The study of the aquatic environment is a vast subject which includes investigations of its physical, chemical and biological components and of how these vary in both space and time. Only the main outlines of study are described in this section. Despite the fact that the aquatic environment, particularly the sea, has been studied intensively, few relationships between fish and their environment have been established which are useful to fisheries biologists. This is probably because the environment is complex and the fish's reactions to it not straightforward, so that simple relationships, for example, between year-class strength and water temperatures, can seldom be expected.
Both for this reason and also for the reason that no single laboratory can undertake to study all aspects of this subject, the director of a laboratory engaged in practical fisheries research must decide what work will give useful results. The decisions will be made on the basis of what aspects of environmental work in either the same or other regions have been shown to be of real value.
Two potentially useful studies are the distribution of fish in relation to environmental features and the basic productivity of the water and its relation to the potential productivity of the water masses. This section will concentrate mainly on these studies.
Sampling programmes must be planned. The pattern of observations required must be drawn up as a station list. A station is defined as a geographical point at which observations are carried out. There are some principles in station planning which are generally applicable: (a) lines of stations (sections) should run at right angles across the expected isolines of the characteristics to be sampled (an isoline is a line along which the characteristic studied e.g. temperature is the same); (b) the distance between stations should be governed by the expected rate of change along the section of the properties to be measured; (c) the frequency of sampling should be related to the expected rate of change with time. The distance between stations should be governed by the expected rate of change in the characteristic being sampled; the greater the expected rate of variation the less distance there should be between stations. In the sea there are standard depth observations which are suitable under most circumstances. These are 0, 10, 20, 30, 50, 75, 100, 150, 200, 300, 400, 500, 600, 800, 1,000, 1,200, 1,500, 2,000 m and at 1,000 m intervals thereafter, where applicable.
Having decided the number of stations and the frequency with which they are to be sampled sufficient equipment, chemicals and sample containers must be put on board.
At sea the person with most navigational experience, usually one of the watch-keeping ship's officers sees that the ship is precisely at the position of the station decided be-forehand by the scientist. This is recorded by the latter together with details of the observations made.
Temperature is the physical characteristic of the environment most widely recorded because it is easy to measure and because it has proved the environmental feature most easily related to fish distribution and aggregation. The easiest way to take a surface water temperature is to fill a bucket with water from the lee side of the ship, away from any cooling water or other outfalls and then read the temperature with a small heat-capacity thermometer.
However, special equipment is required for taking temperatures at depth. The same equipment is often also used for taking water samples.
Pettersen-Nansen bottles (Fig. 7.1) are probably the most convenient sampling instrument in depths up to 200 m. They should not be used below this depth because temperature changes can take place while hauling them from greater depths. Using a single Pettersen-Nansen bottle the temperature at each depth to be sampled is obtained by successively lowering the bottle to each depth, closing it with a messenger (a weight slid down the wire) after it has been at that depth for at least half a minute, and retrieving it. The temperature is then read immediately using a thermometer magnifier.
At depths greater than 200 m temperatures are taken with thermometers fitted in either a reversing frame or attached to reversing water sampling bottles (Fig. 7.1). Up to seven bottles or frames can be used at one time, spaced on the wire to take samples from the required depths. The first is activated by a messenger, which then releases the next messenger and so on. Thermometers are used in pairs, one protected and one unprotected, and often two sets are used, one checking on the other.
A protected thermometer is so called because it is encased in a sealed glass tube which protects it from being squeezed by the pressure of the water. (Fig. 7.2) The unprotected thermometer has no such protection and the pressure of the water squeezes the glass and makes the temperature of the unprotected thermometer read higher than that of the protected. Because the pressure of the water is proportional to depth the true depth at which the sample was taken can be calculated.
A rough estimate of this can be obtained from the angle of the wire to the vertical and the length of wire out. This is satisfactory for wire angles of 5° to several hundred meters but only to 100 m for larger wire angles. The reasons for the differences between calculated depth and actual depth are that the wire does not run in a straight line from the ship to the sampling bottle and that the speed of the currents alters the angle of the water. If current systems overlay each other, running in directions, the wire angle will change appropriately as it runs through them.
The main thermometer has a special construction. It consists of a main expansion bulb (Fig. 7.2) and a smaller one at the other end of the tube. The capillary of the tube has two constrictions (Fig. 7.2). The thermometer is lowered with the main bulb lowermost and the mercury in the main bulb expands to fill the capillary and part of the smaller expansion bulb. When the messenger hits the bottle and reverses the frame the mercury column breaks at the lower constriction leaving a column of mercury which is now isolated from the main expansion bulb and whose volume was dependent upon the temperature at the sampling point. The volume of this mercury and therefore the temperature which is recorded will still depend upon the temperature at which the main thermometer is read so each main thermometer has an associated auxiliary thermometer to record this (Fig. 7.2). The main thermometers are read to an accuracy of 0.01°C and the auxillary thermometer to 0.1°C, usually with a special reader incorporating a light and a magnifying glass.
When taking a series of temperature observations (known as a ‘cast’) the bottom bottle is lowered to its correct depth and held there for at least five minutes to allow each thermometer to attain the temperature of the layer which it is intended to register. The first messenger is then released to close the first bottle. The closure of this and subsequent bottles should be checked by holding the wire, each closure being indicated as a ‘thump’ on the wire. Beyond about 500 m these thumps may not be felt and the messenger should be timed; it travels at a rate of 250 m a minute. When the bottles are retrieved they must be kept vertical, in the reversed position, until the temperatures of all the thermometers have been read. After reading they should be stored with the bulb down, to reduce the danger of air-look formation in the thermometers.
A series of corrections has to be made to the temperature as read on the protected thermometer to obtain the temperature at the point where the sample was taken. These corrections are to allow for
(a) expansion or contraction of the recording column of mercury resulting from differences between the temperature at the point of sampling and that at reading as already described
(b) slight irregularities in the capillary tube called the ‘index correction’; this is obtained by calibrating the thermometer at different temperatures. Most of them have been calibrated by the U.S. Bureau of Standards, the N P.L., or P.T.R. which supply with the thermometer certificates giving the index correction to be made at different readings of the thermometer.
The volume of the main thermometer bulb up to the reading of 0°C on the capillary, known as the degree volume, must also be known. It is given on each thermometer. The total correction to be made to the temperature read on the thermometer is given by:
V0, I and K are constants which are supplied with the thermometer. As K is usually 6,100 or 6,300 and the sum of the other terms approximately 100, the required degree of accuracy (0.01°C) can be obtained using the following formula:
which is the formula normally used.
TW = T¹ + ΔT
in which Tw is the corrected reading of the protected thermometer.
The formula for correcting the temperature of the unprotected thermometer is:
The depth, in metres, at which the temperature was taken (D), called thermometric depth, is given by:
| in which | Q = pressure coefficient of the unprotected thermometer |
| d = mean density of the sea water column above the depth at reversal |
and the other symbols are as previously described.
Q is a constant supplied with the thermometer and d is obtained from tables.
Full details of these formulae and of the methods of processing temperature data are given by LaFond (1951).
A worked example is given in Appendix 7.1.
In many cases it may be advisable to record sea surface temperatures at frequent intervals when the ship is underway because rapid changes usually indicate current divergences and convergences which may be of importance in fish distributions. The surface temperature can be easily measured by taking a sample in a bucket or, in larger ships, by using a thermograph fixed to the condenser intake. The latter gives a continuous record.
Continuous recording of the temperature-depth profile is made with a bathythermograph (Fig. 7.3) which records temperature against depth on a smoked glass slide. It can be either lowered when the ship is stationary or whilst it is underway. The bathythermograph is lowered into the surface water and kept there for 30 seconds to allow the thermometer to adjust to the surface temperature before lowering to the required depth. The amount of wire required to reach any depth will depend on the ship's speed and the wire characteristics. It is normally advisable to do several trial runs to get a wire/depth conversion factor. The surface temperature should also be read with a bucket and thermometer when the bathythermograph is lowered. After retrieving the instrument the slide is preserved by marking it with the necessary identification data, washing it with freshwater, dipping it into amylacetate lacquer and then drying it in a rack. Bathythermograph records are read by placing the slides over a standard grid in the viewer provided with the instrument. The actual temperatures are not read so accurately as with a thermometer, the expected precision being normally about 0.1°C. Bathythermographs provide a continuous record of temperature against depth which is obtained quickly and which, for many purposes, more than compensates for the reduced accuracy. Expendable bathythermographs are now available; these are fired from the ship's side while it is in motion and the temperature-depth record is transmitted back to the ship by wire which automatically disconnects at the ship when all of it has paid out.
Bathythermographs are particularly useful in providing an exact measure of both the depth and the intensity of the thermocline. This is an area of marked temperature discontinuity providing a physical mechanism which separates two layers of water. The water below the thermocline may be de-oxygenated and uninhabitable by fish. In some lakes, such as Lake Kivu, the thermocline is permanent and the lakes always stratified. In freshwater the change in density with temperature is much more important than in sea water, particularly at temperatures above 20°C.
Temperature data find an application to practical fishery problems mainly where the range of distribution of a species is limited by oceanic boundaries or where a species aggregates at these boundaries. Examples of these phenomena are broadly divisible into three categories, those in which:
Perhaps the best example of the first category is the distribution of Barents Sea cod, which is limited by the 2°C isotherm. In early summer, when only a small area of this region is covered by bottom water warmer than 2°C, cod are found concentrated in this warmer water and catch-rates there can be high (Fig. 7.4). Later in the summer, as the warmer water spreads eastward, the cod spread out over this wider area of higher temperatures and catch-rates fall (Fig. 7.5).
There are several examples in the second category. King and Hida (1957) have shown that yellow-fin tuna aggregate in zones of both convergence and divergence of the Pacific equatorial currents and that in these zones, when the divergence is narrow, catches are 2 to 3 times higher than in the main currents (Fig. 7.6). Similarly it has been shown that Atlanto-Scandian herring concentrate at the sharp temperature boundary between warm and cold water off the east coast of Iceland during the summer (Benko and Seliverstov, 1971). In the eastern North Sea in spring, herring are packed on the western edge of the cold Baltic outflow where it meets the warmer North Sea water. In such cases the association between fish aggregation and temperature is probably not direct. In the case of the North Sea herring, the cold low salinity water flowing out of the Baltic provides vertical stability, which in turn leads to an earlier spring outburst of phytoplankton and zooplankton than over the rest of the North Sea. It is probably the high level of food availability in this area, rather than the temperature directly, which results in the herring aggregating. Similar mechanisms may apply in the other cases cited.
The third category, of fish concentrating in areas of upwelling, is best illustrated in the fisheries for anchoveta off the coast of Peru and northern Chile and in the pilchard fisheries off the west coast of Africa. In both cases the fisheries take place in areas where currents well up against the continental shelf, resulting in areas of very rich feeding. Again the association between temperature and fish in these cases is probably an indirect one with food as the direct link.
Quite apart from the circumstances which permit temperature data to be used to predict the areas of highest concentrations of fish and of most profitable fishing, temperature has a direct effect on the metabolism of the fish. Fish can only tolerate temperatures within a certain range and within this range there is a narrower optimum range for growth and reproduction. Normally, in areas where a fish species has been established for a considerable time the temperature will not depart from the tolerance range of the species but variations of temperature outside the optimum range for reproduction can result in variations in the success of recruitment. At West Greenland the cod fishery did not exist before the 1920's because the water was too cold for cod. Now it seems as if the water is again becoming too cold. Year-classes since that of 1968 have been very small and it is doubtful whether the stock will continue to exist.
In the literature there are numerous references to the effects of variations in temperature altering the timing of and the success of spawning. Mankowski (1950) states that low temperatures delay, and high temperatures advance, the spawning of Baltic cod. Simpson (1953) has shown that abnormally low temperatures on the spawning grounds forced North Sea plaice to spawn elsewhere. Woodland (1964) and Galloway (1941) describe the lethal effects of low temperature on fish.
Success of egg and larval survival, as measured by variation in year-class strength, would be expected to show the greatest change with temperature because it would seem that at these stages the species would be most susceptible to departures from the optimum temperature requirements. Kurita (1959) has related variations in year-class strength of Japanese sardine to temperature variations. Chase (1955) has done a similar relationship for Georges Bank haddock, Benko and Seliverstov (1971) for Atlanto-Scandian herring and Postuma (1971) for North Sea herring. There are numerous instances of similar relationships in the literature for a large number of species in many sea areas. But the fact remains that all of these are retrospective correlations with considerable latitude for the original choice of the temperature variable. That the relationships are seldom heard of again, after initial publication, might suggest that their predictive value is small.
There is little direct evidence of mortality associated with abnormal temperatures, either in the adult or larval stages. Galloway (1941) has recorded extensive mortality of fish in the coastal areas of Florida associated with drastic temperature changes whilst Simpson (1953) for the cold winter of 1946-47 recorded mortalities of sole, cod and whiting in the southern North Sea.
Detailed descriptions of the methods of analyses for the wide range of substances in water for which analyses can be carried out are given in the FAO Manual of Methods in Fisheries Biology and in standard textbooks quoted there (e.g. Strickland and Parsons, 1960). In this manual only advice on how samples should be taken and preserved for analysis of the major substances will be described and some of the more important applications of these analyses discussed.
Generally water samples for chemical analysis are taken from either the same or a selection of the stations and depths sampled for temperature. To take samples a piece of rubber tubing with a glass tube about 15 cm long in one end is fastened to the tap of the sampling bottle, the air valve at the top of the bottle is opened and water drawn off. Samples for oxygen determination, if required, are taken first, followed by those for other chemical analysis. The bottles in which the samples are to be stored are first rinsed with a little of the sample water before filling with the sample. Chlorinity analysis is one of the most important in the marine environment. The results are used for deriving the salinity and density of the water which in turn are used for the characterization of water masses and for tracing their origin, movements and mixing. Salinity variations in offshore areas are relatively small but can be considerable in coastal areas due to variations in run-off. These variations can affect the osmotic balance of fish and the buoyancy of pelagic fish eggs.
Dissolved oxygen samples are run into 250 ml dark bottles of accurately known volume fitted with ground glass stoppers. In drawing off the sample the glass tube must be kept close to the bottom of the sample bottle to ensure that no bubbles of air enter the sample, and the water allowed to flow after the bottle is full. The tube is withdrawn and the glass stopper fitted when the bottle is completely full, care being again taken that no air bubbles enter the bottle with the stopper. If these precautions are not followed oxygen from the air will dissolve in the sample and result in a false determination. To fix oxygen samples, 1 ml manganese chloride solution is added near the top of the sample bottle, followed by 2 ml of alkali iodide mixture added by immersing the tip of the pipette to about one inch below the surface. The stopper is replaced immediately and the chemicals mixed very thoroughly for at lease fifteen seconds during which the stopper must be held firmly in place.
In the sea low oxygen content seldom is critical for the survival of fish but this can occur in certain localities in the tropics, where a subsurface oxygen minimum layer is formed, and close to the bottom of badly aerated depressions, such as the deeper layers of the Black Sea. Oxygen depletion is much commoner in fresh water, instratified lakes and some marshes. Because oxygen is produced by plants in photosynthesis the oxygen content of the water can also give a useful, if rather rough, indication of the amount of carbohydrate synthesis which has recently taken place in that body of water.
Salinity samples are run into 200–300 ml bottles fitted with attached stoppers with rubber washers. The bottles are almost completely filled, leaving only 5–10 ml space under the stopper to allow for expansion. These samples require no other treatment before analysis.
Phosphates, nitrates and silicates are important nutrient salts, whose exhaustion in water limit organic production. The amounts of these nutrients in an area indicate the potential fertility of an area and of the biological processes which have taken or are taking place. Samples for phosphate determination are drawn off to fill the bottle to 2 cm below the stopper. Six drops (1/2-1 ml) chloroform are then added, the stopper put on and the bottle shaken vigorously until the sample becomes milky. The samples are then stored in a refrigerator and analysed at the earliest opportunity.
Nitrate samples are drawn off into small bottles with ground glass stoppers, leaving ample space to allow for expansion. These samples are stored without chemical treatment in a deep freeze until analysis. Silicate samples are stored in small plastic bottles and kept in a refrigerator, without preservation, until analysis.
Further details on sea water analyses are given by Strickland and Parsons (1960).
Measurement of light intensity at different depths is valuable because it is related to both the photosynthetic activity of the plants, which provide the primary production, and the ecology and depth distribution of the animals. The diurnal vertical migration of many marine organisms suggests that they are following an optimum light intensity. Moreover, the reaction to fishing gear of some species is partly governed by their ability to see it, and this will depend upon the light intensity at the depth they occupy. Generally, light energy at different depths in the sea is not measured directly but is computed from measurement of the extinction coefficient of the water and the intensity of radiation at the surface. The extinction coefficient can be measured either visually or with optical instruments. The simplest visual method is the Secchi disc which is a white disc 20 cm in diameter. It is lowered into the water on the shadow side of the vessel attached to a wire. The depth at which it disappears from sight is measured in meters. It is lowered 2–3 m deeper and then hauled to the surface, the depth at which it becomes visible again also being noted. The mean of these two depths is converted to an extinction coefficient from the equation X = 1.7/D, where X = extinction coefficient and D = the mean depth of disappearance of the Secchi disc in meters. The extinction coefficient can also be measured by extinction meters which have an electrical light 60–200 cm above a photocell.
Although pollution is a growing problem, particularly in seas surrounding highly industrialized countries, it is not included in this manual for two reasons. First, the efficient monitoring of both heavy metals and toxic organochlorides demands a level of expertise and sophistication of equipment, which is the subject of special publications. Second, there is no evidence to date of any direct effect of pollutants on the productivity of any marine fish stock except for isolated incidents close to a source of pollution. There is considerable evidence that pollution can seriously affect the suitability of fish as food (Ackefors et al., 1970) but this is a problem for public health authorities rather than for the marine biologist.
Because the field of plankton investigations is so extensive it is not covered in detail in this manual, except for two aspects which have the most direct application to practical fishery problems. These are firstly the assessment of primary production and total plankton biomass, which are guides to the potential capacity of an area to support fish stocks, and secondly the application of fish egg and larval surveys to investigate the time and place of spawning of fish and in fish stock assessment.
The simplest and most accurate method of estimating the gross production of an area is to measure the photosynthetic pigments of the plant population. The rate of photosynthesis is then calculated from the chlorophyll concentrations, light and transparency data using a general relationship between light and the rate of photosynthesis. This gives a value for gross production only; to estimate net production a value for the loss due to respiration is required, and this is more difficult to obtain. However, for most practical fishery purposes values of gross production are quite adequate.
The technique consists of taking a sample of 1 to 5 l, depending on the richness of the phytoplankton, from a number of depths within the photosynthetic zone using a Pettersen-Nansen or other suitable water bottle. These samples are then filtered through cellulose membrane filters of 0.4–0.65 um, (um, pronounced mu-em, is 10–6 metres) pore size which have been covered with sufficient finely powdered MgCO3 to give about 10 mg/cm² over the area of the filter. A suction pressure of 2/3 atmospheres applied to the filters speeds up filtration. These filters can be stored, in the dark, over silica gel at 1°C for up to two months, although it is preferable to extract the pigment from the damp filter immediately, and carry out the spectrophotometric measurement without delay.
The pigment is extracted from the phytoplankton by placing the filter in 90% acetone for at least 10 minutes. Extracts should preferably be treated immediately, but can be stored for several hours at room temperature in the dark. Measurement of the pigments is carried out using the spectrophotometer, with a bandwidth of 3 um or less and cells with a light-path of 4–10 cm, reading the extinction at 750, 663, 645 and 630 um against a 90% acetone blank. The extinction at 750 um is then subtracted from those at the other values and the answers divided by the light-path of the cell used in centimetres. If these corrected extinctions are e663, e645, and e630 the concentration of chlorophyll a in the 90% acetone extract is = 11.64e663–2.16e645+0.10e630. If this value is then multiplied by the volume of the extract in cubic millimetres and divided by the volume of the sea water filtered in litres, the concentration of chlorophyll in the sea water is obtained as ug per litre. A full account of the techniques for the determination of plant pigments in sea water is given in “Determination of Photosynthetic Pigments in Sea Water”, Monograph on Oceanographic Methodology, Unesco 1966.
The concentration of chlorophyll a calculated as given above, gives an adequate measure of the standing crop of phytoplankton. If an estimate of gross production is required in terms of weight of carbon produced per day this can be calculated from the equation
| where | P = photosynthesis in g carbon/m²/day |
| R = relative photosynthesis |
as given in a graph in Ryther and Yentsch (1957) which relates relative photosynthetic activity to surface radiation for the appropriate value of surface radiation. K is the extinction coefficient per meter measured as described on page 159 and C = g chlorophyll per cubic meter.
One of the major problems in measuring zooplankton biomass is that no one gear is adequate for sampling the wide size range of organisms which occur in the plankton, ranging from protozoa of about 2u to coelenterates up to 1 metre in diameter. In principle this difficulty could be overcome by using several gears at each station to cover all the major size groups. However, there would then be considerable overlap in the size ranges sampled by the different gears so that when the catches from them were summed to give the total biomass some organisms would be counted more than once. In this situation the best solution is probably to adopt a single gear which gives adequate sampling of the size range of organisms which constitute the bulk of the plankton biomass and accept the fact that the estimate is an underestimate because a proportion of the plankton has been omitted due to mesh selection or evasion.
A zooplankton net with a mesh aperture of 200 um is the best compromise and a suitable net of this mesh size has been described in detail by Working Party 2 of the ICES/SCOR/ UNESCO Working Group (Anon., 1968) which was set up to advise on the standardization of zooplankton sampling. The Working Party recommended that this net should be used for vertical hauls but there seems no reason why it should not be used obliquely if it is towed at about 1.5 knots. However, this is too slow for larger organisms, which will evade the net at this speed. For pelagic decapods and larval fish in the later stages speeds of 4 knots are necessary.
The depth range which it is necessary to sample to measure the zooplankton biomass can be determined only by experiment locally. However, samples taken by either oblique or vertical hauls from a maximum depth of 200 metres to surface will usually give a sufficiently accurate estimate of the zooplankton biomass. The question of the relative merits of vertical and oblique hauls for measuring the abundance of plankton has not been fully resolved. Vertical hauls have the advantage that the volume of water filtered is more easily measured but on the other hand they are very susceptible to the variation produced by small scale plankton patchiness. On balance, it would seem that a more valid estimate is given by oblique hauls because they largely even out plankton patchiness and so give a better average estimate representative of a fairly large area. The meshes of plankton nets will clog if plankton, either phytoplankton or zooplankton, is dense. For this reason plankton nets should always be fitted with flowmeters so that the volume of water filtered can be calculated.
Two sampling instruments are the Gulf III (Fig. 7.7) and the Bongo sampler (Fig. 7.8) which can be towed obliquely. They are fitted with filtering cones of 250 um mesh aperture. Zooplankton samples should be preserved in 4% formalin buffered with either borax or marble chips. To determine the biomass of the sample, the simplest method is to measure the displacement volume by filtering the sample through a piece of fine bolting silk (No. 25). The piece of silk and the retained plankton are then placed on an absorbent paper for 10 minutes after which the sample is put into a graduated cylinder containing a known volume of water. The increase in volume in the cylinder is recorded as the displacement volume of the plankton. The biomass weight (wet weight) can be obtained by transferring the collection to a glass cylinder of known weight which has a base of fine nylon mesh. When the bulk of the preservative has drained off a suction pump is used to remove the remainder. The material is then washed with water under suction, which is continued for a short time to draw air through the plankton before weighing. The results of such estimations are expressed as weight or volume below 1 m² of sea surface from the volume of water filtered by the sampling gear as measured by flow-meter and the depth through which the sampling gear was hauled.
Although these methods give the most accurate and comparable results, useful information can be obtained by towing simple zooplankton nets. If a flow-meter is not used the results are not quantitative but if a standard procedure is followed each time the results are qualitatively comparable. An example of a qualitative study of this type is that by Holden and Green (1960).
The weight of the plankton biomass would be expected to be of greatest relevance to fishery problems in two major spheres. For those pelagic fish species which feed directly on the plankton, a relation might be expected between centres of high zooplankton abundance and concentrations of fish and so of high catch rates. Although in general terms this has been shown to be true for a number of fish species the detailed relationship is normally far from simple and its value as a method of guiding a fishing fleet to the most productive grounds would seem very limited. Two reasons may account for this: first, there may be a time lapse between the build-up in the plankton and the aggregation of the fish on to it; secondly, the areas of highest fish aggregation are probably those where no plankton occurs because it has all been eaten.
The other area in which zooplankton estimation would appear likely to give useful practical fishery results is in prediction of year-class strength. The most widely accepted hypothesis is that the variations in year-class size of the highly fecund telcost fishes result from changes in the amount of available food from year to year, lack of adequate food being held to be the main factor causing mortality of larval fish. Although this seems acceptable on theoretical grounds (Gulland, 1965), the concept raises some problems of interpretation in species which do not show a similar pattern of year-class strength variation (Jones, in press), but which spawn in the same area at the same time and have similar food preferences. Certainly it is true that no one has yet succeeded in showing a convincing long-term relationship between year-class size and the abundance of food for any species. This may be due to the inadequacy of the available data rather than to the underlying premise being untenable.
The other major application of plankton sampling in practical fisheries problems is that of accurately assessing the time and place of spawning and the size of the spawning stock. The timing can be assessed from an examination of the spawning cycle, as discussed in Section 5. It may be possible to determine the area of spawning from the position of capture but fish may remain in a high maturity stage for a considerable period of time before spawning, during which they may move a considerable distance. A more accurate technique is to survey the area for the presence of eggs or larvae of the species under consideration by plankton sampling. Before such a survey as much information as possible about the time and place of capture of fish in spawning condition should be obtained. Use of these data will allow the area and time span of the survey to be reduced to the minimum.
An ACMRR Working Party on Fish Egg and Larval Surveys has recommended that for sampling fish eggs and larvae, other than the larger sizes of the latter, the best available instrument is a 60 cm Bongo Sampler. This is fitted with nets of 0.505 and 0.333 mm mesh size and towed obliquely at from 1.5 to 2.0 knots from the maximum depth at which the eggs and larvae of the species under investigation are found to the surface (Anon., 1971). These speeds are satisfactory for eggs and early larvae which cannot escape the nets. Higher speeds (up to 4 knots) are required to catch late stage larvae. An FAO Manual for fish egg and larval surveys is in course of preparation. Such surveys should be done over a grid of stations, at fixed distances apart, covering the whole of the area which from other data available may contain spawning centres. Similarly, such grids should be repeated at regular time intervals over the total period which again from other sources of information may contain spawning activity.
Before starting egg and larval surveys it is best if the planktonic stages of the species being investigated can be identified so that there is no possibility of confusion with another species with closely similar eggs and larvae. If two species, whose eggs are extremely difficult to separate in the earliest stages, spawn in the same area about the same time a later stage of development which can be definitely allocated to each species must be used. This will introduce some error because the eggs will have drifted from the spawning area and dispersed over a wider area than they initially occupied during the period from spawning to the stage utilized. Similarly, unless the time taken from spawning to the development stage used is known, there will be a bias in the estimate of the start, finish and time of maximum spawning activity.
In the early stages of a fishery when the egg and larval stages have not been identified there may be no option but to group eggs and early larval stages by species. Identification can be done only by rearing eggs and larvae in the laboratory, which is not easy.
The estimation of the abundance of a fish stock from egg and larval plankton surveys makes similar demands to those of the identification of time and location of spawning. The basic concept of assessing stock abundance in this way is comparatively simple. The total production of an egg or larval stage of a species is obtained from a series of surveys covering a spawning season. From a knowledge of the mean fecundity of mature females the total abundance of the mature female stock can then be calculated from the equation:
M = P/F
| where | P = egg production estimate |
| F = mean fecundity of mature females |
The abundance of the exploited stock is obtained from the proportions in the commercial catches of mature female fish to that of mature male and immature fish combined. Since the fecundity of a fish is proportional to its weight, the stock can also be estimated in weight by using the fecundity per unit weight relationship rather than the mean fecundity.
The main problem is to obtain an estimate of egg production over a spawning season. This is done by sampling over the area of distribution of the eggs sufficiently intensively to give an adequate representation of changes in egg density. Normally the total egg counts for each survey are obtained by sampling a grid of stations regularly placed at fixed distances apart. This simplifies the subsequent computation. For maximum efficiency of estimation there is much to be said for sampling at closer distances in the areas of highest concentration, where the variability is normally highest, and at more widely spaced stations at the fringes of the distribution where variability is normally least. It is necessary to sample the total range of depth within which the eggs are distributed by oblique hauls with a Gulf III or Bongo net. This must be fitted with a flow-meter so that the total number of eggs caught can subsequently be converted to the number below 1 m² of surface. The numbers caught at each station are then converted to numbers in the total volume of distribution; for a regularly spaced station grid the mean of the station observations is multiplied by the volume which each station represents; for an irregularly spaced sampling grid isometric lines are drawn, the volume within each isoline measured and this volume multiplied by the number of eggs.
Because egg production varies with time over a spawning season, it is necessary to carry out a number of surveys during the spawning season. To obtain estimates of daily egg production, the total production for each survey is divided by the number of days which it takes for the egg to pass through the stage utilized in the analysis at the temperatures prevailing at the time of that survey. The final result is a series of estimates of the total daily egg production at various times during the spawning season. To obtain the total seasonal production there are two approaches. For many species egg production over a season follows a normal curve; the area under this type of curve can be estimated from a knowledge of the values at any three points on it. Of course in the initial stage of the investigation it is necessary to carry out more than three cruises to determine whether the seasonal egg production is normal.
If the seasonal egg production does not follow a normal curve an estimate of total production is obtained in two ways. One method is to take the total egg production for the cruise and to weight this as follows. Let:
| P | = total egg production for the survey | |
| d | = duration of the survey in days | |
| 2b | = the number of days between this survey and the preceding survey | |
| 2c | = the number of days between this survey and the next survey |
Then the estimated egg production for the period (b+c+d) days is:
For the first and last surveys the egg production should be low and for the first survey b can be taken as zero and for the last survey d likewise. The other method is to plot the survey totals as ordinates against the mid-date of that cruise and to measure the area under the resulting curve. If the curve of seasonal spawning intensity is not normal more surveys will be required to give comparable accuracy than if it is normal. The number required cannot be defined until a pilot survey has been run to determine how spawning does vary with time. A discussion of the use of such surveys to estimate stock size and the confidence limits of such estimates is given in Saville (1964).
Example:
Nine surveys were carried out in March 1971 to sample the abundance of haddock eggs. During each survey 32 stations were sampled, the stations being spaced 20 nautical miles apart in a square grid pattern.
(a) During the survey 10-12 March the required data for each station were as follows:
| Station Number | Number of eggs in Stage I in each sample | Volume of water filtered (cubic metres) | Depth of haul (metres) | Number of Stage I eggs below 1 sq metre |
|---|---|---|---|---|
| A | B | C | A×C/B=D | |
| 1 | 0 | 55 | 10 | 0.00 |
| 2 | 1 | 62 | 31 | 0.50 |
| 3 | 1 | 58 | 42 | 0.72 |
| 4 | 0 | 65 | 63 | 0.00 |
| 5 | 1 | 70 | 64 | 0.91 |
| 6 | 8 | 63 | 63 | 8.00 |
| 7 | 10 | 61 | 42 | 6.89 |
| 8 | 4 | 62 | 31 | 2.00 |
| 9 | 2 | 60 | 20 | 0.67 |
| 10 | 25 | 54 | 30 | 13.89 |
| 11 | 40 | 58 | 40 | 27.59 |
| 12 | 1 | 45 | 45 | 1.00 |
| 13 | 0 | 50 | 52 | 0.00 |
| 14 | 15 | 63 | 45 | 10.71 |
| 15 | 75 | 65 | 47 | 54.23 |
| 16 | 0 | 60 | 25 | 0.00 |
| 17 | 0 | 61 | 21 | 0.00 |
| 18 | 90 | 58 | 36 | 55.86 |
| 19 | 35 | 56 | 48 | 30.00 |
| 20 | 1 | 63 | 63 | 1.00 |
| 21 | 0 | 61 | 58 | 0.00 |
| 22 | 75 | 60 | 43 | 53.75 |
| 23 | 15 | 59 | 40 | 10.17 |
| 24 | 1 | 57 | 22 | 0.39 |
| 25 | 0 | 66 | 30 | 0.00 |
| 26 | 10 | 65 | 38 | 5.85 |
| 27 | 20 | 75 | 50 | 13.33 |
| 28 | 0 | 57 | 61 | 0.00 |
| 29 | 0 | 59 | 83 | 0.00 |
| 30 | 2 | 50 | 75 | 3.00 |
| 31 | 1 | 65 | 62 | 0.95 |
| 32 | 0 | 53 | 71 | 0.00 |
Eggs pass through Stage I in 2.5 days. Calculate the mean daily egg production within the area surveyed during the time this survey was carried out (1 square nautical = 343 × 104 square metres).
The mean number of eggs per square metre in the area surveyed from the data in column D = 9.45.
Mean daily egg production = 9.45/2.5 = 3.77 per square metre.
Each station represents 20² square nautical miles
Area of survey = 400 × 343 × 104 × 32 square metres
= 43904 × 106 square metres
Total daily egg production during period of this survey
= 43904 × 3.77 × 106 = 165518 × 106
(b) Other surveys carried out over this area in 1971 gave the following estimates of daily egg production:
| Period of Survey | Estimated Daily Egg Production | |
|---|---|---|
3–5 | March | 3290 × 106 |
6–8 | " | 49106 × 106 |
10–12 | " | 165518 × 106 |
13–15 | " | 212086 × 106 |
16–17 | " | 243750 × 106 |
19–21 | " | 175208 × 106 |
23–25 | " | 83690 × 106 |
26–28 | " | 22115 × 106 |
29 March-1 April | 925 × 106 | |
Calculate the total annual egg production of the stock.
Plot these values against the mid-date of each survey as in Figure 7.9 on an appropriate scale
The area under the graph is equal to 153.25 large squares.
Each of these represents 2 × 1 × 1010 eggs on the scale used
| Total annual egg production | = 153.25 × 2 × 1010 |
| = 306.5 × 1010 |
(c) The fecundity-length relationship of this stock is F = 1.309L3.2 where F = fecundity and L = length of a female in centimetres.
If the mean length of the spawners is 28 cm and the sex ratio of ripe fish is 1:1 calculate the number of fish in the mature stock.
The mean fecundity of the female mature stock is | = 1.309 × 283.2 |
Similar techniques have been used for species which lay their eggs on the bottom; grab samples are taken over the spawning beds and the area contoured. However, there is no suitable technique for sampling freshwater fish, the majority of which attach their eggs to plants. Also the larvae are not distributed in a pelagic place as are those of marine fish.
Anon., 1971 Report on the ACMRR Working Party on Fish Egg and Larval Surveys. ACMRR 6/71-WP4
Ackefors, H. et al., 1970 A survey of the mercury pollution problem in Sweden with special reference to fish. Oceanogr.mar.Biol., 8:203–22
Benko, Iu.K. and A.S. Seliverstov, 1971 Influence of some factors on the abundance of Atlanto-Scandian herring year-classes. Rapp.P.-v.Réun.Cons.perm.int.Explor.Mer, 160: 153–7
Galloway, J.C., 1941 Lethal effect of the cold winter of 1939–40 on marine fish at Key West, Florida. Copeia, 1941(2):118–9
Gulland, J.A., 1965 Survival of the youngest stages of fish and its relation to year-class strength. Spec.Publ.int.Commn.nthw.Atlant.Fish., (6):363–71
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Fig. 7.1 Pettersen-Nansen (left) and reversing water bottle (right)
Fig. 7.2 Reversing thermometer (pressure protected) and construction of the capillary
Fig. 7.3 Bathythermograph, grid for reading recording and typical recording
Fig. 7.4 Distribution of cod and bottom temperature in Bear Island area in early summer
Fig. 7.5 Distribution of cod and bottom tempoerature in Bear Island area later in summer when warm water has spread eastward
Fig. 7.6 Variations with the current system in yellow-fin catch and zooplankton volume
Fig. 7.7 Dutch modification of Gulf III sampler
Fig. 7.8 Side view of Bongo samplers
Fig. 7.9 Plot of daily egg production against time for haddock egg survey.
This example is taken from Hydrographic Department Professional Paper No. 19, “Temperature and Salinity Measurements using reversing water bottles”, published by the Hydrographic Department, Ministry of Defence (Navy), London, 1967. This publication gives an excellent step-by-step account of how to take such measurements.
T¹ = temperature read from the protected reversing thermometer = 3.40°C
t = temperature read from the auxiliary thermometer = 19.5°C
Vo = degree volume of protected reversing thermometer = 67°C
K = thermal coefficient of protected reversing thermometer = 6100
I = index of correction of protected reversing thermometer = -0.07°C
That is, actual temperature at reversal (Tw) = 3.14°C
T¹u = temperature read from the unprotected reversing thermometer = 13.50°C
tu = temperature read from auxiliary thermometer = 19.7°C
Vo = degree volume of unprotected reversing thermometer = 159°C
K = thermal coefficient of unprotected reversing thermometer = 6300
I = index of correction of unprotected reversing thermometer = -0.15°C
That is, true temperature at reversal plus the effect of temperature is 12.83°C ( = Tu)
To calculate the depth:
d = mean density of sea water at reversal depth = 1.029
Q = pressure coefficient of the unprotected reversing thermometer = 0.00954
The actual depth at which reversal occurred was 993 metres compared with 1000 metres recorded on the meter wheel slowing wire out.
