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Properties of odour plumes from natural baits
Fisheries Research 110 (2011) 459–464
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Properties of odour plumes from natural baits Håkan Westerberg a,∗ , Karin Westerberg b a b
Swedish Board of Fisheries, Box 423, 40126 Göteborg, Sweden Department of Chemical Engineering, Lund University, Box 124, 221 00 Lund, Sweden
a r t i c l e
i n f o
Article history: Received 9 March 2011 Received in revised form 2 June 2011 Accepted 7 June 2011 Keywords: Odour plume Bait Dispersion Mixing Attraction
a b s t r a c t The formation of an odour trail can be broken down into a chain of events: first, the emission of the attractant from the bait, second, a turbulent mixing into the immediate surroundings and third, a downstream advection and dispersion of the attractant by the current flowing past the bait. In this work, a simple analysis of the physical processes governing the first two stages shows that the release of attractant will decline as the inverse of the square root of time. The decline curve will be similar for all solid natural baits within a realistic size range during normal soak times. Experiments were performed to measure typical diffusion constants of odour substances in fish tissue and it is shown that due to the low diffusivity just a thin surface layer will release its content of odour substances during a fishing operation using pieces of fish as bait. Moreover, the initial concentration of attractant when a plume is formed will be independent of the size of a solid bait and only depend on the ratio of the surface area to the cross-section area of the bait. To increase the maximum concentration of odour substance in the plume and utilize a given amount of bait optimally, a solid bait should be cut into smaller pieces. The effect of current speed on the initial odour concentration in the plume is strong. Due to speed variations during the soak time the effectiveness of the attraction will vary in time. To extend the analysis and develop models for optimization of the range of the olfactory signal from a bait it is necessary to know both the concentration of the attractant in the bait and the target species detection threshold. When such data become available it will be possible to estimate the range of attraction of baits under different environmental conditions. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The operation of a fish trap or pot requires that the fish encounters the gear. This can be achieved either mechanically using a leader net or by attraction to a signal which is transmitted from the trap. The properties of the underwater environment are different from terrestrial environments, which hinders intuitive interpretation and understanding of attraction efficiency. The chemosensory and acoustic senses have evolved in the aquatic environment and are better adapted to water than to air. In water, everything which is soluble is a potential olfactory stimulus, whereas only the small class of volatile substances give odour in air. Dispersion in water is slower by five orders of magnitude and an odour trail tends to be detectable over longer time and distances than in air (Csanady, 1973). Acoustic signals spread in all directions and can at the right frequencies be detected at long range, but are not commonly used as a method of attraction. Vision is ineffective other than at very
∗ Corresponding author. Tel.: +46705269956. E-mail addresses: hakan.westerberg@fiskeriverket.se [email protected] (K. Westerberg).
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short ranges in the sea. The by far most common mode of attraction is by smell. The formation of an attracting odour trail can be broken down into a chain of events: First, the emission of the attractant from the bait, second, turbulent mixing into the immediate surroundings and third, downstream advection and dispersion of the attractant by the current flowing past the bait. The emission phase has been studied in several laboratory and field experiments (e.g. Løkkeborg, 1990; Løkkeborg and Johannessen, 1992; Løkkeborg and Pina, 1997). The theory and modelling of plume dilution and dispersion in the third stage is well developed for atmospheric plumes and the general results can be applied also to underwater plumes. Particularities pertinent to odour tracking of fish and crustaceans have been reviewed among others by Atema (1987, 1995), Westerberg (1991) and Moore and Crimaldi (2004). The purpose of the present study is mainly to look at the first and second phase of the development of a plume in a semi-quantitative way, in order to formulate optimal criteria for baits under various conditions. To this end a simple experiment to measure the diffusion coefficient of attractants within natural baits was conducted. The third phase and the aspect of fish behaviour in relation to an odour plume are treated in more general terms.
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H. Westerberg, K. Westerberg / Fisheries Research 110 (2011) 459–464 Table 1 Summary of the experiments. Sample
Dye
Whiting Mackerel Whiting Mackerel Whiting Mackerel Whiting Mackerel Whiting frozen Mackerel frozen Octopus Prawn Whiting frozen Mackerel frozen Prawn
Rhodamin B Rhodamin B Rhodamin B Rhodamin B Methylene blue Methylene blue Rhodamin B Rhodamin B Rhodamin B Rhodamin B Rhodamin B Rhodamin B Rhodamin B Rhodamin B Rhodamin B
Time (s) 3900 3900 14,400 14,400 12,600 12,600 54,300 54,300 24,000 24,000 24,300 24,300 86,100 86,100 85,800
Penetration (mm)
Standard dev
0.963 1.075 1.322 1.348 1.618 2.362 1.832 1.884 1.826 1.994 3.114 2.484 3.110 2.826 2.170
0.107 0.064 0.206 0.038 0.149 0.211 0.054 0.288 0.316 0.187 0.652 0.448 0.505 0.371 0.319
Fig. 1. Cross-section of whiting (left) and mackerel (right) after immersion in dye for 4 h. The penetration depth is measured perpendicular to the original cut surface.
2. Materials and methods Any water-soluble compound in a fish tissue can act as an attracting odour: free amino acids, small peptides, nucleotides or organic molecules. In order to get an estimate of the molecular diffusivities in fish tissues, samples of bait were immersed in a dye solution, the dye molecules acing as model molecules for the odour compounds. The penetration depth of the dye was measured in a cross-section of the bait after varying time intervals. Assuming that adsorption and capillary effects can be neglected the system is described by Fick’s law and the rate of diffusion into and out of the tissue are equivalent. The penetration depth is then equal to the diffusion length Ld and the diffusivity D of the dye can be calculated as: D=
(Ld ) 4t
2
The baits tested were fresh and frozen whiting (Merlangius merlangus) and mackerel (Scomber scombrus), frozen tiger prawn (Penaeus monodom) and frozen small octopus (Octopus dolfusi). Frozen baits were defrosted prior to the experiments. Two dyes were used, 10 g/L Rhodamine B and 5 g/L Methylene blue. According to the Stokes–Einstein relation the diffusion coefficient is inversely proportional to the diameter of the molecule. The molecular weights of Rhodamine and Methylene blue are 380 and 310 g/mole, respectively. This is deemed so close to the molecular weights of free amino acids, ranging between 100 and 200 g/mole, that the diffusivity of the dye can be taken as representative for that of the attractants. The baits were suspended in dye solution and stirred with a magnetic stirrer (250 rpm). The dye was allowed to penetrate the bait for time intervals of 1–24 h. The baits were taken out of the dye bath, rinsed in water and dried on the surface before making an incision perpendicular to the cut surface of the bait. In this crosssection a penetration front of dye was quite clearly distinguishable. The skin of the fish was likely to act as a barrier against mass transport and the penetration depth was therefore only measured from the cut surface of the bait sample, see Fig. 1. The samples were photographed together with a mm scale and the distance from the cut surface to the dye front was measured using the Bersoft Image Analyse software at four to seven points and averaged. 3. Results and discussion 3.1. Release of attractant The odour from natural bait, a piece of fish or mollusc, is a mixture of soluble organic compounds (Carr et al., 1996). Free amino
acids have been identified as important food attractants (Hara, 2006) but other substances such as peptides, guanido compounds, nucleotides, and organic acids may be involved. Inside dead tissue the transport of such substances is by molecular diffusion alone. The magnitude of the diffusion coefficient, D, of organic molecules in water is 10−10 m2 /s (Delgado, 2007) and will probably be smaller in fish muscle tissue due to obstruction effects (Masaro and Zhu, 1999). A measure of how far a substance has propagated in the tissue by diffusion in the gradient direction and the time t is called the diffusion length, Ld , and is defined as √ Ld = 2 Dt A typical size of a bait, Li , is 10−2 to 10−1 m and if the ambient current velocity, U, is 10−2 to 10−1 m/s then the residence time, tr , of water surrounding the bait will fall in the range 0.1 to 10 s. The ratio between Li and Ld at time tr gives a measure of the relative importance of diffusion in the bait and convection on the outside. The square of this ratio is the dimensionless Peclet number, which becomes Pe =
Li U D
With the parameters above we find that 106 < Pe < 108 , which shows that convection outside the bait is much faster than the rate of diffusion. The diffusion within the bait is thus the only process determining the flux of the attractant to the surrounding fluid and we can assume that the concentration is zero at the bait surface in order to calculate the concentration gradient inside the bait. 3.2. Measurements of tissue diffusion coefficient The type of bait, dye used and submersion times are summarised in Table 1 together with the mean and standard deviation of the observed penetration depth. The calculated effective diffusivities are reported in Table 2 and Fig. 2. The calculated diffusivities were higher for the measurements performed over shorter time periods. The uncertainty of those measurements were also larger, and effects such as creeping of dye on the fresh incision surface can be expected to have a larger influence on the experiments with shorter exposure times and therefore smaller penetration depths. Another possible explanation is initial adsorption at the bait surface. The mean value of D found in the experiments ranged between 6 × 10−11 and 3 × 10−11 m2 /s for the bait types that were tested.
H. Westerberg, K. Westerberg / Fisheries Research 110 (2011) 459–464
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Table 2 The experimental results and calculated diffusivities, D, in m2 /s. D+ and D− are values for the diffusion coefficient calculated from the mean penetration depth + and − one standard deviation, respectively. Sample
Dye
Whiting Mackerel Whiting Mackerel Whiting Mackerel Whiting Mackerel Whiting frozen Mackerel frozen Octopus Prawn Whiting frozen Mackerel frozen Prawn
Rhod Rhod Rhod Rhod Meth Meth Rhod Rhod Rhod Rhod Rhod Rhod Rhod Rhod Rhod
Time (s) 3900 3900 14,400 14,400 12,600 12,600 54,300 54,300 24,000 24,000 24,300 24,300 86,100 86,100 85,800
3.3. Release rate Based on the result of the experiment a value 4 × 10−11 m2 /s was used to calculate the diffusion length as a function of time, which is also a measure of the thickness of the layer of a bait which will contribute to forming the odour trail. The result of this calculation is shown in Fig. 3. After 24 h soak time the diffusion length is less than 4 mm. For a cubic piece of bait which the side 3 cm this means that just 50% of the bait has contributed to odour emission. It will take more than a week until the odour substances at the core of the bait reach the surface. Immediately after submersion of the bait, at times ≤tr , the flux of odour substances depends on rapidly changing diffusion processes both inside and outside the bait surface. When t > >tr and while the diffusion is limited to a thin layer inside the bait surface we can neglect the effect of the shape of the bait so that the local flux, J, is proportional to the concentration gradient: J = −D
∂c ∂n
where n is a coordinate in the direction perpendicular to the surface and c is the concentration of the substance. As discussed above, convection will maintain the concentration at zero at the surface. With the assumptions that the bait is large compared to the diffusion length during the whole soaking time the characteristic length of
Fig. 2. The calculated diffusivities for the samples as a function of exposure time. Panel (a) mackerel and panel (b) whithing.
D
D+
D−
5.94E−11 7.41E−11 3.03E−11 3.15E−11 5.19E−11 1.11E−10 1.55E−11 1.63E−11 3.47E−11 4.14E−11 9.97E−11 6.35E−11 2.81E−11 2.32E−11 1.37E−11
7.33E−11 8.32E−11 4.05E−11 3.33E−11 6.19E−11 1.31E−10 1.64E−11 2.17E−11 4.78E−11 4.95E−11 1.46E−10 8.84E−11 3.80E−11 2.97E−11 1.80E−11
4.69E−11 6.55E−11 2.16E−11 2.98E−11 4.28E−11 9.18E−11 1.45E−11 1.17E−11 2.38E−11 3.40E−11 6.24E−11 4.26E−11 1.97E−11 1.75E−11 9.99E−12
the concentration gradient becomes equal to the diffusion length, which means that we can make the approximation c0 c0 =− J ≈ −D Ld 2
D t
where c0 is the initial concentration in the bait tissue. As c0 and D are constants this means that the flux of odour from large baits depends only on time and follows a simple and universal decline with t−1/2 . Fig. 4 show the flux expressed relative to an arbitrary initial value chosen as the flux at 1 min after submersion. This time dependence of the rate of release is also seen in the experiment made by Løkkeborg (1990). Fig. 5 shows his data compared to a decrease calculated with a t−1/2 law. The main conclusion from the analysis is that the release of odour will decline as the inverse of the square root of time. The decline curve will be similar for all solid natural baits over a wide range of bait sizes and soak times. Given the observed diffusion constant a thin surface layer, 3 mm thick, will release its content of odour substances during a 12 h fishing operation. The implication is that a large part of the baits is wasted. The optimal shape of a bait would be one with a large surface to volume ratio and a characteristic cross-section approximately equal to the diffusion length for the intended soak time. The recreational fisherman’s favourite bait earth-worm seems to be close to this optimum.
Fig. 3. The thickness of the surface layer of a natural bait contributing to formation of an odour plume as a function of soak time.
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H. Westerberg, K. Westerberg / Fisheries Research 110 (2011) 459–464
Fig. 4. Relative rate of emission of an odour substance from a natural bait as a function of soak time. The rate at 1 min after immersion is taken as 100%.
Another implication is that the release of odour will increase sharply if a fresh surface is exposed on the bait; if a fish takes a bite out of it for instance. 3.4. Initial dilution of the attractant As found above the release of odour substances is a timedependent phenomenon which for large baits will occur at the same rate J over the whole surface. The total surface area, A, times the flux J gives the source strength or emission, H, from the bait; √ D H = Ac0 √ 2 t The odour will mix continuously into the water current flowing past the bait. The character of this mixing process will vary depending on the turbulent intensity of the current. If the eddy velocity at the length scale of the bait is low compared to the mean current velocity the bait can be considered an obstacle which generates vortices and intensified mixing in the downstream wake. The odours swept off from the bait will feed into this mixing zone and be diluted.
Fig. 5. The observed rate of release of amino acids from a mackerel bait determined by fluorometry in running sea water, solid squares (Løkkeborg, 1990), compared to the theoretical decrease calculated from the observed relative rate at 1 h and following a t−1/2 dependence, open circles. Observed rates are presented relative to the rate 5 min after immersion.
Fig. 6. The ratio of surface to cross-section area (bars) for a bait which is successively cut into smaller pieces (solid squares show number of pieces) and loosely packed in a bait bag. This ratio is proportional to the initial odour concentration in the plume.
With this assumption a simple estimate can be made of the starting value of the diluted odour concentration in the plume. The cross-section of the wake is approximately the same as the crosssection of the bait. The turbulence in the wake of a sphere is fully developed 100–500 diameters downstream (Townsend, 1949). The diameter of the wake grows slowly and is proportional to the cube root of the downstream distance (Johansson and George, 2006). This means that the odour is homogenized by the vortices generated locally behind the bait and that this occurs over a distance on the order of magnitude of 102 to 103 bait diameters. If this cross-section of the bait is denoted B those assumptions and simplifications lead to the following expression for the initial odour concentration ci : H Ac0 ci ∼ = UB 2UB
D t
The initial odour concentration in the plume will thus be proportional to the odour concentration in the bait and inversely proportional to the current velocity. It will decline as t−1/2 . An important observation is that the initial concentration will be independent of the size of the bait and only depend on the ratio of surface to cross-section area of the bait. For a single piece of bait the surface to cross-section ratio, P = A/B, will have a minimum of 4, which is that of a sphere. Any other, more irregular shape has a larger ratio; the ratio for a cube for example is 6. For asymmetric shapes the ratio will also depend on the orientation. However, the most obvious way to increase P is to divide the bait into smaller pieces. This will both increase the concentration of odour in the plume and result in improved utilization of a fixed amount of bait. To illustrate the effect of cutting up the bait we can construct a simple example, where a cube is cut in half, each piece again cut in half across the longest side and so on. The pieces are then randomly packed, giving a package with a somewhat larger cross-section than the original cube, in this example it is assumed to be 20% larger. The result of his exercise is shown in Fig. 6. The calculations show that to increase the concentration of odour in the plume and utilize a given amount of bait optimally the bait should be cut into relatively small pieces. With three cuts, resulting in eight pieces, the surface to cross-section area and consequently the plume concentration have almost doubled. With six cuts the concentration has increased threefold. The diagram shows the 6 first cuts. If the original piece has a side of 6 cm the smallest pieces will have the size 1.5 cm, which still is large compared to the
H. Westerberg, K. Westerberg / Fisheries Research 110 (2011) 459–464
Fig. 7. Time series of relative odour concentration from a bait submerged at the time of minimum tidal current speed (solid curve) or with 1.55 (dashed), 3.1 (dotted) and 4.65 h (dash-dot) delay. The tide is semidiurnal with a 12.4 h period and a tidal ellipse with ε = 0.1.
24 h diffusion length. In situations when the time to lower a bait to great depth is large the loss of attractant during descent should be taken into account and the minimum size of the pieces may be larger. Besides P the other parameters that determine ci is the ambient current velocity and time. The time dependence is a steady decrease of emission following a t−1/2 law as has been discussed in Section 3.1. The initial dilution is inversely proportional to the current velocity; the variability can thus be quite large with weak currents while the dilution becomes less sensitive to fluctuations when currents are strong. Diurnal and semi-diurnal tidal currents tend to dominate the velocity field in the coastal environment where traps and pots are used. Even in areas with weak tides, such as the Mediterranean or the Baltic, currents are seldom steady for long periods. Passing weather systems will generate transient currents and long-period waves, operating on time scales from hours to one day. To illustrate the effect of current variations on the odour concentration in the plume we can calculate the development of ci in time depending on the timing of when the pot is deployed within a semi-diurnal tidal cycle. The result is shown in Fig. 7. In this calculation the current is never allowed to reach zero and the minimum current in the tidal ellipse is put to 10% of the maximum. Nevertheless, the effect of the current speed on the initial dilution is such that the maximum plume concentration occurs at slack tide even if this occurs several hours after the setting of the gear. The effect of current speed on the odour concentration in the plume is strong and the timing of setting the gear can be important. Depending on current variations during the soaking time the effectiveness of the attraction may vary strongly. At the same time, the current speed does not only determine the initial concentration, it is also the determining factor for the downstream advection of the odour and the range of attraction of the bait. 3.5. Formation of the odour plume From the assumption made above that the bait acts as an obstacle to the current it follows that the initial mixing generating a starting value of the concentration of the attractant occurs in the wake a short distance, on the order of magnitude of 102 to 103 bait diameters, downstream of the bait. Subsequent to this the
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odour is subject to a continuous dispersion and diffusion by turbulent eddies of the same or smaller size than the odour cloud and to advection by the mean current and larger eddies. This will result in a meandering and gradually expanding plume with decreasing odour concentration, similar to the smoke plume from a chimney. The detailed development of the plume depends on several widely variable conditions, differing between times and places at which a pot or trap is used. Important factors are the velocity and shear of the current, the turbulence intensity at different spatial scales and whether the fluid is density stratified. There is a vast literature on this subject; some general texts are for example Csanady (1973) and Okubo and Levin (2001). Some observations relevant to understanding odour plumes from baited pots will be given here, but it is outside the scope of this paper to cover plume formation in any detail. Much of the theory and modelling of plumes is based on a steady-state statistical description of the concentration at different points in the plume. Such models are called Gaussian dispersion models and the output is typically a normal distribution of the concentration of the odour in a plane perpendicular to the mean current direction, with the plume centred along a straight axis in the mean current direction. For an individual point downstream from an odour source a Gaussian model describes the long-time average concentration. This has little or no relevance for understanding the detection and orientation of a fish to the plume in the natural environment. What a fish is able to perceive is the instantaneous realization of the plume: a much narrower, meandering odour cloud, which may be broken up into discrete patches (Balkovsky and Shraiman, 2002; Ormerod, 2001). A Gaussian plume model strongly exaggerates the local dilution of the odour concentration at a downstream point in a plume. At a fixed position far enough downstream of the bait the odour signal will be absent most of the time. When an odour patch arrives it will be perceived as a burst with strong concentration fluctuations without a simple relation between the local current direction in the odour patch and the direction to the source. The local concentration in an odour patch will decrease with time due to dispersion. Dispersion can be defined as the combined effect of diffusion and shear. Diffusion is the mixing with the surrounding fluid by turbulent or molecular movements whereas shear is the spatial rate of change of the velocity which causes deformation and stretching of the patch. The rate of dispersion depends on the intensity of mixing and the strength of the shear. In practise baited pots are placed at or close to the bottom. This is a hydrodynamic regime where the current velocity decreases to zero at the bottom and the vertical turbulent eddies are limited in size by the distance to the bottom. This so called benthic boundary layer, BBL, can be divided into a mm-thick viscous and laminar sub-layer at the very bottom and an outer Ekman layer influenced by the rotation of the earth. In between those the flow is unaffected by the rotation of the earth and the mean velocity increases logarithmically with height (Wimbush and Munk, 1970; Boudreau and Jørgensen, 2001). This layer is characterised by a constant vertical shear stress and is called the logarithmic layer. The thickness of the logarithmic layer in coastal waters is typically 1–10 m, which means that this is where odour plumes from baits are usually formed. The vertical eddy diffusion increases linearly with height, h, above the bottom while the current shear decreases with height. It can be shown that the local dispersion in the BBL will decrease with height above the bottom as h−1/2 and increase with time as t3/2 . As a consequence, the dispersion of an odour plume in the BBL is larger for a source close to or on the bottom while the downstream advection is slower the closer the plume is to the bottom.
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4. Conclusions
Acknowledgments
A simple analysis of the physical processes involved in the formation of odour trails from bait was shown to be useful for understanding the criteria for optimal use of baits. It identifies which properties of the bait and its environment are important in defining the properties of the plume. Crucial findings in the analysis were the rapid decline in odour source strength and the low utilization of the active substances in natural baits due to the slow mass transfer in the bait. This emphasizes the importance of developing alternative delivery systems. It should be possible to design artificial baits so that the emission rate is constant or, even better, proportional to the ambient current velocity, which would give a constant initial odour concentration in the plume. To extend the analysis and develop models for optimization of the range of the olfactory signal from a bait it is necessary to know both c0 in the bait and the threshold concentrations where the target species detects and becomes attracted to the odour trail. Much experimental data exist on detection and search activation thresholds to different chemical stimuli (Døving, 1986; Hara, 1994). Measurements of c0 or ci are essentially lacking however. When such data become available it will be fruitful to apply the existing more sophisticated dispersion models for the BBL to analysis of the range of attraction of baits under various environmental conditions. The focus of this paper is on the dispersion of the odour plume. Evidently, the diminishing concentration is decisive for how far downstream an odour can be detected by a fish, but there are other properties that may be equally important. Any information that a fish can extract on the age of the trail or on the distance and direction to the source must be advantageous when deciding if it is worth starting a search. Some work on this question has been done in insects (Murlis et al., 1992) and in crustaceans (Weissburg and Zimmer-Faust, 1994). Essentially, the finding is that the animals integrate some form of chemically modulated orientation (chemotaxis) with a visual or mechanical assessment of flow conditions in order to move upwind (rheotaxis). Across-stream turns, zig-zaging or casting, are typical features of the tracks (Vickers, 2000). It is characteristic that the animal receives short intermittent bursts of odour concentration; temporal analysis of the signal can then be used to infer spatial gradients. No similar detailed experiments have been made on fish behaviour in intermittent plumes under realistic conditions. Such studies would give a better understanding of what properties of an odour plume are the most significant from a fishery technology point of view.
Special thanks go to Kjell Døving for his comments and many years of fruitful discussions on fish chemoreception. References Atema, J., 1987. Distribution of chemical stimuli. In: Atema, J., Fay, R.R., Popper, A.N., Tavolga, W.N. (Eds.), Sensory Biology of Aquatic Animals. Springer-Verlag, NY, pp. 29–56. Atema, J., 1995. Chemical signals in the marine environment: dispersal, detection and temporal signal analysis. Proc. Natl. Acad. Sci. U.S.A. 92, 62–66. Balkovsky, E., Shraiman, B.I., 2002. Olfactory search at high Reynolds number. Proc. Natl. Acad. Sci. 99, 12589–12593. Boudreau, B.P., Jørgensen, B.B., 2001. The benthic boundary layer: transport processes and biogeochemistry. Oxford U.P., Oxford, NY. Carr, W.E.S., Netherton, J.C., Gleeson, R.A., Derby, C.D., 1996. Stimulants of feeding behavior in fish: analyses of tissues of diverse marine organisms. Biol. Bull. 190, 149–160. Csanady, G.T., 1973. Turbulent diffusion in the environment. D. Reidel Pub. Co., Dordrecht. Delgado, J.M.P.Q., 2007. Molecular diffusion coefficients of organic compounds in water at different temperatures. J. Phase Eq. Diff. 28, 427–432. Døving, K.B., 1986. Functional properties of the fish olfactory system. In: Ottoson, D. (Ed.), Progress in Sensory Physiology 6. Springer-Verlag, Berlin, pp. 39–104. Hara, T.J., 1994. The diversity of chemical stimulation in fish olfaction and gustation. Rev. Fish Biol. Fish. 4, 1–35. Hara, T.J., 2006. Feeding behaviour in some teleosts is triggered by single amino acids primarily through olfaction. J. Fish Biol. 68, 810–825. Johansson, P.B., George, W.K., 2006. The far downstream evolution of the highReynolds-number axisymmetric wake behind a disk. J. Fluid Mech. 555, 363–385. Løkkeborg, S., 1990. Rate of release of potential feeding attractants from natural and artificial bait. Fish. Res. 8, 253–261. Løkkeborg, S., Johannessen, T., 1992. The importance of chemical stimuli in bait fishing–fishing trials with soaked bait. Fish. Res. 14, 21–29. Løkkeborg, S., Pina, T., 1997. Effects of setting time, setting direction and soak time on longline catch rates. Fish. Res. 32, 213–222. Masaro, L., Zhu, X.X., 1999. Physical models of diffusion for polymer solutions, gels and solids. Prog. Polym. Sci. 24, 731–775. Moore, P., Crimaldi, J., 2004. Odor landscapes and animal behavior: tracking odor plumes in different physical worlds. J. Mar. Syst. 49, 55–64. Murlis, J., Elkinton, J.S., Cardé, R.T., 1992. Odor plumes and how insects use them. Ann. Rev. Entomol. 37, 505–532. Okubo, A., Levin, S.A., 2001. Diffusion and ecological problems: modern perspectives. Springer, New York. Ormerod, R., 2001. Improving odour assessment by using better dispersion models: some examples. Water Sci. Technol. 44, 149–156. Townsend, A.A., 1949. The fully developed wake of a circular cylinder. Australian J. Sci. Res., Series A: Phys. Sci. 2, 451–468. Vickers, N.J., 2000. Mechanisms of animal navigation in odour plumes. Biol. Bull. 198, 203–212. Weissburg, M.R., Zimmer-Faust, R.K., 1994. Odour plumes and how blue crabs use them in finding prey. J. Exp. Biol. 197, 349–375. Westerberg, H., 1991. Properties of aquatic odour trails. In: Døving, K. (Ed.), Proc. 10:th Int. Symp. Olfaction and Taste, Graph. Com. Syst. Oslo, pp. 45–65. Wimbush, H., Munk, W., 1970. The benthic boundary layer. In: Maxwell, A.E. (Ed.), The Sea, vol.1. Wiley, NY, pp. 731–758.
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Properties of odour plumes from natural baits Håkan Westerberg a,∗ , Karin Westerberg b a b
Swedish Board of Fisheries, Box 423, 40126 Göteborg, Sweden Department of Chemical Engineering, Lund University, Box 124, 221 00 Lund, Sweden
a r t i c l e
i n f o
Article history: Received 9 March 2011 Received in revised form 2 June 2011 Accepted 7 June 2011 Keywords: Odour plume Bait Dispersion Mixing Attraction
a b s t r a c t The formation of an odour trail can be broken down into a chain of events: first, the emission of the attractant from the bait, second, a turbulent mixing into the immediate surroundings and third, a downstream advection and dispersion of the attractant by the current flowing past the bait. In this work, a simple analysis of the physical processes governing the first two stages shows that the release of attractant will decline as the inverse of the square root of time. The decline curve will be similar for all solid natural baits within a realistic size range during normal soak times. Experiments were performed to measure typical diffusion constants of odour substances in fish tissue and it is shown that due to the low diffusivity just a thin surface layer will release its content of odour substances during a fishing operation using pieces of fish as bait. Moreover, the initial concentration of attractant when a plume is formed will be independent of the size of a solid bait and only depend on the ratio of the surface area to the cross-section area of the bait. To increase the maximum concentration of odour substance in the plume and utilize a given amount of bait optimally, a solid bait should be cut into smaller pieces. The effect of current speed on the initial odour concentration in the plume is strong. Due to speed variations during the soak time the effectiveness of the attraction will vary in time. To extend the analysis and develop models for optimization of the range of the olfactory signal from a bait it is necessary to know both the concentration of the attractant in the bait and the target species detection threshold. When such data become available it will be possible to estimate the range of attraction of baits under different environmental conditions. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The operation of a fish trap or pot requires that the fish encounters the gear. This can be achieved either mechanically using a leader net or by attraction to a signal which is transmitted from the trap. The properties of the underwater environment are different from terrestrial environments, which hinders intuitive interpretation and understanding of attraction efficiency. The chemosensory and acoustic senses have evolved in the aquatic environment and are better adapted to water than to air. In water, everything which is soluble is a potential olfactory stimulus, whereas only the small class of volatile substances give odour in air. Dispersion in water is slower by five orders of magnitude and an odour trail tends to be detectable over longer time and distances than in air (Csanady, 1973). Acoustic signals spread in all directions and can at the right frequencies be detected at long range, but are not commonly used as a method of attraction. Vision is ineffective other than at very
∗ Corresponding author. Tel.: +46705269956. E-mail addresses: hakan.westerberg@fiskeriverket.se [email protected] (K. Westerberg).
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short ranges in the sea. The by far most common mode of attraction is by smell. The formation of an attracting odour trail can be broken down into a chain of events: First, the emission of the attractant from the bait, second, turbulent mixing into the immediate surroundings and third, downstream advection and dispersion of the attractant by the current flowing past the bait. The emission phase has been studied in several laboratory and field experiments (e.g. Løkkeborg, 1990; Løkkeborg and Johannessen, 1992; Løkkeborg and Pina, 1997). The theory and modelling of plume dilution and dispersion in the third stage is well developed for atmospheric plumes and the general results can be applied also to underwater plumes. Particularities pertinent to odour tracking of fish and crustaceans have been reviewed among others by Atema (1987, 1995), Westerberg (1991) and Moore and Crimaldi (2004). The purpose of the present study is mainly to look at the first and second phase of the development of a plume in a semi-quantitative way, in order to formulate optimal criteria for baits under various conditions. To this end a simple experiment to measure the diffusion coefficient of attractants within natural baits was conducted. The third phase and the aspect of fish behaviour in relation to an odour plume are treated in more general terms.
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H. Westerberg, K. Westerberg / Fisheries Research 110 (2011) 459–464 Table 1 Summary of the experiments. Sample
Dye
Whiting Mackerel Whiting Mackerel Whiting Mackerel Whiting Mackerel Whiting frozen Mackerel frozen Octopus Prawn Whiting frozen Mackerel frozen Prawn
Rhodamin B Rhodamin B Rhodamin B Rhodamin B Methylene blue Methylene blue Rhodamin B Rhodamin B Rhodamin B Rhodamin B Rhodamin B Rhodamin B Rhodamin B Rhodamin B Rhodamin B
Time (s) 3900 3900 14,400 14,400 12,600 12,600 54,300 54,300 24,000 24,000 24,300 24,300 86,100 86,100 85,800
Penetration (mm)
Standard dev
0.963 1.075 1.322 1.348 1.618 2.362 1.832 1.884 1.826 1.994 3.114 2.484 3.110 2.826 2.170
0.107 0.064 0.206 0.038 0.149 0.211 0.054 0.288 0.316 0.187 0.652 0.448 0.505 0.371 0.319
Fig. 1. Cross-section of whiting (left) and mackerel (right) after immersion in dye for 4 h. The penetration depth is measured perpendicular to the original cut surface.
2. Materials and methods Any water-soluble compound in a fish tissue can act as an attracting odour: free amino acids, small peptides, nucleotides or organic molecules. In order to get an estimate of the molecular diffusivities in fish tissues, samples of bait were immersed in a dye solution, the dye molecules acing as model molecules for the odour compounds. The penetration depth of the dye was measured in a cross-section of the bait after varying time intervals. Assuming that adsorption and capillary effects can be neglected the system is described by Fick’s law and the rate of diffusion into and out of the tissue are equivalent. The penetration depth is then equal to the diffusion length Ld and the diffusivity D of the dye can be calculated as: D=
(Ld ) 4t
2
The baits tested were fresh and frozen whiting (Merlangius merlangus) and mackerel (Scomber scombrus), frozen tiger prawn (Penaeus monodom) and frozen small octopus (Octopus dolfusi). Frozen baits were defrosted prior to the experiments. Two dyes were used, 10 g/L Rhodamine B and 5 g/L Methylene blue. According to the Stokes–Einstein relation the diffusion coefficient is inversely proportional to the diameter of the molecule. The molecular weights of Rhodamine and Methylene blue are 380 and 310 g/mole, respectively. This is deemed so close to the molecular weights of free amino acids, ranging between 100 and 200 g/mole, that the diffusivity of the dye can be taken as representative for that of the attractants. The baits were suspended in dye solution and stirred with a magnetic stirrer (250 rpm). The dye was allowed to penetrate the bait for time intervals of 1–24 h. The baits were taken out of the dye bath, rinsed in water and dried on the surface before making an incision perpendicular to the cut surface of the bait. In this crosssection a penetration front of dye was quite clearly distinguishable. The skin of the fish was likely to act as a barrier against mass transport and the penetration depth was therefore only measured from the cut surface of the bait sample, see Fig. 1. The samples were photographed together with a mm scale and the distance from the cut surface to the dye front was measured using the Bersoft Image Analyse software at four to seven points and averaged. 3. Results and discussion 3.1. Release of attractant The odour from natural bait, a piece of fish or mollusc, is a mixture of soluble organic compounds (Carr et al., 1996). Free amino
acids have been identified as important food attractants (Hara, 2006) but other substances such as peptides, guanido compounds, nucleotides, and organic acids may be involved. Inside dead tissue the transport of such substances is by molecular diffusion alone. The magnitude of the diffusion coefficient, D, of organic molecules in water is 10−10 m2 /s (Delgado, 2007) and will probably be smaller in fish muscle tissue due to obstruction effects (Masaro and Zhu, 1999). A measure of how far a substance has propagated in the tissue by diffusion in the gradient direction and the time t is called the diffusion length, Ld , and is defined as √ Ld = 2 Dt A typical size of a bait, Li , is 10−2 to 10−1 m and if the ambient current velocity, U, is 10−2 to 10−1 m/s then the residence time, tr , of water surrounding the bait will fall in the range 0.1 to 10 s. The ratio between Li and Ld at time tr gives a measure of the relative importance of diffusion in the bait and convection on the outside. The square of this ratio is the dimensionless Peclet number, which becomes Pe =
Li U D
With the parameters above we find that 106 < Pe < 108 , which shows that convection outside the bait is much faster than the rate of diffusion. The diffusion within the bait is thus the only process determining the flux of the attractant to the surrounding fluid and we can assume that the concentration is zero at the bait surface in order to calculate the concentration gradient inside the bait. 3.2. Measurements of tissue diffusion coefficient The type of bait, dye used and submersion times are summarised in Table 1 together with the mean and standard deviation of the observed penetration depth. The calculated effective diffusivities are reported in Table 2 and Fig. 2. The calculated diffusivities were higher for the measurements performed over shorter time periods. The uncertainty of those measurements were also larger, and effects such as creeping of dye on the fresh incision surface can be expected to have a larger influence on the experiments with shorter exposure times and therefore smaller penetration depths. Another possible explanation is initial adsorption at the bait surface. The mean value of D found in the experiments ranged between 6 × 10−11 and 3 × 10−11 m2 /s for the bait types that were tested.
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Table 2 The experimental results and calculated diffusivities, D, in m2 /s. D+ and D− are values for the diffusion coefficient calculated from the mean penetration depth + and − one standard deviation, respectively. Sample
Dye
Whiting Mackerel Whiting Mackerel Whiting Mackerel Whiting Mackerel Whiting frozen Mackerel frozen Octopus Prawn Whiting frozen Mackerel frozen Prawn
Rhod Rhod Rhod Rhod Meth Meth Rhod Rhod Rhod Rhod Rhod Rhod Rhod Rhod Rhod
Time (s) 3900 3900 14,400 14,400 12,600 12,600 54,300 54,300 24,000 24,000 24,300 24,300 86,100 86,100 85,800
3.3. Release rate Based on the result of the experiment a value 4 × 10−11 m2 /s was used to calculate the diffusion length as a function of time, which is also a measure of the thickness of the layer of a bait which will contribute to forming the odour trail. The result of this calculation is shown in Fig. 3. After 24 h soak time the diffusion length is less than 4 mm. For a cubic piece of bait which the side 3 cm this means that just 50% of the bait has contributed to odour emission. It will take more than a week until the odour substances at the core of the bait reach the surface. Immediately after submersion of the bait, at times ≤tr , the flux of odour substances depends on rapidly changing diffusion processes both inside and outside the bait surface. When t > >tr and while the diffusion is limited to a thin layer inside the bait surface we can neglect the effect of the shape of the bait so that the local flux, J, is proportional to the concentration gradient: J = −D
∂c ∂n
where n is a coordinate in the direction perpendicular to the surface and c is the concentration of the substance. As discussed above, convection will maintain the concentration at zero at the surface. With the assumptions that the bait is large compared to the diffusion length during the whole soaking time the characteristic length of
Fig. 2. The calculated diffusivities for the samples as a function of exposure time. Panel (a) mackerel and panel (b) whithing.
D
D+
D−
5.94E−11 7.41E−11 3.03E−11 3.15E−11 5.19E−11 1.11E−10 1.55E−11 1.63E−11 3.47E−11 4.14E−11 9.97E−11 6.35E−11 2.81E−11 2.32E−11 1.37E−11
7.33E−11 8.32E−11 4.05E−11 3.33E−11 6.19E−11 1.31E−10 1.64E−11 2.17E−11 4.78E−11 4.95E−11 1.46E−10 8.84E−11 3.80E−11 2.97E−11 1.80E−11
4.69E−11 6.55E−11 2.16E−11 2.98E−11 4.28E−11 9.18E−11 1.45E−11 1.17E−11 2.38E−11 3.40E−11 6.24E−11 4.26E−11 1.97E−11 1.75E−11 9.99E−12
the concentration gradient becomes equal to the diffusion length, which means that we can make the approximation c0 c0 =− J ≈ −D Ld 2
D t
where c0 is the initial concentration in the bait tissue. As c0 and D are constants this means that the flux of odour from large baits depends only on time and follows a simple and universal decline with t−1/2 . Fig. 4 show the flux expressed relative to an arbitrary initial value chosen as the flux at 1 min after submersion. This time dependence of the rate of release is also seen in the experiment made by Løkkeborg (1990). Fig. 5 shows his data compared to a decrease calculated with a t−1/2 law. The main conclusion from the analysis is that the release of odour will decline as the inverse of the square root of time. The decline curve will be similar for all solid natural baits over a wide range of bait sizes and soak times. Given the observed diffusion constant a thin surface layer, 3 mm thick, will release its content of odour substances during a 12 h fishing operation. The implication is that a large part of the baits is wasted. The optimal shape of a bait would be one with a large surface to volume ratio and a characteristic cross-section approximately equal to the diffusion length for the intended soak time. The recreational fisherman’s favourite bait earth-worm seems to be close to this optimum.
Fig. 3. The thickness of the surface layer of a natural bait contributing to formation of an odour plume as a function of soak time.
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Fig. 4. Relative rate of emission of an odour substance from a natural bait as a function of soak time. The rate at 1 min after immersion is taken as 100%.
Another implication is that the release of odour will increase sharply if a fresh surface is exposed on the bait; if a fish takes a bite out of it for instance. 3.4. Initial dilution of the attractant As found above the release of odour substances is a timedependent phenomenon which for large baits will occur at the same rate J over the whole surface. The total surface area, A, times the flux J gives the source strength or emission, H, from the bait; √ D H = Ac0 √ 2 t The odour will mix continuously into the water current flowing past the bait. The character of this mixing process will vary depending on the turbulent intensity of the current. If the eddy velocity at the length scale of the bait is low compared to the mean current velocity the bait can be considered an obstacle which generates vortices and intensified mixing in the downstream wake. The odours swept off from the bait will feed into this mixing zone and be diluted.
Fig. 5. The observed rate of release of amino acids from a mackerel bait determined by fluorometry in running sea water, solid squares (Løkkeborg, 1990), compared to the theoretical decrease calculated from the observed relative rate at 1 h and following a t−1/2 dependence, open circles. Observed rates are presented relative to the rate 5 min after immersion.
Fig. 6. The ratio of surface to cross-section area (bars) for a bait which is successively cut into smaller pieces (solid squares show number of pieces) and loosely packed in a bait bag. This ratio is proportional to the initial odour concentration in the plume.
With this assumption a simple estimate can be made of the starting value of the diluted odour concentration in the plume. The cross-section of the wake is approximately the same as the crosssection of the bait. The turbulence in the wake of a sphere is fully developed 100–500 diameters downstream (Townsend, 1949). The diameter of the wake grows slowly and is proportional to the cube root of the downstream distance (Johansson and George, 2006). This means that the odour is homogenized by the vortices generated locally behind the bait and that this occurs over a distance on the order of magnitude of 102 to 103 bait diameters. If this cross-section of the bait is denoted B those assumptions and simplifications lead to the following expression for the initial odour concentration ci : H Ac0 ci ∼ = UB 2UB
D t
The initial odour concentration in the plume will thus be proportional to the odour concentration in the bait and inversely proportional to the current velocity. It will decline as t−1/2 . An important observation is that the initial concentration will be independent of the size of the bait and only depend on the ratio of surface to cross-section area of the bait. For a single piece of bait the surface to cross-section ratio, P = A/B, will have a minimum of 4, which is that of a sphere. Any other, more irregular shape has a larger ratio; the ratio for a cube for example is 6. For asymmetric shapes the ratio will also depend on the orientation. However, the most obvious way to increase P is to divide the bait into smaller pieces. This will both increase the concentration of odour in the plume and result in improved utilization of a fixed amount of bait. To illustrate the effect of cutting up the bait we can construct a simple example, where a cube is cut in half, each piece again cut in half across the longest side and so on. The pieces are then randomly packed, giving a package with a somewhat larger cross-section than the original cube, in this example it is assumed to be 20% larger. The result of his exercise is shown in Fig. 6. The calculations show that to increase the concentration of odour in the plume and utilize a given amount of bait optimally the bait should be cut into relatively small pieces. With three cuts, resulting in eight pieces, the surface to cross-section area and consequently the plume concentration have almost doubled. With six cuts the concentration has increased threefold. The diagram shows the 6 first cuts. If the original piece has a side of 6 cm the smallest pieces will have the size 1.5 cm, which still is large compared to the
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Fig. 7. Time series of relative odour concentration from a bait submerged at the time of minimum tidal current speed (solid curve) or with 1.55 (dashed), 3.1 (dotted) and 4.65 h (dash-dot) delay. The tide is semidiurnal with a 12.4 h period and a tidal ellipse with ε = 0.1.
24 h diffusion length. In situations when the time to lower a bait to great depth is large the loss of attractant during descent should be taken into account and the minimum size of the pieces may be larger. Besides P the other parameters that determine ci is the ambient current velocity and time. The time dependence is a steady decrease of emission following a t−1/2 law as has been discussed in Section 3.1. The initial dilution is inversely proportional to the current velocity; the variability can thus be quite large with weak currents while the dilution becomes less sensitive to fluctuations when currents are strong. Diurnal and semi-diurnal tidal currents tend to dominate the velocity field in the coastal environment where traps and pots are used. Even in areas with weak tides, such as the Mediterranean or the Baltic, currents are seldom steady for long periods. Passing weather systems will generate transient currents and long-period waves, operating on time scales from hours to one day. To illustrate the effect of current variations on the odour concentration in the plume we can calculate the development of ci in time depending on the timing of when the pot is deployed within a semi-diurnal tidal cycle. The result is shown in Fig. 7. In this calculation the current is never allowed to reach zero and the minimum current in the tidal ellipse is put to 10% of the maximum. Nevertheless, the effect of the current speed on the initial dilution is such that the maximum plume concentration occurs at slack tide even if this occurs several hours after the setting of the gear. The effect of current speed on the odour concentration in the plume is strong and the timing of setting the gear can be important. Depending on current variations during the soaking time the effectiveness of the attraction may vary strongly. At the same time, the current speed does not only determine the initial concentration, it is also the determining factor for the downstream advection of the odour and the range of attraction of the bait. 3.5. Formation of the odour plume From the assumption made above that the bait acts as an obstacle to the current it follows that the initial mixing generating a starting value of the concentration of the attractant occurs in the wake a short distance, on the order of magnitude of 102 to 103 bait diameters, downstream of the bait. Subsequent to this the
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odour is subject to a continuous dispersion and diffusion by turbulent eddies of the same or smaller size than the odour cloud and to advection by the mean current and larger eddies. This will result in a meandering and gradually expanding plume with decreasing odour concentration, similar to the smoke plume from a chimney. The detailed development of the plume depends on several widely variable conditions, differing between times and places at which a pot or trap is used. Important factors are the velocity and shear of the current, the turbulence intensity at different spatial scales and whether the fluid is density stratified. There is a vast literature on this subject; some general texts are for example Csanady (1973) and Okubo and Levin (2001). Some observations relevant to understanding odour plumes from baited pots will be given here, but it is outside the scope of this paper to cover plume formation in any detail. Much of the theory and modelling of plumes is based on a steady-state statistical description of the concentration at different points in the plume. Such models are called Gaussian dispersion models and the output is typically a normal distribution of the concentration of the odour in a plane perpendicular to the mean current direction, with the plume centred along a straight axis in the mean current direction. For an individual point downstream from an odour source a Gaussian model describes the long-time average concentration. This has little or no relevance for understanding the detection and orientation of a fish to the plume in the natural environment. What a fish is able to perceive is the instantaneous realization of the plume: a much narrower, meandering odour cloud, which may be broken up into discrete patches (Balkovsky and Shraiman, 2002; Ormerod, 2001). A Gaussian plume model strongly exaggerates the local dilution of the odour concentration at a downstream point in a plume. At a fixed position far enough downstream of the bait the odour signal will be absent most of the time. When an odour patch arrives it will be perceived as a burst with strong concentration fluctuations without a simple relation between the local current direction in the odour patch and the direction to the source. The local concentration in an odour patch will decrease with time due to dispersion. Dispersion can be defined as the combined effect of diffusion and shear. Diffusion is the mixing with the surrounding fluid by turbulent or molecular movements whereas shear is the spatial rate of change of the velocity which causes deformation and stretching of the patch. The rate of dispersion depends on the intensity of mixing and the strength of the shear. In practise baited pots are placed at or close to the bottom. This is a hydrodynamic regime where the current velocity decreases to zero at the bottom and the vertical turbulent eddies are limited in size by the distance to the bottom. This so called benthic boundary layer, BBL, can be divided into a mm-thick viscous and laminar sub-layer at the very bottom and an outer Ekman layer influenced by the rotation of the earth. In between those the flow is unaffected by the rotation of the earth and the mean velocity increases logarithmically with height (Wimbush and Munk, 1970; Boudreau and Jørgensen, 2001). This layer is characterised by a constant vertical shear stress and is called the logarithmic layer. The thickness of the logarithmic layer in coastal waters is typically 1–10 m, which means that this is where odour plumes from baits are usually formed. The vertical eddy diffusion increases linearly with height, h, above the bottom while the current shear decreases with height. It can be shown that the local dispersion in the BBL will decrease with height above the bottom as h−1/2 and increase with time as t3/2 . As a consequence, the dispersion of an odour plume in the BBL is larger for a source close to or on the bottom while the downstream advection is slower the closer the plume is to the bottom.
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4. Conclusions
Acknowledgments
A simple analysis of the physical processes involved in the formation of odour trails from bait was shown to be useful for understanding the criteria for optimal use of baits. It identifies which properties of the bait and its environment are important in defining the properties of the plume. Crucial findings in the analysis were the rapid decline in odour source strength and the low utilization of the active substances in natural baits due to the slow mass transfer in the bait. This emphasizes the importance of developing alternative delivery systems. It should be possible to design artificial baits so that the emission rate is constant or, even better, proportional to the ambient current velocity, which would give a constant initial odour concentration in the plume. To extend the analysis and develop models for optimization of the range of the olfactory signal from a bait it is necessary to know both c0 in the bait and the threshold concentrations where the target species detects and becomes attracted to the odour trail. Much experimental data exist on detection and search activation thresholds to different chemical stimuli (Døving, 1986; Hara, 1994). Measurements of c0 or ci are essentially lacking however. When such data become available it will be fruitful to apply the existing more sophisticated dispersion models for the BBL to analysis of the range of attraction of baits under various environmental conditions. The focus of this paper is on the dispersion of the odour plume. Evidently, the diminishing concentration is decisive for how far downstream an odour can be detected by a fish, but there are other properties that may be equally important. Any information that a fish can extract on the age of the trail or on the distance and direction to the source must be advantageous when deciding if it is worth starting a search. Some work on this question has been done in insects (Murlis et al., 1992) and in crustaceans (Weissburg and Zimmer-Faust, 1994). Essentially, the finding is that the animals integrate some form of chemically modulated orientation (chemotaxis) with a visual or mechanical assessment of flow conditions in order to move upwind (rheotaxis). Across-stream turns, zig-zaging or casting, are typical features of the tracks (Vickers, 2000). It is characteristic that the animal receives short intermittent bursts of odour concentration; temporal analysis of the signal can then be used to infer spatial gradients. No similar detailed experiments have been made on fish behaviour in intermittent plumes under realistic conditions. Such studies would give a better understanding of what properties of an odour plume are the most significant from a fishery technology point of view.
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