Introduction

Most cephalopods have rapid growth rates, r-selection strategy, and short life cycles (^{Boyle and Rodhouse 2005}). Their role as keystone species in marine food webs (^{Piatkowski et al. 2001}) and their sensitivity to physicochemical variations in the water column, variability in ocean circulation at different scales, and food availability make them potential indicators of environmental change (^{Moreno et al. 2009}, ^{Camarillo-Coop et al. 2011}, ^{Zaragoza et al. 2014}). The stage in the cephalopod life cycle that is most sensitive to environmental variations is the paralarval phase. Paralarvae occur at the early post-hatching stage in most cephalopods (^{Young and Harman 1988}); they are planktonic and thus depend on the effects of ocean circulation (^{Boyle and Rodhouse 2005}).

In Mexico, mainly off northwestern Mexico and off the Gulf of Tehuantepec, the effects of the regional oceanography and of some physical, chemical, and biological variables on paralarvae distribution have been analyzed at different temporal and spatial scales. The paralarval community structure from Ensenada, Baja California, to Point Abreojos, Baja California Sur, during the 1997-2001 El Niño/Southern Oscillation (ENSO) event was modulated by physical variables associated with the latitudinal extension and retrieval of water masses, which prompted a significant change in community indexes and in the dominance of species whose biogeographic affinities were associated with anomalous warming or cooling events (^{Durazo and Baumgartner 2002}, ^{Granados-Amores et al. 2010}). During 2004-2007, when ENSO influence was not significant in the Gulf of California, 3 paralarval assemblages were recurrently found in surface water masses, indicating that tropical species were transported into the gulf and enriched the resident community (^{De Silva-Dávila et al. 2015}). On the other hand, size distribution and oceanic and coastal paralarval communities in the Gulf of Tehuantepec were influenced by 2 eddies, an upwelling front, and the hydrodynamics over the continental shelf (^{Aceves-Medina et al. 2017}). The southwestern coast of Baja California Sur is one of the least studied regions in northwestern Mexico; thus, the seasonal effect of ocean dynamics on paralarval communities there is mostly unknown.

The Sthenoteuthis oualaniensis-Dosidicus gigas com plex (SD complex) is a frequent and abundant component of paralarval communities off the Pacific coasts of Mexico. This species complex is formed by paralarvae, measuring ≤3.0-4.0 mm mantle length (ML), that cannot be morphologically differentiated to species (^{Staaf et al. 2011}, ^{De Silva-Dávila 2013}). This complex has been positively correlated with high temperatures at the 3 above mentioned regions, despite the different environments, and paralarvae measuring ≤2.0 mm ML evidenced the occurrence of spawning events in all these regions (^{Granados-Amores et al. 2010}, ^{De Silva-Dávila 2013}, ^{Aceves-Medina et al. 2017}), exhibiting the reproductive plasticity and opportunistic nature of these species (^{Hoving et al. 2013}, ^{Hernández-Muñoz et al. 2016}).

Findings on paralarval abundance are based on studies using only one sampling gear. However, the use of different net towing methods can affect the structure of collected zooplankton communities, and cephalopod communities are not an exception (^{Saito 1994}, ^{Staaf et al. 2013}). The use of different complementary sampling gears can provide a stronger database for the analysis of cephalopod paralarval communities (^{De Silva-Dávila et al. 2015}).

In spring the California Current appears as an intense equatorward flow that runs parallel to the coast from Ensenada to Point Abreojos. In autumn this flow weakens and tends to form sinuous meanders (^{Durazo 2015}). The oceanic region off the southwestern coast of Baja California Sur (SWC) is the southernmost portion of the California Current System, where temperate water of subarctic origin and warm water of subtropical origin seasonally converge (^{Durazo and Baumgartner 2002}). In April the equatorward flow is constant, with a meander flowing offshore south of Magdalena Bay; in October, the effect of the California Current decreases and the dominant flow has a poleward direction, passing near the coast up to the Gulf of Ulloa (^{Zaitsev et al. 2014}).

The objective of this study was to determine the seasonal differences between the communities of cephalopod paralarvae collected with 2 types of net tows off the SWC and to relate them to changes in oceanographic conditions during spring and autumn 2003. The information produced here, especially information on paralarvae of the SD complex, which includes the fishery resource D. gigas, will improve knowledge for better assessment of this resource on both sides of the Baja California Peninsula.

Materials and methods

Eighty-five zooplankton samples were collected during 2 oceanographic campaigns aboard the R/V Río Suchiate of the Secretaría de Marina. These campaigns were carried out off the SWC, from Magdalena Bay to Cape San Lucas (Fig. 1a), between 30 April and 14 May (spring) (Fig. 1b) and between 30 September and 11 October (autumn) 2003 (Fig. 1c).

Figure 1 Study area off the southwestern coast of the Baja California Peninsula, Mexico (a), and sampling stations during spring (b) and autumn (c) 2003. 

At each sampling station, CTD casts (SBE 19 SeaCAT, maximum depth 500 m) were performed to determine the environment in the water column (^{Durazo and Baumgartner 2002}, ^{Avendaño-Ibarra et al. 2010}). Zooplankton was collected by making simultaneous surface and oblique tows with CalCOFI nets (60-cm diameter and 505-µm mesh) equipped with calibrated flowmeters, following ^{Kramer et al. (1972)}. Maximum depth was 1 m for surface tows and 223 m for oblique tows. Samples were fixed in buffered 4% formaldehyde (^{Kramer et al. 1972}) and displacement volumes were determined (^{Beers 1976}). Cephalopod paralarvae were separated from the unfractionated samples and then identified and measured (^{Roper and Voss 1983}, ^{Sweeney et al. 1992}, ^{De Silva-Dávila 2013}) using a Stemi SV-11 stereoscope equipped with a calibrated ocular micrometer. Abundance was standardized to number of paralarvae per 1,000 m3 of filtered water (PL·1,000 m^-3) (^{Fleminger 1964}).

A nonparametric analysis (Mann-Whitney U test, P <0.05) was used to record significant differences between abundances, and traditional ecological indexes (richness, diversity, and dominance) (^{Margalef 1982}) were used to analyze community structure between seasons and types of tow. The biogeographic affinity (temperate, tropical-subtropical, and cosmopolite) of paralarvae was determined based on the biogeographic affinity of adults (^{Jereb and Roper 2010}, ^{Jereb et al. 2014}).

Taxon abundance distribution was determined with a two-way cluster analysis for each type of tow using the Bray-Curtis dissimilarity index (β = -0.25) and the PC-ORD 6.0 software package (^{McCune and Mefford 2011}). Two primary abundance matrices per taxon (surface tow and oblique tow) with log10 (x + 1) transformed data were used, and 2 secondary matrices were made with the following qualitative variables: season (spring/autumn) and identified water mass at each sampling station. The species-environment relationship was explored by performing a canonical correspondence analysis per type of tow with CANOCO 4.5, and a Monte Carlo test (999 permutations) (^{Ter Braak 1994}). Two primary abundance matrices and 2 secondary environment matrices (surface tow and oblique tow) were used, excluding species that were present at only one sampling station. The environment matrix for surface tows included the variables temperature, salinity, and zooplankton volume at 1 m depth; and the environment matrix for oblique tows included temperature at 10 m depth, salinity at 10 m depth, and zooplankton volume in the oblique tows. Chlorophyll a concentrations (mg·m^-3) obtained from biweekly satellite images (resolution: 4 km²) during each sampling season (http://coastwatch.pfeg.noaa.gov/erddap/index.html) were used for both matrices.

Results

Environmental variables

In spring conditions were cold (average = 18.5 ºC), with a coast-ocean surface temperature gradient (minimum = 13.5 ºC) from Magdalena Bay to the north of El Pescadero, and maximum temperature (22.5 ºC) was recorded to the south off Cape San Lucas (Fig. 2a). Transitional Water and Subarctic Water were recorded near the coast and off the coast, respectively, and a Subtropical Surface Water intrusion was recorded in the southern area (Fig. 2b). Chlorophyll a concentrations showed an inverse gradient, with highest values (3.2 mg·m^-3) in the coastal zone off Magdalena Bay and El Salado and lowest values (0.01 mg·m^-3) in the oceanic zone (Fig. 2c). Zooplankton volumes at the surface were highest (1,500 mL·1,000 m^-3) to the north of Magdalena Bay and to the south, off El Conejo (Fig. 2d). By contrast, conditions in autumn were warm. Sea surface temperature reached 29.2 ºC near the coast, with an average that was 9 ºC higher than the average for spring (Fig. 2e). In autumn 2 Subtropical Surface Water intrusions in the oceanic zone, Transitional Water in the coastal and central oceanic zones, and a small Tropical Surface Water intrusion in the oceanic zone to the south were identified (Fig. 2f). Chlorophyll a concentrations were low throughout the study area (average = 0.11 mg·m^-3) (Fig. 2g), and surface zooplankton volumes were lower (maximum = 600 mL·1,000 m^-3) than the volumes observed for spring (Fig. 2h).

Figure 2 Distribution of environmental variables during spring (a, b, c, d) and autumn (e, f, g, h). Abbreviations: SST, sea surface temperature; S, salinity; Chla, chlorophyll a concentration; ZV, zooplankton volume; SAW, Subarctic Water; TrW, Transitional Water; StSW, Subtropical Surface Water; TSW, Tropical Surface Water. Locations: 1, Cape San Lázaro; 2, Magdalena Bay; 3, El Salado; 4, El Conejo; 5, El Pescadero; 6, Cape San Lucas. 

Statistical analysis

The cluster analysis for surface tows showed the presence of 2 species-stations groups only during autumn in Transitional Water and Subtropical Surface Water, one characterized by the abundance and high frequency of Abraliopsis sp. 1 and the other by the low frequency of Argonauta sp. 3 and Japetella diaphana. A third group characterized by the SD complex was present during both seasons in all the identified water masses (Fig. 3a). Clustering for oblique tows showed a similar pattern. The first group, which included mainly stations sampled in autumn, was characterized by the presence of the SD complex in warm water masses and at 3 stations with Subarctic Water. The pelagic octopus J. diaphana and the jumbo squid D. gigas made up the second group, which was present in Transitional Water and Subarctic Water, and the third and fourth groups included species with low-frequency appearances in both seasons (Fig. 3b).

Figure 3 Two-way cluster analysis for surface (a) and oblique (b) tows. Water masses: SAW, Subarctic Water; TrW, Transitional Water; StSW, Subtropical Surface Water; TSW, Tropical Surface Water. Sampling station code is noted after the hyphen. For species acronyms, see Table 1. Rectangles with broken lines indicate the species-stations groups.  

The canonical correspondence analysis for surface tows explained 92.6% of the cumulative variance in the species-environment relation on the first 2 axes. Chlorophyll a concentration (0.624) and zooplankton volume (0.899) on axes 1 and 2, respectively, showed the highest correlation. Similarly, for oblique tows the cumulative variance explained 71.4% of the species-environment relation, with zooplankton volume at 10 m depth (-0.769) on axis 1 and chlorophyll a concentration (0.884) on axis 2 showing the highest correlations (Table 3). The canonical correspondence analysis triplots per type of tow showed 2 species- stations groups (Fig. 4a, b). Group 1 included only spring sampling stations in a productive environment (chlorophyll a concentrations = 0.5-2.0 mg·m^-3 and zooplankton volumes = 256-1,500 mL·1,000 m^-3) with low temperature (16-18 ºC) and salinity (33.2-33.6) in Subarctic Water. Group 2 included all autumn sampling stations and 2 additional spring sampling stations in oblique tows. In comparison with group 1, these stations were associated with a less productive (chlorophyll a = 0.5-1.0 mg·m^-3 and zooplankton volume = 1,024 mL·1,000 m^-3), warmer (18-22 ºC), more saline (33.6-34.2) environment in different water masses (Subarctic Water, Transitional Water, and Subtropical Surface Water). At the surface, paralarvae of Onychoteuthis horstkottei (group 1) and of Abraliopsis sp. 1 and Abraliopsis sp. 2 (group 2) showed good correlations with chlorophyll a, high surface temperature, and low chlorophyll a concentration, respectively. In oblique tows, Octopus veligero (group 1) correlated with chlorophyll a, while 3 species from the genus Argonauta correlated with low zooplankton volumes and high temperature (Fig. 4a, b).

Table 3 Summary of the canonical correspondence analysis applied to abundance and environmental data from surface and oblique tows. Significant correlations for surface tows: axis 1, F = 1.511, P < 0.590; remaining axes, F = 0.656, P < 0.854. Significant correlations for oblique tows: axis 1, F = 1.500, P < 0.344; remaining axes, F = 0.995, P < 0.444. 

	Surface				Oblique
	Axis	Axis 2	Axis 3		Axis	Axis 2	Axis 3
Eigenvalues	0.366	0.100	0.032		0.568	0.481	0.301
Species-environment correlations	0.712	0.380	0.251		0.853	0.823	0.694
Cumulative variance (%)
Species data	11.3	14.4	15.4		8.9	16.5	21.2
Species-environment relation	72.7	92.6	99.0		38.7	71.4	92.0
Environmental variables
Temperature	-0.464	0.325	-0.664		-0.342	-0.146	-0.121
Salinity	0.128	0.200	-0.761		-0.220	0.159	-0.179
Zooplankton volume	0.498	0.899	0.163		-0.769	0.462	-0.384
Chlorophyll a	0.624	0.034	0.471		-0.070	0.884	0.461

Figure 4 Triplots from the canonical correspondence analysis of surface (a) and oblique tows (b). Environmental variables: ZV, zooplankton volume; Chla, chlorophyll a; T1, temperature at 1 m depth; S1, salinity at 1 m depth; T10, temperature at 10 m depth; S10, salinity at 10 m depth. For species acronyms, see Table 1. Letters A-J followed by numbers indicate the sampling station after the corresponding symbol. 

Discussion

This is the first time the analysis of seasonal changes in the community of cephalopod paralarvae off the SWC under normal conditions (without ENSO) is presented. The analysis of 2 types of simultaneous tows showed substantial differences in the type of information that is obtained from each tow and their utility.

Integral studies (abundance-environment) of cephalopod paralarvae in areas of high biological productivity, such as upwelling systems, have increased worldwide due to the growing commercial demand for fishery resources. This study, conducted on the SWC during 2003, a year with no anomalous ENSO-induced thermal conditions (http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso stuff/ensoyears.shtml), allowed us to determine, for the first time, the influence of seasonal changes on the community of cephalopod paralarvae in the southernmost region of the California Current upwelling system.

^{Durazo (2015)} determined that the region to the north of Point Eugenia (28ºN) was homogeneous, with low temperature and salinity during winter and spring and predominating subarctic conditions throughout the year, whereas to the south subarctic conditions were predominant in winter and spring and tropical-subtropical conditions in summer and autumn. In the region to the north of our study area, anomalous intrusions of warm and cool water during the 1997-2001 ENSO event prompted drastic changes in different marine communities (e.g., fish and zooplankton larvae and adults, including paralarvae) (^{Lea and Rosenblatt 2000}, ^{Lavaniegos et al. 2002}, ^{Granados-Amores et al. 2010}, ^{Jiménez-Rosenberg et al. 2010}).

^{Granados-Amores et al. (2010)} indicated that during the 1997-1998 ENSO event, the northward extension of paralarvae of tropical species reached the region off Punta Baja, Baja California (^{Camarillo-Coop 2006}, ^{Granados-Amores et al. 2010}). During the El Niño-La Niña transition, temperate species showed a 2-fold abundance increase with distribution in Subarctic Water, and tropical species showed a 50% abundance decrease with distribution in the warm water mass to the south; the continued presence of Subarctic Water from Ensenada to the south of Point Abreojos, in the period from autumn 1998 to late summer 1999, determined de disappearance of taxa with tropical-subtropical and cosmopolite affinities, giving rise to a temperate community dominated by the Gonatidae family (^{Granados-Amores et al. 2010}). In comparison, the seasonal change with no ENSO effects off the SWC were significantly different. Though samples were collected in seasons with contrasting oceanographic conditions (^{Durazo 2015}), the biogeographical affinity of paralarvae was predominantly tropical-subtropical and the greatest change was given not by the dominant biogeographical affinity or by taxon distribution in a particular water mass, as reported for the north, but by the restructuring of the community as richness of tropical taxa almost doubled in autumn when Subarctic Water retreated and the warm environment expanded with the appearance of 3 water masses: Tropical Surface Water, Subtropical Surface Water, and Equatorial Subsurface Water (100-225 m) (^{Avendaño-Ibarra et al. 2010}). The change in community structure in autumn was also modulated by the decrease in the abundance of paralarvae, in particular of the SD complex (dominant taxon), which together with the arrival of new species to the region, prompted an increase in diversity and a decrease in dominance. ^{Avendaño-Ibarra et al. (2010)} analyzed the same survey data examined in the present study and recorded a significant increase in the number and abundance of fish larvae of tropical species during autumn, similar to our results; however, fish larval abundances were 3 times higher in autumn and paralarval abundance decreased during this season.

The latitudinal movement of water masses and the formation of cephalopod paralarval assemblages on the SWC were also observed in the Gulf of California during 2004-2007 when there were no significant ENSO effects (^{De Silva-Dávila et al. 2015}). In the Gulf of California there were 3 associations that were determined by temperature and salinity, but also by zooplankton volumes and chlorophyll a concentrations, as was observed on the SWC. These 3 associations showed latitudinal boundaries that changed with the extension or retrieval of surface water masses that were present year-round (^{De Silva-Dávila et al. 2015}). On the SWC the groups determined by the canonical correspondence analysis were associated with the 2 predominant environments: the productive cool environment in spring and the less productive warm environment in autumn. However, most taxa (group 2) were not exclusively distributed in a single water mass, indicating the absence of correlations with temperature and salinity, and the characteristic oceanographic variability with typically 3 warm water masses, a high variability compared to that recorded for the region off the central portion of the peninsula and further north (^{Lynn and Simpson 1987}, ^{De Silva-Dávila et al. 2002}). A wide variety of individual species responses to environmental seasonality was expected because every species or group of species (neritic, oceanic) has different reproductive strategies (^{Moreno et al. 2009}). In tropical areas with only one warm surface water mass, such as the Gulf of Tehuantepec, there were no recorded changes in the community of paralarvae and their segregation was the result of mesoscale processes (^{Aceves-Medina et al. 2017}).

The species that characterized each group in the SWC were different from the ones reported for the Gulf of California. Onychoteuthis horstkottei (group 1, surface tow) is distributed in the tropical equatorial Pacific (^{Bolstad 2010}). However, O. horstkottei paralarvae have been recorded in warm environments from the Gulf of California to the Gulf of Tehuantepec (^{De Silva-Dávila et al. 2015}, ^{Aceves-Medina et al. 2017}). Their presence in a cold environment on the SWC in spring could be associated with their northern geographic range limit or with a broader geographic range than the previously reported. The same goes for paralarvae of Abraliopsis sp. 1 and Abraliopsis sp. 2 (group 2, surface tow), probably corresponding to Abraliopsis falco and Abraliopsis affinis, respectively (^{Jereb and Roper 2010}). Octopus veligero (group 1, oblique tow) is an octopus species that, in the adult stage, is well known to be distributed in waters off the region from Point Eugenia to the tip of the Baja California Peninsula and into La Paz Bay, in the Gulf of California (^{Jereb et al. 2014}). Its paralarvae, on the other hand, have been recorded in the Gulf of Tehuantepec (at 29-34 ºC and 33.1-34.4 salinity) (^{Alejo-Plata et al. 2013}). Despite the large number of zooplankton surveys on the Pacific coasts of Mexico (^{Granados-Amores et al. 2010}, ^{De Silva-Dávila et al. 2015}, ^{Aceves-Medina et al. 2017}), our study on the SWC was the first to report the presence of this species in Subarctic Water off northwestern Mexico. This first report confirms the known geographic range of this species and provides information on the species biology and ecology, which is mostly unknown (^{Jereb et al. 2014}). With respect to paralarvae of the genus Argonauta (group 2, oblique tow), which were recorded only in autumn, its biogeographic affinity (pantropical-oceanic) (^{Nesis 2003}) and high abundance in the southern portion of the Gulf of California (23-26 ºC) (^{De Silva-Dávila 2013}) were associated with increasing temperatures during this season. On the other hand, paralarvae of the SD complex, which characterized the tropical association in the Gulf of California, were widely distributed on the SWC in both seasons but showed no significant correlation with any environmental variable, nor did they characterize a specific group.

In the early life stages, paralarvae of some species are associated with productive zones, where chlorophyll a concentrations and zooplankton volumes provide large nutrient quantities for development (^{González et al. 2005}, ^{Moreno et al. 2009}, ^{Vidal et al. 2010}), suggesting the coupling of species to regional productivity pulses (^{De Silva-Dávila et al. 2015}, ^{Aceves-Medina et al. 2017}). Though chlorophyll a concentrations have been associated with paralarval abundance, most paralarvae, however, do not directly feed on phytoplankton because they become voracious predators soon after hatching (^{Chen et al. 1996}). In particular, hatched Ommastrephidae paralarvae, including the SD complex, have a typical rhynchoteuthion shape with fused tentacles that form a proboscis that is not useful for capturing prey until it separates, hindering food capture (^{O’Dor et al. 1986}, ^{Shea 2005}). The mucus on the bodies of paralarvae (<4.0 mm ML) allows growth of microorganisms, which paralarvae can collect and ingest with their proboscis (^{Vidal and Haimovici 1998}) and are probably their main source of energy during this stage of development. After the proboscis separates, paralarvae are considered juveniles and can more efficiently make use of the available zooplankton for their development (^{Shea 2005}, ^{De Silva-Dávila 2013}), since they can then feed on calanoid copepods and on larger and more diverse prey species (amphipods, euphausiids, cephalopods, and fish) as they grow (^{Vidal and Haimovici 1998}, ^{Camarillo-Coop et al. 2013}). Zooplankton volumes could thus be used as reliable indicators of the food available for paralarvae.

The California Current upwelling system off the Baja California Peninsula has all the necessary features to cover the energy requirements of highly voracious organisms such as squid. To the north of Point Abreojos and further north to Ensenada, zooplankton volumes recorded during the 1997-2001 ENSO event were typical of the region, in spite of the anomalous warm conditions (^{Lavaniegos et al. 2002}). By contrast, zooplankton volumes in the spring and autumn oblique tows were higher during normal conditions on the SWC (Table 4). Both maximum total abundance of paralarvae and the abundance of the SD complex during the ENSO transition phase (July 1998) were much higher on the northwestern coast than on the SWC, but the average values obtained with only 2 surveys for both coasts were very similar. This suggests that during the 1997-1998 period the anomalous warming of the system was more favorable than were zooplankton volumes to the presence of highly abundant paralarvae, particularly paralarvae of the Ommastrephidae family (D. gigas, S. oualaniensis, and the SD complex), which dominated the community (^{Camarillo-Coop 2006}). The high abundance of the SD complex that brought on a spawning event off Point Eugenia when conditions were anomalously warm is evidence of the reproductive plasticity of these species and their opportunistic nature (^{Camarillo-Coop 2006}, ^{Hoving et al. 2013}, ^{Hernández-Muñoz et al. 2016}). In the Gulf of California, zooplankton volumes (oblique tows) were mostly higher than those recorded for the northwestern coast and the SWC, but total paralarval abundance in May (spring) was 1.5 and 4.8 times higher than maximum total abundances for the northwestern coast and SWC, respectively (Table 4a). With respect to paralarvae of the SD complex in May, the number of paralarvae in the gulf nearly reached the maximum number recorded for the northwestern coast (transition phase) and was twice the maximum number recorded for the SWC (Table 4a).

On the other hand, the paralarvae of the SD complex collected in the surface tows were significantly more abundant than those collected in the oblique tows on the SWC, with 8.5 times more paralarvae for this taxon (Table 4). The same thing occurred in the Gulf of California. The abundances obtained from cruises that carried out both types of tows (S-195 and S-207) showed 8.6 times more paralarvae of this complex in surface tows (Table 4). The average value (817 PL·1,000 m^-3) for paralarvae collected with surface tows on the SWC is the second highest recorded for the region off northwestern Mexico, after the average for the Gulf of California. This result is important because D. gigas, which is part of this complex, supports the most important cephalopod fishery on the Pacific coast of Mexico, with landings of 36,381 metric tons in 2013 (^{CONAPESCA 2014}, ^{Zepeda-Benites et al. 2017}). In spite of this, there is no sampling program for obtaining the necessary data to identify the species hatching areas and seasonal patterns. The presence of naturally deposited egg masses of D. gigas at less than 20 m depth when temperature was 24-27 ºC (^{Staaf et al. 2008}, ^{Birk et al. 2017}) and the efficiency of surface tows for catching paralarvae of the SD complex in the tropical equatorial Pacific (^{Staaf et al. 2013}), the Gulf of California (^{De Silva-Dávila 2013}), and waters on the SWC (this study) prove that this type of tow, which is of low cost and easily executed in short operation times, could be the best sampling option to start a monitoring and assessment program for squid off northwestern Mexico. The recorded differences when surface and oblique tows are analyzed separately could lead to different inferences and conclusions on the entire community of paralarvae.