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Variacion diaria en la distribucion vertical de larvas de peces forzada por filamentos de surgencia frente a Punta Angamos (norte de Chile).

Diel variation in the vertical distribution of fish larvae forced by upwelling filaments off Punta Angamos (northern Chile)

INTRODUCTION

The interaction between vertical distribution, migration and hydrodynamic processes is of special importance to the horizontal distribution of fish larvae in coastal upwelling regions and adjacent areas (Parrish et al., 1981; Norcross & Shaw, 1984; Olivar, 1990; Smith & Suthers, 1999). Recent studies in the Gulf of California have shown the importance of the thermocline and the mixed layer for the vertical distribution of fish larvae (Danell-Jimenez et al., 2009; Inda-Diaz et al., 2010). Fish eggs and larvae with near-surface distribution are more susceptible to the offshore transport in the Ekman layer associated with coastal upwelling (John & Re, 1995; Smith & Suthers, 1999; Rojas et al., 2002). In the vertical plane, current speed is of a magnitude that allows fish larvae and other planktonic organisms to conduct vertical migrations through the water column (Munk et al., 1989; Pringle, 2007). As the intensity of currents can vary along the water column, the vertical distribution may affect the horizontal dispersal of fish larvae and thus the drift of larval cohorts to their nursery areas.

Fish larvae migrate vertically at two temporal scales: they accompany most of zooplankton in its diel or subdiel (e.g., tidal) migration and their mean preferred depth also seems to change as they develop (Fortier & Harris, 1989; Paris & Cowen, 2004; Leis et al., 2005). The most common pattern of larval migration is the movement toward the top of the water column during the night, and the descent toward deeper waters during daytime (Tsukamoto et al., 2001). However, the reverse pattern has also been seen (Brodeur & Rugen, 1994), as well as fish larval disperse during the night and aggregate in daytime (Brewer & Kleppel, 1986; Munk et al., 1989).

The vertical distribution of zooplanktonic organisms has been observed to correlate with many environmental factors, such as light intensity (Job & Bellwood, 2000; Guizien et al., 2006), temperature (Annis, 2005), or the depth of pycnocline (Munk et al., 1989), thermocline (Boehlert et al., 1992; Annis, 2005) and chlorophyll maximum (Lampert et al., 2003). In most cases, these correlations relate to diel or subdiel movements and presumably result from a trade-off among reaching high concentrations of food near the surface or around the clines (Munk et al., 1989) and avoiding surface-dwelling visual predators (Fiksen & Giske, 1995), and cold water at depth which slows down growth (Lampert et al., 2003). The advantages of the vertical migration are generally proposed to be light-related predator avoidance (Yamashita et al., 1985), the pursuit of zooplankton prey (Fortier & Leggett, 1983; Munk et al., 1989), facilitation of larval transport in varying tidal currents (Hare & Govoni, 2005), optimization of the energetic advantage gained by larvae at certain depths in thermally stratified water (Neilson & Perry, 1990), pursuit of optimum light conditions for larval survival (Hurst et al., 2009), rhythms of swim bladder inflation (Stenevik et al., 2007), and a strategy to guarantee their retention in shallow waters, the latter of crucial importance in upwelling regions. In upwelling areas, the diel vertical position of fish larvae determines if they are retained in shallow and productive waters or advected offshore, and larvae with near-surface distributions are more susceptible to offshore transport associated with coastal upwelling than deeper distributions that render larvae to shoreward transport (Rodriguez, 1990; HernandezLeon et al., 2007).

Eggs and fish larvae may be subjected to intense alongshore and cross-shelf flow in upwelling ecosystems, such that the ability to develop or adopt retention mechanisms near the upwelling centres may play a relevant role to maintain local populations (Wing et al., 1998; Pringle, 2007) and hence successful recruitment. Among proposed retention mechanisms, diel vertical migration (DVM) through the use of reversing vertical flows (Peterson, 1998; Morgan & Fisher, 2010) has been hypothesized. At the coastal zone of the Peninsula de Mejillones and Bahia Mejillones (23[degrees]05'S), it is usual the presence of coastal upwelling of cold-waters rich in nutrients (Marin et al., 1993, 2001). However, this upwelled nutrient-rich cold waters also produces dispersal of herbivorous planktivore populations, increasing the fish larval transport offshore to areas with reduced food availability. Associated to northern Chile upwelling system, a shallow oxygen minimum layer (<0.5 mL [L.sup.-1]) within the euphotic zone might impose a strict physiological limit for vertical movement (Morales et al., 1996; Giesecke & Gonzalez, 2004), forcing aggregation of planktonic organisms in the upper 50 m layer (Escribano & Hidalgo, 2000).

The local upwelling focus off Peninsula de Mejillones generates changes in the circulation patterns that modify the advective environment, inducing the mixing and transport of mero- and holoplanktonic organisms (Olivares, 2001; Rojas et al., 2002). Thus, the development of an upwelling plume also induces abrupt changes on physical conditions of the water column. Along with a deepening of the mixed layer, surface waters are subjected to an abrupt cooling, which is accompanied by low oxygen concentration. These changes, taking place mostly inside the upwelling plume, might also influence larval vertical distribution. However, the inherent characteristics of each species may show inter-specific and ontogenic larval differences. The objective of this article is to evaluate the role of diel vertical distributions and ontogenetic migrations of fish larvae as behaviours responsible of nearshore retention during cold water filaments development off Punta Angamos, northern Chile.

MATERIALS AND METHODS

Sampling design and procedures

Oceanographic data and plankton samples were collected during September 1998 and January 1999 off Punta Angamos (23[degrees]29'S, 70[degrees]59'W) (Fig. 1). A grid of 23 stations was quasi-synoptically sampled (26 h) on board the R/V Purihaalar from the Universidad de Antofagasta. At each station, vertical profiles of temperature ([degrees]C), salinity and dissolved oxygen (mL [L.sup.-1]) were carried out from surface to 200 m depth with a SeaBird SBE-19 conductivity temperature depth (CTD), equipped with a calibrated YSI Beckman oxygen sensor. Current profiles were measured with the vessel anchored at each station using a Doppler sensor current meter (Aanderaa RCM-9). In addition, fluorescence profiles were obtained from 100 m to surface, using a Wetstar fluorometer attached to an Ocean Sensor CTD. Fluorescence units were converted to Chl-a concentration by a linear algorithm derived from an in situ calibration (Escribano & McLaren, 1999). Four fixed stations were used for ichthyoplankton sampling: two inside and two outside the upwelling plume, in accordance with a satellite NOAA image of sea surface temperature (SST) obtained one day before the start of the sampling, on September 9 (1998) and January 11 (1999), respectively (Fig. 1). Detailed information on satellite-derived SST is described in Marin et al. (2001). The following two days after the oceanographic grid was finished, fish larvae were sampled again at four stations during two daytime and two night-time periods. This design considered upwelling/non-upwelling conditions as the main treatment (i.e., inside/outside the cold plume) and variability introduced by daytime/night-time effect, as well as temporal changes after 24 h. Diel changes in vertical distribution of fish larvae were evaluated by sampling three depth strata each time: 200-80, 80-20 and 20-0 m. According with Escribano et al. (2001), during upwelling events the upper 20 m of the water column represent the Ekman layer, the 80-20 m stratum shows a reversal flow, and the 200-80 m stratum is a deep and more stable layer.

Ichthyoplankton sampling and analysis

Ichthyoplankton was captured using a vertically towed Hensen net with 0.5 m opening, 200 [micro]m mesh, equipped with a double opening-closing system, and a calibrated General Oceanic flowmeter. Samples were preserved in 4% buffered formalin. To detect changes in the water column during the ichthyoplankton sampling, the CTD and fluorometer were also deployed as described above at each of the four stations in the next two days. In the laboratory, all fish larvae were removed for counting and identified to the lowest taxonomic level using descriptions by Fahay (1983), Moser (1996), and Neira et al. (1998). Larvae were separated into two ontogenetic groups according to the bending of the notochord, preflexion and postflexion larvae (larvae in flexion stage were included in postflexion larvae). The number of individuals collected in the different sampling strata was standardized to number [unit.sup.-1] of volume of filtered water (densities): 1000 [m.sup.3] for fish larvae (densities). Average vertical distributions were calculated using all the stations. Fish larvae caught in the different sampling strata were also integrated to obtain the number of individuals 10 [m.sup.-2] of sea surface (abundances).

Data analysis

The effect of diel distribution of selected taxa of larval fish was analyzed through weighted mean depth (WMD) as the density centre according (Heath et al., 1991; Gronkjaer & Wieland, 1997):

WMD = [n.summation over (i = 1)][p.sub.i][Z.sub.i] = [n.summation over (i = 1)][C.sub.i][Z.sub.i]/[n.summation over (i = 1)][C.sub.i]

where [p.sub.i] and [C.sub.i] are, respectively, the proportion and the concentration fish larvae (number 1000 [m.sup.-3]) in the ith stratum, and [Z.sub.i] is the mid-depth of the ith stratum.

The non-parametric Kruskal-Wallis test (Sokal & Rohlf, 1985; Siegel & Castellon, 1988) was used to assess the effects of upwelling vs non-upwelling locations (inside and outside the upwelling plume), the statistical significance of differences in total larval abundance between daytime and nighttime, and among the different depth strata. Because of low occurrence for many species, Kruskal-Wallis test was not used for each separated taxa. To assess short-term variability, we analysed changes in oceanographic conditions and total larval abundance at stations separated by 12 h intervals.

A nonparametric multivariate procedure (BIO-ENV) was used to analyze the relationship between select environmental variables and larval community. The details of the BIO-ENV algorithm and its suitability for use in analyzing the interactions of biological and environmental data are described by Clarke & Gorley (2001) and Clarke & Warwick (2001). A similarity matrix of depth-stratified samples by larval taxa (61 samples*15 taxa) was also performed. This matrix was analyzed in association with three environmental variables: mean depth (m), mean temperature ([degrees]C), and mean dissolved oxygen (mL [L.sup.-1]) of each depth-stratified sample. The BIO-ENV analyses were performed by using the Spearman rank correlation method on the normalized Euclidean distance similarity matrices of the log (n + 0.1)-transformed, nonstandardized environmental variables by depth-stratified samples (Clarke & Gorley, 2001). All diversity, cluster and BIO-ENV analyses were performed by using PRIMER statistical software.

RESULTS

Mesoscale features off Punta Angamos during austral spring and summer

The NOAA satellite images showed, in September 1998, a well-developed upwelling filament with cold surface waters below 14[degrees]C (Fig. 1a). Conversely, in January 1999 a weak upwelling filament was seen with surface temperatures near 15[degrees]C (Fig. 1b). Chlorophylla showed spatial heterogeneity off Punta Angamos. In September 1998, high concentrations of Chl-a (>2.0 mg [m.sup.-3]) were recorded southward of Punta Angamos. In January 1999, high chlorophyll levels (>5.0 mg [m.sup.-3]) were found northward from upwelled focus. In September 1998, the average values of SST showed significant differences ([F.sub.[1,6]] = 40.39; P < 0.01) between upwelled and non-upwelled zones (inside and outside the filament). Similarly, significant effects ([F.sub.[1,6]] = 9.60; P < 0.05) were seen in surface chlorophyll values in both locations. Contrary, the average values of SST in January 1999 showed non significant differences ([F.sub.[1,6]] = 0.346; P > 0.05) between both sites (inside/outside), nor in the average surface chlorophyll values ([F.sub.[1,6]] = 0.089; P > 0.05).

In September 1998, velocity profiles showed a southward flow throughout the water column, and a typical structure across shore of an upwelling event composed by three layers, a westward surface layer (Ekman layer, 0-25 m depth), a subsurface eastward layer (25-50 m) and a deep layer (50-200 m) flowing westward (Fig. 2a). In January 1999 the water column showed the predominance of a southward and westward flow throughout the water column (Fig. 2b). In both cruises low speeds (<0.2 m [s.sup.-1]) were measured between surface and 20 m depth, increasing to values of ~0.3 m [s.sup.-1] below that depth.

In September 1998 differences of ~4[degrees]C were seen in SST maps inside and outside the upwelling filament (Fig. 3a). Temperature profile at upwelled waters showed smooth vertical gradients in surface waters (0.1[degrees]C [m.sup.-1] in the first 10 m of the water column), while outside the filament (non-upwelling) thermocline creates strong stratification in the first 50 m depth. Differences in the concentration of dissolved oxygen in surface waters were also evident inside and outside the upwelling filament (<1.5 mL [L.sup.-1] vs 2.0-6.0 mL [L.sup.-1], respectively, Fig. 3b). From the 50 m depth oxygen values decreased below ~0.2-0.4 mL [L.sup.-1] in nonupwelling waters. Inside the filament, subsurface waters (50-100 m) showed an increase in its dissolved oxygen concentration (~2.0 mL [L.sup.-1]) (Fig. 3b). Chlorophyll-a profiles showed differences of ~2.0 mg [m.sup.-3] at the surface layer inside and outside the upwelling filament; a subsurface chlorophyll peak (>3.0 mg [m.sup.-3]) was detected in the filament; however, in the mixed layer the concentration of Chl-a was larger out the filament (3.0-5.2 mg [m.sup.-3]). Below 50 m depth chlorophyll concentration decreased to ~2.0 mg [m.sup.-3] in both water types (Fig. 3c).

In January 1999, SST maps showed differences less than 2[degrees]C inside and outside the upwelling filament, and temperature profiles were similar in the vertical structure but with higher temperatures in nonupwelling surface waters (Fig. 3a). Water column outside the upwelling filament showed higher amount of dissolved oxygen in the top 75 m (Fig. 3b) and chlorophyll in the top 25 m, compared with values obtained inside the filament during austral summer (Fig. 3c).

Taxonomic composition of ichthyoplankton

During the oceanographic survey in September 1998, 42 positive samples (from a total of 48) of fish larvae were collected which represent an occurrence frequency of 93.3%. A total of 189 larvae were separated, 13 taxa identified, representing 68.9% of total larvae (Table 1). Dominant species were Diogenichthys laternatus (28.6%), Bathylagus nigrigenys (21.8%), Engraulis ringens (17%) and Diogenichthys atlanticus (6.9%). In January 1999, only 30 positive samples of fish larvae were collected out of a total of 48 samples, representing an occurrence frequency of 62.5%. A total of 350 larvae were separated, and 11 taxa identified, representing 34.5% of total larvae (Table 1). Again, larval taxa collected correspond to D. laternatus (28.5%), E. ringens (21.7%), B. nigrigenys (11.2%), S. sagax (9.6%) and D. atlanticus (3.8%).

Larval distribution

Fish larval abundance showed differences at both locations. In September 1998, higher larval fish abundance was collected in stations located outside the upwelling filament (i.e., mean [+ or -] SD: E3 = 74 [+ or -] 101 larvae 10 [m.sup.-2]; E4 = 47 [+ or -] 47 larvae 10 [m.sup.-2]) (Fig. 4). In January 1999, higher larval concentrations were found outside the filament (Fig. 4), and in a station near of it (i.e., E2 = 210 [+ or -] 356 larvae 10 [m.sup.-2]; E3 = 162 [+ or -] 303 larvae 10 [m.sup.-2] and E4 = 62 [+ or -] 47 larvae 10 [m.sup.-2]).

Kruskal-Wallis test only showed significant effects ([F.sub.[1,116]] = 8.05; P < 0.01) for larval abundance during day and night, as well as among depth strata during September 1998 ([F.sub.[1,116]] = 4.64; P < 0.05), this difference in abundance is explained by the high variability observed in the deep layer (80-200 m) (P = 0.005; Tukey post-hoc). In January 1999, significant effects were found inside and outside the upwelling filament as well as among strata (P < 0.05) and significant effects were observed in the interactions between factors (upwelling*strata) ([F.sub.[2,107]=] 4.02; P < 0.05) and (strata*h) ([F.sub.[2,107]] = 3.22; P < 0.05) on larval fish abundance. Interactions suggest that the location of the sampling station together with the effect of daynight explains a significant percentage of the observed variability.

Vertical distribution of fish larvae

In September 1998, higher fish larvae abundance was found during the night in the mid (20-80 m) and deep stratum (80-200 m) outside the upwelling filament (Fig. 5). In January 1999, the highest larval abundance was collected during the night in the 20-80 m stratum in both zones, and in deeper waters inside the filament larvae were collected only during dayligth (Fig. 5). Low fish larvae abundance (~200 larvae 1000 [m.sup.-3]) was observed in the upper and middle strata (0-20 and 2080 m) in upwelled and non-upwelled waters during daytime sampling.

In September 1998, the mid-depth layer (20-80 m) showed higher number of taxa (12), mainly associated with non-upwelling waters (Table 2). Larval D. laternatus, B. nigrigenys and Diogenichthys sp. were collected in all strata. However, only D. laternatus was found throughout the sampled water column inside and outside the upwelling plume. Myctophids Hygophum reinhardti and Protomyctophum crockeri were scarcely collected and found in mid-depth of non-upwelling waters. Larval mote sculpin Normanichthys crockeri were also collected in mid-depth of the upwelled waters. In January 1999, myctophids D. laternatus, D. atlanticus, Diogenichthys sp., Diaphus theta, Lampanyctus sp. and small pelagic E. ringens and S. sagax were collected in all three strata. However, only S. sagax was found in all three layers both inside and outside of the upwelling plume (Table 2). N. crockeri larvae was found only in the upper (0-20 m) and middepth strata of upwelled waters, while Myctophum nitidulum, Merluccius gayi and Ceratoscopelus townsendi were poorly represented in this cruise.

Daily changes in the vertical distribution of fish larvae

The three most abundant species were collected on surface waters (WMD was located between 10 and 72 m (range = 62 m) at night and deeper (WMD was located between 14 and 139 m (range = 125 m) during daylight. Late stage larval (postflexion) showed wider amplitude in vertical distribution during day and night (WMD was located between 10 and 96 m (range = 86 m). Small pelagic larvae (E. ringens) during spring showed significant differences ([F.sub.[1,48]] = 58.85; P < 0.05) among depth during day/night sampling as well as differences at depth for early and late-stage larvae ([F.sub.[1,48]] = 5.80; P < 0.05) (Fig. 6). In summer larval E. ringens showed significant differences ([F.sub.[1,48]] = 89.61; P < 0.05) among depth strata only during day/night catch. Larval myctophid D. laternatus showed significant differences among depth during sampling hours (day/night) (K-W test; P < 0.05), and inside/outside upwelled plume at this cruise (Fig. 6). Larval B. nigrigenys collected at spring showed significant differences ([F.sub.[1,49]] = 78.11; P < 0.05) over depth during day/night sampling. Finally, B. nigrigenys larvae collected during summer showed significant differences ([F.sub.[1,36]] = 17.55; P < 0.05) at vertical distribution, being found at shallower waters at day and deeper at nighttime (Fig. 6).

Environmental relationships

The similarity dendrogram (~82 %) shows three groups of fish larvae of different habitats in the adult stage (Fig. 7a). Group 1 was represented by two coastal pelagic species (Engraulis ringens and Sardinops sagax). Additionally, this group included myctophid Diaphus theta of epi-mesopelagic origin. All these species were found at stations under the influence of upwelling filament and coastal front. Group 2 was represented mostly by myctophids epi-mesopelagic, as well as some species of meso-bathypelagic habits (Bathylagus nigrigenys) and bathydemersal (Merluccius gayi). These species were found exclusively associated with upwelling front warm waters. Finally, Group 3 was represented by a single demersal species N. crockeri present in a zone influenced by cold water from upwelling filament. Cluster analyses also indicated the presence of two larval depth assemblages: <100 m and >100 m (Fig. 7b).

BIO-ENV and correlation analyses revealed significant relationships among several environmental factors and larval average concentrations. A depthstratified BIO-ENV analysis, which included mean temperature ([degrees]C), mean dissolved oxygen (mL [L.sup.-1]) and chlorophyll (mg [m.sup.-3]) of each depth-stratified sample (i.e., 0-20; 20-80 and 80-200), showed that depth alone explained 44% in mean larval concentrations. No multiple factor combination explained more variability in larval concentration data. Pairwise correlation analyses revealed significant negative correlations between temperature and mean concentrations of small pelagic E. ringens (three strata), S. sagax (only upper water) and Myctophid D. atlanticus (upper/midwater). Significant negative correlations (P < 0.05) were seen between B. nigrigenys (midwater) and S. sagax (deepwater) with dissolved oxygen and chlorophyll (Table 3). Average concentrations of N. crockeri (three strata) and D. laternatus (upper/midwater) larvae, were also positively correlated with temperature and negatively correlated with chlorophyll, although the correlations were not significant (P > 0.05).

DISCUSSION

Physical conditions

The distinctive feature of the Humboldt Current System in northern Chile is the strong oceano-graphic mesoscale activity in the region, registered from satellite images during the plankton samples collection (Marin et al., 2001), and from changes in hydrographic conditions during upwelling process. Normally, an increase wind stress about sea surface layer favors the cold filament formation, as extends until 60 km offshore, clearly visible from sea surface temperature satellite images. This structures type, as well as meanders of cyclonic eddies and upwelling shadows have been described previously (Escribano & Hidalgo, 2000; Rojas et al., 2002; Marin et al., 2003).

Community structure in the upwelled filaments

The highest fish larval density (96%) was found in the first 100 m of the water column, a feature documented by other studies (Auth & Brodeur, 2006; Landaeta et al., 2008). The ichthyoplanktonic taxocenosis found off Punta Angamos (Peninsula de Mejillones) showed a mixture of coastal and oceanic species (epipelagic, epimesopelagic and mesopelagic). However, it is not possible to see a spatial gradient distribution, due to presence of epi-mesopelagic oceanic species nearshore (i.e., D. laternatus and D. atlanticus) and some coastal species (i.e., E. ringens and S. sagax). A dense aggregation of phytoplankton and zooplankton associated to the upwelling filament formation suggests that areas near it are suitable habitat for fish larvae feeding, increasing their survival probability (Escribano & Hidalgo, 2000). However, results do not show a clear relationship among larval abundance with high chlorophyll concentrations, because coastal fronts may modify the chlorophyll spatial distribution toward Bahia Mejillones. This suggests that the relationship between coastal species with high primary productivity areas would be too fragile if fish larval have not developed tactics and/or mechanisms that avoid or reduce offshore horizontal transport, facilitating their retention in shallow areas nearshore (Pringle, 2007; Morgan & Fisher, 2010). Usually, E. ringens is found in shallow areas nearshore (Castro et al., 2000), however the low abundance of anchoveta larvae in the study area, suggests that a large number have been transported from inshore to adjacent zones of the upwelling filament, condition which was also seen in fish larvae in the Canary Upwelling System (Brochier et al., 2011).

In both cruises the highest larval densities were found in non-upwelling waters (middle/deep strata) below the Ekman layer (>20 m), habitat continually affected by short-term variations as well oceanographic conditions changes due to circulation in the area (Marin et al., 2001). Myctophids D. laternatus, D. atlanticus, Triphoturus oculeus, D. theta, among others, have been previously reported by several authors in the area (Rojas et al., 2002; Rodriguez-Grana & Castro, 2003) and are associated with warm-water increasing their abundance. The significant presence of myctophid fish larvae found in the warm epipelagic layer (Sassa, 2001; Sassa et al., 2002a, 2004b; Moku et al., 2003), likely coincide with spawning periods during most of the year (Gjosaeter & Kawaguchi, 1980; Olivar & Beckley, 1994). Small pelagics (E. ringens and S. sagax) generally associated with cold upwelled water (Loeb & Rojas, 1988; Becognee et al., 2009) were seen preferentially at warm waters, probably because of the mechanical effect exercised by cold water upwelling which facilitates larval transport nearshore (Rodriguez et al., 1999).

From an ecological perspective, the occurrence of D. laternatus and D. atlanticus larvae in upwelling/ non-upwelling conditions might be the result of successive adaptations to this environment. B. nigrigenys and M. nitidulum larvae showed preferences for warmer waters due to larval abundance in nonupwelling water. On the contrary, N. crockeri was found exclusively in cold upwelled water. Effects of ontogeny, temperature, and ligh might explain partially the changes in migration patterns of fish larvae (Hurst et al., 2009). However, the distribution patterns in adult species vary according to eating habits and life history (Watanabe et al., 1999).

Diel variation and ontogenetic migration

Early stages of fish in presence of cold water upwelled showed a minimum difference in the average depth during night. Fish larvae were found in shallow layers (~40 m) where the drift effects would be less intense. However, during daylight only some late-stage larvae (postflexion) were found in deeper layers (~70-80 m) that match with compensatory flow coastward. Our hypothetical approach suggests that larvae with a morphological advanced development are located deepwater during daytime that facilitate eventually the return of fish larval to inshore zones using a compensatory deep flow. Thus, results obtained from cruises on 9 September (1998) and 11 January (1999) showed a trend towards a diel vertical migration and/or ontogenetic migration; this behavior would be independent of condition type (upwelling/non-upwelling) as well as from hydrographic conditions observed during active upwelling events off Punta Angamos. However, in another regions with long periods of active upwelling the traditional migratory model (down at day, up during the night) would be partially affected, as evidenced the change in the migratory behavior (down at night, up during the day) seen in some myctophids larvae (Rodriguez et al., 2006; Auth et al., 2007). In general, it is well documented that a large number of fish larvae are visual predators concentrating its feeding rhythms during the daylight hours or during dusk and dawn hours (Hunter, 1984). Thus, behavior migratory of some myctophid larvae collected for both periods off Punta Angamos seems given by daily feeding cycles, with higher ingestion of food during daylight hours (Rodriguez-Grana & Castro, 2003; Rodriguez-Grana et al., 2005; Sassa & Kawaguchi, 2005).

The cross-shelf currents derived from the upwelling process may cause significant offshore transport of fish larvae as well as mortality due to low food availability (Pizarro et al., 1998). Off Punta Angamos, especially during the active upwelling phase in September (1998), the mechanical effect generated for cold-water upwelling and the current velocity (~0.1 m [s.sup.-1]) of the surface layer (~0-20 m) may transport offshore large numbers of larvae, decreasing their survival probability. On the contrary, a significant number of fish larvae could remain there either by staying deep in the water column or by undertaking diel vertical migrations between surface and bottom currents (Roughan et al., 2005). This evidence may partially explain the presence of fish larvae in the focus of the upwelling during both periods. We believe that ichthyoplankton sampling should be conducted primarily at night if at all possible to eliminate any potential bias due to net avoidance. Net avoidance should at least be factored into any model estimating abundances and depth distributions of larvae collected during both day and night. Although this study was not designed to determine the underlying causes of the DVM of larval fish, it does provide some evidence for type-I DVM (up at night, down during the day) for E. rigens, D. laternatus and B. nigrigenys larvae during active events of upwelling off Punta Angamos, despite not finding evidence for either type of DVM for larval E. ringens in previous studies (Rojas et al., 2002; Rodriguez-Grana & Castro, 2003). This type-I DVM could help to partially explain the retention of larval fish assemblages, in particular E ringens larvae, close to recruitment areas along and inshore from the shelf due to the ability of larvae to regulate their position in the water column and to take advantage of selective Ekman transport (Sakuma & Larson, 1995; Stenevik et al., 2007; Morgan et al., 2009; Morgan & Fisher, 2010) related to water flows generated during active and/or relaxation phase (i.e., spin-up/spin-down phases) of upwelling events (Send et al., 1987) off Punta Angamos.

CONCLUSIONS

In this study the vertical distribution of fish larvae during coastal upwelling in September 1998 and January 1999 off Punta Angamos showed changes during day/night suggesting diel vertical migration as a strategy to avoidance of the offshore Ekman layer during upwelling events; this migratory behavior would be independent of the condition type (upwelling/nonupwelling) as well as from hydrographic conditions registered at both oceanographic surveys. Early stages of fish larvae during daylight were found at an average depth which coincides with the zone boundary among the Ekman layer and the polar flow that dominates the circulation, while some late-stage larval were found at an average depth that coincide with the Ekman layer. The mixed coastal and oceanic species of fish larvae found off Punta Angamos indicated the importance of this local upwelling focus for the development of larval fish assemblages in northern Chile.

DOI: 103856/vol42-issue3-fulltext-2

Received: 17 August 2012; Accepted: 12 April 2014

ACKNOWLEDGEMENTS

The author acknowledges the cooperation and logistical support provided by crew the R/V Purihaalar. Special thanks to Dr. Mauricio Landaeta for their valuable contributions and comments on earlier drafts of the manuscript. This work has been funded by FONDECYTChile, Grant 198-0366 adjudicated to R. Escribano and V. Marin.

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Pablo M. Rojas (1)

(1) Division de Investigacion en Acuicultura, Instituto de Fomento Pesquero (IFOP) Balmaceda 252, Puerto Montt, Chile

Corresponding author: Pablo M. Rojas (pablo.rojas@ifop.cl)

Table 1. Total fish larvae collected (inside/outside) for upwelling
filaments off Punta Angamos in September 1998 and January 1999. The
number corresponds to larvae per 10 [m.sup.2]. Occurrence corresponds
to number of positive samples (total samples = 48) in which the
species was present.

                                              September 1998
                                              Upwelling
Family             Taxa                       Maximum     Occurrence
                                              abundance

                   Diogenichthys atlanticus   54          13
                   Diogenichthys latematus    76          39
                   Triphoturus oculeus        -           -
                   Diaphus theta              -           -
                   Lampanyctus sp.            -           -
                   Diogenichthys sp.          36          4
                   Protomyctophum crockeri    -           -
                   Hygophum reinhardti        -           -
                   Myctophum nitidulum        -           -
                   Ceratoscopelus townsendi   -           -
                   Unidentified sp. 1         82          20
                   Unidentified sp. 2         -           -
Engraulidae        Engraulis ringens          28          4
Clupeidae          Sardinops sagax            28          9
Bathylagidae       Bathylagus nigrigenys      54          17
Normanichthyidae   Normanichthys crockeri     41          13
Merlucciidae       Merluccius gayi            27          4
Macrouridae        Unidentified sp.           76          4

                                              September 1998
                                              Non-upwelling
Family             Taxa                       Maximum     Occurrence
                                              abundance

                   Diogenichthys atlanticus   70          22
                   Diogenichthys latematus    545         44
                   Triphoturus oculeus        28          4
                   Diaphus theta              -           -
                   Lampanyctus sp.            84          22
                   Diogenichthys sp.          88          32
                   Protomyctophum crockeri    84          4
                   Hygophum reinhardti        28          4
                   Myctophum nitidulum        35          13
                   Ceratoscopelus townsendi   -           -
                   Unidentified sp. 1         -           -
                   Unidentified sp. 2         195         40
Engraulidae        Engraulis ringens          38          4
Clupeidae          Sardinops sagax            92          26
Bathylagidae       Bathylagus nigrigenys      397         40
Normanichthyidae   Normanichthys crockeri     -           -
Merlucciidae       Merluccius gayi            38          9
Macrouridae        Unidentified sp.           16          4

                                              January 1999
                                              Upwelling
Family             Taxa                       Maximum     Occurrence
                                              abundance

                   Diogenichthys atlanticus   19          4
                   Diogenichthys latematus    123         13
                   Triphoturus oculeus        27          4
                   Diaphus theta              72          17
                   Lampanyctus sp.            19          4
                   Diogenichthys sp.          -           -
                   Protomyctophum crockeri    -           -
                   Hygophum reinhardti        -           -
                   Myctophum nitidulum        -           -
                   Ceratoscopelus townsendi   -           -
                   Unidentified sp. 1         49          17
                   Unidentified sp. 2         -           -
Engraulidae        Engraulis ringens          1119        39
Clupeidae          Sardinops sagax            93          35
Bathylagidae       Bathylagus nigrigenys      34          9
Normanichthyidae   Normanichthys crockeri     47          9
Merlucciidae       Merluccius gayi            -           -
Macrouridae        Unidentified sp.           -           -

                                              January 1999
                                              Non-upwelling
Family             Taxa                       Maximum     Occurrence
                                              abundance

                   Diogenichthys atlanticus   135         35
                   Diogenichthys latematus    269         30
                   Triphoturus oculeus        79          13
                   Diaphus theta              101         13
                   Lampanyctus sp.            158         39
                   Diogenichthys sp.          -           -
                   Protomyctophum crockeri    -           -
                   Hygophum reinhardti        -           -
                   Myctophum nitidulum        25          4
                   Ceratoscopelus townsendi   26          4
                   Unidentified sp. 1         -           -
                   Unidentified sp. 2         173         39
Engraulidae        Engraulis ringens          1364        39
Clupeidae          Sardinops sagax            148         43
Bathylagidae       Bathylagus nigrigenys      48          13
Normanichthyidae   Normanichthys crockeri     -           -
Merlucciidae       Merluccius gayi            27          4
Macrouridae        Unidentified sp.           42          4

Table 2. Location of different taxonomic groups, per strata depth,
in September 1998 and January 1999 (inside/outside upwelling
filament). The abundance corresponds to (N = number per 1.000
[m.sup.3]) and occurrence (%) represents the number
of cases (samples = 48) in which the species was present.

                                         September 1998
                                         Upwelling   Non-upwelling
Stratum (m)   Taxa                       N     %     N      %

              Diogenichthys laternatus   195   9     65     9
              Diogenichthys atlanticus   -     -     -      -
              Diogenichthys sp.          -     -     27     4
              Lampanyctus sp.            -     -     -      -
              Diaphus theta              -     -     -      -
0-20          Mictophum nitidulum        -     -     34     4
              Engraulis ringens          -     -     -      -
              Sardinops sagax            -     -     -      -
              Bathylagus nigrigenys      -     -     478    13
              Normanichthys crockeri     41    4     -      -
              Myctophidae Unidentified   332   30    198    13
              Diogenichthys laternatus   114   17    1179   30
              Diogenichthys atlanticus   78    9     198    13
              Triphoturus oculeus        -     -     28     4
              Myctophum nitidulum        -     -     64     9
              Hygophum reinhardti        -     -     28     4
              Diaphus theta              -     -     -      -
              Diogenichthys sp.          36    4     201    17
20-80         Protomyctophum crockeri    -     -     84     4
              Lampanyctus sp.            -     -     189    17
              Engraulis ringens          28    4     38     4
              Sardinops sagax            -     -     38     4
              Bathylagus nigrigenys      54    4     641    22
              Normanichthys crockeri     25    4     -      -
              Merluccius gayi            27    4     38     4
              Myctophidae Unidentified   199   17    650    22
              Macrouridae Unidentified   76    4     -      -
              Diogenichthys laternatus   98    17    250    22
              Diogenichthys atlanticus   24    4     81     9
              Triphoturus oculeus        -     -     -      -
              Diaphus theta              -     -     -      -
              Diogenichthys sp.          -     -     49     9
              Ceratoscopelus townsendi   -     -     -      -
80-200        Lampanyctus sp.            -     -     55     4
              Engraulis ringens          -     -     -      -
              Sardinops sagax            -     -     41     9
              Bathylagus nigrigenys      92    17    152    17
              Normanichthys crockeri     19    4     -      -
              Merluccius gayi            -     -     16     4
              Myctophidae Unidentified   106   22    165    17
              Macrouridae Unidentified   -     -     16     4

                                         January 1999
                                         Upwelling   Non-upwelling
Stratum (m)   Taxa                       N     %     N      %

              Diogenichthys laternatus   162   9     -      -
              Diogenichthys atlanticus   -     -     32     4
              Diogenichthys sp.          -     -     -      -
              Lampanyctus sp.            -     -     32     4
              Diaphus theta              121   9     101    4
0-20          Mictophum nitidulum        -     -     -      -
              Engraulis ringens          82    4     32     4
              Sardinops sagax            227   18    217    18
              Bathylagus nigrigenys      -     -     -      -
              Normanichthys crockeri     47    4     -      -
              Myctophidae Unidentified   134   13    50     4
              Diogenichthys laternatus   19    4     675    17
              Diogenichthys atlanticus   19    4     237    17
              Triphoturus oculeus        27    4     27     4
              Myctophum nitidulum        -     -     25     4
              Hygophum reinhardti        -     -     -      -
              Diaphus theta              50    9     27     4
              Diogenichthys sp.          -     -     -      -
20-80         Protomyctophum crockeri    -     -     -      -
              Lampanyctus sp.            19    4     162    17
              Engraulis ringens          64    9     39     4
              Sardinops sagax            236   22    290    9
              Bathylagus nigrigenys      -     -     354    18
              Normanichthys crockeri     19    4     -      -
              Merluccius gayi            -     -     27     4
              Myctophidae Unidentified   19    4     282    13
              Macrouridae Unidentified   -     -     -      -
              Diogenichthys laternatus   -     -     391    13
              Diogenichthys atlanticus   -     -     114    13
              Triphoturus oculeus        -     -     101    9
              Diaphus theta              -     -     22     4
              Diogenichthys sp.          -     -     -      -
              Ceratoscopelus townsendi   -     -     26     4
80-200        Lampanyctus sp.            -     -     252    17
              Engraulis ringens          91    4     -      -
              Sardinops sagax            54    9     26     4
              Bathylagus nigrigenys      -     -     -      -
              Normanichthys crockeri     -     -     -      -
              Merluccius gayi            -     -     -      -
              Myctophidae Unidentified   -     -     158    22
              Macrouridae Unidentified   -     -     42     4

Table 3. Correlation coefficients for the fish larvae collected in
stratified sample with three oceanographic variables during the day
and night (inside/outside) in upwelling plume off Punta Angamos in
September 1998 and January 1999: water temperature ([degrees]C),
dissolved oxygen (mL [L.sup.-1]) and chlorophyll-a (mg [m.sup.-3]),
and log-transformed densities (number per 1000 [m.sup.3]) of fish
larvae present in all three strata. * P < 0.05.

Stratum (m)   Taxa                       Temperature   Dissolved
                                                       oxygen

              Diogenichthys laternatus   0.10          0.06
              Diogenichthys atlanticus   -0.66 *       0.11
0-20          Engraulis ringens          -0.85 *       -0.52
              Sardinops sagax            -0.70 *       0.01
              Bathylagus nigrigenys      -0.78 *       -0.30
              Normanichthys crockeri     0.52          0.06
              Diogenichthys laternatus   0.23          0.07
              Diogenichthys atlanticus   -0.51         -0.39
20-80         Engraulis ringens          -0.83 *       -0.83 *
              Sardinops sagax            -0.45         -0.45
              Bathylagus nigrigenys      -0.65 *       -0.54 *
              Normanichthys crockeri     0.45          -0.45
              Diogenichthys laternatus   -0.29         0.20
              Diogenichthys atlanticus   -0.78 *       -0.81 *
80-200        Engraulis ringens          -0.70 *       -0.80 *
              Sardinops sagax            -0.68         -0.78 *
              Bathylagus nigrigenys      0.17          -0.10
              Normanichthys crockeri     0.48          -0.48

Stratum (m)   Taxa                       Chlorophyll-a

              Diogenichthys laternatus   -0.11
              Diogenichthys atlanticus   -0.17
0-20          Engraulis ringens          0.41
              Sardinops sagax            -0.16
              Bathylagus nigrigenys      0.37
              Normanichthys crockeri     -0.06
              Diogenichthys laternatus   -0.40
              Diogenichthys atlanticus   -0.54 *
20-80         Engraulis ringens          -0.60 *
              Sardinops sagax            0.47
              Bathylagus nigrigenys      -0.61 *
              Normanichthys crockeri     -0.47
              Diogenichthys laternatus   -0.40
              Diogenichthys atlanticus   0.68
80-200        Engraulis ringens          0.53
              Sardinops sagax            -0.52 *
              Bathylagus nigrigenys      0.32
              Normanichthys crockeri     -0.39
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