International Council for the Exploration of the Sea Pelagic
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-----------"'~:::::---------.[-,-5-' · ) 31 International Council for the C.M. 1985/H:~/Ref B Exploration of the Sea Pelagic Fish Committe ACOUSTIC ABUNDANCE ESTIMATION OF MACKEREL, PILCHARD IN CANARY ISLANDS WATERS. APRIL 1984. by 1 x. Pastor ) and A. Delgado de Molina2) , 1) Instituto Espanol de Oceanografia Centro Costero de Baleares - Muelle de Pelaires, s/n 07015 Pal~a de Mallorca, Spain 2) Instituto Espanol de Oceanografia Centro Costero de Canarias P.O. Box 1373 38080 Santa Cruz de Tenerife, Spain ABSTRACT , e Results of the acoustic survey "Canarias AI 484" are reported in this paper. This survey was carried out on board the research vessel "Cornide de Saavedra" in April 1984 in the waters off Canary Islands. The ~ain objective of the sur vey was to estimate the abundance by year class for mackerel (Scomber japonicus), and p~}Chard (Sardina pilchardus). Abundance by length class for bogue (Boops boops) was also estimated. Canary Islands are of volcani~ origin, and there is not continental shelf aroundthem. Therefore, the surveyed area covered was the first five miles around each island. Total biomass estimate for small pelagic species was 73.000 tons, of wich 38.000 tons correspond to mackerei, 29.000 tons to bogue and 6.000 tons to pilchard. The areas of highest abundance were found around the islands of Fuerte'lentura and Lanzarote, ant those of lowest abundance were around the islands of Hierro and La Palma. I Ir\.' b' --_.--....._._-- 2 RESUHE Les resultats de la campagne acoustique "Canarias AI 484" sont presentes dans ce papier. Cette campagne a ete realisee au bord du B/R "Cornide de Saavedra" a april du 1984 dans les eaux des Iles Canar~5 Les principaux objectifs de cette campagne on ete obtenir l'estimation de llaboundance par classes dlage pour ma quereaux et sardine. Llaboundance par classe de taille pour bogue ha ete aussi estimee. L'aire couverte.ha ete les cine milles au large de la c6te de les iles on il n'y a que du talud insulaire. La biomase total estimee a ete pour les petits poissons pelagiques 73.000 tons, dans les quelles 38.000 tons sont de maqueraux, 29.000 tons bogue et 6.000 tons sardine. Les aires du major aboundanee ont ete ren- ~ eontres autour des iles Fuerteventura et Lanzarote, et les d'aboundance mineure autour des iles Hierro et La Palma. INTRODUCTION The resources of small pelagic species of the CanaryIslands are mainly formed.by populations of mackerel, horse mackerel, pilchard, bogue and anchovy. These spe- eies constitute the second fishing resource of major industrial importance in the islands. They are exploited by a loca~ fleet of small purseiners with the help of small boats with artificial light. ~. The annual catches of these small pelagic species are approximately 5.000 tons, of wich 3.000 are caught in the western islands (the provinee of Sta. Cruz de Tenerife) and the remainder in the eastern islands (province of Las Palmas). The tunna fishery is the main fishing resource of Canary Islands. The devel~p ment of this fishery is bound to the availability of the coastal pelagic species, because the live beat used in this activity is basically formed by mackerel and pilchard. Of these speci~ the mackerel constitutes approximately 60 % of the total catches. From the biological point of view, these species have been studied with certain regularity (OELGADO OE MOLINA et al., 1983), and some pr~liminary experiments of acoustic evaluations have been carried out (BRAVO OE LAGUNA, personal eommunica tion) and (ANON, 1984). 3 .. During the spring of 1984 a survey was carried out on board R/V "Cornide de Saa vedra" ·.... i th the aim of obtaining an acoustic estimate of the biomass and distri':' bution of those resources. The cruise was carried out during the period between 16th and 26th of April. The survey tracks run by the vessel covered the area within the first five miles nearest to the coast, with the exception of the'half mile closest to the coast, due to safety reasons during navigation. The shoals near the archipielago (Concepci6n, el Banquete and Amanay) were also covered. MATERIAL AND METHODS Figures 1, 2 and 3 show the area covered by the "Cornide de Saavedra" during the cruise, the survey grid and tge pelagic fishing stations. The Canary Islands, of volcanic origin, practically lack continental shelf, • and at a short distance from the coast the depth reaches some hundred metres. For this reason, the usual limit of the isobats of 200 of 500 m. was not used in this survey. In its place, an area of five miles·around each one of the is lands was stablished as the survey area; where it is foreseeable that most of the coastal pelagic resources are concentrated. This hypothesis was confirmed, in general, during the courses navigated between the islands. 125 survey tracks were established in a zig-zag grid. The vertices of the tracks were 10 nautical miles apart of each other. The most interna! points of the tracks were p1aced as near of the coast as possible for the vessels size, at a distance which ranged between 0.1 and 0.5 miles. Despite this, the first half mile area around each island must be,considered as poorly covered. The area surveyed was of 3.643 nm2 • The acoustic equipment used during the survey was formed by an E~ 400 SIMRAD - - echosounder of 38 kHz and a QD digital integrator~he settings of the instru- --------------------------ments during the cruise are shown in table I. Integration values were recorded at the end of each mile. The number of miles navigated in the surveyed area was 1.024. The value of the coverage index, N/V-S- = 17.0 is considered accep- 4 table, and minimizes the errors wich can be made with respect to the estimation of the indexes of relative abundance (AGLEN, 1983). The fishing stations were carried out with a pe1agic gear of 10 m. of vertical opening and at a towing speed of 3 or 4 knots. Fishing stations were used for echoes identification and biological sampling. They were carried out whenever fish concentrationes appeared. Table II shows the characteristics and the re sults of the fishing stations. Prior to the start of the survey, the acoustic instruments were calibrated (( I]., by means of an standard target (coppersphere of 60 cm 0) (FOOTE, 1981). Results .=- I. ' of this calibration are shown in anexe I. For the biomass estimation of the surveyed area, this was divided in 9 subareas, corresponding to each one of the seven major islands of the archipielago and_to the shoals of Amanay and el Banquete, in the south of Fuerteventura. The shoal of La Concepci6n, situated 55 miles north of Lanzarote, was not considered. Despite it having been surveyed, it did not provide any integration value copres ponding to small pelagic fish. The transformation of the values of relative echointegration to tons of biomass corresponding to each one of the species considered- was carried out through the ~spec:ivefconversionfactors (9)~ The procedure used to calculate these factors is shown in annexe II. The absolute density per length class was estimated for the three species that made up almost the totality of the catch in the fishing stations: mackerel, bogue and pilchard. The following expression was used: k .. M = __~~~J _ p (DALEN AND SMEDST~, 1979) ,ij k .. ~J 1: I: j i 2 where P represents the density (tons/mn )for the length class i of the species j, ij kij is its proportion in the catches of the fishing stations carried out in that subarea. M is the total of the integration va1ue attributed to pe1agic fish in that subarea. C.. is the value of C corresponding ~o length-i of species j. J.J ~"-_._- ---- -_ .. _-_. --------------_.------ 5 The densities per length class and species calculated in this way for each subarea are converted into biomass using the expression: B .. ~J = where A is the su-face for each subarea, in square nautical miles. In order to transform the biomass in tons per length class into number of indi viduals per length class, the corresponding length/weight relations were applied: mackerel w= 0.0025 L3.46146 (OELGADO OE MOLINA et al. , 1983) bogue w= 0.0071 L3.21690 (GIRAROIN, 1978) pilchard w= 0.0065 L3.08539 (OELGADO DE MOLINA et al. , 1983) Abundances per length class were converted into number of individuals per age class in the case of mackerel and pilchard. Length/age relation obta~ned from thesamples taken in the ~ishing stations during the survey was applied for mackerei. This relation is shown in table III. The number of pilchard otholits per length class obtained during the survey did not permit the calculation of a reliable ~r~th/age relation for this spe cies and this cruise. Therefore, the length/age relation obtained by DELGADO OE MOLINA et al., (1983) from sampies carried out over the commercial catches of the local fishing fleet was used. A length/age relation was not available for the Canary Islands bogue. For this ~ reason, the abundance in number of individuals per length class could not be converted into abundance in individuals per age class. RESULTS The total biomass estimated for small pelagic fishes during the survey was 73.00~ tons, of-wich approximately 53 % correspond to mackerel (Scomber japonicus),- 40 % to bogue (Boops boops) and 7 % to pilchard (Sardina pilchardus). Figure 4 shows the general distribution of abundance in the archipielago. Figu res 1, 2 and 3 show the survey tracks, fishing-stations and abundance distribu tion for each of the three areas into wich the archipielago was divided for car- --_._-,-,--------- 6 tographic purposes.