BIOVOLÚMENES DE ZOOPLANCTON, DISTRIBUCIÓN Y ABUNDANCIA DE ICTIOPLANCTON, EUFÁUSIDOS Y PARALARVAS DE CEFALÓPODOS, VERANO 2020 ZOOPLANKTON BIOVOLUMES, DISTRIBUTION AND ABUNDANCE OF ICHTHYOPLANKTON, EUPHAUSIIDS, AND CEPHALOPOD PARALARVAE (SUMMER 2020)

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INTRODUCTION
Researching the early life stages of commercially significant species, like Engraulis ringens, and other pelagic resources holds immense importance.These studies provide crucial data for delineating and monitoring spawning zones, fundamental aspects of fisheries management.
Se obtuvieron 260 muestras de zooplancton recolectadas con una red Hensen de 300 micras de abertura de malla, mediante lances Zooplankton investigations offer insights into the presence of biological indicators within water masses.Taxonomic groups such as copepods and holoplanktonic mollusks serve as valuable indicators, helping characterize oceanographic conditions during the collection period.
In recent years, there has been a growing focus on cephalopod paralarvae studies in Peru.These studies not only enhance our understanding of the biodiversity and abundance of this group but also shed light on the dynamics linked to the seasonality of cephalopod species concerning water masses.This knowledge is vital, especially given the potential commercial exploitation of these species, exemplified by the case of Argonauta nouryi.
Imarpe, through the Fisheries Oceanography project, has been gathering ichthyoplankton and zooplankton data during various research expeditions.This report unveils the outcomes related to the composition, abundance, and distribution of ichthyoplankton and specific zooplankton groups, such as euphausiids and cephalopods.These findings were derived from the Hydroacoustic Pelagic Resources Cruise conducted between February and March (summer) of 2020, spanning from Punta Sal (3°S) to Ilo (18°S).

MATERIAL AND METHODS
The biological samples utilized in this study were collected during the Hydroacoustic Pelagic Resources Cruise 2020-0203, conducted aboard the R/V Olaya, Humboldt, and Flores, spanning from February 22 to March 25, 2020.Sampling activities were conducted at distances ranging from 2 to 100 nm offshore, covering the extensive stretch from Punta Sal (3°S) to Ilo (18°S) (18°S).
A total of 260 zooplankton samples were obtained using a Hensen net with a mesh opening of 300 microns.Vertical tows were conducted from a depth of 50 m to the surface verticales desde 50 metros de profundidad hacia la superficie en barco parado (Fig. 1).Las muestras fueron fijadas con formaldehído al 2%.
Sample analyses were performed at Imarpe's Zooplankton and Secondary Production Laboratory (Headquarters).Zooplankton biovolumes were estimated using the displacement method as outlined by Kramer, Kalin, Stevens, Thrailkill, & Zweifel (1972).Prior to measurements, gelatinous organisms, large crustaceans exceeding 2 cm, and juvenile/adult fish were excluded.
The resulting data were expressed in mL.m -3 .Quantitative analysis encompassed ichthyo-
Data tabulation was carried out using Excel, and distribution and abundance maps were created using Surfer v.13.
The distribution of biovolume ranges exhibited a consistent pattern (Table 1).

Engraulis ringens
E. ringens eggs were consistently observed within the continental shelf, stretching from the northern region of Punta La Negra to Huacho.Particularly dense concentrations were noted off Punta La Negra and Malabrigo.Nevertheless, from Callao to Ilo, the eggs were scattered with generally low densities.Larvae exhibited a similar distribution pattern to the eggs, albeit with broader coverage extending north of Punta La Negra, reaching up to the border with Ecuador.In contrast, larval abundance levels were significantly lower than those of eggs, with a noticeable cluster observed in the coastal zone south of Punta La Negra (Fig. 3).

Vinciguerria lucetia
The spatial distribution of V. lucetia eggs and larvae extended beyond the Peruvian continental shelf, tracing the country's coastline.The majority of samples displayed values of less than 500 eggs and fewer than 100 ind.m -2 of larvae.Areas with higher egg abundance (exceeding 1,000 ind.m -2 ) were identified off Punta La Negra, Malabrigo, and Atico.Additionally, an evident concentration of larvae (>500 ind.m -2 ) was observed in the oceanic zone off Huarmey (Fig. 4).
The less common and less abundant species of euphausiids, such as Nematoscelis flexipes and Nematoscelis sp., were primarily concentrated in the northern region.N. flexipes was observed with adults off the coast of Paita, while Nematoscelis sp. was found with adults ranging from Paita to Chimbote, and its juveniles were scattered as far as San Juan de Marcona.
Similarly, both adults and juveniles of Nyctiphanes simplex exhibited a specific distribution, occurring between Talara and Chimbote, with a higher concentration near the continental shelf break in Pimentel.In contrast, adults of Stylocheiron affine were found in two distinct zones-one in the north and the other in the central part of the surveyed area.Juveniles of this species were more evenly spread from Talara to Huacho, with some concentrated areas off Pisco and Atico (Fig. 7).
The early developmental stages of euphausiids, (calyptopis and furcilia) were identified but could not be definitively assigned to specific species.These stages were classified separately and showed distinct distribution patterns.Higher concentrations were observed in the northern region, extending from Talara to Chimbote, while comparatively lower concentrations were found in the southern area, ranging from south of Pisco to Ilo.These distributions spanned from coastal areas to more oceanic regions (Fig. 8).

Paralarvae
Ten cephalopod paralarvae species representing ten distinct families were identified.
Argonauta spp.exhibited a broad spatial distribution, with significant concentrations observed off the shores of Paita, Chicama, and Callao.In contrast, Abraliopsis sp. had a more localized presence, ranging from Talara to Callao and extending beyond the continental shelf.
Other noteworthy species such as Ancistrocheirus sp. and Chtenopteryx sicula displayed similar distribution patterns, spanning the northern, central, and southern regions (Fig. 9).

DISCUSSION
The spatial distribution of zooplankton biovolumes during the summer directly corresponds to the continental shelf, closely mirroring the coastal distribution of E. ringens.This correlation indicates a significant predation impact on zooplankton, as proposed by Ayón, Criales-Hernandez, Schwamborn & Hirche, 2008.Biovolumes below 0.25 mL.m -3 were mainly found within 30 nm and aligned with the occurrence of cold coastal waters (ccw) extending from Pimentel to Ilo.This pattern mirrors the distribution of E. ringens  res > 0,5 mL.m -3 estuvieron relacionados a las Aguas Subtropicales Superficiales (ASS) desde el norte de Punta La Negra hasta el sur de San Juan de Marcona; así como también con las Aguas Ecuatoriales Superficiales (AES) frente a Punta Sal.Finalmente, los valores > 1 mL.m -3 estuvieron en la misma zona donde las acf (frente a Punta Sal hasta Salaverry), y las ASS estuvieron distribuidas (frente a la zona oceánica de Callao y San Juan de Marcona).Además, la distribución de los biovolúmenes de zooplancton para el verano 2020 fue parecida a la distribución en el verano del 2004 (Nakazaki, 2012), ya que hubo una distribución de masas de agua similar en ambos períodos (Imarpe, 2004).
Con respecto a las demás especies de ictio-  (Imarpe, 2020).Biovolumes exceeding 0.5 mL.m -3 were associated with Surface Subtropical Waters (SSW) from north of Punta La Negra to the south of San Juan de Marcona, as well as Equatorial Surface Waters (ESW) off Punta Sal.Conversely, values exceeding 1 mL.m -3 were primarily observed in areas where both ccw (off Punta Sal to Salaverry) and SSW were prevalent, notably offshore Callao and San Juan de Marcona.Moreover, the zooplankton biovolume distribution in the summer of 2020 closely resembled the pattern observed in the summer of 2004 (Nakazaki, 2012), indicating similar water mass distributions during both periods (Imarpe, 2004).
E. ringens eggs and larvae displayed a distribution pattern near the coastline, closely linked to ccw.A significant concentration of spawning adults was particularly observed off Punta La Negra (Imarpe, 2020).Regarding eggs, there was also a noticeable concentration in the mixing area off the coastal zone of Huacho and Malabrigo, although its distribution within the continental shelf was limited (Fig. 10).This pattern differed from that observed in the summer of 2018 when high concentrations of E. ringens eggs were found between 100 and 120 nm offshore (Correa & Nakazaki, 2019).
As for V. lucetia, a mesopelagic species, its eggs and larvae exhibited a characteristic distribution pattern in the oceanic zone, extending along the coastline beyond the continental shelf.The most significant concentrations were noted off Paita and from Malabrigo to San Juan de Marcona, a pattern reminiscent of the summer of 2018 (Correa & Nakazaki, 2019) (Fig. 10).
Referente a los eufáusidos, las especies E. mucronata y E. eximia tuvieron distribución definida con relación a la plataforma continental, más no con la distribución de masas de agua.En cambio, E. lamelligera, N. simplex y gran parte de S. affine, se encontraron distribuidas en las acf tanto en adultos como juveniles, mientras que N. flexipes se encontró asociada a las AES.En cuanto a los estadios de caliptopis y furcilia no determinados, se observan dos patrones de distribución diferentes, por lo que podría inferirse que se trate de dos especies diferentes, una relacionada a las acf al norte y otra con ASS al sur.
commercially valuable species like Scomber japonicus were notably scarce, marking a contrast to the summer of 2017 when their abundances surged due to the presence of coastal El Niño (Correa et al., 2018 2 ).The absence of T. murphyi might be attributed to its primary spawning occurring between winter and spring (Ayón & Correa, 2013).
In terms of euphausiids, the distribution of species such as E. mucronata and E. eximia was closely linked to the continental shelf rather than specific water masses.Conversely, species like E. lamelligera, N. simplex, and much of S. affine were found in ccw both as adults and juveniles.N. flexipes, on the other hand, was associated with ESW.Additionally, undetermined calyptopis and furcilia stages displayed distinct distribution patterns, suggesting the possibility of two separate species.One seemed related to ccw in the northern regions, while the other was associated with SSW in the south.
Concerning cephalopod paralarvae, the genus Argonauta was the most abundant and widely distributed along the entire Peruvian coast, a pattern consistent with observations made during the summers of 2014 (Orosco, 2016) and 2018 (Correa & Nakazaki 2019).This prevalence can be attributed to its direct association with SSW, with its presence within the continental shelf resulting from the intrusion of this water mass into coastal areas, as observed during the summer of 2020.Similarly, Abraliopsis sp.demonstrated a specific oceanic distribution, not necessarily tied to specific water masses, being associated with ccw in the north and SSW in the central region.

CONCLUSIONS
The zooplankton biovolumes ranged from 0.005 to 3.118 mL.m -3 , averaging 0.626 mL.m -3 with a median of 0.468 mL.m -3 .The highest values were observed along the north-central coast of Peru, specifically from Punta Sal to Salaverry.
In terms of ichthyoplankton, E. ringens eggs exhibited a relative abundance of 43.3 %, reaching densities of up to 44,802 eggs.m -2 , primarily concentrated in the coastal zone within 30 nm.Larvae displayed a relative abundance of 29.8 %, with densities reaching up to 1,173 larvae.m - .
In the realm of cephalopod paralarvae, the most abundant and frequent species were Argonauta spp.(810 ind.m -2 ) and Abraliopsis sp.(186 ind.m -2 ).The former was associated with SSW, while the latter was linked to oceanic conditions.

Figure 10 .
Figure 10.Occurrence of eggs and larvae of E. ringens and V. lucetia in relation to sea surface salinity.Hydroacoustic Pelagic Resources Cruise 2020-0203.R/V Olaya, Flores, and Humboldt

Table 4 .
Composition, frequency, and abundance of cephalopod paralarvae.Hydroacoustic Pelagic Resources Cruise 2020-0203.R/V Olaya, Flores, and Humboldt terms of spatial distribution, both adult and juvenile Euphausia species, including E. eximia, E. mucronata, and E. distinguenda, were ubiquitous along the entire Peruvian coastline.E.eximia and E. distinguenda predominantly inhabited the continental shelf's edge and beyond, while E. mucronata exhibited a more coastal distribution.E.lamelligera was prevalent in the northern zone, spanning from Talara to Huacho and off Pisco, with the highest abundance noted off Paita.Conversely, E. tenera exhibited a more scattered and less abundant presence, primarily off Paita and from Chimbote to Ilo.Juvenile Euphausia sp.displayed a widespread distribution, encompassing both within and outside the continental shelf (Fig.6).