Distribution and Age Structure of Mackerel (Scomber Scombrus, L.) and Horse Mackerel (Trachurus Trachurus, L.) in the Northern Coast of Spain, 1989-1994*

SCI. MAR., 61(3): 345-366 SCIENTIA MARINA 1997

Distribution and age structure of mackerel (Scomber scombrus, L.) and horse mackerel (Trachurus trachurus, L.) in the northern coast of Spain, 1989-1994*

B. VILLAMOR1, P. ABAUNZA1, P. LUCIO 2 and C. PORTEIRO3 1Instituto Español de Oceanografía, Centro Oceanográfico de Santander, Apdo. 240, 39080 Santander, Spain. 2AZTI, Instituto de Investigación y Tecnología para la Pesca, Oceanografía y Alimentación, 48395 Sukarrieta, Vizcaya, Spain. 3Instituto Español de Oceanografía, Centro Oceanográfico de Vigo, Apdo. 1552, 36080, Vigo, Spain.

SUMMARY: This paper describes the seasonal and geographical distribution of mackerel (Scomber scombrus, L.) and horse mackerel (Trachurus trachurus, L.) in the northern coast of Spain (ICES Division VIIIc and Sub-division IXa North), based on commercial catches and bottom trawl surveys. Mean monthly commercial catches were mapped by ICES statistical rectangle, and the quarterly distribution by age group (0,1,2 and >2 years) and by Sub-division (Sub-division VIIIc East, VIIIc West and IXa North) are presented for the period 1989 to 1994 in the case of mackerel, and 1990 to 1994 for horse mackerel. Series of catch per unit effort by age are also analysed for some commercial fleets.The age group distribution (0,1,2 and >2 years) in autumn, based on information from bottom trawl surveys, is presented for the whole area studied.The data analysis reflects clear seasonality of mackerel in the northern coast of Spain. Nevertheless, in the case of horse mackerel in the same area, seasonality is not so evident. These results are discussed with respect to spawning and feeding migrations of these species in the Northeast Atlantic.

Key words: Scomber scombrus, Trachurus trachurus, distribution, age structure, migrations, ICES Division VIIIc, ICES Division IXa, the north of Spain, South of Bay of Biscay.

RESUMEN: DISTRIBUCIÓN Y ESTRUCTURA DE EDAD DE LA CABALLA (SCOMBER SCOMBRUS L.) Y EL JUREL (TRACHURUS TRACHU- RUS L.) EN LAS COSTAS DEL NORTE DE ESPAÑA, 1989-1994. – Este trabajo describe la distribución estacional y geográfica de la caballa (Scomber scombrus, L.) y del jurel (Trachurus trachurus, L.) en las costas del norte de España (División VIIIc y Sub-división IXa norte), basado en las capturas comerciales y en las campañas de arrastre de fondo.Se han mapeado las capturas medias comerciales mensuales por rectángulo estadístico del ICES, así como la distribución trimestral por grupos de edad (0,1,2 y >2 años) y por Sub-divisiones (Sub-división VIIIc este, VIIIc oeste y IXa norte), para los años 1989 a 1994 en el caso de la caballa y de 1990 a 1994 para el jurel. También se analizan series de captura por unidad de esfuerzo por edad de algunas flotas comerciales. Basada en las campañas de arrastre de fondo, se presenta la distribución por grupos de edad (0,1,2 y >2 años) durante el otoño en el area de estudio.El analisis de los datos refleja una clara estacionalidad de la caballa en las costas del norte de España. Sin embargo, en el caso del jurel, la estacionalidad no es tan evidente en esta área. Se discuten estos resultados en relación con las migraciones de puesta y de alimentación que realizan estas especies en el Atlántico Nororiental.

Palabras clave: Scomber scombrus, Trachurus trachurus, distribución, estructura edad, migraciones, División VIIIc del ICES, División IXa del ICES, Sur del Golfo de Vizcaya.

*Received August 23, 1996. Accepted May 14, 1997.

DISTRIBUTION AND AGE STRUCTURE OF MACKEREL AND HORSE MACKEREL 345 FIG. 1. – Study area showing the ICES rectangles, Divisions and Sub-divisions.

INTRODUCTION Discards of these species coming about in the Bay of Biscay (Pérez et al., 1995) and vary greatly The mackerel (Scomber scombrus, L. 1758) and as a function of the fleet, season of the year and horse mackerel (Trachurus trachurus, L. 1758) fish- length of fishes. eries are two important traditional fisheries in the In the area studied (northern coasts of Spain), Northeast Atlantic particularly in volume of catches, mackerel is only a target species of the handline fleet with estimated mean landings of 715,000 and (41% of total landings) during the spawning season 430,000 tonnes respectively in the years from 1989 (March and April), and is a by catch of other fleets to 1994 (Anon., 1996). (purse seine (40%), trawl (17%) and gillnet (2%)). In the area of the Bay of Biscay (ICES Subarea Horse mackerel is mainly caught by purse seine (61%) VIII), the mackerel and horse mackerel fisheries are and trawl fleets (38%) and to a lesser extent by hook made up of different fishing gears (purse seine differ- (0.5 %) and gillnet fleets (0.5%). This fishing strategy ent trawling methods, long-line and gillnet). Accord- did not vary in the period of study (Anon., 1996). ing to the data estimated for the period from 1992 to To improve the management of these fisheries, 1994 (Anon., 1993, 1995a, 1996) mean annual land- the stocks (considered as fisheries management ings of mackerel from the Bay of Biscay were 22,300 units) for each of these species were established tonnes, while those of horse mackerel were 48,000 t. between the seventies and the eighties, referring to Around 80% of mackerel landings corresponded to zones of their distribution areas in the Northeast Spain. In the case of horse mackerel landings, more Atlantic: the North Sea Stock (in ICES Divisions than 55% were made by Spain, 25% by the Nether- IIIa, IVbc and part of IVa); the Western Stock (Divi- lands and about 8% by France and Denmark. Anoth- sions IIa, the rest of IVa, Subareas VI and VII and er 1,700 t of mackerel and 8,700 t of horse mackerel Divisions VIIIabd), and the Southern Stock (Divi- annually must be added to these figures, from Divi- sions VIIIc and IXa, that is to say, almost exclusive- sion IXa North (entirely caught by Spain). ly restricted to waters around Spain and Portugal).

346 B. VILLAMOR et al. FIG. 2. – The monthly distribution of mackerel (1983-1994) and horse mackerel (1989-1994) landings (t) in the Spanish VIIIc and IXa fishery.

The delimitation of stocks was made, partly The aim of the present paper is to describe the based on biological indications available from seasonal and geographical distribution of these two research at that time, but also in part based on polit- species in the southernmost part of the Bay of Bis- ical/administrative convenience to facilitate the cay (Division VIIIc), although due to its proximity management of these fisheries (Lucio and Villamor, reference will also be made to the northern part of in preparation). Nevertheless, in the case of macker- Division IXa. It must be pointed out that 75% of all el at least, the results of research mainly carried out estimated international mackerel landings and 52% in the nineties involving fish tagging, the study of of those of horse mackerel, corresponding to the egg distribution and abundance, and the analysis of Bay of Biscay (Subarea VIII), come from the south- the characteristics of the fisheries in areas adjacent ern area (Division VIIIc). For this reason the study to the Western and Southern Stocks (that is to say, in will be fundamentally based on the data of Spanish the south of the Bay of Biscay), the veracity of this commercial fisheries (which, in practice, are the distinction was placed in doubt, considering in only ones which fish in these waters), but other Anon., (1996) that the Northeast Atlantic mackerel complementary information will be taken into constituted a single stock. account with respect to this area. In the area of study Mackerel and horse mackerel are migratory there is very little information on spatial and sea- species and migrations have been reported, particu- sonal distribution of these species. Only Villamor et larly off the coasts of Northern Europe (Macer, al. (1994) can be mentioned, who present a mean 1977; Lockwood, 1988; Walsh and Martin, 1986; distribution of mackerel from 1983 to 1993, and Eltink et al., 1986; Anon., 1990a; Walsh, 1994; Anadón (1950); Lozano Cabo (1952) with a general Borges et al., 1995), and in the case of mackerel also description of horse mackerel fishery in Sub-divi- in the Bay of Biscay (Uriarte, 1995). sions VIIIc West and IXa North.

DISTRIBUTION AND AGE STRUCTURE OF MACKEREL AND HORSE MACKEREL 347 FIG. 3. – The monthly distribution of mackerel landings (t) by year and ICES Sub-division (Sub-division IXa North, VIIIc West and VIIIc East) for the period 1989-1994, including total Spanish annual landings by Sub-division.

348 B. VILLAMOR et al. FIG. 4. – Mean monthly landings (t) of mackerel by statistical rectangle in the South of the Bay of Biscay, for the period 1983-1994.

MATERIAL AND METHODS defined statistical rectangles, commonly used in the ICES area. In order to search for possible dis- The area of study is divided into ICES divi- tribution changes as a function of age in the case sions (VIIIc and IXa) and sub-divisions (VIIIc of mackerel, Sub-division VIIIc East was divided East, VIIIc West and IXa North) (Anon., 1992a into Sub-division VIIIc East 1 and VIIIc East 2 and 1992b), which are in turn divided into pre- (Fig. 1).

DISTRIBUTION AND AGE STRUCTURE OF MACKEREL AND HORSE MACKEREL 349 Data of monthly landings were obtained from fishing vessel owners’ associations and fishermen’s associations through the existing information net- work of the IEO and AZTI (advisory organisations to fisheries and oceanography administration) in all ports of the Cantabrian and Galician coast. The series of catches per unit effort (CPUE’s) of the commercial fleets are analysed quarterly to determine with greater precision and corroborate whether there are abundance changes in the area. For mackerel, the hook (handline) fleet of the port of Santoña, which operates in Sub-division VIIIc East, the trawl fleet of La Coruña, operating in Sub-division VIIIc West, and the operating results of the purse seine fleet of Vigo and Riveira, which fishes in Sub-division IXa North, are analysed. For horse mackerel, the trawl fleets of the ports of Avilés and of La Coruña, which work in Sub-division VIIIc East and VIIIc West respectively, are analysed. Data used to obtain the CPUE’s are the quarterly landings, number of fishing trips and some characteristics of the fleets (motor power in HP and number of vessels). These were obtained from the daily sales sheets supplied by fishing vessel owners’ associations and fishermen’s associations. Fishing effort in the case of purse seine fleets is given as the number of effective fishing trips, in the case of trawl as HP*fishing days*10-2 and in the case of the handline fleet as the total numbers of fishing trips of this fleet. Effort for the different fleets was calculated by month and quarter. In order to describe the distribution of juvenile and adult concentrations, the quarterly catches in number and percentage by Sub-division and by age groups are presented, considering ages 0,1 and 2 to be juveniles FIG. 5. – Mackerel catch per unit effort series (1989-1994) in com- (Lockwood and Johnson, 1977; Cort, 1982; Cort et mercial fisheries by quarter. al., 1986; Kerstan, 1988; Anon., 1990b; Lucio, 1990a and 1990b) and mainly adults those of age >2 (3 and more years). To determine age structure of mackerel To find the spatial and seasonal distribution of and horse mackerel catches, half year length age keys Spanish landings of mackerel and horse mackerel in were applied to the length distributions of catches the northern coast of Spain, mean monthly landings obtained by month and quarter. Otoliths were used to were mapped by ICES statistical rectangle from designate ages according to the reading criteria estab- 1983 to 1994 in the case of mackerel and from 1989 lished in the ICES area for mackerel (Anon., 1987; to 1994 in the case of horse mackerel. Landings by Anon., 1995b) and horse mackerel (Eltink and Kuiter, ICES statistical rectangle were obtained from infor- 1989; Anon., 1991a). The monthly total of length dis- mation relating to the fishing grounds of monthly tributions was obtained by carrying out length sam- samples of mackerel and horse mackerel landings pling by fishing gears in the main ports of the obtained by the fleet in the ports. These samples, Cantabrian and Galician coast at monthly intervals. distributed by statistical rectangle, are pondered to Otoliths are obtained from biological sampling in the the total of landings at each port, and each year the laboratory, as well as from trawl and acoustic surveys total monthly landings are obtained by statistical performed in the area of study, and a full representa- rectangle for the whole of the area of study. tion of the whole length range was achieved.

350 B. VILLAMOR et al. FIG. 6. – The monthly distribution of horse mackerel landings (t) by year and ICES Sub-division (Sub-division IXa North, VIIIc West and VIIIc East) for the period 1989-1994, including total Spanish annual landings by Sub-division.

DISTRIBUTION AND AGE STRUCTURE OF MACKEREL AND HORSE MACKEREL 351 FIG. 7. – Mean monthly landings (t) of horse mackerel by statistical rectangle in Division VIIIc and Sub-division IXa North, for the period 1989-1994.

352 B. VILLAMOR et al. FIG. 8. – Horse mackerel catch per unit effort series (1990-1994) in commercial fisheries by quarter.

Data from bottom trawl surveys are presented in RESULTS number of specimens per trawl hour, by age groups (0, 1, 2 and >2) and by Sub-division. Data presented Spatial and seasonal distribution of landings in are those from the surveys carried out in autumn weight (September-October) in the whole of the area studied (Division VIIIc and Sub-division IXa North) Mackerel from 1989 to 1994. The methodology of these surveys is described in detail by various authors (Fariña The mackerel landings of the Spanish fishery in et al., 1985; Pereiro and Piñeiro, 1985; González- the south of the Bay of Biscay to the north and Gurriarán and Olaso, 1987; 1990; Sánchez et al., north-west of the Iberian Peninsula by months for 1991). These surveys used a kind of bottom trawl the period 1983-1994 (Fig. 2) show clear seasonali- gear called “baca” (similar to the gear normally used ty, constant year after year, with predominant land- in these waters by the commercial trawl fleet) which ings in spring. Mackerel landings in this area show targets demersal species. The limitations of this kind great stability year after year, with a mean annual of survey must consequently be borne in mind when total of 17,000 t, caught by a fleet which has fol- considering the validity of abundance indices of lowed a practically constant fishing strategy pelagic species. With these reservations, the data throughout these years (Anon., 1996). from these surveys may provide some indications of The monthly distribution of mackerel landings by the distribution and abundance of mackerel and Sub-division from 1989 to 1994 indicates that landings horse mackerel in autumn. in Sub-division VIIIc East make up 70% of total land-

DISTRIBUTION AND AGE STRUCTURE OF MACKEREL AND HORSE MACKEREL 353 FIG. 9a. – Catch at age distribution of mackerel by quarter (1st and 2nd quarter), year and Sub-division as percentage of total catch in number for the whole area in the period 1989-1994, including catch in weight (t) and catch in number (‘000) for the whole area. ings in weight of the whole area studied, with more than second half of the year, the peak catch varying from 80% of landings in this Sub-division coming in the July to October over the years studied. (Fig. 3) months of March and April. Landings in Sub-division The distribution of mean monthly landings from VIIIc West make up 20% of total landings, mainly com- 1983 to 1994 by ICES statistical rectangle shows an ing in April. Landings in Sub-division IXa North make inter-related spatial and seasonal variation. The fish- up 10% of total landings, and are made mainly in the ery advances from east to west during spring. (Fig. 4)

354 B. VILLAMOR et al. FIG. 9b. – Catch at age distribution of mackerel by quarter (3rd and 4th quarter), year and Sub-division as percentage of total catch in number for the whole area in the period 1989-1994, including catch in weight (t) and catch in number (‘000) for the whole area.

The series of CPUE’s of the commercial fleets second half. In Sub-division IXa North, where the indicate the existence of seasonality in the macker- level of catches in weight is very small, CPUE’s el population in the area studied. In Division VIIIc, increase progressively throughout the year, reach- the CPUE’s of the commercial fleets are higher in ing their highest levels in the second half of the the first half of the year, and practically zero in the year. (Fig. 5)

DISTRIBUTION AND AGE STRUCTURE OF MACKEREL AND HORSE MACKEREL 355 TABLE 1. – Annual catch in number of 0-group, 1-group, 2-group and >2 group mackerel in millions. Figures in brackets are annual catch of 0-group, 1-group, 2-group and >2 group as percentage of all groups, in each Sub-division.

Sub-Division 1989 1990 1991 1992 1993 1994

0-group IXa North 27.37 5.98 3.06 40.51 5.46 24.35 (73.8%) (44.8%) (40.8%) (95.6%) (61.5%) (94.4%) VIIIc West 6.43 3.53 0.01 0.00 0.12 0.00 (44.1 %) (16.8%) (0.1 %) (0.0%) (0 9%) (0.0%) VIIIc East 1 0.22 3.91 1.46 0.57 0.02 0.33 (2.0%) (21.1 %) (7.9%) (3.8%) (0.1%) (1.7%) VIIIc East 2 0.00 0.00 0.00 0.00 0.00 0.00 (0.0%) (0.0%) (0.0%) (0.0%) (0.0%) (0.0%)

Total 34.02 13.42 4.53 41.08 5.59 24.68

1-group IXa North 8.68 5.45 1.22 0.67 2.02 0.04 (23.4%) (40.8%) (16.3%) (1.6%) (22.7%) (0.1%) VIIIc West 2.19 11.86 2.50 0.91 6.14 0.51 (15.0%) (56.5%) (15.0%) (16.4%) (47.8%) (3.6%) VIIIc East 1 0.82 5.18 3.92 3.35 0.41 0.14 (7.5%) (28.0%) (21.3%) (22.2%) (2.8%) (0.7%) VIIIc East 2 0.09 0.01 0.14 0.02 0.06 0.02 (0.6%) (0.1%) (0.7%) (0.1%) (0.3%) (0.1%)

Total 11.78 22.50 7.79 4.95 8.63 0.71

2-group IXa North 0.67 1.13 1.35 0.24 0.63 0.07 (1.8%) (8.5%) (18.1%) (0.6%) (7.1%) (0.3%) VIIIc West 1.72 1.38 2.75 1.24 3.60 4.04 (11.8%) (6.6%) (16.5%) (22.5%) (28,1%) (28.7%) VIIIc East 1 0.40 0.48 1.10 1.50 1.02 0.70 (3.7%) (2.6%) (6.0%) (10.0%) (6.9%) (3.7%) VIIIc East 2 0.33 0.37 1.16 0.09 0.80 0.63 (2.3%) (2.1%) (5.7%) (0.5%) (3.8%) (2.7%)

Total 3.12 3.36 6.36 3.08 6.04 5.45

>2-group IXa North 0.37 0.79 1.86 0.97 0.77 1.34 (1.0%) (5.9%) (24.8%) (2.3%) (8.7%) (5.2%) VIIIc West 4.24 4.19 11.37 3.37 2.98 9.56 (29.1%) (20.0%) (68.4%) (61.0%) (23.2%) (67.7%) VIIIc East 1 9.46 8.97 11.95 9.65 13.28 17.98 (86.8%) (48.4%) (64.8%) (64.1%) (90.2%) (93.9%) VIIIc East 2 14.02 17.23 19.00 16.64 20.10 22.62 (97.1%) (97.9%) (93.6%) (99.3%) (95.9%) (97.2%)

Total 28.09 31.18 44.17 30.64 37.17 51.49

356 B. VILLAMOR et al. TABLE 2. – Distribution and abundance of mackerel by age group in number per hour trawl in autumn 1989-1994 from Spanish bottom trawl survey.

Sub-Division 1989 1990 1991 1992 1993 1994

0-group

IXa North 0 0 0 13 3 0

VIIIc West 0 0 0 0 0 0

VIIIc East 1 1 7 2 60 1 0

VIIIc East 2 0 1 1 6 1 21

Total 1 8 3 79 5 21

1-group

IXa North 0 0 0 1 34 0

VIIIc West 0 1 1 0 2 0

VIIIc East 1 0 18 5 6 16 2

VIIIc East 2 0 0 0 0 0 0

Total 0 19 6 7 52 2

2-group

IXa North 0 0 0 0 13 0

VIIIc West 0 1 1 1 2 0

VIIIc East 1 0 1 4 1 10 2

VIIIc East 2 0 0 0 0 0 0

Total 0 2 5 3 25 2

>2-group

IXa North 0 0 0 0 0 0

VIIIc West 0 0 5 2 0 3

VIIIc East 1 1 0 15 3 2 3

VIIIc East 2 0 0 0 0 0 0

Total 1 0 20 5 2 7

Horse mackerel about in the months between spring and the end of autumn, peaks of catches varying from one year to The mean landing of horse mackerel in the area another. The main peaks appear most often in June- studied is 30,000 tonnes annually, a level which has July and September, although November is also a remained stable throughout the period analysed noteworthy month for landings. During the winter (Anon., 1996). months landings are smaller, although they are Stability of landings is also observed during sometimes outstanding, as observed in the years much of the year. The most abundant landings come 1989 and 1990. (Fig. 2)

DISTRIBUTION AND AGE STRUCTURE OF MACKEREL AND HORSE MACKEREL 357 TABLE 3. – Annual catch in number of 0-group, 1-group, 2-group and >2 group horse mackerel in millions. Figures in brackets are annual catch of 0-group, 1-group, 2-group and >2 group as percentage of all groups, in each Sub-division.

Sub-Division 1990 1991 1992 1993 1994

0-group

IXa North 32.60 6.80 0.10 0.93 7.88 (31.2%) (11.6%) (0.2%) (1.9%) (7.7%)

VIIIc West 0.04 0.06 0.16 0.03 0.56 (0.0%) (0.1%) (0.1 %) (0.0%) (0 3%)

VIIIc East 0.08 17.34 2.31 1.33 0.81 (0.1%) (27.2%) (2.3%) (1.6%) (1.1%)

Total 32.72 24.20 2.57 2.28 9.25

1-group

IXa North 50.56 28.46 41.39 11.63 32.13 (48.3%) (48.4%) (69.1%) (23.5%) (31.2%)

VIIIc West 42.09 10.24 86.74 9.39 36.49 (27.3%) (8.1%) (51.6%) (9.3%) (21.8%)

VIIIc East 36.89 3.75 59.13 13.19 8.95 (46.7%) (5.9%) (58.4%) (16.2%) (12.5%)

Total 129.53 42.45 187.27 34.22 77.56

2-group

IXa North 8.37 7.43 8.64 17.66 43.49 (8.0%) (12.6%) (14.4%) (35.7%) (42.3%)

VIIIc West 17.09 18.74 10.03 33.98 93.15 (11.1%) (14.9%) (6.0%) (33.5%) (55.8%)

VIIIc East 6.30 23.74 2.26 14.17 21.95 (8.0%) (37.2%) (2.2%) (17.4%) (30.6%)

Total 31.76 49.91 20.93 65.80 158.60

>2-group

IXa North 13.09 16.09 9.73 19.28 19.40 (12.5%) (27.4%) (16.3%) (38.9%) (18.9%)

VIIIc West 95.03 96.80 71.01 57.90 36.87 (61.6%) (76.9%) (42.3%) (57.2%) (22.1%)

VIIIc East 35.7 18.93 37.61 52.92 40.01 (45.2%) (29.7%) (37.1%) (64.9%) (55.8%)

Total 143.89 131.82 118.36 130.11 96.28

358 B. VILLAMOR et al. TABLE 4. – Distribution and abundance of horse mackerel by age group in number per hour trawl in autumn 1990-1994 from Spanish bottom trawl survey.

Sub-Division 1990 1991 1992 1993 1994

0-group

IXa North 47 0 13 198 0

VIIIc West 1 0 0 119 49

VIIIc East 5 62 214 278 3290

Total 53 62 227 595 3339

1-group

IXa North 0 0 0 22 0

VIIIc West 000062

VIIIc East 27 20 111 96 194

Total 27 20 111 118 256

2-group

IXa North 00030

VIIIc West 00002

VIIIc East 4 7 2 10 69

Total 4 7 2 13 71

3+ -group

IXa North 9 21 22 80 80

VIIIc West 8 24 179 129 82

VIIIc East 168 36 198 172 85

Total 185 81 399 381 247

Horse mackerel fishing takes place along the ty of yields in the first three quarters of the year, and whole of the continental shelf. Sub-division VIIIc later fall in the fourth. In Sub-division VIIIc East the West is where the highest volume of landings is lowest yields are observed in the third quarter. (Fig. 8) achieved, always making up over 43% of the landed total (Figs. 6 and 7). Spatial and seasonal distribution of catches at A slight spatial-seasonal variation exists in horse age mackerel landings in a western direction as the year progresses. Landings in Sub-division VIIIc East are Mackerel somewhat higher in the first half of the year, while in Sub-division IXa North they are higher at the end The catch of mackerel juveniles of age group 0 in of the summer and in autumn (Figs. 6 and 7). the area studied takes place in the second half of the The catch per unit effort series of the trawl fleets year. They are mainly caught in Sub-division IXa in Sub-division VIIIc West reflect a certain instabili- North, though they appear in Sub-division VIIIc West

DISTRIBUTION AND AGE STRUCTURE OF MACKEREL AND HORSE MACKEREL 359 FIG. 10a. – Catch at age distribution of horse mackerel by quarter (1st and 2nd quarter), year and Sub-division as percentage of total catch in number for the whole area in the period 1990-1994, including catch in weight (t) and catch in number (‘000) for the whole area.

and VIIIc East 1 in some years. The proportion of the of the year, mainly in March and April. In the sec- catch of age group 0 varies from one year to another. ond half of the year they disappear drastically from In Sub-division VIIIc East 2 there is no catch of this the area. This pattern of spatial and seasonal distrib- age group. (Table 1 and Figs. 9a and 9b ) ution of catches is constant year after year (Figs. 9a Age groups 1 and 2 appear during the whole and 9b). year throughout the area, with great variability in In Sub-division VIIIc East 2 the catch of adults the catch in number among years and Sub-divi- makes up between 94% and 99% of the annual catch sions. The catches of these age groups are insignif- in number for the years studied. Meanwhile, in Sub- icant in Sub-division VIIIc East 2. (Table 1 and division IXa North the percentage of the annual Figs. 9a and 9b) catch of juveniles makes up between 75% and 97%. Adult fishes (>2 years) are caught throughout (Table 1) Division VIIIc, although mainly in Sub-division The data of the bottom trawl surveys (number of VIIIc East. Adult catches diminish gradually specimens per trawl hour) carried out in autumn of towards the west of Division VIIIc in both number the years 1989 to 1994 indicate a scarce presence of and percentage. (Table 1 and Figs. 9a and 9b) this species, the catch consisting mainly of juveniles These adult specimens are caught in the first half (Table 2).

360 B. VILLAMOR et al. FIG. 10b. – Catch at age distribution of horse mackerel by quarter (3rd and 4th quarter), year and Sub-division as percentage of total catch in number for the whole area in the period 1990-1994, including catch in weight (t) and catch in number (‘000) for the whole area.

Horse mackerel of adults in the catch with respect to the total catch of the area is significantly lower during the fourth Fish at age 0 are partially recruited to fishable quarter when compared with that of the first and sec- part of the stock from the third quarter of the year, ond (Figs. 10a and 10b). In the catches of Sub-divi- and are caught fundamentally in Sub-divisions VIIIc sion VIIIc West, the fall in the percentage of adults East and IXa North. Ages 1 and 2 are already fully in 1994 is noteworthy (Table 3). recruited in any of the Sub-divisions or seasons of The variability in the catch of juveniles among the year (Table 3 and Figs. 10a and 10b). As with years and the presence of adults throughout the year age 0, great differences are found in catch levels of is corroborated by the data analysis of the catch per these age groups, among years, Sub-divisions and unit effort at age of the trawl fleets of Avilés and La quarters, with no clearly observed trend to describe Coruña (Figs. 11 and 12). In the figures it can also these variations (Fig. 10b). In Sub-division IXa be observed that age 0 has not been accessible to North, the catch in number is mainly made up of these fleets, due either to the gear or to the area juveniles, as in the case of mackerel, at over 80% of (owing, for example, to fish being close in to the the catch in this Sub-division (Table 3). coast), or has been discarded by them and that yields Adults are found in all Sub-divisions throughout of adults descend for the fleet of La Coruña during the year. In Sub-division VIIIc East the percentage the fourth quarter.

DISTRIBUTION AND AGE STRUCTURE OF MACKEREL AND HORSE MACKEREL 361 FIG. 11. – Horse mackerel catch per unit effort at age series (1990- FIG. 12. – Horse mackerel catch per unit effort at age series (1990- 1994) of the trawl Avilés fleet (Sub-division VIIIc East). 1994) of the trawl La Coruña fleet (Sub-division VIIIc West).

The results of the autumn bottom trawl surveys DISCUSSION reflect that the highest yields come about in Sub- division VIIIc East for all age groups. On the other Mackerel hand, the presence of adults is patent in all Sub-divisions, to the contrary of what was found in macker- The results show that the mackerel fishery in the el, in which the presence of adults is very scarce, as area studied presents strong seasonality, and takes is the extreme inter-annual variability in juvenile place mainly in the first half of the year in Division numbers (Table 4). VIIIc over the continental shelf. Mackerel spawning

362 B. VILLAMOR et al. in these areas takes place in spring, from February to juvenile area of the area studied. Upwelling phe- June, reaching a peak in April (Solá et al., 1990, nomena occur in waters of the continental shelf of 1996) over the continental shelf and away from it, a Galicia (VIIIc West and IXa North) between April great abundance of eggs appearing to the south of and September, making it an area of high primary 44º 30’N in the central and western areas of Divi- production (Cabanas et al., 1992; Varela, 1992), thus sion VIIIc (Lago de Lanzós et al., 1993; Solá et al., playing an important role in recruitment processes 1996). The peak of mackerel catches in Division (Cushing, 1995). VIIIc comes in March and April during the spawn- The behaviour and bathymetric distribution of ing season and consists of spawning mackerel of age mackerel renders the information obtained on the >2 years (Cort, 1982; Cort et al., 1986; Lucio, species from bottom trawl surveys to be of little use 1990a; Villamor et al., 1994; Anon., 1996). In the in the estimation of real abundance of the resource second half of the year adults disappear from the (Sánchez et al., 1994). With these reservations, it is area, and neither are they to be found in Division possible to obtain some idea of mackerel distribu- IXa (Anon., 1996). tion and abundance in autumn, which indicate the The series of CPUE’s of the commercial fleets scarce presence of mackerel in the second half of the indicate that there is seasonality in mackerel abun- year, juveniles making up most of the catch, which dance in the area studied, and confirm that seasonal takes place mainly in south Galician waters (Sub- and spatial variation of the mackerel fishery is relat- division IXa North) and the central part of the ed to the seasonal and spatial variation of the abun- Cantabrian Sea (Sub-division VIIIc East 1). This is dance of this species in the area. in agreement with Villamor et al., (1994) who show Typically migratory populations are character- the mean distribution of mackerel juveniles and ized by a biological cycle in which spawning adults from autumn bottom trawl surveys for the grounds, juvenile areas and feeding areas are inter- years 1983 to 1993. These data indicate two main related by a migratory pattern which, in temperate centres of distribution for age group 0, a large one in seas, tends to be constant over the years (Cushing, Sub-division IXa North and a smaller one in the cen- 1975). This migratory pattern may be the reason tral part of Division VIIIc (Sub-division VIIIc East why seasonality in catches or a spatial-seasonal vari- 1). Ages 1 and 2 are mainly concentrated in Sub- ation in age compositions of catches are obtained. In division IXa North and the data of adult mackerel in the case of mackerel, these two aspects are clearly autumn from 1983 to 1993 indicate that abundances observed and furthermore, recent tagging experi- are very scarce and are mainly concentrated in North ments (Uriarte, 1995) show that spawning mackerel, Galician waters (Sub-division VIIIc West) and the which are the target of the spring fishery in the western part of Sub-division VIIIc East 1. Cantabrian Sea, migrate at the end of spring and In view of the results, it can be concluded that summer towards feeding areas off the Norwegian adult mackerel go to the South of the Bay of Biscay coast, where they spend the autumn before returning to reproduce in the first half of the year, entering the the following spring to the spawning grounds of the area by the south-eastern part, advancing to the west western component of the North-eastern Atlantic of the Cantabrian Sea through the months of spring mackerel. in the same direction as the weak anti-cyclonic cur- The results also show that the mackerel fishery in rents in the Bay of Biscay (Pingree, 1993). After these areas advances from east to west through spawning, these mackerel return towards the north spring in the years studied, coinciding with Uriarte to the feeding areas. Doubts remain about the route (1995), who points to an initial migration of mack- by which they make this return, whether it is by the erel to the west along the coast of the Cantabrian north-west of the Spanish coast, crossing the ocean Sea, and Villamor et al. (1994) who also find a dis- towards the north-west of the Bay of Biscay or fol- placement of the fishery to the west. lowing the same route by which they came to return Mackerel growth in the first months is very fast, along the continental shelf of the bay. Uriarte (1995) reaching 22 cm by the end of the year in which they suggests the former on the basis of the initial migra- were born while still in age group 0. At this size and tion found towards the west of the Cantabrian Sea, less (mackerel of 15-20 cm) they are exploited by during which spawning takes place in the western the commercial fisheries in the second half of the part of the continental shelf of the Cantabrian Sea year mainly in Sub-division IXa North, as are mack- and outside it (Lago et al., 1993). Furthermore, this erel of ages 1 and 2, this being the main mackerel may be supported by Priede et al. (1995), who found

DISTRIBUTION AND AGE STRUCTURE OF MACKEREL AND HORSE MACKEREL 363 adult spawning mackerel in June throughout the be due to two factors: a greater presence of adults in shelf area and beyond, to the West of Ireland in the area by spawning migration or a greater avail- waters of over 3000 m depth. In the second half of ability to the fishing gears of the resource owing to the year mackerel found are those born the same horse mackerel behavioural characteristics in the year (age 0) and mackerel juveniles of ages 1 and 2 spawning season, or a combination of both. appear mainly in the north-western part of the Iber- Spawning migration may take place from the ian Peninsula (IXa North), although they may also North of the Bay of Biscay and Celtic Sea in the be found in the central (VIIIc East 1) and western same direction as the weak anti-cyclonic currents (VIIIc West) parts of the North of Spain in much from east to west in the Bay of Biscay (Pingree, smaller quantities. The few adults found in the sec- 1993). With respect to greater availability, it must be ond half of the year are concentrated in North Gali- taken into account that horse mackerel is a serial cian waters (VIIIc West). spawner with a very long spawning period (Macer, Thus, Sub-division IXa North is mainly an area 1974; Borges and Gordon, 1991). Pitcher (1995) of recruitment. In the spawning season, adults con- argues that a partial spawning process forces pelag- centrate mainly in Division VIIIc, although there is ic fish to spend more time in the aggregation phase, also a presence of eggs (Solá et al., 1990, 1996) and becoming more vulnerable to fishing gears. Never- spawning adults (Villamor et al., 1994) in Sub-divi- theless, Macer (1977) and Lockwood and Johnson sion IXa North. (1977) describe horse mackerel behaviour as being of greater aggregation in the winter and greater dis- Horse mackerel persion in the spawning season. The increase in yields of the fleet of La Coruña (Sub-division VIIIc Certain stability in horse mackerel catches in West) in the second and third quarters and the weight and in age composition throughout the year, greater percentage of adults in the first half of the particularly in the adult phase when considered as a year in Sub-division VIIIc East are in accordance whole, leads us to consider that the horse mackerel with both lines of reasoning. population appears more stable and stationary than The fact that greater catches are obtained in Sub- typically migratory, above all in the westernmost division VIIIc West is chiefly explained by the size part of the area of study. This stability was of the fishing area and effort applied. In this Sub- described by Anadón (1950) on studying different division, the continental shelf is much greater than pelagic fish species in Sub-divisions VIIIc West in the rest, and the number of vessels which work and IXa North. there is also greater. There are also variations, although of small mag- Once the spawning season has concluded, a nitude. To study the nature of these with greater pre- feeding migration may be one interpretation of cision, the series of catch per unit effort (CPUE) what is observed in the fall in fishing yields, as hap- were analysed, although this analysis also presents pens in the case of mackerel. Nevertheless, in horse limitations. Hilborn and Walters (1992) define mackerel the fall in yields which comes about is CPUE as a relative abundance index which is not small and, indeed, in some Sub-divisions, such as always directly proportional to the abundance of the IXa North, catches increase. It may be that a slight resource. The greatest yields observed in the Avilés fall in yields of the fleet of La Coruña (Sub-division trawl fleet, and in the volume of the total catch in VIIIc West) in the fourth quarter is compensated by Sub-division VIIIc East in the first half of the year, an increase observed in the catches of Sub-division coincide with the spawning season of horse macker- IXa North. Horse mackerel in the South of the Bay el, which takes place mainly during late spring in of Biscay do not seem to need to make long migra- this area (Lucio and Martin, 1989). Horse mackerel tions in search of food. The upwelling characteris- in the area of study reaches sexual maturity at 21 cm tics in Sub-divisions VIIIc West and IXa North in males and 22 cm in females (Abaunza et al., (Fraga, 1981) and in the central part of Sub-division 1995a). Females reach maturity at between three VIIIc East (Botas et al., 1990) bring about a high and four years of age (Lockwood and Johnson, primary production (Varela, 1992). This would 1977; Kerstan, 1988; Lucio, 1990b). Given the pres- ensure, through the food-chain, the feeding of ence of eggs in the area of study in this season (Solá mainly zooplanktivorous species like horse macker- et al., 1990; Porteiro et al., 1993; Franco et al., el, although the area is not really characterized by 1993; Solá et al., 1996), the increase in yields may an high zooplankton biomass index (Valdés et al.,

364 B. VILLAMOR et al. 1990). The good catches observed in November and ACKNOWLEDGEMENTS the results of the series of trawl surveys, on reflect- ing a presence of adults in all Sub-divisions which The authors wish to thank all those who worked is repeated year after year, would support this in the ICES fisheries team at the Instituto Español de hypothesis. The feeding areas even seem to remain Oceanografía (I.E.O.) and Instituto de Investigación stable, with scarce changes between Sub-divisions y Tecnología para la Pesca, Oceanografía y Ali- in the search for food, as deduced by the differences mentación (AZTI). found in Anisakis simplex infestation levels among the different Sub-divisions (Abaunza et al., 1995b). However, the differences observed in adult catches REFERENCES in the second half of the year in the easternmost Abaunza, P., A.C. Fariña and P. Carrera. – 1995a. Geographic vari- area of Division VIIIc seem to suggest more ations in sexual maturity of the horse mackerel, Trachurus tra- marked migratory behaviour of adults in these churus, in the Galician and Cantabrian shelf. Sci. Mar., 59: 211- 222. waters. Abaunza, P., B. Villamor and J.R. Pérez. – 1995b. Infestation by The lengths corresponding to the youngest ages larvae of Anisakis simplex (Nematoda: Ascaridata) in horse mackerel, Trachurus trachurus, and Atlantic mackerel, are below 14 cm for age 0 and up to 18 cm for age Scomber scombrus, in ICES Divisions VIIIb, VIIIc and IXa (N- 1 (Anon., 1992b). The great variations found in the NW of Spain). Sci. Mar., 59: 223-233. Anadón, E. – 1950. Sobre la sustitución alternativa en el litoral gal- catch of these ages, both in surveys and commercial lego de los llamados peces emigrantes (sardina, espadín, catches, may be due to several reasons. Firstly, anchoa y jurel). Bol. Inst. Esp. Oceanogr. 24: 1-20. Anon. – 1987. 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366 B. VILLAMOR et al.