Elsevier

Fisheries Research

Volume 160, December 2014, Pages 60-68
Fisheries Research

Larval condition of Merluccius hubbsi (Marini, 1933) in the northern Patagonian spawning ground

https://doi.org/10.1016/j.fishres.2013.11.009Get rights and content

Abstract

Argentinean hake, Merluccius hubbsi, is one of the most important fishery resources of the South-western Atlantic Ocean, but its spawning biomass decreased in the last fifteen years, mainly due to the increased fishing pressure. It is widely accepted that to understand recruitment variability is necessary to study the factors that determine survival of early stages of development. Nutritional condition indexes have been widely used to determine starvation in wild larvae. In the present investigation, condition of M. hubbsi larvae was estimated employing field collected material by means of RNA/DNA index. This is the first attempt to use this index in this species. The change in the RNA/DNA index showed a clear ontogenetic pattern: increasing from preflexion to postflexion stages, showing an apparent decrease at the end of postflexion stage and a conspicuous increase once transformation stage was achieved. This pattern could be indicating that the transition between postflexion and transformation stages might represent a critical phase along larval development. RNA/DNA index also showed significant differences between areas from the spawning grounds characterized by different chlorophyll a concentration and abundance of potential prey. The study of nutritional condition represents a useful tool for identifying favorable nursery areas, providing valuable information for a comprehensive management of a population subject to overfishing.

Introduction

Argentinean hake, Merluccius hubbsi, is one of the most important fishery resources of the Argentine Sea, with annual catches between 170,000 and 370,000 t since 2000 (Sánchez et al., 2012). It inhabits the southwestern Atlantic Ocean waters from Cabo Frío in Brazil (22° S) to southern Argentina (55° S) at depths ranging from 50 to 500 m (Cousseau and Perrota, 1998). There are two fishing stocks, north and south of 41° S. Southern or Patagonian stock, which was the subject of this study, is the most important fishing resource, accounting for 85% of the total estimated biomass for this species. During the last years of the 90s decade the spawning biomass of both stocks dramatically decreased, which was mainly attributed to increased fishing pressure during previous years (Aubone et al., 2000). Lately the Patagonian stock also started to show changes in fish density, sex composition, size structure and spawning location. Historically, the reproductive activity of this group took place near Isla Escondida (43°30′ – 44° S and 65° W), mainly between November and April with a peak in December and January (Ehrlich, 1998, Macchi et al., 2004, Macchi et al., 2007). From 1998, studies showed variations in the reproductive pattern of this species, suggesting the movement of reproductive adults offshore toward deeper waters and a decrease in the density of the shoals (Ehrlich et al., 2000, Macchi et al., 2010).

The hydrography of the Patagonian shelf is characterized by the formation of a frontal system, produced as a result of the dynamics of tides (average position of the tidal front indicated in Fig. 1a). This tidal front separates homogeneous coastal waters from stratified waters (Glorioso, 1987). This frontal structure is not permanent and its formation occurs during the austral spring and summer.

Tidal fronts are characterized by high availability of nitrate typical from areas of high biological productivity caused by phytoplankton blooms and large aggregations of copepods (Ramírez et al., 1990). This scenario creates a variety of suitable habitats for adult fish spawning and nursery areas for their eggs and larvae (Sánchez and Ciechomski, 1995). Bakun and Parrish (1991) characterized Patagonian tidal frontal area as a successful spawning region. Thus, hake traditional spawning area would fulfill with Bakun's “fundamental triad” principles (Bakun, 1996): surface layers enrichment due to the rise of deep rich in nutrients waters, the concentration of planktonic organisms that are potential prey for larvae and the retention of eggs and larvae in favorable areas. The combination of these processes ensures survival of fish early life stages.

The observed changes in hake spawning pattern causes that hatching takes place in sites with different environmental features to those found close to frontal zones, and probably less favorable for larval growth and survival. The questions arising from the above are: What happens to larvae that hatch away from high-productivity traditional spawning areas? Will they have the same survival chances? Will they be more affected by starvation?

Nutritional condition indexes have been widely used to determine the importance of starvation in larvae caught in the ocean (Buckley, 1984, Clemmesen, 1994, O’Connell, 1980, Robinson and Ware, 1988, Theilacker, 1986). Different criteria have been developed to assess nutritional condition of fish larvae based on the differences that starvation produces in body form, condition factor, chemical cell constituents and histological integrity. These methodologies were extensively reviewed by Ferron and Leggett (1994). They divided condition indices into three main categories according to the main organization levels: organism, tissular and cellular. These indices operate at different time scales. Typically, the higher the organization level of the index, the longer it takes to respond to an environmental change. Morphometric techniques or lipid content determination allows to assess physiological changes on a weekly scale, growth rates (RNA/DNA, otolith growth, cell cycle studies) or indices of starvation (histological indexes such as intestinal epithelial cells height) give information about changes on a scale of days and rates of feeding (e.g. stomach fullness or trypsin activity) on a scale of hours. Many studies suggest that RNA/DNA index is one of the best indicators of the nutritional status of various marine organisms (e.g. Bailey et al., 1995, Clemmesen, 1994, Folkvord et al., 1996) and is currently the most widely used biochemical indicator of nutritional condition of fish larvae (Caldarone et al., 2003, Clemmesen, 1994, Grote et al., 2012, Malzahn et al., 2007, Meyer et al., 2012, Paulsen et al., 2013). The RNA/DNA index is an ecophysiological indicator that provides a measure of the capacity for synthesis of cells and usually correlates with the nutritional status of individuals (Buckley et al., 1999, Ferron and Leggett, 1994). The theoretical principle of using RNA/DNA ratio is based on the fact that DNA content is virtually constant in somatic cells, so that tissue concentrations reflect the number of cells and is independent of nutritional status, while the amount of cellular RNA, mainly ribosomal RNA (rRNA) available in the tissues is directly proportional to the level of protein synthesis (Clemmesen, 1994). RNA/DNA ratio varies with age, stage of development, size of the individual and changing environmental conditions (Bulow, 1970). It has also proved to be susceptible to environmental changes that affect the physiology of organisms, such as a low concentration of prey (Chícharo and Chícharo, 1995, McGurk et al., 1992). Thus, the organisms in good condition usually have higher values of the ratio RNA/DNA than those which are in a poor condition (e.g. Clemmesen, 1994, Robinson and Ware, 1988).

Due to the overfishing that hake has suffered in recent years, studies on reproduction and recruitment of this species have been intensified. There is detailed information on spatial and temporal distribution of the species, breeding and rearing, egg production and reproductive parameters (e.g. Macchi et al., 2004, Macchi et al., 2006, Macchi et al., 2007, Macchi et al., 2010, Pájaro et al., 2005, Renzi et al., 2000, Renzi et al., 2002). However, up to now, no research has been conducted on the nutritional condition of larvae. Thus, this study represents the first approach to this issue and the first time RNA/DNA determinations are made employing this species.

The information obtained from this study will determine whether all areas of the nursery grounds are equally favorable for larval survival, and thus, will provide tools to explain the variations in recruitment and may be used for the management of this species under severe fishing pressure.

Section snippets

Sampling and larval source

Larvae were collected during a research survey carried out in northern Patagonian coasts during January–February 2011, within the spawning peak of M. hubbsi southern stock. Samples were taken employing a 300 μm mesh Bongo net performing oblique tows from about two meters from bottom to surface. Additionally samples were taken according to the acoustics records with a 500 μm mesh Rectangular Midwater Trawl (RMT) and when the acoustic signal was close to the bottom a 1000 μm mesh epibenthic sampler

Hydrography

A clear separation between the coastal area, characterized by well-mixed waters (Fig. 1c), and the offshore stratified region was observed within traditional spawning area. A sharp thermocline of varying thickness at 30–40 m depth separated the upper and bottom waters in most of the sampled stations: offshore stations of traditional spawning area and external central area (Fig. 1d), as well as external northern (Fig. 1e) and southern (Fig. 1f) areas.

Ontogenetic changes in nucleic acid content and RD index

Ontogenetic changes in RNA and DNA contents

General considerations

Many studies have shown evidence that the RNA/DNA ratio is one of the best indicators of the nutritional condition and growth of several marine organisms (e.g. Bailey et al., 1995, Clemmesen, 1994, Folkvord et al., 1996). To date, it is the most widely used biochemical index to determine larval condition, especially for clupeids larvae since they are abundant and easily obtained. Nevertheless, for the many different hake species around the world, there is only one work available on this issue,

Acknowledgments

We wish to thank BIP E. Holmberg personnel for their help during sample collection and specially Paola Betti and Ezequiel Leonarduzzi for carefully collecting hake larvae herein employed. To María Delia Viñas and Brenda Temperoni for providing zooplankton abundances, and Daniel Cucchi Colleoni for chlorophyll data. This work was supported by the INIDEP and the CONICET, Fellowship PIP 112 200801 00815. This is INIDEP contribution N° 1846.

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