© 2006 International Council for the Exploration of the Sea
Larval distribution and growth of the rockfish, Sebastes capensis (Sebastidae, Pisces), in the fjords of southern Chile
Laboratorio de Oceanografía Pesquera y Ecología Larval (LOPEL), Departamento de Oceanografía, Universidad de Concepción Casilla 160-C, Correo 3, Concepción, Chile
*Correspondence to M. F. Landaeta: tel: +56 041 207029; fax: +56 041 256571. e-mail: mlandaeta{at}udec.cl.
The distribution and the growth of larval rockfish Sebastes capensis were studied during two oceanographic cruises carried out in a sector of the Chilean fjords (43°30'S–46°S) during spring 2002 (November) and winter 2003 (August). Abundance (up to 600 per 10 m2) of pre- and post-flexion larvae was higher during the austral spring than in winter (20 per 10 m2). Larvae were smaller principally above the halocline in the first 50 m of the water column throughout the channels and fjords; in contrast, post-flexion larvae were more abundant seawards and were totally absent in the freshest waters. Analysis of the otoliths indicated that larvae between 3.3 and 12.2-mm standard length collected during November grew 0.15 mm d–1. Additionally, it showed that older larvae from the outer part of the channels and over the shelf laid down wider otolith increments than larvae found in the interior waters, coinciding with the area of greatest abundance of larger calanoid copepods. Ontogenetic differences in distributions suggest that different habitats are utilized during the early life stages of S. capensis, and that they seem to be shared with several other Sebastes species. However, the mechanisms for transport in different areas vary widely, making the question of the evolutionary forces driving these ontogenetic changes in distribution even more intriguing.
Keywords: larval growth, otolith, Patagonian fjords, rockfish, Sebastes capensis, southern Chile
Received 8 April 2005; accepted 23 January 2006.
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Introduction |
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 Top Introduction MethodsResultsDiscussionReferences |
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The rockfish, Sebastes spp., are highly diversified, viviparous fish with about 72 species that are mostly endemic to the North Pacific (Nelson, 1994), where they sustain fisheries that constituted nearly 20% of the total commercial groundfish fishery in 2000 (Laidig et al., 2004). In the South Pacific Ocean, there are considerably fewer Sebastes species. Along the Chilean coast, for instance, only one species has been recorded (Kong, 1985): Sebastes capensis (Gmelin 1788). It supports a small artisanal fishery, 171 t being caught in 1999, but just 45 t during 2002 (SERNAPESCA, 2002).
Sebastes capensis is a bottom-dwelling fish that inhabits shallow and rocky areas between 5 and 15 m deep. Females retain the eggs until maturation, when embryos are extruded throughout most of the year (Sabatés and Olivar, 1990). Pelagic larvae have been collected in coastal waters all along the Chilean coast (Loeb and Rojas, 1988; Castro et al., 2000; Rodríguez-Graña and Castro, 2003), and are particularly abundant at the southern tip of South America (Balbontín and Bernal, 1997; Bernal and Balbontín, 1999), where fjords and channels dominate the coastline.
The Chilean fjord zone (41°S–55°S) is a highly heterogeneous system consisting of several microbasins (sensu Antezana, 1999), forming one of the largest estuarine areas in the world. The ecosystem is more than 1600 km long and covers an area of about 240 000 km2 (Palma and Silva, 2004). Surface forcing is dominated by strong poleward winds, heavy precipitation (2.5 m year–1; Strub et al., 1998), freshwater run-off, and river discharge (Guzmán and Silva, 2002). The complex fjord coastline is sheltered and increases the influence of the tidal forcing (amplitudes from 1.5 to 8.0 m; Strub et al., 1998; Valle-Levinson et al., 2001). Two-layer estuarine circulation patterns are found in many of the fjords (Strub et al., 1998). Although larvae of S. capensis are among the most abundant taxa in ichthyoplankton samples throughout the area (Balbontín and Bernal, 1997), there is no information on its ecology in southern Chile.
This study examines the early life history of the rockfish Sebastes capensis in the Patagonian fjord waters of southern Chile. Based on data from two oceanographic cruises (winter and spring), our objectives were (i) to obtain information on adult reproductive locations and seasonality (winter and spring) by examining potential differences in larval distribution and abundance, (ii) to identify potential ontogenetic changes in larval distribution within and between seasons, and (iii) to determine if differences existed in environmental characteristics between the inner and outer (seaward) zones of the channels during the main reproductive season that could be related to the changes observed in larval distribution and back-calculated birthdates estimated from otoliths.
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Methods |
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TopIntroductionMethodsResultsDiscussionReferences |
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Two oceanographic cruises were conducted during 16–27 November 2002 and 9–22 August 2003 in the austral zone of Chile (Figure 1) on board the "Vidal Gormaz". The cruise track was similar during each cruise and consisted of sampling one channel orientated in a north–south direction (Moraleda Channel) and two transverse channels orientated in an east–west direction (Ninualac and Pulluche Channels). During November 2002, the two northernmost stations were not sampled; in August 2003, a middle station in the Ninualac Channel and the outer station of the Pulluche Channel were not sampled. At each oceanographic station, hydrographic casts were performed to 300 m with a Seabird SB-19 CTD (Conductivity–Temperature–Depth) profiler. Ichthyoplankton sampling was carried out using an opening–closing 1-m2-mouth Tucker trawl, equipped with three 300-µm mesh nets and a flowmeter mounted in the frame. The net was deployed obliquely to a maximum depth of 100 m or to near the seabed, and two or three strata were sampled (0–20, 20–50, and 50–100-m depth), depending on bottom depth. After recovery, the nets were rinsed and the ichthyoplankton samples preserved in 4% buffered formaldehyde (strata) and 95% ethanol (integrated samples).
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All fish larvae were removed from each sample and identified to the lowest possible taxon; larval rockfish were identified following Sabatés and Olivar (1990). Larvae were classified according to their developmental stage as pre- or post-flexion (inflexion larvae were included in post-flexion). All undamaged larvae were measured to the nearest 0.1 mm (notochord length, NL, or standard length, SL). No attempt was made to correct lengths for the effects of preservation. Larval rockfish counts were converted to density (number 1000 m–3) for each stratum, and the integrated abundance of rockfish larvae in the water column (number 10 m–2) was estimated for each sampling station.
Otolith analysis was only performed for rockfish larvae collected during November 2002. The limited number collected in August 2003, as well as their narrow size range, prevented any analysis. Otoliths were removed from 102 larvae (3.3–12.2 mm SL), and ages were determined following the procedures given in Laidig et al. (1991). Otoliths were embedded in epoxy resin on a glass slide. Growth increments of sagittal otoliths were counted and measured under a light microscope at a magnification of x1000 using an oil immersion lens. Otolith radius, distance of the extrusion check from the primordium, and otolith area were measured using a Sony CCD-IRIS video camera attached to a microscope connected to a PC with Optimas® 6.1 software.
Counting and measurement of growth increments began with the conspicuous dark mark (extrusion check), which has also been observed in otoliths of other larval Sebastes (Penney and Evans, 1985; Kokita and Omori, 1998; Laidig and Sakuma, 1998; Plaza et al., 2003; Laidig et al., 2004). Although we did not attempt to validate the daily periodicity of growth increments in S. capensis, other authors have confirmed the daily nature of growth increments in S. melanops (Yoklavich and Boehlert, 1987), S. jordani (Laidig et al., 1991), S. thompsoni (Kokita and Omori, 1998), S. paucispinis, S. goodie, S. entomelas, and S. flavidus (Woodbury and Ralston, 1991).
The birthdate composition of all larvae was estimated as follows. First, a larval length-at-age key was obtained for the entire period of sampling. The length frequency distribution of larvae at each station was then converted to an age–frequency distribution using the length-at-age key, from which the birthdate composition was back-calculated. The birthdate distribution for each sampling day was weighted according to the standardized abundance of the catch per station, then summed to obtain the distribution in the population.
Hydrographic cross-sections were generated from CTD profiles using contouring functions in Surfer 8. In order to detect potential effects of depth (above and below 50 m), time of day (day/night samples), and salinity (above or below 30) on the abundance of pre- and post-flexion S. capensis larvae during the austral spring cruise of November 2002, a three-way ANOVA was performed on log-transformed larval density data from the positive stations. The analysis was performed separately for pre- and post-flexion rockfish. Factors were fixed, except salinity (above or below 30), which was a random factor. The salinity value represented the average for the depth strata. Vertical migration of larvae was examined through an interaction term for the effects of depth and day/night stations in the ANOVA analysis (Table 1).
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To assess whether differences existed in the potential food environment of S. capensis larvae in the upper 50 m between the inner and the seaward ends of the two transverse channels (Pulluche and Ninualac) during the month of maximum abundance (November, see Results), we quantified the abundance of two size fractions (300–500 µm and >500 µm total body length) of calanoid and cyclopoid copepods, and of metanauplii of Rhincalanus nasutus, a calanoid copepod with large (>300 µm) naupliar stages. The size ranges considered may be somewhat larger than that required for first-feeding larvae. However, the aim was to observe whether a coincidence may have occurred between larval size and the size of potential food particles we would expect the larvae to eat as they grow. Finally, under the assumption that smaller larvae might benefit from developing in a food-rich and more stable environment, we calculated and compared the Brünt–Väisälä frequency (
, where g = gravity (9.8 m s–2)) as a measure of water stability at the innermost and the outermost (seaward) stations of both channels. |
Results |
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Seasonal changes in abundance and distribution
During the austral spring (November 2002), 1505 S. capensis larvae were collected (1327 pre- and 178 post-flexion). Standardized integrated abundance among stations ranged between 35.8 and 664.2 per 10 m2 (Figure 1b). Pre-flexion larvae were collected at all stations sampled, although abundance was greatest in the northern part of the Moraleda Channel, high throughout the Ninualac Channel, and high also towards the outer end of the Pulluche Channel (Figure 1b). Post-flexion larvae were absent in the southern part of the Moraleda Channel and inside the Pulluche Channel. Towards the outside of the Ninualac and Pulluche Channels and in the north of the Moraleda Channel near the Corcovado Gulf, densities of post-flexion larvae were highest (Figure 1c).
In August 2003 just 172 larvae were caught, all of them pre-flexion, with densities ranging from 0.9 to 19.9 per 10 m2. At that time, pre-flexion larvae were found only in the Ninualac Channel, in the southern part of the Moraleda Channel, and off the Pulluche Channel (Figure 1d). Abundance was therefore significantly less in winter than in spring (Mann–Whitney U-test, p < 0.001).
Vertical distribution of larvae
The vertical distributions of pre- and post-flexion Sebastes capensis larvae differed during November 2002. Pre-flexion larvae were encountered in all strata sampled in all channels (Figure 2a–c), and their abundance was significantly greater in the upper 50 m (ANOVA, F = 229.83, p < 0.05) in the Pulluche and Moraleda Channels. In the Ninualac Channel (Figure 2b), pre-flexion larvae were evenly distributed throughout the water column only inshore. No differences associated with diel vertical migration or salinity were detected in pre-flexion larvae (Table 1). Post-flexion larvae collected in November had a different vertical distribution: most were in the upper (<50 m) layer to seawards, and there was a significant reduction in abundance in surface water where the salinity was <30 (ANOVA, F = 12.35, p < 0.01; Table 1). This distribution was particularly evident in the inner part of the channels (Figure 2d–f), where post-flexion larvae were collected mainly in deeper, more saline water.
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In August 2003, when only a few pre-flexion larvae were collected, there was no distinct pattern in vertical distribution. Only a slight broadening in the vertical distribution range was noted from the inner to the outer areas of the Ninualac and Pulluche Channels (Figure 2g–i).
Age and growth of larvae
The larval size distribution was broader during spring 2002 (2.9–13.4 mm SL) than in winter 2003 (3.9–5.8 mm SL). Consequently, larvae collected in spring were in the pre- and post-flexion stages (smaller and larger than 7 mm SL, respectively), but those collected in winter were only pre-flexion (Figure 3). A t-test for independent samples (p < 0.0001) confirmed that the mean larval size was larger during November 2002.
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The extrusion check observed in otoliths of larval Sebastes capensis collected in November ranged from 10.2 to 16.8 µm (mean ± s.d.: 12.5 ± 1.5 µm; n = 67). Larval growth rate was estimated by a linear model of least squares fitted to the relationship between SL and age (coefficient of determination r2 = 0.92, n = 102; Figure 4a). This model estimated a growth rate of 0.15 mm d–1 and a larval extrusion length of 3.78 mm. Otolith radius and standard length were related linearly (Figure 4b), and otolith area was related exponentially with Sebastes larvae SL (Figure 4c).
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In November 2002, larvae collected in the seaward part of the channels were significantly older (up to 66 days old) than larvae from the inner areas (<40 days old; Kolmogorov–Smirnov test, p < 0.001; Figure 5a). Back-calculated birthdates for larvae collected in November 2002 indicated that they had been extruded from September through November of the same year. Interestingly, those larvae born early in the season (September) were caught only at the seaward stations (Figure 5b).
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There was no significant difference between the mean increment width of otoliths (in µm) of pre- and post-flexion larvae collected inside and outside the channels (Kolmogorov–Smirnov test, p > 0.05) during the first 20 days of larval life. However, older than 20 days, the otolith increments in larvae captured outside the channels were wider than those of larvae collected in the inner part of the channels (Kolmogorov–Smirnov test, p < 0.05; Figure 6).
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Hydrography
In November, seawater temperature ranged from 10 to 11°C throughout the channels and throughout the water column down to 150 m (Figure 7a–c). Temperatures were lower in the deepest strata right along the channels, including their innermost parts. Among channels, Moraleda was the coldest, with temperatures below 10.5°C immediately below the surface layer (down to 25 m), followed by Ninualac (<10.5°C below 50 m), and Pulluche (<10.5°C below 60–70 m). Strong salinity gradients were observed along all the three channels (Figure 2a–c). Salinity was at its minimum (<28) at the head of the channel head (its inner end), showing the influence of the large freshwater input from rainfall and ice melting in the mountains, as well as from the rivers located east of the channels, and the San Rafael Lake and Glacier (46°40'S 74°W) that drains at the head of the Moraleda Channel. Maximum salinity (>33) was at the outer part (seawards) of the Ninualac and Pulluche Channels, and at the Moraleda Channel mouth.
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In winter (August), the overall temperature range (9.5–10.1°C) was lower than that in November, and temperature was generally inverted throughout the area (Figure 7d–f). Temperatures were lowest in the inner parts of the channels but, in contrast with the situation in November, maximum temperatures were at mid-depth (Moraleda Channel) or deeper (Ninualac and Pulluche Channels). Similar to the spring conditions (November), strong salinity gradients were observed along all three channels in winter, with minimum values (<29) in the inner part of the channels and maximum values (>33) deeper and to seaward (Figure 2g–i).
Potential larval feeding environment in the Ninualac and Pulluche Channels
Copepod abundance (number m–3) along the Ninualac Channel (Figure 8a, b) varied between both size fractions. While smaller copepods were abundant throughout the Pulluche Channel, larger copepods and metanauplii were more abundant towards the outer side of this channel (Figure 8b). Smaller calanoid copepods, although variable in abundance throughout the channel, also tended to be more abundant towards the channel's seaward end. Cyclopoid copepods were scarce throughout.
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In the Pulluche Channel (Figure 8c, d) there was an increase in abundance of small copepods towards the channel's head, particularly smaller calanoid copepods and cyclopoids, the latter occurring only close to this channel's head. Metanauplii were more abundant to seaward.
High values (>0.02) of the Brünt–Väisälä frequency, N, denoting greater stratification, were observed in the upper layer (0–40 m) at the innermost and outermost stations of the Pulluche Channel (Figure 9a). Lower values (<0.01) were deeper in the water column in the outer channel. In the Ninualac Channel's inner zone, high (but more variable) Brünt–Väisälä frequency, N, values were found in shallow water (0–60 m), and lower values (0.01 or less) were observed throughout the water column to seaward (Figure 9b).
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The inner and seaward stations along the Ninualac Channel had similar total numbers of smaller copepods, high stratification levels, and large numbers of pre-flexion S. capensis larvae. Alternatively, to seaward in this channel, the greater abundance of larger copepods and the older, post-flexion larval stages coincided. In contrast, in the Pulluche Channel, although smaller copepods were more abundant in the high stability areas (at the inner part of the channel), smaller S. capensis larvae were distributed throughout the channel. However, most of the older, post-flexion larvae were to seaward, where copepods were larger and the waters were more stable.
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Discussion |
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Fjord and channel systems in Scandinavia, Iceland, Greenland, British Columbia, Alaska, and Chile form some of the largest estuarine areas in the world (Cameron and Pritchard, 1963). These systems are utilized for spawning, nursery, and recruitment by many marine fish with contrasting life histories, varying from mesopelagic (Lopes, 1979) and pelagic (Brown, 2002) to demersal fish (Marteinsdottir et al., 2000; Boje, 2002). The fjords of southern Chile are less well studied, but reports on ichthyoplankton indicate that fish eggs and larvae of small pelagic fish such as sardine (Strangomera bentincki), mesopelagic fish such as lightfish (Maurolicus parvipinnis), and economically important demersal fish such as Merluccius australis and Macruronus magellanicus are abundant within the fjords and channels (Balbontín and Bernal, 1997; Bernal and Balbontín, 1999). Of the shallow-water species, Sebastes capensis has been reported to be the most abundant (Balbontín and Bernal, 1997).
In our study, rockfish larvae were found during August and November, although abundance was greater in November. Previous work on Chilean fjords also found abundant rockfish larvae throughout the region during spring (Balbontín and Bernal, 1997; Bernal and Balbontín, 1999), including in the northern part of the Moraleda Channel, where we missed two stations in November. In contrast, in central Chilean coastal waters (33°S–36°S), larval S. capensis are most abundant earlier in the year (i.e. mid to late winter, Castro et al., 2000; Hernández-Miranda et al., 2003). In southern Chile, there is a marked seasonality in primary production and microphytoplankton composition and abundance (Toro et al., 1999), winter phytoplankton being characterized by low salinity diatoms and spring and summer phytoplankton by marine diatoms (Cassis et al., 2002). This change in microplankton composition and production is coincident with the increase in abundance of larval S. capensis in November (Figure 1b–d). Our results, therefore, suggest a potential delay in peak reproduction of this species as latitude increases.
In November, we noted a different spatial distribution of larvae according to ontogenetic stage and age (Figures 1b, c and 2). While pre-flexion larvae were very abundant throughout the channels, post-flexion larvae were found mainly in the larger Moraleda Channel and in the seaward (outer) part of the transverse channels and over the shelf. There were also differences in hydrographic conditions and in the distribution of potential food particles between the inner and the outer parts of the channels. Areas of marked water column stability (higher Brünt–Väisälä frequency) tended to coincide with greater concentrations of smaller copepods. However, the association between higher stability, smaller copepods, and smaller S. capensis larvae was not clearcut, because smaller (pre-flexion) larvae were caught throughout the channels. Our nets may have underestimated the abundance of smaller particles (i.e. copepod eggs and small nauplii), but such a sampling inefficiency should have not affected selectively some areas over others (e.g. inner vs. outer stations). Further, larger S. capensis larvae always occurred in the outer part of the channels and over the shelf, where the largest copepods were also found, suggesting an association between potential large size of larval food particles and larger rockfish larvae. Previous studies on other Sebastes species have demonstrated that as rockfish larvae grow, their foraging ability also increases, and they prey on a wider range of particle sizes, from copepod eggs to copepodites, and even selecting between calanoid and cyclopoid species (Anderson, 1994). In agreement with this scenario, two sequential non-exclusive hypotheses might be proposed: (i) the smaller particle size of available prey inside the channels associated with lower salinity and temperature might result in poorer condition of larger larvae (which may explain their scarcity and the narrower width increment in the otoliths), and (ii) larger larvae migrate or are transported from the inner part of the channels towards the shelf, where food particles are larger and where faster growth may be achieved (as suggested by the larger numbers of rockfish larvae and the wider rings in their otoliths). The few post-flexion larvae collected in the inner part of the channels were younger than those collected offshore and were located in deep layers, suggesting that they remained inshore in less favourable environmental conditions. Although limited data are currently available to test these hypotheses, they do agree with observations on other species of Sebastes on the Scotian Shelf (Drinkwater et al., 2000), as well as with the estuarine circulation pattern observed in the area, in which a shallow layer of low salinity water flows from the head to the outer part of the channels, and subsurface Sub-Antarctic Surface Waters (SSW) enter the channels and fjords at a deeper layer (30–150-m depth; Silva et al., 1998).
We found a linear relationship between otolith radius and SL, similar to that described for S. saxicola (Laidig et al., 1996), S. rastrelliger (Laidig and Sakuma, 1998), and Sebastes spp. from the North Atlantic (Penney and Evans, 1985). Our estimate of the extrusion check radius of S. capensis in the Patagonian fjords (10.2–16.8 µm) is in the range observed for other species of the Northeast Pacific, which vary from 10.5 µm in S. wilsoni (Laidig et al., 2004) to 16.9 µm in S. jordani (Laidig and Ralston, 1995). Interestingly, species from the Northwest Pacific, particularly off Japan, show larger extrusion check radii (17.6 ± 0.3 µm in S. thompsoni, according to Kokita and Omori, 1998; 26.1 ± 1.4 µm in S. inermis, according to Plaza et al., 2001), probably because larvae from the latter species extrude at larger sizes than eastern Pacific species.
Larval S. capensis showed growth rates during spring (0.148 mm d–1, Figure 4a) within the range reported for other Sebastes species. The growth rate observed during the first 40 days in S. saxicola was 0.125 mm d–1 (Laidig et al., 1996), Sakuma and Laidig (1995) reported growth rates of 0.135 mm d–1 for S. goodie, and Laidig et al. (1991) documented a growth rate for S. jordani in its first 20 days of approximately 0.165 mm d–1. Older larvae grow faster. Laidig et al. (1996), Kokita and Omori (1998), and Plaza et al. (2003) observed that growth rates of S. saxicola, S. thompsoni, and S. inermis, respectively, increased from 0.32 to 0.47 mm d–1 in larvae and juveniles. Whether or not this increase in growth rate with age also occurs in S. capensis is unknown. However, we suspect that the attainment of faster growth by older larvae is probably associated with coastward migration and later settlement, as described for other Sebastes species.
In this study, older larvae were found in water of higher salinity, usually to seaward of the channels and mostly within the upper 50 m. Other Sebastes spp. larvae found offshore also concentrate in the mixed layer above the pycnocline (Yoklavich et al., 1996; Sakuma et al., 1999; Drinkwater et al., 2000; Moser and Pommeranz, 2000). Several oceanographic processes have been proposed as potential mechanisms to retain larvae adjacent to their original coastal area: upwelling shadows (Wing et al., 1998), stratified Taylor columns (Dower and Perry, 2001) and frontal displacements (Bjorkstedt et al., 2002). Sebastes larvae have also been observed associated with drifting seaweed, which they apparently use as a shelter against predation (Kokita and Omori, 1998). Their return to a nearshore juvenile habitat takes place when they attain a larval size of 30–90 mm SL (Wing et al., 1998). We found very few post-flexion larvae in the inner part of the channels, and those we did catch were principally in deeper water. Therefore, it is possible that either those few larvae were retained within the channels as they developed, or they were transported into the channels by means of the deeper, more saline, east-moving layer, which is part of the general estuarine circulation described for the entire region by Silva et al. (1998).
The changes in distribution observed in larval S. capensis suggest that different habitats are being occupied during the early life stages. This change in ontogenetic larval distribution is probably similar in essence to the changes reported for other Sebastes species around the world. However, this similarity is intriguing given that the environmental conditions in the different areas are so different, varying from warmer coastal waters of the tropics (Rodríguez-Graña and Castro, 2003) to high-latitude areas such as the Chilean fjords and channels (this study). The evolutionary forces that drive such habitat change during early ontogeny, and which seem so common for a large number of even non-related taxa (i.e. decapod larvae), remain to be explained.
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Acknowledgements |
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We thank the officers and crew of the "Vidal Gormaz" and the scientific personnel on board for their help in the collection of the zooplankton samples. Hydrographic data were provided by N. Silva and M. Cáceres. We also thank Claudia Bustos and Cristian Chandía for their help in sorting fish larvae and copepods, respectively. During this study the first author was supported by a CONICYT scholarship for doctorate students, and funding for the study was provided by a CIMAR 8 FIORDOS (CONA) project to LRC.
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References |
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