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provided by Aquatic Commons SPONSORS Hydrobiologists from East, Central and West Africa with substantial support from other African countries, Fishery Scientists in the United States, Canada, U.K., Europe, Soviet Union and Israel. OBSERVATIONS EDITOR SIZE ON BOTTO Dr. J. Okedi, Director, E.A.F.F.R.O., Jinja, Uganda. WITH EMPHASI: EDITORIAL BOARD Mr. M. Abolarin, Co-Manager, Kainji Lake Professor W.B. Banage, Makelere Univer Project, Lagos, Nigeria. sity, Kampala, Uganda. ALMO J. CORDONE Al UNDP(SF)/ FAO Lake Vici Mr. J. Kambona, Chief Fisheries Officer, Mr. R.E. Morris, Director, E.A.M.F.R.O., Freshwater Fisheries Resel Dar es Salaam, Tanzania. Zanzibar. Mr. J. Mubanga, Director, Fisheries Division, Dr. T. Petr, Senior Lecturer in Hydrobiology, Chilanga, Zambia.. Makerere University, Kaflpala, Uganda. Dr. L. Obeng, Director, Institute of Aquatic Professor Mohamed Hyder, University of Biology, Achimota, Ghana. Nairobi, Kenya. Mr. N. Odero, Director ofFisheries, Nairobi, Professor, A. F. De Bont, Universite de Kenya. Kinshasa, Kinshasa XI, Republique Demo Mr. S.N. Semakula, Chief Fisheries Officer, cratique du Zaire Entebbe, Uganda.
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REPRINTS The Lake Victoria Fishe Authors will receive 60 reprints frce of charge. Extra reprints may be procured on cost. ject and the East A PUBLISHER Fisheries Research Orga East African LiteratllIc Bureau. P.O. Box 30022. Nairobi. Kenya. on a co-operative explora ing survey of Lake Vic1 ISSUES objectives of this progran The Journal consists of one volume a year, consisting of two issues with approximately tain the relative abundan eighty pages each. of the major commerci evaluate bottom trawlin~ SUBSCRIPTION fishing technique. The j Annual subscription within East Africa Sh. 35. Outside East Africa. East African Sh. 70, bottom trawling survey h US $ IO,(Xl 1968, and terminated in F
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.a ._,,~ ..
;' POSSIBLE FISH STOCK SIZE ASSESSMENT AND AVAILABLE PRODUCTION SURVEY AS DEVELOPED ON LAKE KARIBA
EUGENE K. BALON UNDP/FAO, Central Fisheries Research Institute, Chilanga, Zambia
This paper is an outline of methods practically useful for the evaluation of ichthyomass, fish abundance, available production and yield in lakes and rivers. Terms and concepts are reviewed, and difficulties stemming from the use of "predetermined" mathematical models are discussed. Sampling with toxicants in blocked-off areas was found to be the most practical method and is described in detail. For the total estimation of ichthyomass the spatial ranges of fish distribution must be determined; the results of echo-sounding surveys for horizontal, vertical, topographical, seasonal and diel fish distribution are given. Some of the most important methods for computing available production are listed and applied to Lake Kariba as an example. In particular, a method based on the balance between the main predator and prey species is reviewed. The ecological production survey concept is finally stressed as applied to multi species fish stocks.
Present address: Department ofZoology, University ofGuelph, Guelph, Ontario, Canada.
"It cannot be over-emphasized that no defined In biology (e.g. MARR 1957; matter how the data are tabulated MAYR 1963). A stock is usually considered (machine, desk calculator or by hand), as a topographical part but not an isolated the tabulations can be no more accurate part of the population (the fish stock of a than the original observations. Erroneously cove is part ofthe whole lake fish population). or sloppily collected data will always be In an ecological sense stock may be equal to erroneous and inaccurate no matter how taxocene (CHODOROWSKI 1959; HUT refined the methods of tabulating or CHINSON 1967; HANDLER 1970) in analyzing the data are. Statistical results which case the stock includes members from can never be better than the data they are a number of species. The terms spawning based on" (LAMBOU 1959). stock, usable stock, fishable stock, adult stock There are different meanings for terms and unit stock (e.g. HOLT 1958; RICKER developed in fish population analysis (HOLT 1958b; GULLAND 1967, 1969) always refer et al. 1959; HOLT 1960). A stock is part of a to some definite part of all the fish inhabiting population and both terms have been clearly a certain area; they must be defined precisely 46 EUGENE K. BALON PI and differentiated by a special adjective. precision (CHAPMAN 1968; NIKOLSKIJ Stock or population sample is different from 1965). Nevertheless it is the simplest method catch sample. Catch and landing are equal so far available to estimate the production as neither to ecological production nor to yield. meant by Ivlev's definition. They represent the part of production and From the fishery management point of yield which is caught and landed by fisher view we may consider the part of production men. The quantitative values of stock or represented by the total elaboration of fish population represent abundance of fish and tissue during any time interval A t by fish ichthyomass (if applied to all fish taxa surviving to the end of A t. [n case the ratio present) or standing crop or standing stock. between the instantaneous growth and morta If applied to single species we may prefer the lity rates (GjZ) is constant and no immigra terms biomass or mass of species under tion or emigration of fish occurs in the study. The meaning of other terms will be evaluated stock, this part also represents clear from the following text (Fig. I). We production in the sense of Ivlev (RICKER also simplify the terms applied in this fish 1958a; NEESS and DUGDALE 1959). This production survey, as opposed to rather well production value ca~ be estimated "by defined uses by some ecologists. multiplying individual weight increases by Parallel with the development of the fish the mean number of fish present in each production concept new terms were invented, interval of interest" (CHAPMAN 1967). defined and modified by various adjectives It was used by BURMAKIN and ZHAKOV such as: actual, whole, gross, net, global, (1961), HOLCIK (I970a, I970b) and by c primary, potential, real, total, secondary, myself. o community, effective, etc. Such proliferation "Production" here is akin to the "net .-...., may finally lead to misunderstanding and production" of some ecologists and would -~ misinterpretation of the production concept, equal the "available production" of a a.::::s' if the latter is used without explanation. population, untouched by death, predation or o Recwtly the most correct interpretation of exploitation during the specified time interval. D. fish production is that "Production is defined Normally it is a part of Ivlev's production. as the total elaboration of fish tissue during The latter incorporates also the various any time interval At, including what is increments of the tissue of fish that have formed by individuals that do not survive died from natural causes, or have been to the end of A t (IVLEV 1945)" or "regard eaten by predators, or caught by fishermen at less of the ultimate fate of that tissue" different times within the time interval of (CHAPMAN 1967, 1968). It is formulated in interest. The available production is that simplified from by RICKER (1946): mass of fish which may be taken through P = GB (I) increased exploitation in addition to existing Here P is production per unit time, G is a levels. Thus yield can be interpreted as coefficient of biomass growth, and B is the being a harvestable part of production. mean biomass during the unit time interval. More work has been done on ocean or Based on this formula the original numerical anadromous fish population analysis with estimation of production proposed by Ricker hypothetical mathematical models (BARA was later developed into a simple graphical NOV 1918; DERZHAVIN 1922; RICKER method by ALLEN (1951). Although many 1954, 1958a, BEVERTON and HOLT 1957; authors have used this method of estimating DOl 1959; HEMPEL and SAHRHAGE fish production, it is also subject to some 1961; GULLAND 1962, 1965, 1968a, 1968b, criticism on the question of realism and 1969; IVANOV 1961; LARKIN and HOUS- FISH STOCK. SIZE ASSESSMENT AND AVAILABLE PRODUCTION SURVEY 47
NIKOLSKIJ mplest method : production as nent point of : of production lration of fish ~ l to al A t by fish Cl. I case the ratio Ql ::0 'fth and morta ...to VI Ql d no immigra > l- occurs in the to I - Iso represents lev (RICKER ~E 1959). This :stimated "by increases by 'esent in each IMAN 1967). / mdZHAKOV ~ 170b) and by c: c Ql E o Ql .- I to the "net ..., .:::t. U c: -ci ,ts and would 1'3 u Ql ;:l'" :ction" of a -; - B » bIl Q. Vl 1 0 1, predation or "0 o c l time interval. 0. '§ 's production. 0: E o I- "0 the various III C III U c
L.. ~ ll) HOLT 1957; U ro .s::; Ii ;AHRHAGE a> a. r-- a::::: Vl 1968a, 1968b, ..; rand HOUS- ~ 48 EUGENE K. BALON
TON 1964; PELLA and TOMLINSON 4. Completion of field data collection. population an 1969; FOX ]970) than has been done on 5. Tests of models thought to be appropriate. the total stock fresh water fish populations. The reason 6. Formulation of theoretical generalization representative for this is that the ocean environment is and construction of models that will yield to sample fish more difficult to sample than a smaller predictions. devices used to fresh water area (REGIER 1970a). The The rigorous purist would argue that a they are higl open water habitat of a large lake and that model is to be constructed separately for history of thei of an ocean are similar, however, and every case of data collection and in respect practical if use sampling methods for population analysis to the specific situation. If not, the influence gear's selectivit used in ocean fishery research are equally of predetermined mathematical models on requires speci. valid for shoaling open water fish without data collection and resulting calculation can undertaken for respect to salinity or the size of environment. lead to quite erroneous conclusions; empiric Some devices Furthermore, methods of fresh water fish real data cannot be selected ultimately, specific cases population al)alysis stressed here are fully but must be collected. Similar views have estimates, e.g. applicable for lagoons, reefs, or inshore been expressed by·VARLEY, PEARSALL, single species ~ marine populations (BALON and SENES GRAHAM (1962), NIKOLSKIJ (1965), The capture- 1967; HOLCJK 1970b). KNIGHT (1968) and others. Furthermore, We may identify the role of models and no model constructed in advance is able are biased in t~ the stock (BAS distinguish between roles as follows. to predict species inter-relationships and 1969) selected Methodological models are those which such model construction for a single or a few DeLury and 0 help to programme the research and state equalized species is the source of unrealistic MAN 1948) in advance the number of samples required results and erroneous conclusions (NIKOL smaller stream 1 for certain variability coefficients and confi SKU 1964; HOLCIK; 1970a). by gear selecti dence limit's. These models are clearly useful. Difficulties with overly simplified mathe production by Models that seek to define the interrelation matical models may be illustrated by the the latter meth< ships of a series of variables rather precisely, trend in estimates of maximum sustained data for prac as opposed to general semi-quantitative yield of the Peruvian anchoveta, Cetengrau/is (HOLClK I' research concepts, should not be assumed mysticetus as more data become available Combined wit uncritically (VARLEY'S comment, 1962), (SCHAEFER 1967, 1970). Alternative esti (BREDER 19 particularly in work which tries to generalize mates could have been obtained employing some cases re and predict theoretical conclusions. A model simple calculations and evaluations ofempiric The Lake 1< must not be accepted and applied before the variables from the fish biology, e.g. relation based upon the collection of sufficient data to indicate its of mortality rates (man and birds) to fecun more, it has t degree of realism. The steps might be formu dity and growth increases of the exploited important asp lated as follows: anchoveta population. But this may mean development iI 1 Field data collection (measurements) on more field data sampling. "It is clear that our evaluation of tl variables thought to be closely related to mathematical models are not yet adequate a well planned the question under consideration. for the complexity of natural systems" was yield of fish (I 2. Summary calculations (perhaps with the the conclusion of a panel of ecologists under would be not] help of computers, e.g. GERKING 1965; A. D. Hasler's chairmanship. It continued as processing tee ALLEN 1966; SWINGLE and SWINGLE follows: "Computer simulation is a promising size and com] ]966; LEEPER, STERN and LAMBOD technique for understanding such matters, scope of the 1958; LAMBOU 1959, recognizing that but computer models need both adequate systems, etc. D computers cannot improve bad data or formulation, or programming, and higher and fishing ef replace missing data). quality data ..." (HANDLER 1970). estimates rela1 3. Construction of working models. The usual starting point in a quantitative nor its relati<
D FISH STOCK SIZE ASSESSMENT AND AVAILABLE PRODUCTION SURVEY 49 ta collection. population analysis is to count or estimate potential catches early in the development It to be appropriate. process. Available mathematical models pro tical generalization the total stock or, more commonly, to obtain representative samples of it. If devices used posing to do so may yield only very approxi ldels that will yield to sample fish for such purpose are the same mate estimates applicably only to dominant species. uld argue that a devices used to capture fish for food or sport, they are highly selective and the whole It was decided that the main data to be ted separately for collected were: relative abundance of fish ion and in respect history of their use proves them to be un and relative biomass by species, relation f not, the influence practical if used uncritically. To estimate the gear's selectivity to hmits of usable precision ships and balance between various species, natical models on age composition and mortality rates of ing calculation can requires special studies, that have been various species, and annual weight growth mclusions; empiric undertaken for only a few species and gears. Some devices can be used successfully in increments. ~lected ultimately, ,imilar views have specific cases without explicit selectivity One of the most adaptable survey methods ,EY, PEARSALL, estimates, e.g. use of the purse seine with for shallow waters is sampling by eradicants (OLSKIJ (1965), single species stocks with schooling habits. in blocked-off areas. Since TARZWELL's lers. Furthermore, The capture-mark-recapture methods also (1940) first trials in 1939 in the Tennessee are biased in that they evaluate only part of Valley, and ESCHMEYER'S (1943) pro I advance is able .relationships and the stock (BASTL, HOLCIK and KRUPKA gramme, the method has been greatly lr a single or a few 1969) selected by gear in use. The Leslie improved and has been in general use in DeLury and other similar methods (CHAP freshwater reservoir population studies urce of unrealistic elusions (NIKOL MAN 1948) are applicable mainly for (CARTER 1958; CHANCE 1958); and 195~; lOa). smaller stream habitats and are also affected LAMBOU and STERN SWINGLE simplified mathe by gear selectivity. In some cases where 1958; SURBER 1959). Somr experiments illustrated by the production by juveniles can be ignored, have also been started in the sea (HIATT the latter method can yield sufficiently good iXimum sustained and STRASBURG 1960; RANDALL 1963). data for practical management purposes )veta, Cetengraulis After this method was used for the first (HOLClK 1970a; MARTEN 1970). time in Africa on Lake Kariba, the results become available Combined with juvenile capture devices . Alternative esti of an experiment on the Douglas Reservoir )tained employing (BREDER 1960) this method could in in the U.S.A. were published which discuss some cases replace chemical sampling. luations ofempiric the significance of such sampling for an The Lake Kariba programme has been logy, e.g. relation entire lake (HAYNE, HALL and NICHOLS based upon the above ~onsiderations. Futher d birds) to fecun- 1967). The Lake Kariba work has utilized more, it has been assumed that the most of the exploited experience with different toxicants in popula important aspect of fishery research and t this may mean tion studies on the Elbe and Danube riverine development in early stages should be the lakes of Czechoslovakia (OLIVA 1955, 1960; It is clear that our evaluation of the protein reserves, employing not yet adequate BALON 1963, 1966a, 1966b, I966c, 1967, a well planned study of the production and lral systems" was 1968b); and this experience was adapted yield of fish (HOLT 1967). Otherwise there f ecologists under to African conditions. would be nothing to guide the gear and p. It continued as The selection of the method of application processing technologists in planning the ion is a promising depends upon the topography and maturity size and composition of fish catches, the of the reservoir treated. If the lake is shallow 19 such matters, scope of the processing plants, marketing d both adequate and old with well developed open water systems, etc. Data on fish catches or landings ling, and higher ichthyofauna, the sampling area should be ER 1970). and fishing effort statistics cannot provide established away from the shore by blocking estimates related to ecological production in a quantitative off square or circular areas. This should nor its relationship to the magnitude of also be the case on most natural lakes, such
D 50 EUGENE K. BALON as Lake Mweru. Some deep lakes, however, about 2 km. Around islands with steep shores have more or less separate inshore and open fish were found only in coves or in strips water fish fauna. This is the case of Lake a few meters wide close to shore. In areas Tanganyika, Lake Victoria, etc. Here blocked of the lake shallower than 15 m but far away offshore areas as well as coves should be from shore, fish were recorded only occasion sampled. At Lake Kabira our original ally, as was the case in the deeper open water presumption was that the entire fish popula areas. These observations are partly con tion was of riverine origin and that there firmed by COKE'S (1968) results with had not yet been time to develop an open gillnets set in different depths. water fish fauna. After testing this by echo It is estimated, therefore, that the main sounding and giIlnets set in different are-as average area of Lake Kariba inhabited by it became evident that cove and inshore fish in 1968 and 1969 was 33,422 ha, which sampling should be representative enough. corresponded to the ~20 m depth area at a Stream and river sampling is simpler between normal water level of 485 m (COCHE 1968, two blocking nets and the chemical can be and personal communication). This repre neutralized downstream. sents 6.2 % of the total lake surface, 5,364 2 km • ECHO-SOUNDING SURVEY FOR RANGES 'OF FISH DISTRIBUTION SAMPLING METHOD To test whether the fish fauna was essenti Lake Kariba, which has numerous bays ally an inshore one, and to find ranges of and coves along the entire shoreline and fish distribution, a large scale echo-sounding which presents a fish distribution as men survey was begun on Lake Kariba. With tined above, is one of the best impound few exceptions, records from open water ments for cove and shore-line sampling. areas showed practically no fish. Closer to On the average this method should provide shore, in depths less than 30 m, the abundance representative samples of the whole fish Fig 2. Setting of ~ of fish appeared to increase, the maximum population. In a three year period several of Chikanka IshlJ being at 5 m during daytime and about 10m 0.5 to 5 ha coves, stream estuari.es and at night. This generalization was equally three-sided blocked shore-line areas were iveness, harmlc valid for cleared bottom areas and for selected and treated. The blocking nets of cost may be ta areas with submerged trees. Without ex 8 mm mesh were set at different hours of 1966; HERR, ception the records showed that the main the day (Fig. 2). This was done to avoid 1967; POWEll fish distribution was limited to shallow water fencing off the same concentration of fish concentrated ! close to shore and mainly to bays and coves. in the cove or along the shore-line and to boat through Fish seemed to concentrate in the evening to avoid fencing off the highest evening original 5 gallo along the shoreline and especially in bays concentration each time. The blocking nets outboard engi and coves, being distributed in remarkably should be, and usually were, checked by a pump through high density from the surface to 15 m depth diver, especially along the bottom leadrope. similar to tha with maximum density at 6 to 7 m depth. Emulsifiable rotenone at 5 %concentration, water volume Only at places shallower than 15 m was the and 75 % emulsifiable toxaphene sold locally estimated rOUI evening concentration of fish distributed as a cattle dip (firm name: Altik), were used necessary amo from the surface to the bottom. At dawn in (HEMPHILL 1954; HOOPER and lethal dose ( the same places the major concentrations GRZENDA 1957). The kind of eradicant 1966) was exe, offish had changed to a less dense distribution used is, however, not important and avail to kill all fish extending further offshore to a distance of ability (MALAISSE 1969) as well as effect- fish on the bo, FISH STOCK SIZE ASSESSMENT AND AVAILABLE PRODUCTION SURVEY 51
with steep shores aves or in strips the third or fourth day. In present tempera ) shore. [n areas ture conditions, the fish which appeared later 5 m but far away had become so rotten that it was difficult ed only occasion to lift and measure them. Fast killing was leeper open water also essential in order to limit the bias are partly con created by scavenging by birds, mammals, i8) results with and occasionally crocodiles and monitor 1S. lizards. An average of 200 litres of Altik ~, that the main was distributed per hectare or per 20,000 m3 iba inhabited by water which represented 7 ppm toxaphene. 33,422 ha, which Even so, some Clarias were dying only the :1 depth area at a third day. The overdosing has a limited n (CaCHE 1968, effect outside the cove. One month later the Ion). This repre treated area was inhabited again and in one ke surface, 5,364 recorded case had a higher abundance and ma&s of fish (although by a smaller number of species) than it had prior to treatment. Fish 'Aere collected in th,e deepest part by dip nets and along the sh9reline by hand. • numerous bays All fishes including the smallest juvenile:> re shoreline and were collected. On some of the larger fish 'ibution as men the necessary determinations and me.:tsure ~ best impound ments were made on the spot. The majority :e-line sampling. were preserved in 4-10% formalin for later i should provide treatment in the laboratory. the whole fish Fig 2. Setting of a double blocking net in the mouth Before the end of the sampling and the r period several of Chikanka Island cove at Lake Kariba. lifting of the blocking net, the entire cove n estuaries and was measured using plane table and sound line areas were iveness, harmlessness to environment, and ings (first by lead-and-line, later by echo )Iocking nets of cost may be taken into account (LENNON sounding). A detailed bathymetric map was fferent hours of 1966; HERR, GRESELIN and CHAPPEL made. The area, length of shoreline, maxi done to avoid 1967; POWERS and BOWES 1967). The mum and average depths and volume of :ntration of fish concentrated solution was distributed by water were then calculated exactly. Finally lore-line and to boat through a 5 mm hole made in the the blocked cove bottom was checked by highest evening original 5 gallon drum and was mixed by the a diver for residual fish which were counted Le blocking nets outboard engine propeller or by motor for predetermined strips of bottom area. ~, checked by a pump through perforated hose~in a manner lttom leadrope. similar to that of CHANCE (1958). The PR[MARY LABORATORY PROCESS !,; concentration, water volume within the blocked area was ING OF FIELD SAMPLES lene sold locally estimated roughly in order to calculate the lltik), were used necessary amount of toxicant. The minimum The majority of fish sampled, and parti 'DOPER and lethal dose (HENEGAR 1966; MAHDI cularly the smallest ones, were transported ld of eradicant 1966) was exceeded because it was essential to the laboratory in formalin solution. °tant and avail to kill all fish at once and get the residual Every specimen was identified and standard s well as effect- fish on the bottom to surface not later than length recorded. The weight and the total 52 EUGENE K. BALON
.<: length were also recorded for the first few sented the whole lake fish population, o hundred fish of every species and correction corrections had to be applied for all possible ~
Table I. Conversion table of Hydrocynus viffafus observed mean standard length, calculated total length and geometric intercepted mean weights.
Standard Total Weight " Standard Total Weght \ Standard Total Weight I Standard Total Weight I Standard Total Weight length in \ length length in length length in length length in length length in length "'l g inmm inmm g , inmm inmm g mmm in mm g in mm in mm g inmm inmm I iii ::c --- 575 4600 195 74 295 377 530 435 548 1900 724 [fJ 15 18 0.045 155 -l 20 24 0.11 160 .202 80 300 378 555 440 554 1950 580 731 4800 0 2050 585 737 4850 ("") 25 30 0.23 165 208 86 305 384 580 445 561 ~ 567 2100 590 743 5000 30 36 0.40 170 214 94 310 390 620 450 [fJ 2200 595 750 5200 35 42 0.66 175 221 105 315 397 650 455 573 N 5400 t'!l 40 48 1.0 180 227 110 320 403 675 460 579 2300 600 756 2350 605 762 5500 » 45 54 1.3 185 233 125 325 409 730 465 586 [fJ 768 5600 [fJ 50 60 2.0 190 239 130 330 416 740 470 592 2400 610 t'!l 615 775 5700 [fJ 55 66 2.6 195 246 142 335 422 790 475 598 2500 [fJ 605 2600 620 781 5800 s:: 60 72 3.5 200 252 155 340 428 820 480 m 2700 625 787 6000 Z 65 78 4.5 205 258 172 345 435 850 485 611 -l 2800 630 794 6200 70 84 5.6 210 265 180 350 441 900 490 617 » 2850 635 800 6350 75 91 7.2 215 272 198 355 447 950 495 624 z 806 6500 t:i 80 97 8.8 220 278 210 360 453 1030 500 630 2950 640 636 3100 645 813 6600 » 85 103 10.2 225 285 228 365 460 1080 505 < 466 1120 510 642 3200 650 819 6800 » 90 109 12.5 230 291 240 370 ~ 655 825 7000 95 121 14.6 235 297 260 375 472 1170 515 649 3250 » 660 831 7200 o:l 100 128 17.5 240 304 280 380 479 1220 520 655 3400 t'"' 665 838 7400 t'!l 105 134 21.0 245 310 298 385 485 1270 525 661 3500 3600 670 844 7600 ~ 110 140 24.0 250 316 310 390 491 1330 530 668 3650 675 850 7700 S 115 147 28.5 255 321 335 395 498 1400 535 674 v 3800 680 857 7800 c: 120 153 30 260 327 350 400 504 1440 540 680 n 3900 685 863 7900 -l 125 160 36 265 334 370 405 510 1500 545 687 690 869 8000 C 130 166 40 270 340 400 410 516 1550 550 693 4000 z 275 346 425 415 523 1650 555 699 4100 695 876 8200 [fJ 135 170 44 c: ~29 560 705 4250 700 882 8500 140 176 52 280 353 450 420 1700 , lIl' 712 4400 705 888 8800 < 145 183 56 285 359 475 425 535 1760 565 m 718 4500 710 894 9000 >< 150 189 62 290 365 500 430 542 1830 570 I
VI w
ittrIi t . ~;~W:'.:l'-'r Of .. ,. I V> Table 2. Abundance of fish stock and ichthyomass m cove of Lake Kariba Chikanka Island (No. 16-surface 12100 m', shore line 680m) on 21 June 1968 ~ Standard Average Relative Relative
length length Number Number Number Average Total ·1Weights I Weights tt1 of 1 sp. of fish fish weight weight , in kg in kg c:: 28 Species ranges of of Cl tt1 specimens I of 1 sp. in g I: inmm inmm per I ha per 100m per ha per 100m Z shore line in g shore line t'T1 I I . ABUNDANCE ICHTHYOMASS
1;0'" -~;;;;i: ~--I- ;l> rendalli gefuensis Thys, 1964 II --37---4-25- 582 481 85 177 103281 85.35 15.19 t"' C Clarias gariepinus (Burchell, 1822) 30-710 334 'I 89 73 13 812 72246 59.70 10.62 Z Tilapia mossan;/ ica mortimeri Trewavas, . 1966 35-299 101 479 396 70 78 37350 30.86 5.49 Hydrocynus viUatus Castelnau, 1861 48-420 131 296 245 43 126 37264 30.79 5.48 Marcusenius discorhynchus (Peters, 1852) 33-185 125 329 272 48 42 13706 11.32 2.01 Heterobranchus longifilis Val, 1840 103-'l63 400 7 6 I 1927 13487 11.14 1.98 Sargochromis codril1gtoni (Boulenger, 1908)1 71-270 157 45 37 7 205 9247 7.64 1.36 Synodontis zambezensis Peters, 1852 I 26-278 158 60 49 9 129 7721 6.38 1.13 Distichodus schenga Peters, 1852 63-230 191 28 23 4 227 6367 5.26 0.94 Labeo altivelis Peters, 1852 57-315 210 10 8 1 569 5690 4."10 0.84 Alestes lateralis (Boulenger, 1900) 22-60 40 1759 1454 259 1.3 2271 1.87 0.33 Haplochromis carlouae (Boulenger, 1905) 200-250 225 4 3 0.5 542 2167 1.78 0.32 Malapterurus electricus (Gmelin, 1789) 380 380 1 0.8 0.1 1272 1272 1.05 0.19 Serranochromis macrocephalus (Boulenger, 1899) 162-225 189 4 3 0.5 260 1039 0.85 0.15
I 34-121 94 34 28 5 21 I 711 0.58 0.10 60-150 103 II 9 2 45 495 0.40 0.07 18-179 74 30 25 4 13 391 0.32 0.06 24-88 42 165 136 24 2.3 386 0.31 0.06 35-58 47 79 65 12 2.5 199 0.16 0.03
165-191 178 2 2 0.3 • 100 201 0.16 0.03 75-139 100 13 11 2 15 190 I 0.15 0.03 36-53 44 44 36 6 1.9 83 0.07 0.01 I 24-58 I 42 22 18 3 2.8 61 0.05 0.01 109 109 1 0.8 0.1 64 64 0.05 0.009 1 28-35 31 52 43 8 0.7 38 0.03 0.005 92 92 1 0.8 0.1 20 20 0.02 0.003 70 70 1 0.8 0.1 9.5 9.5 0.008 0.001 53I 53 1 0.8 0.1 3.4 I 3.4 0.003 0.0005 -- i I~~- Total I i 4149 3427 I 609 I 261 46
~ .. --".
~. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ cT Pi" ::tl 0 c ti) g. (" t/) g" 8" !:ii' 8 ...,'E!;f c < c. '" -ri' ...... < ~ ~ -e;,- ~rt 2. ><:;; "Ij ~~~.gG ~3 ~.?"::rr;r g" g" :;. 3 M g" ~ ~.g" rt 2" 5"~.~.g- ~. g 0 g ~ ~ <: .... -Oo.C C _0 ~ Z"d < Il' -
FISH STOCK SIZE ASSESSMENT AND AVAILABLE PRODUCTION SURVEY 55
Table 3. Number of cove samples necessary to produce confidence limits ± 20% of the mean with given oo888§§ I values of coefficient of variability ~ 6666666 I Coefficient of variability Statistical confidence level used I 00 80% 95% 99% 00000r-- ." ." ""' N 8 8""' 6666666 0.20 3 7 11 0.55 14 32 55 "'~I 1.00 42 100 169 ~~~~~O\M I ~ I~ various relationships and "balance" between production and yield and these indices become ____I: the various species, e.g. by applying the well established, much labour can be eliminated ratios FIC, Y/C, AT and the values AF, and the data needed for initial research and 0\1Xl r--- "''''''" "':N~OOo\M IF' SF and E as proposed by SWINGLE development obtained and applied more '0 N (1950). Together with echo-sounding records rapidly and less expensively. showing vertical and horizontal fish distribu tion, they can lead directly to conclusions AGE COMPOSITION, GROWTH AND valuable for fishery management and direct MORTALITY RATES attention to locations and species sizes to be fished. SWINGLE (1961) has proposed the eq For estimating age composition, growth 0Cl. 0Cl. 0Cl..0Cl '0000,,",000 uation Y= l51.I +0.638AT +6.2l2FIC+2.79 and mortality rates, it is necessary to know ,,",- ~ SF (2) for the estimation of ichthyomass the correct time of the annulus appearance (standingcrop) in some reservoirs ofthe U. S.A. (BERG and GRIMALDI 1967). In north The value Y expresses pounds per acre, AT the temperate regions many authors find the ~N_N _ ~N ." percentage of the total weight of a fish stock annulus to be formed in April to July, less composed of fish of harvestable size, Fie often in March to August. Findings in is the ratio of the total weight of all forage subtropical regions to date have pointed to fishes to the total weight of all piscivorous a time of annulus formation between fishes in a stock, and SF the percentage of December and April (EL ZARKA 1961; total weight of forage species composed of VAN RENSBURG 1966a; BALON 1971c), fish available to the average-size piscivorous after the rainy season had gotten well under fish. So far the estimates of ichthyomass way and when the main spawning period derived from the mentioned equation have of many species was observed (JOHNELS been in reasonably good agreement with the 1952). In tropical regions with two rainy sampling averages feund by SWINGLE and seasons and repeated spawning in the same SWINGLE (1968). But more data and tests year the situation seems- to be more com are needed to estimate a corresponding plicated. 1 suspect that there will be one equation for Zambia or other waters. Further main rainy season and one main spawning elaboration of the concepts involved period, to which the annulus appearance (SWINGLE 1968) would appear to be can be related (VAN SOMEREN 1950, useful. 1952). Repeated spawning is also known in Further tests of the general validity of fish oftemperate regions. The time of annulus our direct estimates can be sought by applying formation in all regions seems to be connected RYDER's (1965) morpho-edaphic index or with the main spawning period and with similar approaches which may be elaborated the main vegetation growth period. later (JENKINS 1968). If the relationship Many authors have found that the annulus between dirf,ct estimates of ichthyomass, appearance is delayed as age increases 56 EUGENE K. BALON
(ZAWISZA 19 '0 cases, therefor, '0 ,.,.,+ '0 ,.,.,+ ~ fish are not I "0 E + ... ~ " ,.. manner to the ... ~ E '" ~ '" ~ growth nomen ,.,., + ..,J.'" _~ N calendar year, __I check careful1y
"0 annulus appeal '0 groups should + '0 + ~ N '0 N + ::: ~ E" N for example G N N E ~ :> fish within the + ,.,.,'? + N spawning fish I means only half tropical late Spi '0 • tll) "0 for some early e: ~ + region it may 01 +1"2~ ~ ~+ E" E next age-groups\ - >, • ~ <:5 <:5 N include a full ~ o age-group pI from a Decemb ... ~ few weeks later <.!..'" some authors I :l o o 0
(ZAWISZA 1951; BALON 1955). In most that "There is first of all the presence of cases, therefore, the growing seasons of different kinds of marks which are not fish are not related in the most simple necessarily present on all scales of the same manner to the calendar year. To bring the fish, nor on all scales of the same species. growth nomenclature of fish under the Secondly, the time of the year that the marks calendar year, it is always necessary to are laid down may be different from one check carefully the time of capture with species to another, from region to region annulus appearance (Table 4). The age for the same species, and variable from one groups should agree with calendar years; year to another. There are nearly as many for example Group 0 normally includes explanations proposed as cases studied. The fish within the year of hatching. For late main factor involved may be one or a spawning fish of the temperate zone this combination of the following: drop in means only half a year, for brown trout and temperature, decrease in available food, tropical late spawners an entire year, and photoperiod, reproduction, migration, for some early spawners of the tropical internal physiological rhythm, etc." But still region it may only mean a few weeks. The I have the impression that all this is valid for next age-groups I, II, Ill, IV, etc., always both cold and warm regions. We encounter include a full calendar year. To avoid the the problem of false annuli in the American o age-group paradox when fish juveniles or European pikes as well as in the sub from a December spawn are graduated a tropical tiger-fish. They are, however, always few weeks later from 0 to the I age-group, differentiable by their singular appearance some authors prefer to use the symbols and especially by their incomplete course 0+, 1+, 2+, 3+, 4+ etc. in age-group as described in details elsewhere (CHUGU designation, according to the number of NOVA 1952, 1959; BALON 1963). annuli present. Therefore, J believe it more reasonable to For the time being no unbiased evidence consider the marks observed on tropical fish is available to oppose the objections made scales as similar to the annuli .of temperate by DE BONT (1967) on the validity of the fish and of possible use as year marks if annuli, especially with tropical fish, so it there is no strong evidt;nce to the contrary. is necessary to rely on experience with the In order to speculate about the intervals aging of fish in temperate zones (BALON in organism development (BALON 1960, 1953, 1955, 1957, 1962a, 1962b, 1965). 1971a) it can be assumed that the combination Moreover, it is necessMy to differentiate the of the quoted factors in tropical regions also real tropical areas where the formation of affects the intervals of seasonal development annuli is still questionable (BERTRAM, and that also here the "planetary yearly BORLEY and TREWAVAS 1942; DE system" is recorded on the structure of the BONT 1950; JOHNELS 1952; HOLDEN scales. As it was correctly remarked by 1955; GARROD 1959; FRYER 1961; DE BONT (1967) not even in temperate OKEDI 1969) from the sub-tropical areas regions do we yet know which of the factors with more pronounced seasons where evi or which combination of them causes the dently annuli are formed (LOWE 1952; phenomenon of the annulus formation. JENSEN 1957; KOURA and EL BOLOCK It is obvious that the "complete cessation of 1958; EL BOLOCK and KOURA 1960, growth during winter" cannot be generalized 1961; EL-ZARKA 1961; VAN RENSBURG as the only reason of annulus formation 1966b) as in temperate regions. Consequently, (BERG and GRIMALDl 1967). I can only repeat with DE BONT (1967) In the Lake Kariba studies, the key scales 58 EUGENE K. BALON
(Fig. 3) or vertl length-frequency age determinatil been adjusted t( mens present in ,L ~,.-...... ,,- MORMYRIDAE - - --.. / ...... (Table 5) were u _~ -..z_/ ?J /0 .' ~ ~. / ) ,,/1)--..J _ - __ .... -<_~7' Morrr.yrops ---- ') -./ \\-,;--~~\ Table 5. Age com~ \J ! ,,./ -~ (YPhomyrus relation by age dete
Gnathonemus Standard ~ ------~-~~ {"( 0 --- -).------length 5---·-~ groups in n -~vr:s~,j Mormyrus mm
41- 50 3 • 51- 60 26 CYPRINIDAE 61- 70 30 71- 80 23 /8 )-- _ --l~ Barbus 81- 90 27 /'/ ---- 91-100 18 ~~-=---•.--7>. _~." Labeo !~/,\ 101-110 23 111-120 24 121-130 17 131-140 7 4=~ CHARACIDAE 141-150 2 -... 151-160 5 E~),,,;_~ - --:4',\ Hydrocyntts 161-170 6 -~'\'~~ A/estes 171-180 2 '-" 181-190 1 IV 191-200 4 201-210 3 /- 211-220 1 221-230 ~~.. _~ 231-240 I 3 CITHARIN IDAE .) 241-250 251-260 /<\ Distichodus \, \', __ <''' I 261-270 !\! "': 271-280 \j "'-.J 281-290 291-300 CICHLfDAE 301-310 "'.-/-~ -- - // - ----<::. /~/' 311-320 Ti/apia 321-330 331-340 (:O \- - :>".-.(7=-] SargochromlS 341-350 ~~ .- ~~--= Serranochromis 351-360 Haplochromis 381-390 411-420 501-510 511-520 I ~~I~
Fig 3. Key scales sampling scheme of Lake Kariba fishes. FISH STOCK SIZE ASSESSMENT AND AVAILABLE PRODUCTION SURVEY 59
(Fig. 3) or vertebrae sampled following the rates. The instantaneous rate of total length-frequency distribution were used for mortality (Z) can be calculated according age determination. The latter, after having to RICKER 1958a); the seasonal total been adjusted to the total number of speci mortality rate (A) and the survival rate (S) mens present in the sample of each species have to be determined for the given Z (Table 5) were used for calculating mortality (i for RICKER) from his tables ofexponential
Table 5. Age composition of Hydrocynus vittatus, Chikanka Cove sampled stock. Calculated from the same relation by age determined part (n = 160)
Standard I o I IV VI II I ~n length I groups in n % n % n % n % n % mm
41- 50 3 1.0 3 51- 60 26 8.6 26 61- 70 30 10.0 30 71- 80 23 7.6 23 81- 90 27 9.0 27 91-100 18 6.0 18 101-110 23 7.6 23 111-120 24 I 8.0 24 121-130 17 5.6 17 131-140 7 2.3 I 7 E 141-150 2 0.7 2 151-160 5 1.6 5 161-170 6 2.0 I 6 171-180 2 0.7 2 181-190 1 0.3 I 191-200 4 1.3 4 201-210 3 1.0 3 211-220 1 0.3 4 1.3 5 221-230 II 3.6 11 ,E 231-240 3 1.0 7 2.3 10 241-250 • 4 1.3 4 251-260 4 1.3 5 1.6 9 261-270 2 0.7 3 1.0 5 271-280 2 0.7 3 1.0 2 0.7 7 281-290 3 1.0 3 1.0 I 6 291-300 5 1.6 5
301-310 I 2 0.7 I' 2 311-320 I 0.3 I I I 321-330 2 0.7 2 331-340 3 1.0 3 341-350 I 0.3 I 0.3 I 0.3 3 351-360 2 0.7 2
381-390 1 0.3 I 1 411--420 1 0.3 1
501-510 I 1 I 0.3 I 0.3 2 511-520 I 0.3 1 ~~I~I~I~-12---;-1-8-1~1-4-I-I-I~--;-~~~ 60 EUGENE K. BALON functions and derivatives. The estimate of seasonal and yearly rhythm of scale growth mean seasonal survival survival rate CS-) in relation to the body growth of fish must should be calculated according to be empirically determined" (BALON 1963). (5 Any method which follows this view is, .... ~ n2+n3+n4+n5+ ... +nj S= ~ therefore, correct (e.g. MONASTYRSKY ~ n1 +n2+n3+n4+ ... +n _ i 1 1926; SEGERSTRALE 1933; HILE 1941, ~ where there are i age groups and nj fish in ~ 1970; RICKER and LAGLER 1942; VOVK :::: the ith group. The estimate of mean seasonal =- 1956). However, the effect of ~ize selective total mortality (A) can be derived from :xi I:::: mortality bias has to be taken into account ... ;;. Ricker's table and the mean daily rate of ..2 (RICKER 1969). g Z mortality is 365- (HOLCJK 1966). (4) For the calculation of production we need & I;> l"l to know the annual increment of growth for til t: S and A, however, can be directly read from all species. As the determination of age is the semilogarithmic diagram of the stock part of the gro\fth study it can be used to E curves (BALON 1971c, Fig. 9, BALON establish the age compostition and the ~ [I;;. I 1971d, Fig. 14) and estimated by some kind mortality rates of each studied taxa stock ~ Q) I- of mechanical means from the resulting (Table 6). This has provided some very ~ slope if a certain relationship of units on interesting results. The mean seasonal total '":::I ordinate and abscissa is maintained. This mortality rate for tiger-fish was calculated o~ I;;.~ approach is arithmetically well elaborated as 71 % and the mean seasonal survival by REGIER's (1962) models of mortality rate as 29 %. Using rough mortality values ~ .5 coefficieJ,lts under various fishing pressures. it was calculated that in order to produce "0 Howeve~, empirical data so far obtained for one 6-year-old tiger-fish weighing more ;
different population curves present a propor than 2 kg, 4,000 specimens were required in § 0\ tional distribution of mortality in annual the first growth season, weighing 640 kg '"Q) I::::.... ~ intervals. It can be infLrred that the coefficient total (Table 7). In the same way it was C; of natural mortality is not constant, as assu determined how many juveniles were neces .~ t med by REGIER (1962) but changing with sary to produce one first mature female, :::I every change of fishing mortality as some or male, etc. There seems to be no need to ,-..'" $ kind of density dependant, etc. feedback. It use mortality rates for correction of pro ~ I=: ...... explains the proportional total mortality duction sensu lato. It was proved that in ~ rates within successive years in a "stable" normal conditions the recruitment and o population, in spite of varying actual catch mortality are in balance (OLIVA 1960; :.; '§ (ROBSON and CHAPMAN 1961). ROBSON and REGIER 1968; HOLCIK E Concerning the methods of back calcula 1970a); and we meant by 'normal' when C; t'l'" tion of growth of fish from annuli measure the population's adaptive mechanisms are § C; not yet "pushed beyond the range of their c:: ments on scale and bones it can be repeated o that "Each method is accurate which effectiveness and systems will become un ~ ~ calculates corrections based on one of the stable and will begin to transform" (REGIER Ii 0 ~ objectively determined relations of body 1970a). The calculation of production dates, .... length to the observed scale-radius measure however, should take into consideration :~-I ment. It is impossible to make methods more the time difference between the annulus d E'o __ exact because the chief cause of error, as formation and sampling date (HOLCIK ~ indicated previously, lies in the subjective personal communication February 18, 1971). OIl '"Co I inaccuracy of measurements. There is, hence, We cannot use the increments from back -< :l J ~ ~ "C only a single further course to follow; the calculated growth rates which are related to :0 ~ ~ ~ -( I « -- r;n :T' ~ 0 ....., ~ - d S ;::; ~ ~ - ~,U"f ;:r ~"O ~;:r ~ ~ ':J ~ ~ ~::r!"l n ...' ..... 0 -::...... _. '" 0" G :;, '<...... g. ~ ....g-::I 0 a 0 :::.: (l> S • 'D'-" n ~ rllrll(JQrllrllrllO"'~rllrllO".n~s:: ~O~~O"'~~O ~ Vi !"l rll _ n g- S !"l g. 0 00 to"' E. 0 0". rll ~ '" ::r nl ciCi·...... g !"l ~ 0. 0. 0 g o' 0 g ~ -,""' •. Z ;l>::r...., ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~...., rll ::r 0 n g" !"l .... •• .... (ii 0 s:: S· .... ::r S !"l '" I;; '" o' ~. ;l> ~ to"' 0 ~ ~ ~ ~ S' ::0:r:: q> .....'0 rll =.,-..n (l> ::I S <: S 0. 0 nl (l> ~..c(JQ (l> S' 0 :=:~!"l- n 0 O!"l 0" o.:l::l ~~~ .... CIl"'O""'!"l .... 0 ...... ·0;:00 ...·!"l ;l>rll .... t-l;>~~~...... o."'!"l~rll""'m>o«Z", o.Orll....,'" o< s::- ~!!...<: ;:0 o' .....::r o.:l CIl~'D :::i ~.~....s:: ~!"l ~1:l 0. ~~~0 ~.E:' (ii ~ ~ ::r ~ n(l>O ...... O 8..g" 0 E. ;'!"l ;. 0 8..g g-2- 0 5 8.. 0"I::l~ ~ ~ ~~ ~ ~ ~ ~ O,Q 'D n - """ s:: 0 !:!!..... (l> """ 0 .... 0 0 !"l >;"...... n (l> !"l (l> S .... n ::r .... 0' rll s:: <' <: ~ ~ .... w (:: ~ o l';" ';-' '" '" ::I. "'" I .... rll ~ "'.. 0. ~ I O. I '" (JQ ~ (l> (l> '" _ 0. _ '< >;" rll 0 '" .... 0. ~ (l> ~ -:- - ------..0 Table 6. Age compositions, seasonal total mortality rates (A), survival rates (S) and instantaneous rates of total mortality (Z) for H. vi/a/us Age Groups ( -I' 0 I I II I III IV -I V VI VII I 1: Aged---n- 114 I 23 II 'I 9 I I 2 /--160- part %n I 71.2 I 14.4 6.9 5.6 I 0.6 I 1.2 I 100 "!1 Chikanka A 0.7981 0.5229 0.1813 0.8892 0.4984 I 'I 0.711 Ui Island I S I 0.2019 0.4771 0.8187 0.1108 0.5016 0.289 :I: stock Z 1.60 0.74 • 0.20 2.20 0.69 I 1.24 Total I n 1225 I 35 I 25 I 13 2 30- 1 /---1 -1-- 1--1- ~ stock I %n I 74.7 11.6 8.3 I 4.3 i 0.3 \ 0.7 I 100 1 r.Jl Chikanka A I 0.8443 0.2882 0.4780 0.9227 0.4984 1 I 0.747 N Island ,S I 0.1557 0.7118 0.5220 0.0773 0.5016 I 0.253 trI 7 > r.Jl r.Jl 1.811 23 0.3(,4 15 0.65 21 2'~1614 0.6 7 1.3)1 i-:- ,' ,I 11 trI r.Jl All II14 204 r.Jl studied %n I 55.9 11.3 7.3 10.3 6.9 4.4 [ 3.4 0.5 100 ::: l 1 1 samples A I 0.7981 0.3495 0.2882 0.3297 0.3560 0'2212 0.8577 0.557 trI ~ Sinazongwe I S 0.2019 0.6505 0.7118 0.6703 0.6440 I 0.7788 0.1423 0.443 area I Z I 1.60 0.43 0.34 0.40 0.44 0.25 1.95 0.59 > ~ > -< > t=: Table 7. Relative mass of Hydrocynus villa/us after mean seasonal total mortality rate (A) and mean seasonal survival rate (s) > t:C I"" trI Number of Number of Standard length in cm of I Weight in g of Mass of survivals specimens surviving one specimen I one specimen Weight of growth season "l:I Growth seasons dead after specimens , I kg " mea~1 A = 75% _____ S = 25% 1 mean II range range mean I minimum-maximum ~ I 3000 1000 I 18 9.3-29 I 160 II 14- 460 160 I ~ 2 750 250 I 28 15-:-39 420 60--1150 105 I 15-287 r.Jl 3 188 62 II 35 22-45 I 800 II 193-1800 50 12-112 ~ 4 46 15 39 34-48 1150 750--2100 17 I 11-31 -< 5 11 4 I 48 47-50 'I' 2110 I 2050--2480 8 8-10 ~ 6 3 1 I 49 2300 2 I 0"1.... ~ 62 EUGENE K. BALON the time of annulus formation and multiply it and the index of population weight growth Table 8. Availabl simply by the assessed abundance, which is re intensity Age groups 1 lated to the time of sampling. We have to ij+a adjust the abundance at time of capture (N) to o C w = L CWj (9) the time of the annulus formation (NI) i W = I according to the mean daily rate of mortality From the relative indices of growth o (instantaneous rate of total mortality) as mentioned it is possible to determine the I .Z) mean age of first maturity of the species II NI = N +_t (I (5) stock or population (BALON 1964, 1968a). III 365 IV where t expresses the number of days elapsed But for the evaluation of the seasonal V from the time of annulus formation to the changes in species stock it is more useful VI date of sampling of the given locality, Z is to calculate the relative condition index (LE CREN 1951). Relative the mean instantaneous mortality rate for • Relative the species in a particular locality and N is Relative the number of fish in an age-group. Futher PRODUCTION AND YIELD more, some corrections should be made for For illustrating the results we have selected the same time differences in fish weight here one of the main predator species equals at th( and to young-of-year fish, which exist at (Hydrocynus vittatus) and one main prey ichthyomass time of the sample but not in the time of species (Alestes lateralis) living in Lake Kari biomass (Tab annulus formation (HOLCIK 1971). ba. The calculations are based onthe methods tiger-fish habl In a study of fish production it is useful previously mentioned. The calculated lengths to calculate relative values ofgrowth (BALON for previous years of life are used to estimate fish distribUj 6.2 % ofthe I 1968a). For example, we may calculate the mean lengths and increments (BALON 1971c, for the lake absolute increment Fig. 10, Table 8). The minimum harvestable From this size in relation to the first spawning and espec hi = Ii - li_l (6) or quantity 011 ially to economical increments can be yearly by fis! and the index of the species average size established from the crossing point of length size and wid ij+a increment in percent of length of the first present stod 0H = L hi growth season curve and length in percent of determine th~ i= I (7) length of the final growth season curve in order to separate the fish into classes of (BALON 1971c). HOLCIK (1970 a) assumed average to t] about I kg 0 economically preferredspecies (0H > 5), partly furthermore that from "the relation of pro HOLCIK's exploited secondary species (5)0H > 2), and duction to ichthyomass it is possible to production unexploited accompanying species (0H < 2). compute the best minimum size if we subtr 22.5 kgjha. I rn these formulae 1 is the fish length during act the amount equal to the total production annually a td various ith years of growth, j for juvenile from the total ichthyomass by age groups". In fish more. T and a for adult period (BALON 197Ia). our case both methods gave similar results. 70% of the Weight indices are obtained by replacing According to the population estimated fish from the the symbol 1 by w. and the growth analysis the yearly available Alestfs latE To compare the growth rates of different production2 of tiger-fish in Lake Kariba 59% of the species or the same species in different lakes its share in it is best to calculate the specific weight rate 2. The production values are given the simplest way. No corrections on annulus formation and sampling little over ofgrowth time differences were made. As already stated duction has Wi - Wi_~ all our production values express the annual CW = .100 6.2 % inha w·-I (8) biomass increments of surviving fish only-the I available production. 169 metric tl FISH STOCK SIZE ASSESSMENT AND AVAILABLE PRODUCTION SURVEY 63 ion weight growth Table 8. Available production computation of Hydrocynus villatus in Lake Kariba Age groups Number of Sample curve Mean weight Mean weight Total incre- fish n/ha intercepted in g of one increments in g ments of age (9) \ i'i./ha specimen of one speci- group per year I W men h in kg h i'i./ha ------ldices of growth 0 186 116 140 140 16.2 to determine the I 29 55 590 450 24.7 ity of the species II 21 27 1050 460 12.4 JON 1964, 1968a). III II 12 1530 480 5.3 IV I 6 2000 470 I 2.8 of the seasonal 2 2400 400 V I 0.8 it is more useful VI 2 2 3000 600 1.2 condition index ------Relative production in kg/ha/yr PA 63.9 Relative production in percent of biomass (30.8 kg) PA/B I 207.5 Relative production for 33,422 of fish inhabited lake area FA 2119 metric 'IELD I I tons ts we have selected predator species equals at the time recorded 12 of the that this species is the first to colonize the lone main prey % ichthyomass and 207.5 of the tiger-fish free open lake area, this production value ving in Lake Kari % biomass (Table 8). If we consider that the might be underestimated. From the analysis sed onthe methods tiger-fish habitat corresponds with general of the length frequency, the largest specimens calculated lengths fish distribution recorded (33,422 ha or of A. lateralis are small enough to be eaten re used to estimate 6.2 ofthe lake surface) its yearly production by tiger-fish in the most numerous size Its (BALON 1971c, % for the lake would be 2,119 metric tons. range. A. lateralis will obviously also be limum harvestable From this estimated production the yield eaten by other predators, but on this issue )awning and espec or quantity oftiger-fish which can be removed there is no knowledge up to now. As we have :rements can be yearly by fishing in the most economical been treating only the data for A. lateralis .ng point of length size and without deleterious effect on the and tiger-fish, let us consider the inter ength of the first present stock, can be calculated. If we relation of these fish as an example of yield mgth in percent of determine the harvestable size to belong in estimates for prey species. .vth season curve average to the II-age group (mean weight How is the calculated annual production : (1970 a) assumed about 1 kg or 1.3 k~and 38 em according to of A. lateralis related to the food ration of e relation of pro HOLCIK's method), the yield or annual the main predator? In a paper on tiger-fish it is possible to production of harvestable size would be (BALON 1971c) attention' was drawn to the n size if we subtr 22.5 kg/ha. It is possible., therefore, to crop similarity in many aspects of the biology .e total production annually a total of 742 metric tons of tiger by age groups". In of the tiger-fish and the European pike. fish more. This tonnage represents already Here-because of shortage of values for e similar results. 70 of the prtCsent annual harvest of all the tiger-fish we shall try to use the values ulation estimated % fi:>h from the Zambian part of Lake Kariba. found for the annual food ration of the pike le yearly available Alestes lateralis is the most abundant fish: in the Volga Ddta mentioned by POPOVA in Lake Kariba 59 ~~ of the total. Because of its small size (1967); an aVt.rage value of 3 kg per kilogram ven the simplest way. its share in the ichthyomass is low, only a of body weight. This lies roughly between ,mation and sampling little over 0.7 %. Its annual available pro POPOVA's (1967) and MUNRO's (1967) :. As already stated duction has been calculated for the same "daily ration" 5 ml of food per kg oftiger-fish express the annual 'iving fish only-the 6.2 % inhabited area of Lake Kariba as (calculated as "annual ration" it is 5 x 365 = 169 metric tons (Table 9). As we later found 1,825 ml/kg). The mass of tiger-fish per 64 EUGENE K. HALON os .... inhabited 6, ~ ...c:: 0 - t - _ so the anm ~ '" ,-0 :h 00 A. lateralis c 0"- J:: (Ij 00 £ <:: .<:: E requirement ' ;:;" '-'~ 0 "0 j- --- 0------tion. MATTI E ~ os "" the stomach ( .5 j ~ lateralis, whi( for such spe lID same time it -~ ="i"Je" Oll~ Q, ,<::- ~"" "!". 0 00~ ..... iV o.,,-~ N lID..I Ol- E FISH STOCK SIZE ASSESSMENT AND AVAILABLE PRODUCTION SURVEY 65 inhabited 6.2 % area of Lake Kariba was in view of the newly developed interactions calculated as 1,056,000 kg. This quantity in relation to the available nutrients. of tigerfish requires 3,168,000 kg of food, If there is a shortage oftime the production so the annual production of 169 tons of evaluation of the economically preferred A. lateralis covers 5.3 % of the annual food species and of the secondary f>pecies should requirement of the Kariba tiger-fish popula be sufficient (Table 10); often the accompany tion. MATTHES (1968) found that 5.3 % of ing species do not represent an appreciable the stomach contents ofthe tiger-fish was A. part of the ichthyomass and production. lateralis, which is a very striking agreement The fish population is a complicated for such speculative calculations. At the community. Different species are not equal. same time it probably proves that in this Their inter-and intra-relationships are as case available production estimation is different as those taxonomical characters equal to the total production in Ivlev's from one species to another. Furthermore, meaning (see p. 44). every species is related in various ways to To calculate uniform data for every species the environmental factors. We are able to living in the studied impoundment (Lake recognize several types of single species Kariba with 40 species) is a very long lasting stock, eg.: project indeed. It could only be effectively accomplished by the cooperation of several I.A:X=a:y institutions working on individual species II. A:Y = a : x production and yield to obtain the total III. B:X = b : y annual fish production and yield. TV. B:Y = b : x (11) If the maximum sustainable yield is obtained from the natural balanced fish where A is a high and B a low amount of population the fishery management should food within easy reach, X is the high and Y then look for ways to increase the production the low stock density, a is the fast and b the and yield by changing the species composi slow growth rate, x is the early and y the tion, their relationship, and balance following late sexual maturity (BALON 1963). By changing the above mentioned relationship SWINGLE's (1950) 011 others criteria, accord (e.g. by fishing or dam building), one species ing to the carrying capacity of the lake. In stock change over to anotber species stock Lake Kariba the Tanganyika sardine Limno type. Not always does fishing result in a thrissa miodon was successfully introduced reduction of the abundance or the ichthyo (BELL-CROSS and BELL-CROSS 1971; mass. KESTEVEN (1962) was not precise BALON 1971e) to utilize the uninhabited on this point even when he later dt-scribed open lake area, and a large eel stock inhabiting the situation very well: "The reduction is the 2Q--45 m depth area was later discovered not equal to the catch, since part of what (BALON 1971 b). Assessment of these stocks is caught would have died from natural is done with different methods. The sardine causes. The reduction of community biomass is studied using a combination of determined may be even less than catch-minus-some echo-traces, ring-net hauls and underwater proportion-of-it that would have gone to photography, the eels with capture-mark natural mortality, since removal of the catch recapture method. Both will add to the means presumably greater survival of the already estimated ichthyomass and pro organisms on which the caught fish had fed, duction of the shore-line fish population, except that the surviving members ofthe exp though the addition may not be so simple loited population may each eat slightly more E 66 EUGENE K. BALON Fli Table 10. Recent (1968-1971) list of Lake Kariba fish species in economical order according to the species and take up the average size, abundance, topographical frequency of occurrence and mean percentage of total number of that have been ca specimens. surviving fish cou offset the remov Order I ECONOMICALLY PREFERRED SPECIES Frequency of I Mean percentage number occurrence of total number if the fishing reme ----- potential and effi 1 Ti/apia mossambica mortimeri Trewavas, 1966 8 10.00 of the population 2 Ti/apia rendalli gefuensis Thys, 1964 8 3.62 of the ichthyomal 3 Hydrocynus vittatus Castelnau, 1861 8 2.48 immediate result l 4 C/arias gariepinus (Burchell, 1822) 6 2.00 I 5 Sargochromis codringtoni (Boulenger, 1908) 6 1.10 extend beyond "11 6 Heterobranchus longiji/is Val., 1840 6 0.09 ability). Even re 7 Mormyrops deliciosus (Leach, 1818) 5 0.47 preferred species ~ 8 Labeo altivelis Peters, 1852 5 0.18 a reduction of t~ 9 Mormyrus longirostris Peters 1852 4 0.05 1970). More 011' 10 Distic.hodus schenga Peters, 1852 3 0.28 II Labeo congoro Peters, 1852 3. 0.02 increase of prod 12 Ti/apia andersonii (Castelnau, 1861) 2 0.85 living and early 13 Sargochromis giardi (Pellegrin, 1904) 2 0.16 very long contin' 14 Haplochromis car/ottae (Boulenger, 1905) 1 0.10 methods leading 15 Serranochromis robustus jallae (Boulenger, 1896) 1 0.04 reduce the ichthy, 16 Serranochromis macrocephalus (Boulenger, 1899) I 0.04 17 Distichodus mossambicus Peters, 1852 1 0.03 River (high pop centuries to 0 SECONDARY SPECIES 21.51 huso, maturing fi 18 Alestes latera/is (Boulenger, 1900) 8 58.78 of age (BALD' 19 Hippopotamyrus discorhynchus (Peters, 1852) 6 7.94 short-living and 20 E~tropius depressirostris (Peters, 1852) 5 0.74 found to be im 21 Marcusenius maerolepidotus (Peters, 1852) 5 0.12 22 Malapterurus electricus (Gmelin, 1789) 5 0.07 have to agree 23 Schilbe mystus (Linneaus, 1762) 3 0.13 usually bad lao> 24 Anguilla nebulosa labiata Peters, 1852 fishing techniqu 25 Limnothrissa miodon (Boulenger, 1906) as overfishing". ACCOMPANYING SPECIES 67.78 If vve taJce i complicated rela' Haplochromis darlingi (Boulenger, 1911) 8 5.20 26 I 27 Barbus fasciolatus GUnther, 1868 5 4.64 species in a st, I I 28 Barbus unitaeniatus GUnther, 1866 I 8 1.80 factors (e.g. H Hemihaplochromis philander (Weber, 1897) I 7 1.28 29 I usually find as. 30 Synodontis zambezensis Peters, 1852 I, 6 1.02 community, therq 31 Micralestes acutidens (Peters, 1852) 6 0.41 ,I influence to be ~ 32 Synodontis nebulosus Peters, 1852 5 0.33 I I 33 Brachyalestes imberi imberi (Peters, 1852) 5 0.28 subsistence fishel I 34 Aplocheilichthys johnstonii (GUnther, 1893) 5 0.04 expectt'd (and Wl 35 Labeo cy/indricus Peters, 1852 3 0.01 1961; BALON l~ 36 Barbus /ineomaculatus Boulenger, 1903 2 0.02 pollution vvhere i 37 Barbus poechii Steindachner, 19l1 2 0.02 Furthermore, thi; 38 Labeo lunatus Jubb, 1963 I 0.02 39 Barbus paludinosus Peters, 1852 - - r intensive fishing iI 40 Barilius zambezensis (Peters, 1852) - - maximum sustai 15.07 fishing usually a catch before danger of ex FISH STOCK SIZE ASSESSMENT AND AVAILABLE PRODUCTION SURVEY 67 ~ording to the species and take up the spare food left by those become a perpetual interest of exploiting fe of total number of that have been caught. By eating more, the insititutions and they may be effective if surviving fish could grow more, and further no other factors interfere. As the latter is ~ Mean percentage offset the removal by the catch. Again, usually the case (e.g. pollution and lampreys of total number if the fishing removes older fish, the growing in the Laurentian Great Lakes, river bank potential and efficiency of food utilization regulations and dam building on Europe 10.00 of the population can increase". A reduction main streams) preventive measures to forbid 3.62 2.48 of the ichthyomass is then usually not the overfishing are vital. As empiric fish produc 2.00 immediate result of fishing (if this does not tion data and maximum sustainable yield data 1.10 extend beyond "normal" population adapt are lacking, intensive research should be 0.09 ability). Even reduction of economically developed in this direction. 0.47 preferred species does not necessarily mean As REGIER (I970b) expressed so 0.18 0.05 a reduction of total fish production (TSAI excitingly in his address to the Centennial 0.28 1970). More often fishing results in an Meeting of the American Fisheries Society 0.02 increase of production, especailly in short in relation to pollution: "We have fought 0.85 living andearly maturing fish species. It takes to have corrective technology applied to 0.16 very long continuous fishing with intensive this or that case of pollution. We have won 0.10 0.04 methods leading to a high catch increase to many small battles, but have been losing the reduce the ichthyomass. Even in the Danube war". Similar statements have been made in I 0.04 J 0.03 River (high populated shores) it has taken relation to water buildings equally by myself 21.51 centuries to overfish easy-to-catch giant (BALON 1967). But still COUSTEAU huso, maturing for the first time at 20 years (1964) is right in stating: "In our times the 58.78 of age (BALON 1967). Overfishing of technocrats are at the summit. Almost 7.94 0.74 short-living and early-maturing bleak was everywhere they have powerful contact 0.12 found to be impossible (ENTZ 1952) and I with politicians. It is clear that this will 0.07 have to agree with MESECK (1962) that change again. Biological sciences will again 0.13 usually bad landings "caused by a wrong be in the fore because without life even fishing technique are frequently explained science cannot exist". Let's hope it will as overfishing". not be too late then. If we like to find out 67.78 If we take into consideration all the how our recent fish production studies are 5.20 complicated relationships between different related to the much preferred fish yield 4.64 species in a stock-and the environmental predictions, "then the onl¥ logical conclusion 1.80 factors (e.g. HANDLER 1970) where we is that the biomass of the future is likely to 1.28 usually find a strong trend to balance the be just so much more or less than it is now, 1.02 0.41 community, there is really very little fishing as man makes it to be" KESTEVEN 1962). 0.33 influence to be expected especially in a 0.28 subsistence fishery. More harm is to be 0.04 expected (and was proved, e.g. MILLER ACKNOWLEDGEMENTS: The outline presented in this paper developed and benefited greatly from 0.01 1961; BALON 1967) from dam building and 0.02 discussions held with Dr. K. F. Lagler, Dr. A. G. 0.02 pollution where irreversible changes occur. Coche and Dr. J. Holcik. For their critical review 0.02 Furthermore, this stands also in case of of the manuscript and valuable comments I wish intensive fishing industries with catches over to express my appreciation to Dr. A. G. Coche, maximum sustainable yield. Continuous over and Dr. L. E. Mawdesley-Thomas and especially to Dr. H. A. Regier. To the last my thanks are also 15.07 fishing usually terminates the economy of due for numerous useful comments, recent literature a catch before the species affected is in information, preprints of his papers and a rigorous danger of extinction. Then regulations editing. Some corrections and amendments were 68 EUGENE K. BALON FI~ incorporated after the discussions at the FAa Stock Balon, E. K. (l966b). Contribution to the knowledge pretation of the Assessment Working Party held in Jinja, Uganda, of balance of the fish stocks in the inundation determination of in 1970, for which seminar this paper was originally waters of the Danube [in Slovak]. Biologia Men. 1st. 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