ACOUSTIC SURVEYS IN THE SPANISH MEDITERRANEAN SHELF (1982-1993) ACOUSTIC ESTIMATION OF VOLUME AND DISTRIBUTION OF APHIA MINUTA (PISCES: GOBIIDAE) IN ALCUDIA BAY (MAJORCA ISLAND, SPAIN) M. IGLESIAS. J. MIQUEL, P. OLIVER & F. ALVAREZ Centro Oceanogrifico de Baleares, lEO, Spain M. IGLESIAS. J. MIQUEL & F. ALVAREZ Centro Oceanogritfico de Baleares, lEO, Spain Since 1982. the Spanish Institute of Oceanography (lEO) has been doing systematic hydro-acoustical surveys to evaluate coastal pelagics in the Spanish Mediterranean shelf (20-200 m depth), covering the area between Punta Europa and Cabo de Creus. Five zones have been considered: Albonin, Vera, Alicante, Valencia and Catalufia. Although sardine (Sardina pi/chardus) and anchovy (Engraulis encrasicholus) are the target species, other important species are round sardinella (Sardinella aurita), horse mackerel (Trachurus trachurus; T. mediterraneus and T. picturatus) and mackerel (Sc·omber scombrus and S. japonicus ). These species are mixed and the echograms and catches are normally multispecific, being sometimes very difficult to allocate the echointegration. Acoustic survevs have been carried out on R1V "Cornide de Saavedra", a 67 m long, 2500 HP ste;:, trawler. Prior to each survey, the SIMRAD echosounder system was calibrated using a 60 mm copper standard target sphere with a TS= -33.6 dB. Biological data were collected using a pelagic trawl gear with a 20 mm codend and provided with a Simrad FR-500 netsonder. The average values by sector and strate were elaborated by means of NAKKEN & DOMMASSNES method (1975). Table I summarizes the cruise characteristics. 1985 1987 1988 1990 1991 1992 1993 10 10 10 10 10 20 20 20 Oct Jul :V:ay-Jun Oct-Nov Oct-Nov EK-500 EK-500 EK-500 EK-500 Oct-Nov Oct-Nov m m m 12504 m 16199 16163 14352 2177 1550 1628 3610 3236 9327 m m 28~5 3140 1885 16199 m 10709 m A specialized fishery directed at pelagic gobiids is conducted off Majorca island during wintertime. The target species is the transparent goby (Aphia minuta, Risso 181 0) and by-catch species are Pseudaphia jerreri and Cristallogobius linearis (IGLESIAS & MARTORELL, 1988). The main catches are taken into the bays; Alcudia Bay is the area where most of the fleet work. The importance of this fishery is due to the local appreciation and high market value of transparent goby. The fishery is directed at the aggregations that these pelagic gobiids form near the bottom during its reproduction time, being shoals detected with the aid of fish finders. This characteristic allows the use of acoustic methods to determine its localization and abundance. This study has been the first time that acoustic methods have been used to evaluate Aphia minuta abundance and distribution. In January 1993, "Jonquillo-93" acoustic survey was carried out on RJV "Xarifa'', a 7 m long, 9,9 HP boat, covering the area between Cape Pinar and Cape Farrutx (Alcudia Bay, figure 1). A Skipper 815 paper echosounder with a towed body was used for data collection, using as the log interval one nautical mile. The survey consisted of nine tracks going from 5 meters depth near the coast to the mouth of the Bay (figure 1). The number of nautical miles tracked was 54 and the covered area comprise 47.48 mn2. The distance between tracks was one nautical mile. Aphia minuta records on paper echosounder were identified by means of fishermen aid. The scrutinizing of echograms were made by four differents groups. Only the coincident readings were considered. The stock size of Aphia minuta was estimated by the method of enumeration and volume estimation of shoals recordings (FORBES & NAKKEN, 1972). A simply enumeration of number of shoals by nautical mile give a first relative index of abundance of this species in the area. Since size of shoals is very different, the estimation improves if we consider its volume. Aphia minuta was abundant in January 1993 in Alcudia Bay with a total volume estimate of 12.902.751,44 m3 This result agrees with the catches obtained during the fishing period 1992-1993. The distribution of Aphia min uta was related to depth and type of substrate, the shoals were concentrated on the center of the Bay where the bottoms are mainly flat rocks and sand. Also this area is characterized by clean waters. Shoals have been detected from 8 to 45 meters depth. being more abundant between 30 and 35 meters depth (65%). Volume of shoals runs from 12,7- 84077,4 m3, with the more abundant volumes between 0-1000 m3 (60/%), followed bv volumes between 1000-10000 m3 (34%), 10000-50000 m3 (3,4%) and bigger than 50000 m3 (3%). The acoustic evaluation of Aphia minuta by means of an echosounder has shown to be a valid method to determine its abundance and distribution. Further improvements of the method would require to use an echointegration method and to determine the TS (target strength) value of the species, which may allow to determine number and biomass of the species. Table l. Acoustics surveys in Spanish Mediterranean shelf. After 10 years, the method has been fully standardized and has therefore produced a time series of sardine and anchovy abundances comparable over the last years. The results of the evaluations (biomass) and the catches in these years are as follows: SARDINE BIOMASS (1000 TM) =C~A"-T'HES(:_10cO.M) 1982 83 84 85 86 105111 109 419 174 _ _ _3=:6:____4_c3:___::3:_:5_ __c4:_:5_4c:_7 87 88 89 273121 182 40 41 39 90 91 92 93 175 227 455 322131 38 45 50 46 (1) Understimated, Valencia and Catalufia not surveyed (2) Understimated, Catalufla not surveyed (3) Understimated, Vera and Alicante not surveyed. Figure I. Tracks survey and distribution of Aphia minuta in Alcudla Bay in January 1993 These data suggest a lack of pressure on this stock, which is probably due to low market demand. According to this, catches could be considered independent of existent biomass. ANCHOVY 1982 83 BIOMASS (1000 TM) 60(11 47 _CATHES~(10cM) 84 44 _ _ _5_0_ _3_8_ 25 85 12 86 87 16 4(2) 28 14 20 88 89 17 90 91 92 93 34 17 24 20 23 19 18131 17 This species is the target of the purse seiner fleet, and its commercial value is much higher than the sardine one. The table shows a high fishing exploitation in accordance with this high demand. REFERENCES ABAd. R .. J. MIQUEL and M. MILLAN. 1991. Resultados de Ia campafia de evaluaci6n actistica ECOMED 90 (22 Octubre-22 Noviembre 1990)./nf Tee. /nst. Esp. Oceanogr., 104: 63 pp. ABAD. R .. J. MIQUEL, M. MILLAN and M. IGLESIAS. 1992. Resultados de Ia campafia de evaluaci6n acUstica ECOMED 91 (21 Octubre-21Noviembre 1991)./nf Tee. lnst. Esp. Oceanogr., 131:77 pp. ALVAREZ, F .. X. PASTOR. P. OLIVER & J. MIQUEL. 1983. Resultados de Ia campafia de cvaluaci6n actistica "MEDITERRANEO 83''. MIQUEL, J., J. BRUNO. R. ABAD & A. GIRALDEZ. 1987. lnforme preliminar de Ia campana de cvaluaci6n de stocks de peccs pel<igicos "MEDITERRANEO 87". MIQUEL. J. & R. ABAD. 1988. lnforme pre1iminarde la campaii.a "'ECOMED 88". NAKEN, 0. & A. DOMMASSNES. 1975. The application of an cchointcgration system in investigations on the stock stregth of the Barents sea capelin (Mallotus villosus) 1971-1974. ICES CMIB: 25. OLIVER. P .. X. PASTOR. F. ALVAREZ AND A. ASTUD!LLO. 1982. Acoustic assessment of the coastal pelagic fish stocks of the Spanish Alboran Sea, with special attention to the sardine (May 1982). C.G.P.M. Tech. Consult. Acoust. Meth. Fish Detect. & Abund. Estimat. OLIVER, P. & F. ALVAREZ. 1984. Estimaci6n por metodos hidroactisticos de la biornasa de peces pel<igicos costeros del Meditemineo espafiol (Octubre 1984). A o-1 ooom 3 • 10 000-50 000 m3 + 1 000-10000m3 • >50000m3 REFERENCES FORBES, S.T. & 0. NAKKEN, 1972. Manual de metodos para el estudio y Ia cvaluacion de los recursos pesquerows. Parte 2. Utilizacion de instrumentos ac£sticos para Ia localizacion de peces y la estimacion de su abundancia. Manual de la FAO de Ciencias Pesqueras n" 5. FIRM/MS. IGLESIAS, M. & J.M. MARTORELL, 1998. La pesqueria Jitoral de las Islas Balcares. proyecto cooperativo IEOICAIBICEE. XIVBJ/871812840. 199 pp. a Rapp. Comm. int. Mer Medit., 34, (1995). 246 Rapp. Comm. int. Mer Medit., 34, ( 1995).