nitrate removal rate of 2.35 mg NH₄⁺‐N L^?1 hr^?1 per OD, 9.64 mg ‐N L^?1 hr^?1 per OD and 0.75 mg ‐N L^?1 hr^?1 respectively. The gas chromatography–isotope ratio mass spectrometry results indicated that N₂O was emitted when ¹⁵NH₄Cl, Na¹⁵NO₂ or Na¹⁵NO₃ was used. Additionally, N₂ was also produced when Na¹⁵NO₂ was used. Single‐factor experiments suggested that the optimal conditions for NH₄⁺‐N and ‐N removal were glucose as a carbon source, C/N 15, initial pH 7.5, 30 g/L NaCl, 28°C and a shaking speed of 160 rpm. Orthogonal tests showed that the optimal conditions for NH₄⁺‐N removal were C/N 15, pH 9, 10 g/L NaCl and shaking speed 160 rpm when ammonium chloride was used as the substrate. The optimal conditions for ‐N removal were C/N 10, pH 6, 10 g/L NaCl and a shaking speed of 160 rpm when sodium nitrite was used as the substrate. In summary, B. subtilis strain H1 had highly efficient aerobic nitrifying–denitrifying ability and high adaptability, suggesting that it is potentially valuable to marine aquaculture.     

Application of logistic regression analysis to predict cannibalism in orange‐spotted grouper Epinephelus coioides (Hamilton) fry

Cannibalism is frequently observed in larviculture of orange‐spotted grouper Epinephelus coioides. Previously, based on measurements of morphometric characters, a linear equation of total length (TL) of prey to cannibals was proposed: TL_prey = 0.80 TL_cannibal – 1.50. To verify the reliability of the equation, experiments were performed with pairs of fish with different TLs. Cannibalism occurred only when the cannibal‐prey size ratios were equal to or larger than that predicted by the equation. To predict the probability of cannibalism among the grouper of known TLs, a logistic regression model was proposed. The logistic regression model is: The logistic regression model showed the following: when either TL_prey or TL_cannibal is constant, the probability of cannibalism increases with increase in the cannibal‐prey size ratios; if given a constant cannibal‐prey size ratio, probability of cannibalism is lower in early stages than in later stages. The prediction contrasts with that calculated from the linear equation where the minimum cannibal‐prey size ratios decrease with size of the cannibal. However, the prediction matches observed rearing experiences: cannibals prefer smaller prey to larger ones and the cannibalism rate declines as fry age.     

Pharmacokinetics and tissue residues of marbofloxacin in crucian carp (Carassius auratus) after oral administration

Pharmacokinetics and residue elimination of marbofloxacin (MBF) were studied in crucian carp (Carassius auratus, 250±30 g) kept at two water temperatures of 15 and 25 °C. Marbofloxacin concentrations in plasma and tissues were analysed by means of high‐performance liquid chromatography using an ultraviolet detector. The limits of detection were 0.02 μg mL^?1, 0.02 μg g^?1, 0.025 μg g^?1, 0.02 μg g^?1 and 0.025 μg g^?1 in plasma and muscle, skin, liver and kidney respectively. Fish were administered orally at a single dosage of 10 mg kg^?1 body weight in the PK group. The data were fitted to two‐compartment open models at both temperatures. At 15 °C, the absorption half‐life () and distribution half‐life (t_1/2α) of the drug were 0.36 and 4.48 h respectively. The corresponding values at 25 °C were 0.23 and 0.87 h respectively. The elimination half‐life (t_1/2β) was 50.75 h at 15 °C and 25.05 h at 25 °C. The maximum MBF concentration (C_max) differed little between 15 (6.43 μg mL^?1) and 25 °C (8.36 μg mL^?1). The time to peak concentration was 1.74 h at 15 °C and 0.78 h at 25 °C. The apparent volume of distribution (V_d/F) of MBF was estimated to be 1.36 and 0.87 L kg^?1 at 15 and 25 °C respectively. The area under the concentration–time curve (AUC) was 301.80 μg mL^?1 h at 15 °C and 182.80 μg mL^?1 h at 25 °C. The total clearance of MBF was computed as 0.03 and 0.05 L h^?1 kg^?1 at 15 and 25 °C respectively. After repeated oral administration at a dosage of 10 mg kg^?1 body weight per day for 3 days, the results showed that the elimination half‐lives () of MBF from all tissues at 15 °C were longer than that at 25 °C. Therefore, water temperature is an important factor to be considered when deciding a reasonable withdrawal time.     

Effect of cyclical feeding on compensatory growth,nitrogen and phosphorus budgets in juvenile Litopenaeus vannamei

This study assessed the effect of cyclical feeding on compensatory growth, nitrogen (N) and phosphorus (P) budgets of juvenile Litopenaeus vannamei. A 36‐day growth trial was performed with four different feeding protocols. The control group (S0) was fed to satiation twice every day during the whole experimental period; treatment groups S1, S2 and S3 were fed by the 9, 5 and 3 cycles of 1:3, 2:5 and 3:9 (fasting days:feeding days) respectively. Fasting in S1, S2 and S3 groups did not change the specific growth rate in wet weight (SGR_w), but the feed conversion efficiency (FCE) and protein efficiency ratio (PER) were significantly increased (P < 0.05) in comparison with control. The N and P consumed per unit wet weight gain of shrimp in S1, S2, S3 groups were significantly lower (P < 0.05) than control group by 15.39%, 15.96%, 19.33% for N, and 15.16%, 15.98%, 19.26% for P respectively. The total discharge of N and P (including N and P discharged by faeces (F_N/P), non‐faecal excretions (U_N/P) and exuviations (E_N/P); ) was significantly lower (P < 0.05) in the experiment groups by 19.91–22.07% for N and 18.68–26.37% for P respectively. Overall, the results suggest that the L. vannamei can reach completely compensatory growth, and the total discharge of N and P per unit wet weight gain of L. vannamei significantly decreased by cyclical feeding, which could have a positive effect on the reduced of environmental N and P loading due to the cultured of L. vannamei.     

Experimental evaluation of rainbow trout Oncorhynchus mykiss predation on longnose dace Rhinichthys cataractae

Laboratory and in‐stream enclosure experiments were used to determine whether rainbow trout Oncorhynchus mykiss influence survival of longnose dace Rhinichthys cataractae. In the laboratory, adult rainbow trout preyed on longnose dace in 42% of trials and juvenile rainbow trout did not prey on longnose dace during the first 6 h after rainbow trout introduction. Survival of longnose dace did not differ in the presence of adult rainbow trout previously exposed to active prey and those not previously exposed to active prey ( = 0.28, P = 0.60). In field enclosures, the number of longnose dace decreased at a faster rate in the presence of rainbow trout relative to controls within the first 72 h, but did not differ between moderate and high densities of rainbow trout (F_2,258.9 = 3.73, P = 0.03). Additionally, longnose dace were found in 7% of rainbow trout stomachs after 72 h in enclosures. Rainbow trout acclimated to the stream for longer periods had a greater initial influence on the number of longnose dace remaining in enclosures relative to those acclimated for shorter periods regardless of rainbow trout density treatment (F_4,148.5 = 2.50, P = 0.04). More research is needed to determine how predation rates will change in natural environments, under differing amounts of habitat and food resources and in the context of whole assemblages. However, if rainbow trout are introduced into the habitat of longnose dace, some predation on longnose dace is expected, even when rainbow trout have no previous experience with active prey.