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Spatial and temporal distribution and abundance of European hake, Merluccius merluccius, eggs and larvae in the Catalan coast (NW Mediterranean)
Fisheries Research 60 (2003) 321–331
Spatial and temporal distribution and abundance of European hake, Merluccius merluccius, eggs and larvae in the Catalan coast (NW Mediterranean) M.P. Olivar∗ , G. Qu´ılez, M. Emelianov Institut de Ciències del Mar (CMIMA-CSIC), Passeig Maritim 37-49, 08003 Barcelona, Spain Received 15 January 2002; received in revised form 14 June 2002; accepted 27 June 2002
Abstract Plankton hauls were conducted on five surveys from November 1998 to November 1999 to study the spatial distribution of eggs and larvae of European hake off Catalonia (NW Mediterranean Sea). Also CTD casts to record hydrographic parameters were carried out on a closely spaced station grid. Merluccius merluccius eggs and larvae appeared mainly in late spring, summer and autumn surveys and were very scarce in winter. Strong differences in terms of egg and larval densities were observed between the two November surveys, which could be attributed to the anomalous hydrographic situation during November 1998. M. merluccius egg and larvae were mainly distributed over the continental shelf, with peak abundances between the 100 m isobath and the edge of the shelf. On the evidence of larval size frequency distributions in the different sampling sectors and the closely overlapping distribution patterns for the eggs and the adult spawning stock, drifting of hake eggs and larvae was not a major factor. The larval distribution extended only slightly further offshore than the egg distribution. Using the hydrographic information and the larval distribution data, an attempt was made to relate the different seasonal productivity levels over the spawning period and the distribution of the larvae. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Merluccius merluccius; Hake eggs; Hake larvae; Distribution patterns; NW Mediterranean
1. Introduction The European hake, Merluccius merluccius, is a moderately large fish widely distributed in the eastern North Atlantic Ocean and Mediterranean Sea (Alheit and Pitcher, 1995). The species inhabits the shelf and shelf break zones throughout the western Mediterranean (Oliver and Massut´ı, 1995). It is the main commercial demersal species caught off Catalonia
∗ Corresponding author. Tel.: +34-93-230-95-56; fax: +34-93-230-95-55. E-mail address: [email protected] (M.P. Olivar).
(NW Mediterranean), with annual landings of 2000 t (Mart´ın, 1991). A number of studies dealing with various aspects of the biology of this species and its fishery in the Atlantic have been published over the past decade (Casey and Pereiro, 1995; Ramos and Fernández, 1995; Álvarez et al., 2001; Fives et al., 2001), but knowledge of these aspects in the Mediterranean is more scanty (Recasens, 1994), particularly for the early developmental stages. Hake larvae have been poorly represented in plankton samples in the Mediterranean (Sabatés, 1990; Sabatés and Olivar, 1996), and there have been no previous studies dealing specifically with hake egg distribution and abundance.
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The first half of the year has been described as the spawning season for this species in the Atlantic, although ripe specimens are caught all year round (Pérez and Pereiro, 1985; Casey and Pereiro, 1995). Analysis of adult gonadal development in the NW Mediterranean has also indicated the presence of year-long reproductive activity, the most pronounced peak being in autumn (Recasens et al., 1998). During this period females tend to aggregate towards the edge of the shelf (Recasens et al., 1998). The aim of this study was to analyse spatio-temporal patterns of M. merluccius egg and larval distribution in the NW Mediterranean in relation to different environmental conditions. The study was based on five cruises performed in the different seasons over an annual cycle (November 1998–November 1999). This paper reports the vertical profiles of temperature, salinity and fluorescence for the several surveys and discusses how variability in hydrographic structure may affect the eggs and larval abundance.
2. Materials and methods A smaller spatial sampling resolution scale than previously used for other species, particularly in the areas with the highest concentrations of spawning adults, was considered necessary for this study. Earlier studies on seasonal fecundity and spawning carried out (Recasens et al., 1998) were used as a guide in planning the ichthyoplankton survey schedule. To ascertain the spatial distribution patterns, three main surveys (November 1998, September 1999, and November 1999) were planned to coincide with the months of maximum gonadal development in
Fig. 1. Map of the study area showing transects and sampling stations.
summer–autumn (Recasens et al., 1998). In addition, extra ichthyoplankton samples with reduced spatial coverage were collected in February and June 1999, to provide information on the four seasons of the year (Table 1). The sampling grid consisted of very closely spaced stations located on cross-shelf transects roughly perpendicular to the shore 5–10 miles distant from each other (Fig. 1). The stations along the transects ranged from 40 to 500–1000 m isobaths. In the region with the highest adult spawning population concentration, stations were separated by a distance of only 2.5 miles, while in the more outlying areas the separation was 5 miles. The purpose was to compile information with
Table 1 Sampling details Sampling dates 4–9 November 1998 12–15 February 1999 1–6 June 1999 4–6 September 1999 20–25 November 1999
No. of stations
Latitudinal range
Bathymetric range (m)
77 50 31 64 86
41◦ 36 –40◦ 31 N
34–1181 27–926 70–281 35–640 33–925
41◦ 28 –40◦ 40 N 41◦ 20 –40◦ 55 N 41◦ 15 –40◦ 35 N 41◦ 20 –40◦ 35 N
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a sufficiently fine spatial resolution covering the principal spawning area while also delimiting the egg and larval distributions inshore and offshore. The supplementary ichthyoplankton hauls effected in the June 1999 survey were situated mainly in the region of the shelf (Table 1). At each station a CTD cast to 500 m (depth permitting) was performed to collect data on temperature, salinity, density (sigma-t), and fluorescence throughout the water column. Double oblique hauls were carried out at each station using a Bongo gear with a 60 cm mouth diameter and 500 and 300 m mesh nets. Vessel towing speed was 1 m/s. The plankton hauls covered the water column from surface to 200 m (depth permitting). On board, the plankton samples were concentrated and fixed in 5% formalin buffered with borax. The volume of water filtered through the meshes was estimated using a flow meter placed in the mouth of the Bongo gear, and the maximum depth of the haul was measured with a depth sensor (Minilog). Hake egg identification was based on morphological, morphometric, and pigmentation features and on application of the surface adhesion test (Porebski, 1975). The number of eggs and larvae collected in each haul was standardised to the number under 10 m2 of sea surface based on the volume of water filtered through the meshes and the real depth of each haul. Eggs were staged according to the four developmental stages described by Coombs and Mitchell (1982). Mean bathymetric distribution (WMD) of eggs and larvae was calculated as the weighted mean depth, taking into account the proportion of eggs or larvae per station (Pi ) and station depth (Di ): WMD =
n
Pi D i
i=1
Larvae were measured under a binocular microscope to a precision of 0.1 mm, and abundance by size class was also standardised to the number of larvae/10 m2 . Size frequency distributions were produced by grouping shelf (≤200 m) and offshore stations (>200 m). The Kolmogorov–Smirnov test was used to compare the size structure between regions (Sokal and Rolhf, 1981).
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3. Results 3.1. November 1998 The vertical temperature and salinity distributions showed intense gradients between surface and 50 m at all stations (Fig. 2). Sea surface temperature was between 18 and 20 ◦ C, high for this time of year; below 50 m the temperature was lower than 15 ◦ C. Fluorescence profiles still showed the importance of the deep fluorescence maximum (DFM) between 30 and 60 m, and very low values at the surface, as in summer (Fig. 2). Few hake larvae and eggs were collected on this cruise and appeared in 18 and 32% of the stations, respectively (Table 2). Throughout the entire study area they were mainly located at the shelf (≤200 m) (Fig. 3). Maximum egg densities of 26 eggs/10 m2 were recorded. All eggs in stages I–III were collected inside the 200 m isobath, while some stage IV eggs were found above deeper bottoms. A few hake larvae were present in the study area (a maximum of nine larvae/10 m2 ). All were preflexion larvae ranging in size from 2.75 to 5.75 mm (Fig. 4). 3.2. February 1999 The main features on this survey were the uniform temperature (ca. 13 ◦ C) and salinity values throughout the water column, with the highest fluorescence values in the upper levels of the water column (Fig. 2). Few eggs or larvae were collected on this survey. Larvae were present at only one station and very low numbers of eggs at five stations (Fig. 3) (Table 2). 3.3. June 1999 The vertical profiles for the hydrographic parameters revealed the seasonal thermocline and halocline (Fig. 2). Temperature ranged from 20 ◦ C at the surface to 15 ◦ C at 30 m. Peak phytoplankton concentrations were located below the thermocline and the DFM was situated between 30 and 60 m in depth (Fig. 2). This survey was carried out with the aim of obtaining complementary information on the temporal occurrence of hake eggs and larvae. Stations were placed at the shelf region (Table 1). Both eggs and larvae were more abundant than in previous surveys.
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Fig. 2. Mean vertical temperature (◦ C), salinity (psu) and fluorescence (V) profiles during the five surveys. Horizontal lines indicate standard deviation.
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Table 2 Summary of hake egg and larval abundance in several surveysa Larvae
Eggs
Eggs in stage I
November 1998 Mean density (number/10 m2 ) Standard deviation Maximum density (number/10 m2 ) Percentage of positive stations WMD (m)
0.9 2.1 9.3 18.2 183.9
2.5 5.2 25.6 32.5 128.8
0.9 2.9 15.1 13.0 113.9
February 1999 Mean density (number/10 m2 ) Standard deviation Maximum density (number/10 m2 ) Percentage of positive stations WMD (m)
0.1 – 5.0 2.0 69.0
0.5 – 7.0 10.0 174.9
June 1999b Mean density (number/10 m2 ) Standard deviation Maximum density (number/10 m2 ) Percentage of positive stations WMD (m)
4.7 6.0 20.4 60.0 146.9
September 1999 Mean density (number/10 m2 ) Standard deviation Maximum density (number/10 m2 ) Percentage of positive stations WMD (m) November 1999 Mean density (number/10 m2 ) Standard deviation Maximum density (number/10 m2 ) Percentage of positive stations WMD (m) a b
Eggs in stage II
Eggs in stage III
Eggs in stage IV
0.1 0.7 3.8 3.9 81.7
1.2 2.9 13.2 19.5 106.7
0.2 0.9 4.3 6.5 330.7
0.4 – 7.0 8.0 109.3
0.4 – 0 0 0.4
0.1 – 4.0 2.0 400.0
0.0 – 0 0 0
5.9 10.1 37.6 53.3 132.5
3.9 9.5 35.1 20.0 136.0
0.7 1.9 7.0 13.3 147.7
0.9 1.7 6.0 26.7 101.7
0.4 1.16 4.78 13.33 142.5
2.8 5.1 23.0 35.9 263.7
3.7 7.1 37.3 40.6 147.5
1.7 4.1 24.3 26.6 155.9
0.2 1.0 5.1 6.3 162.8
1.7 5.5 37.3 17.2 139.9
0.2 0.9 5.0 6.3 169.7
3 3.5 13 49 283.3
5 9.0 59 48 196.6
2 5.8 35 29 141.4
1 2.5 13 19 198.6
2 2.7 11 31 249.4
0 1.1 7 6 361.3
Percent of positive stations denotes frequency of occurrence and WMD denotes mean bathymetric distribution. Data not directly comparable due to the narrow sampling bathymetric coverage.
In spite all the stations being placed in a narrow band (near the shelf break), there was considerable variability between neighbouring stations (Fig. 2) (Table 2). Peak abundance values for eggs and larvae were 20 and 38 individuals/10 m2 , respectively. Most of the eggs belong to the first development stage. Larval size classes collected on this survey ranged from 2.75 to 6.75 mm. Abundance levels for larvae between 2.75 and 5.25 mm SL were fairly similar (Fig. 4). 3.4. September 1999 The main hydrographic features during this cruise were typical for the season, with a very pronounced
thermocline and halocline (Fig. 2). Temperature values were higher than 25 ◦ C in the upper 40 m and lower than 15 ◦ C below 50 m throughout the study area. Maximum fluorescence occurred below the thermocline at 50–70 m in depth (Fig. 2). Hake egg distribution covered the area from Barcelona to the transect located just north of the Ebro River Delta (Fig. 3). Nearly all the eggs were collected at the shelf stations, the weighted mean depth of occurrence being <200 m for all the egg stages (Table 2). The larval distribution was similar in a north–south direction, but the bathymetric distribution showed some more larvae further offshore (Fig. 3) (Table 2).
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Fig. 3. Horizontal distribution of hake, M. merluccius, eggs and larvae during the five surveys. Plus symbols indicate ichthyoplankton hauls. Circle size indicates abundance (number/10 m2 ); circle size values are listed in the first map.
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Fig. 4. Size frequency distribution of hake, M. merluccius, larvae in November 1998, June 1999, September 1999, and November 1999.
Peak larval abundance was 23 larvae/10 m2 , the size ranged from 2.25 to 6.25 mm. The length frequency distribution for these larvae yielded a modal size class at 2.75 mm (Fig. 4). The possibility of inshore–offshore larval drift was checked by analysing the length frequency distributions for larvae collected at the shelf stations (≤200 m) and those collected at the slope stations (Fig. 5). The size of larvae collected over the shelf ranged from 2.25 to 4.25 mm with a modal size class at 2.75 mm, while those collected further offshore spanned the size classes of 2.25–6.25 mm, with two modal size classes at 2.75 and 4.75 mm. Comparing the size structure of the two regions with the Kolmogrov–Smirnov test, no significant differences emerged (for nine degrees of freedom, the maximum value of the difference d, was 0.3441, which is lower than the critical value for α = 0.05). 3.5. November 1999 In contrast to the strong stratification found in the previous November, hydrographical conditions of the present survey showed a well-mixed upper layer between the surface and around 100 m at all the sta-
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Fig. 5. Size frequency distributions of hake, M. merluccius, larvae on the shelf and slope on the September and November 1999 surveys.
tions (Fig. 2). Temperature in the upper levels (about 16 ◦ C) was fairly uniform throughout the region. Fluorescence values were higher in the upper levels of the water column, from the surface to 50 m (Fig. 2). Hake egg distribution extended from Barcelona to the southern sector, although near Barcelona abundance values were very low (Fig. 3). The distribution on this survey was broader than on previous surveys both inshore and offshore, with 48% positive stations (Table 2). Weighted mean depth for eggs was 196.6 m, and most of the eggs located offshore of the 200 m isobath belonged to stages III and IV (Table 2). The frequency of occurrence of larvae was similar to that of the eggs (49% of the stations) (Table 2). They were distributed on the entire study area, with higher concentrations in the shelf break zone and a more offshore extension than the egg distribution (Fig. 3). Peak larval density was 13 larvae/10 m2 . Abundance near Barcelona was also lower than on the other transects. Hake larvae ranged from 2.4 to 9.1 mm in size, with a modal size class at 3.25 mm and a second mode at 4.75 mm (Fig. 4). The length frequency distributions for the larvae collected in the shelf and slope regions
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were fairly similar, with the two modal size classes evident in both regions (Fig. 5). The largest larvae collected, 9.1 mm, was caught at a slope station. Comparing the size structure of the two regions with the Kolmogrov–Smirnov test, no significant differences emerged (for 15 degrees of freedom, the maximum value of the difference d, was 0.1931, which is lower than the critical value for α = 0.05).
4. Discussion 4.1. Relationship with environmental conditions Two main ecological periods are distinguished in the NW Mediterranean: May–October, characterised by warm temperatures and a well-defined seasonal thermocline, and November–April, characterised by lower temperatures and mixing of the water column (Margalef, 1984). The fertilisation of shelf waters in the study region has a strong dependence on the stratification (Estrada et al., 1985; Salat et al., 2002). During the warm season the concentration of nutrients in the upper layers is very low and a deep chlorophyll and zooplankton maxima develop (Alcaraz, 1985; Estrada, 1985). Colder months are the most productive due to the mixing processes which make nutrients available to the photic layers (Margalef, 1984; Salat, 1996; Salat et al., 2002). The hydrographical structure of the water column observed in the four cruises carried out in 1999 was fairly typical of each season, while the strong vertical gradients observed in November 1998 did not represent the autumn pattern. Weather conditions in October and November 1998 were not typical of autumn, both air and sea surface temperatures were higher than usual for the season, and autumn storms had not developed. Additionally, the area was near the influence of an anticyclonic eddy of waters of Atlantic origin (warmer and less saline than the Mediterranean waters) which contributed to strengthen vertical gradients (Pascual et al., 2000; Olivar et al., 2002). The noticeable seasonal change in environmental variables in the NW Mediterranean is important for defining and reorganising the composition of local populations of phytoplankton and zooplankton (Estrada et al., 1985; Fernández de Puelles et al., 1997; Calbet et al., 2001). Although there are no data on phytoplankton and
zooplankton populations developing during November 1998, it is likely that the lengthy stratified period had affected these populations and consequently the next trophic levels. The few hake larvae collected in November 1998 indicate either that spawning during the survey and in the preceding month was low or that egg and larval mortality was high due to the unusual weather conditions. The proportion of prespawning and spawning individuals in November 1998 was not significantly different from the proportion in the two preceding months (more than 87% in both cases) (Recasens, pers. commun.). In consequence, survival of the early stages of hake in these environmental conditions was presumably quite low. The situation during the second autumn survey in November 1999 was more seasonal. The upper 100 m of the water column was uniform because of the vertical mixing processes, which enriched the upper layers with nutrients, thereby contributing to new production in the euphotic zone. Hake egg and larval frequency and abundance in the area on the November 1999 survey were consistent with this more favourable situation. The environmental conditions recorded on the late spring and late summer surveys (June and September 1999) revealed the important effect of vertical gradients on both hydrography (temperature and salinity) and biology (productivity at the lower trophic levels, and fluorescence). Strong hydrographic gradients were located between the surface and 50 m, and the principal fluorescence values were located relatively deep in the water column (DFM). Both these features are typical of these seasons (Estrada and Salat, 1989; Salat, 1996). The vertical distributions for M. merluccius eggs and larvae have been reported in the literature to be located mainly at subsurface levels (50–60 m and down to 150 m) (Coombs and Mitchell, 1982; Motos et al., 2000). This vertical range is similar to that of the larvae of other hake species around the world (Kendall and Naplin, 1981; Olivar and Fortuño, 1991; Moser and Pommeranz, 1999). The only available study on feeding ecology of hake larvae corresponds to Merluccius productus (Sumida and Moser, 1980), and indicates that they are zooplankton feeders (mainly on copepods of different stages). The subsurface distribution pattern of larvae would place them near the layers of maximum productivity (deep chlorophyll and zooplankton maxima) when the water column is
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stratified in spring and summer. In autumn, at the onset of mixing, appropriate conditions for phytoplankton and zooplankton growth occur (Estrada et al., 1985) and there are no vertical gradients that may hinder vertical displacements to seek for their preys. 4.2. Spatial egg and larval distribution patterns The main area of hake egg distribution described in the present study coincided with the principal fishing grounds for spawning adults, the shelf edge (Recasens et al., 1998). The spawning grounds for this species in the Atlantic are also mainly located in the vicinity of the continental shelf edge, at the 100–200 m isobaths (Horstman, 1988; Valdés et al., 1996; Álvarez et al., 2001; Fives et al., 2001). The close overlap of eggs and adult spawners suggests that drift is not a particularly important factor for hake eggs. Nevertheless, the slight offshore spread in the of larval distributions in relation to the pattern for the eggs and the slight increase in larger larvae offshore indicates that, though not major, some offshore drift occurs during these early life stages of hake. Additionally, the main larval distributions were located oceanwards of the main distributions of juveniles that have been reported (Demestre and Sánchez, 1998), which implies that shoreward displacement of postlarvae must take place later. According to the larval growth data of other Merluccius species (Bailey, 1982; Jeffrey and Taggart, 2000) and from preliminary analysis of the otoliths of the present larvae (Palomera, pers. commun.), the larvae collected in the ichthyoplankton hauls should be younger than 1 month. Information on the development of hake larvae reared in the laboratory by Bjelland and Skiftesvik (2000) indicated that larvae undergo metamorphosis around 30 days after hatching and that more rapid increase in length occur thereafter. The postlarvae are more developed and are thus more likely to be capable of moving to those areas where juveniles tend to concentrate. 4.3. Temporal egg and larval distribution patterns A review of the literature on the spawning of Mediterranean hake (Oliver and Massut´ı, 1995; Papaconstantinou and Stergiou, 1995; Recasens et al., 1998) indicated that reproduction in this species goes
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on all year, with peak spawning taking place in different months depending on the region. Based on gonadal studies (Recasens et al., 1998) and the egg and larval distributions obtained in the present study, it can be concluded that this species has a protracted spawning period in the Catalan Sea, although winter can be regarded as a resting period for a large proportion of the population. Furthermore, high variability in egg and larval abundance was observed depending on hydrographic conditions. This temporal spawning pattern differs from the pattern for this species off the Atlantic coast of the Iberian Peninsula, where winter–early spring have been reported to be the peak spawning periods (Casey and Pereiro, 1995; Álvarez et al., 2001), compared with a spring–early summer peak around the British Isles (Coombs and Mitchell, 1982; Horstman, 1988; Fives et al., 2001). Latitudinal differences have also been found in the temporal spawning patterns for M. merluccius in the North Atlantic, and this phenomenon has been related to the different temperature regimes (Álvarez et al., 2001). In conclusion, the present study shows that hake spawning off Catalonia tends to be concentrated within a narrow band at the shelf edge. The main season runs from late spring to autumn. Based on the known vertical distribution of M. merluccius larvae, we postulate that during the spawning period potential food availability for larvae is ensured by partial overlap between the principal vertical distributions of both larvae and their food sources. To verify this the hypothesis, future efforts should be directed at the simultaneous assessment of the vertical distribution of hake larvae, the micro- and mesozooplankton and larval feeding regimes.
Acknowledgements The authors are very grateful to B. Aguilera, B. Fontanet, B. Mol´ı and G. Sanz for their help in collecting and sorting the samples, and to the rest of participants on the surveys for their cooperation and assistance on board ship. Special thanks goes to J. Lleonart, I. Palomera and L. Recasens for their helpful comments and advice. Mr. R. Sacks prepared the English translation. This study was funded by grants under the following research programmes: “Fil”
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M.P. Olivar et al. / Fisheries Research 60 (2003) 321–331 Recasens, L., 1994. La merluza del Mediterráneo. La comunidad de que forma parte. Aspectos biológicos de la especie: alimentación, ciclo vital y reproducción. Explotación: pesquer´ıa. In: González-Garcés, A., Pereiro, F.J. (Eds.), Estado actual de los Conocimientos de las Poblaciones de Merluza que habitan la Plataforma Continental atlántica y mediterránea de la Unión Europea con especial atención a la Pen´ınsula Ibérica, Instituto Español de Oceanograf´ıa and European Community Programme Air, pp. 21–32. Recasens, L., Lombarte, A., Morales-Nin, B., Torres, G.J., 1998. Spatiotemporal variation in the population structure of the European hake in the NW Mediterranean. J. Fish. Biol. 53, 387–401. Sabatés, A., 1990. Distribution pattern of larval fish populations in the northwestern Mediterranean. Mar. Ecol. Prog. Ser. 59, 75–82. Sabatés, A., Olivar, M.P., 1996. Variation of larval fish distribution associated with the variability in the location of a shelf-slope front. Mar. Ecol. Prog. Ser. 135, 11–20.
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Salat, J., 1996. Review of hydrographic environmental factors that may influence anchovy habitats in the northwestern Mediterranean. Sci. Mar. 60 (2), 21–32. Salat, J., Garc´ıa, M., Cruzado, A., Palanques, A., Ar´ın, L., Gomis, D., Guillén, J., de León, A., Puigdefàbregas, J., Sospedra, J., Velásquez, Z.R., 2002. Seasonal changes of water mass structure and shelf slope exchanges at the Ebro shelf (NW Mediterranean). Cont. Shelf Res. 22, 327–346. Sokal, R.R., Rolhf, F.J., 1981. Biometry. The Principles and Practice of Statistics in Biological Research. Freeman, New York. Sumida, B., Moser, H.G., 1980. Food and feeding of Pacific hake larvae, Merluccius productus, off southern California and northern Baja California. CalCOFI Rep. 21, 161–165. Valdés, L., Lago de Lanzós, A., Solá, A., Franco, C., Sánchez, P., Álvarez, P., 1996. Hake, mackerel and horse mackerel distribution of eggs and larvae in relation to geostrophic circulation in the Bay of Biscay. ICES C.M. 1996/S, p. 16.
Spatial and temporal distribution and abundance of European hake, Merluccius merluccius, eggs and larvae in the Catalan coast (NW Mediterranean) M.P. Olivar∗ , G. Qu´ılez, M. Emelianov Institut de Ciències del Mar (CMIMA-CSIC), Passeig Maritim 37-49, 08003 Barcelona, Spain Received 15 January 2002; received in revised form 14 June 2002; accepted 27 June 2002
Abstract Plankton hauls were conducted on five surveys from November 1998 to November 1999 to study the spatial distribution of eggs and larvae of European hake off Catalonia (NW Mediterranean Sea). Also CTD casts to record hydrographic parameters were carried out on a closely spaced station grid. Merluccius merluccius eggs and larvae appeared mainly in late spring, summer and autumn surveys and were very scarce in winter. Strong differences in terms of egg and larval densities were observed between the two November surveys, which could be attributed to the anomalous hydrographic situation during November 1998. M. merluccius egg and larvae were mainly distributed over the continental shelf, with peak abundances between the 100 m isobath and the edge of the shelf. On the evidence of larval size frequency distributions in the different sampling sectors and the closely overlapping distribution patterns for the eggs and the adult spawning stock, drifting of hake eggs and larvae was not a major factor. The larval distribution extended only slightly further offshore than the egg distribution. Using the hydrographic information and the larval distribution data, an attempt was made to relate the different seasonal productivity levels over the spawning period and the distribution of the larvae. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Merluccius merluccius; Hake eggs; Hake larvae; Distribution patterns; NW Mediterranean
1. Introduction The European hake, Merluccius merluccius, is a moderately large fish widely distributed in the eastern North Atlantic Ocean and Mediterranean Sea (Alheit and Pitcher, 1995). The species inhabits the shelf and shelf break zones throughout the western Mediterranean (Oliver and Massut´ı, 1995). It is the main commercial demersal species caught off Catalonia
∗ Corresponding author. Tel.: +34-93-230-95-56; fax: +34-93-230-95-55. E-mail address: [email protected] (M.P. Olivar).
(NW Mediterranean), with annual landings of 2000 t (Mart´ın, 1991). A number of studies dealing with various aspects of the biology of this species and its fishery in the Atlantic have been published over the past decade (Casey and Pereiro, 1995; Ramos and Fernández, 1995; Álvarez et al., 2001; Fives et al., 2001), but knowledge of these aspects in the Mediterranean is more scanty (Recasens, 1994), particularly for the early developmental stages. Hake larvae have been poorly represented in plankton samples in the Mediterranean (Sabatés, 1990; Sabatés and Olivar, 1996), and there have been no previous studies dealing specifically with hake egg distribution and abundance.
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The first half of the year has been described as the spawning season for this species in the Atlantic, although ripe specimens are caught all year round (Pérez and Pereiro, 1985; Casey and Pereiro, 1995). Analysis of adult gonadal development in the NW Mediterranean has also indicated the presence of year-long reproductive activity, the most pronounced peak being in autumn (Recasens et al., 1998). During this period females tend to aggregate towards the edge of the shelf (Recasens et al., 1998). The aim of this study was to analyse spatio-temporal patterns of M. merluccius egg and larval distribution in the NW Mediterranean in relation to different environmental conditions. The study was based on five cruises performed in the different seasons over an annual cycle (November 1998–November 1999). This paper reports the vertical profiles of temperature, salinity and fluorescence for the several surveys and discusses how variability in hydrographic structure may affect the eggs and larval abundance.
2. Materials and methods A smaller spatial sampling resolution scale than previously used for other species, particularly in the areas with the highest concentrations of spawning adults, was considered necessary for this study. Earlier studies on seasonal fecundity and spawning carried out (Recasens et al., 1998) were used as a guide in planning the ichthyoplankton survey schedule. To ascertain the spatial distribution patterns, three main surveys (November 1998, September 1999, and November 1999) were planned to coincide with the months of maximum gonadal development in
Fig. 1. Map of the study area showing transects and sampling stations.
summer–autumn (Recasens et al., 1998). In addition, extra ichthyoplankton samples with reduced spatial coverage were collected in February and June 1999, to provide information on the four seasons of the year (Table 1). The sampling grid consisted of very closely spaced stations located on cross-shelf transects roughly perpendicular to the shore 5–10 miles distant from each other (Fig. 1). The stations along the transects ranged from 40 to 500–1000 m isobaths. In the region with the highest adult spawning population concentration, stations were separated by a distance of only 2.5 miles, while in the more outlying areas the separation was 5 miles. The purpose was to compile information with
Table 1 Sampling details Sampling dates 4–9 November 1998 12–15 February 1999 1–6 June 1999 4–6 September 1999 20–25 November 1999
No. of stations
Latitudinal range
Bathymetric range (m)
77 50 31 64 86
41◦ 36 –40◦ 31 N
34–1181 27–926 70–281 35–640 33–925
41◦ 28 –40◦ 40 N 41◦ 20 –40◦ 55 N 41◦ 15 –40◦ 35 N 41◦ 20 –40◦ 35 N
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a sufficiently fine spatial resolution covering the principal spawning area while also delimiting the egg and larval distributions inshore and offshore. The supplementary ichthyoplankton hauls effected in the June 1999 survey were situated mainly in the region of the shelf (Table 1). At each station a CTD cast to 500 m (depth permitting) was performed to collect data on temperature, salinity, density (sigma-t), and fluorescence throughout the water column. Double oblique hauls were carried out at each station using a Bongo gear with a 60 cm mouth diameter and 500 and 300 m mesh nets. Vessel towing speed was 1 m/s. The plankton hauls covered the water column from surface to 200 m (depth permitting). On board, the plankton samples were concentrated and fixed in 5% formalin buffered with borax. The volume of water filtered through the meshes was estimated using a flow meter placed in the mouth of the Bongo gear, and the maximum depth of the haul was measured with a depth sensor (Minilog). Hake egg identification was based on morphological, morphometric, and pigmentation features and on application of the surface adhesion test (Porebski, 1975). The number of eggs and larvae collected in each haul was standardised to the number under 10 m2 of sea surface based on the volume of water filtered through the meshes and the real depth of each haul. Eggs were staged according to the four developmental stages described by Coombs and Mitchell (1982). Mean bathymetric distribution (WMD) of eggs and larvae was calculated as the weighted mean depth, taking into account the proportion of eggs or larvae per station (Pi ) and station depth (Di ): WMD =
n
Pi D i
i=1
Larvae were measured under a binocular microscope to a precision of 0.1 mm, and abundance by size class was also standardised to the number of larvae/10 m2 . Size frequency distributions were produced by grouping shelf (≤200 m) and offshore stations (>200 m). The Kolmogorov–Smirnov test was used to compare the size structure between regions (Sokal and Rolhf, 1981).
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3. Results 3.1. November 1998 The vertical temperature and salinity distributions showed intense gradients between surface and 50 m at all stations (Fig. 2). Sea surface temperature was between 18 and 20 ◦ C, high for this time of year; below 50 m the temperature was lower than 15 ◦ C. Fluorescence profiles still showed the importance of the deep fluorescence maximum (DFM) between 30 and 60 m, and very low values at the surface, as in summer (Fig. 2). Few hake larvae and eggs were collected on this cruise and appeared in 18 and 32% of the stations, respectively (Table 2). Throughout the entire study area they were mainly located at the shelf (≤200 m) (Fig. 3). Maximum egg densities of 26 eggs/10 m2 were recorded. All eggs in stages I–III were collected inside the 200 m isobath, while some stage IV eggs were found above deeper bottoms. A few hake larvae were present in the study area (a maximum of nine larvae/10 m2 ). All were preflexion larvae ranging in size from 2.75 to 5.75 mm (Fig. 4). 3.2. February 1999 The main features on this survey were the uniform temperature (ca. 13 ◦ C) and salinity values throughout the water column, with the highest fluorescence values in the upper levels of the water column (Fig. 2). Few eggs or larvae were collected on this survey. Larvae were present at only one station and very low numbers of eggs at five stations (Fig. 3) (Table 2). 3.3. June 1999 The vertical profiles for the hydrographic parameters revealed the seasonal thermocline and halocline (Fig. 2). Temperature ranged from 20 ◦ C at the surface to 15 ◦ C at 30 m. Peak phytoplankton concentrations were located below the thermocline and the DFM was situated between 30 and 60 m in depth (Fig. 2). This survey was carried out with the aim of obtaining complementary information on the temporal occurrence of hake eggs and larvae. Stations were placed at the shelf region (Table 1). Both eggs and larvae were more abundant than in previous surveys.
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Fig. 2. Mean vertical temperature (◦ C), salinity (psu) and fluorescence (V) profiles during the five surveys. Horizontal lines indicate standard deviation.
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Table 2 Summary of hake egg and larval abundance in several surveysa Larvae
Eggs
Eggs in stage I
November 1998 Mean density (number/10 m2 ) Standard deviation Maximum density (number/10 m2 ) Percentage of positive stations WMD (m)
0.9 2.1 9.3 18.2 183.9
2.5 5.2 25.6 32.5 128.8
0.9 2.9 15.1 13.0 113.9
February 1999 Mean density (number/10 m2 ) Standard deviation Maximum density (number/10 m2 ) Percentage of positive stations WMD (m)
0.1 – 5.0 2.0 69.0
0.5 – 7.0 10.0 174.9
June 1999b Mean density (number/10 m2 ) Standard deviation Maximum density (number/10 m2 ) Percentage of positive stations WMD (m)
4.7 6.0 20.4 60.0 146.9
September 1999 Mean density (number/10 m2 ) Standard deviation Maximum density (number/10 m2 ) Percentage of positive stations WMD (m) November 1999 Mean density (number/10 m2 ) Standard deviation Maximum density (number/10 m2 ) Percentage of positive stations WMD (m) a b
Eggs in stage II
Eggs in stage III
Eggs in stage IV
0.1 0.7 3.8 3.9 81.7
1.2 2.9 13.2 19.5 106.7
0.2 0.9 4.3 6.5 330.7
0.4 – 7.0 8.0 109.3
0.4 – 0 0 0.4
0.1 – 4.0 2.0 400.0
0.0 – 0 0 0
5.9 10.1 37.6 53.3 132.5
3.9 9.5 35.1 20.0 136.0
0.7 1.9 7.0 13.3 147.7
0.9 1.7 6.0 26.7 101.7
0.4 1.16 4.78 13.33 142.5
2.8 5.1 23.0 35.9 263.7
3.7 7.1 37.3 40.6 147.5
1.7 4.1 24.3 26.6 155.9
0.2 1.0 5.1 6.3 162.8
1.7 5.5 37.3 17.2 139.9
0.2 0.9 5.0 6.3 169.7
3 3.5 13 49 283.3
5 9.0 59 48 196.6
2 5.8 35 29 141.4
1 2.5 13 19 198.6
2 2.7 11 31 249.4
0 1.1 7 6 361.3
Percent of positive stations denotes frequency of occurrence and WMD denotes mean bathymetric distribution. Data not directly comparable due to the narrow sampling bathymetric coverage.
In spite all the stations being placed in a narrow band (near the shelf break), there was considerable variability between neighbouring stations (Fig. 2) (Table 2). Peak abundance values for eggs and larvae were 20 and 38 individuals/10 m2 , respectively. Most of the eggs belong to the first development stage. Larval size classes collected on this survey ranged from 2.75 to 6.75 mm. Abundance levels for larvae between 2.75 and 5.25 mm SL were fairly similar (Fig. 4). 3.4. September 1999 The main hydrographic features during this cruise were typical for the season, with a very pronounced
thermocline and halocline (Fig. 2). Temperature values were higher than 25 ◦ C in the upper 40 m and lower than 15 ◦ C below 50 m throughout the study area. Maximum fluorescence occurred below the thermocline at 50–70 m in depth (Fig. 2). Hake egg distribution covered the area from Barcelona to the transect located just north of the Ebro River Delta (Fig. 3). Nearly all the eggs were collected at the shelf stations, the weighted mean depth of occurrence being <200 m for all the egg stages (Table 2). The larval distribution was similar in a north–south direction, but the bathymetric distribution showed some more larvae further offshore (Fig. 3) (Table 2).
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Fig. 3. Horizontal distribution of hake, M. merluccius, eggs and larvae during the five surveys. Plus symbols indicate ichthyoplankton hauls. Circle size indicates abundance (number/10 m2 ); circle size values are listed in the first map.
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Fig. 4. Size frequency distribution of hake, M. merluccius, larvae in November 1998, June 1999, September 1999, and November 1999.
Peak larval abundance was 23 larvae/10 m2 , the size ranged from 2.25 to 6.25 mm. The length frequency distribution for these larvae yielded a modal size class at 2.75 mm (Fig. 4). The possibility of inshore–offshore larval drift was checked by analysing the length frequency distributions for larvae collected at the shelf stations (≤200 m) and those collected at the slope stations (Fig. 5). The size of larvae collected over the shelf ranged from 2.25 to 4.25 mm with a modal size class at 2.75 mm, while those collected further offshore spanned the size classes of 2.25–6.25 mm, with two modal size classes at 2.75 and 4.75 mm. Comparing the size structure of the two regions with the Kolmogrov–Smirnov test, no significant differences emerged (for nine degrees of freedom, the maximum value of the difference d, was 0.3441, which is lower than the critical value for α = 0.05). 3.5. November 1999 In contrast to the strong stratification found in the previous November, hydrographical conditions of the present survey showed a well-mixed upper layer between the surface and around 100 m at all the sta-
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Fig. 5. Size frequency distributions of hake, M. merluccius, larvae on the shelf and slope on the September and November 1999 surveys.
tions (Fig. 2). Temperature in the upper levels (about 16 ◦ C) was fairly uniform throughout the region. Fluorescence values were higher in the upper levels of the water column, from the surface to 50 m (Fig. 2). Hake egg distribution extended from Barcelona to the southern sector, although near Barcelona abundance values were very low (Fig. 3). The distribution on this survey was broader than on previous surveys both inshore and offshore, with 48% positive stations (Table 2). Weighted mean depth for eggs was 196.6 m, and most of the eggs located offshore of the 200 m isobath belonged to stages III and IV (Table 2). The frequency of occurrence of larvae was similar to that of the eggs (49% of the stations) (Table 2). They were distributed on the entire study area, with higher concentrations in the shelf break zone and a more offshore extension than the egg distribution (Fig. 3). Peak larval density was 13 larvae/10 m2 . Abundance near Barcelona was also lower than on the other transects. Hake larvae ranged from 2.4 to 9.1 mm in size, with a modal size class at 3.25 mm and a second mode at 4.75 mm (Fig. 4). The length frequency distributions for the larvae collected in the shelf and slope regions
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were fairly similar, with the two modal size classes evident in both regions (Fig. 5). The largest larvae collected, 9.1 mm, was caught at a slope station. Comparing the size structure of the two regions with the Kolmogrov–Smirnov test, no significant differences emerged (for 15 degrees of freedom, the maximum value of the difference d, was 0.1931, which is lower than the critical value for α = 0.05).
4. Discussion 4.1. Relationship with environmental conditions Two main ecological periods are distinguished in the NW Mediterranean: May–October, characterised by warm temperatures and a well-defined seasonal thermocline, and November–April, characterised by lower temperatures and mixing of the water column (Margalef, 1984). The fertilisation of shelf waters in the study region has a strong dependence on the stratification (Estrada et al., 1985; Salat et al., 2002). During the warm season the concentration of nutrients in the upper layers is very low and a deep chlorophyll and zooplankton maxima develop (Alcaraz, 1985; Estrada, 1985). Colder months are the most productive due to the mixing processes which make nutrients available to the photic layers (Margalef, 1984; Salat, 1996; Salat et al., 2002). The hydrographical structure of the water column observed in the four cruises carried out in 1999 was fairly typical of each season, while the strong vertical gradients observed in November 1998 did not represent the autumn pattern. Weather conditions in October and November 1998 were not typical of autumn, both air and sea surface temperatures were higher than usual for the season, and autumn storms had not developed. Additionally, the area was near the influence of an anticyclonic eddy of waters of Atlantic origin (warmer and less saline than the Mediterranean waters) which contributed to strengthen vertical gradients (Pascual et al., 2000; Olivar et al., 2002). The noticeable seasonal change in environmental variables in the NW Mediterranean is important for defining and reorganising the composition of local populations of phytoplankton and zooplankton (Estrada et al., 1985; Fernández de Puelles et al., 1997; Calbet et al., 2001). Although there are no data on phytoplankton and
zooplankton populations developing during November 1998, it is likely that the lengthy stratified period had affected these populations and consequently the next trophic levels. The few hake larvae collected in November 1998 indicate either that spawning during the survey and in the preceding month was low or that egg and larval mortality was high due to the unusual weather conditions. The proportion of prespawning and spawning individuals in November 1998 was not significantly different from the proportion in the two preceding months (more than 87% in both cases) (Recasens, pers. commun.). In consequence, survival of the early stages of hake in these environmental conditions was presumably quite low. The situation during the second autumn survey in November 1999 was more seasonal. The upper 100 m of the water column was uniform because of the vertical mixing processes, which enriched the upper layers with nutrients, thereby contributing to new production in the euphotic zone. Hake egg and larval frequency and abundance in the area on the November 1999 survey were consistent with this more favourable situation. The environmental conditions recorded on the late spring and late summer surveys (June and September 1999) revealed the important effect of vertical gradients on both hydrography (temperature and salinity) and biology (productivity at the lower trophic levels, and fluorescence). Strong hydrographic gradients were located between the surface and 50 m, and the principal fluorescence values were located relatively deep in the water column (DFM). Both these features are typical of these seasons (Estrada and Salat, 1989; Salat, 1996). The vertical distributions for M. merluccius eggs and larvae have been reported in the literature to be located mainly at subsurface levels (50–60 m and down to 150 m) (Coombs and Mitchell, 1982; Motos et al., 2000). This vertical range is similar to that of the larvae of other hake species around the world (Kendall and Naplin, 1981; Olivar and Fortuño, 1991; Moser and Pommeranz, 1999). The only available study on feeding ecology of hake larvae corresponds to Merluccius productus (Sumida and Moser, 1980), and indicates that they are zooplankton feeders (mainly on copepods of different stages). The subsurface distribution pattern of larvae would place them near the layers of maximum productivity (deep chlorophyll and zooplankton maxima) when the water column is
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stratified in spring and summer. In autumn, at the onset of mixing, appropriate conditions for phytoplankton and zooplankton growth occur (Estrada et al., 1985) and there are no vertical gradients that may hinder vertical displacements to seek for their preys. 4.2. Spatial egg and larval distribution patterns The main area of hake egg distribution described in the present study coincided with the principal fishing grounds for spawning adults, the shelf edge (Recasens et al., 1998). The spawning grounds for this species in the Atlantic are also mainly located in the vicinity of the continental shelf edge, at the 100–200 m isobaths (Horstman, 1988; Valdés et al., 1996; Álvarez et al., 2001; Fives et al., 2001). The close overlap of eggs and adult spawners suggests that drift is not a particularly important factor for hake eggs. Nevertheless, the slight offshore spread in the of larval distributions in relation to the pattern for the eggs and the slight increase in larger larvae offshore indicates that, though not major, some offshore drift occurs during these early life stages of hake. Additionally, the main larval distributions were located oceanwards of the main distributions of juveniles that have been reported (Demestre and Sánchez, 1998), which implies that shoreward displacement of postlarvae must take place later. According to the larval growth data of other Merluccius species (Bailey, 1982; Jeffrey and Taggart, 2000) and from preliminary analysis of the otoliths of the present larvae (Palomera, pers. commun.), the larvae collected in the ichthyoplankton hauls should be younger than 1 month. Information on the development of hake larvae reared in the laboratory by Bjelland and Skiftesvik (2000) indicated that larvae undergo metamorphosis around 30 days after hatching and that more rapid increase in length occur thereafter. The postlarvae are more developed and are thus more likely to be capable of moving to those areas where juveniles tend to concentrate. 4.3. Temporal egg and larval distribution patterns A review of the literature on the spawning of Mediterranean hake (Oliver and Massut´ı, 1995; Papaconstantinou and Stergiou, 1995; Recasens et al., 1998) indicated that reproduction in this species goes
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on all year, with peak spawning taking place in different months depending on the region. Based on gonadal studies (Recasens et al., 1998) and the egg and larval distributions obtained in the present study, it can be concluded that this species has a protracted spawning period in the Catalan Sea, although winter can be regarded as a resting period for a large proportion of the population. Furthermore, high variability in egg and larval abundance was observed depending on hydrographic conditions. This temporal spawning pattern differs from the pattern for this species off the Atlantic coast of the Iberian Peninsula, where winter–early spring have been reported to be the peak spawning periods (Casey and Pereiro, 1995; Álvarez et al., 2001), compared with a spring–early summer peak around the British Isles (Coombs and Mitchell, 1982; Horstman, 1988; Fives et al., 2001). Latitudinal differences have also been found in the temporal spawning patterns for M. merluccius in the North Atlantic, and this phenomenon has been related to the different temperature regimes (Álvarez et al., 2001). In conclusion, the present study shows that hake spawning off Catalonia tends to be concentrated within a narrow band at the shelf edge. The main season runs from late spring to autumn. Based on the known vertical distribution of M. merluccius larvae, we postulate that during the spawning period potential food availability for larvae is ensured by partial overlap between the principal vertical distributions of both larvae and their food sources. To verify this the hypothesis, future efforts should be directed at the simultaneous assessment of the vertical distribution of hake larvae, the micro- and mesozooplankton and larval feeding regimes.
Acknowledgements The authors are very grateful to B. Aguilera, B. Fontanet, B. Mol´ı and G. Sanz for their help in collecting and sorting the samples, and to the rest of participants on the surveys for their cooperation and assistance on board ship. Special thanks goes to J. Lleonart, I. Palomera and L. Recasens for their helpful comments and advice. Mr. R. Sacks prepared the English translation. This study was funded by grants under the following research programmes: “Fil”
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