L'anchois européen et la sardine européenne sont sujets à de fortes pressions de pêche et les stocks sont soit pleinement exploités soit surexploités. Cette situation tient au partie au fait qu'un grand nombre de pêcheries d'anchois et de sardines sont gérées sur la base de zones géographiquement délimitées par le CIEM, le CGPM ou d'autres organisations régionales sans qu'il y ait nécessairement de véritable cohérence entre ces limites administratives et les processus biologiques. Dans ce contexte, l'objectif principal de la thèse a été d'explorer à partir de l'analyse de la forme de l'otolithe la structuration des populations de sardines et d'anchois à petite (régionale) et grande (aire de répartition) échelles spatiales. À petite échelle spatiale, nous avons essayé d'analyser comment les structures océanographiques et géographiques comme les fronts hydrologiques et les détroits peuvent affecter la structure des populations. À grande échelle spatiale, les structures des populations mises en évidence par l'analyse de la forme de l'otolithe sont comparées et discutées avec les résultats des études génétiques. Les résultats montrent des structurations plus complexes chez l'anchois que chez la sardine. Contrairement à la sardine, les caractéristiques hydrologiques comme le front hydrologique Almeria-Oran (AOF) et le détroit de Gibraltat constituent des barrières à la dispersion et au mélange des anchois. En matière de gestion des stocks de sardines et d'anchois, nos résultats proposent de nouveaux découpages et suggèrent une révision des limites des stocks actuellement retenus. Les populations de petits poissons pélagiques sont connues pour être particulièrement sensibles aux fluctuations de l'environnement. La deuxième partie du travail de thèse a été consacrée à l'étude de l'écologie alimentaire de la sardine et de l'anchois à grande échelle spatiale en Méditerranée et en Atlantiqu NE. Anchois et sardines sont essentiellement zooplantonophages. Ils consomment majoritairement des copépodes (59.4% des proies identifiées pour la sardine et plus de 78% chez l'anchois). La comparaison des régimes alimentaires suggère un faible chevauchement entre les deux espèces particulièrement dans les zones de fortes productivités biologiques. Toutefois, en Méditerranée, où les eaux sont connues pour être oligotrophiques et donc peu productives, il peut y avoir chevauchement des niches trophiques des 2 espèces. / The European anchovies and sardines are subject to heavy fishing pressure and their stocks are either fully exploited or overexploited. This is partly because many anchovy and sardine fisheries are managed on the basis of geographical areas bounded by ICES, GFCM and other regional organizations without necessarily true coherence between these administrative boundaries and the biological processes. In this context, the main objective of the thesis is to explore the population structure of sardines and anchovies at small (regional) and large (distribution range) spatial scales from the analysis of the otolith shape. At a smaller spatial scale, we tried to analyze how oceanographic and geographic structures, such as, hydrological fronts and Strait, can affect population structure. At larger spatial scales, population structures revealed by the analysis of otolith shapes were compared and discussed with the results of genetic studies. The results showed more complex population structure in anchovies than in sardines. Unlike sardines, hydrological characteristics, such as, the hydrological Almeria-Oran Front (AOF) and the Strait of Gibraltar constitute barriers that limit the dispersal and mixing of anchovies. In managing stocks of sardines and anchovies, our results suggest new division and suggest a readjustment of stocks currently held. Population of small pelagic fish are known to be particularly sensitive to changes in the environment. The second part of the thesis is devoted to studying of the feeding ecology of sardines and anchovies at large spatial scale in the Mediterranean and North-eastern Atlantic. Anchovies and sardines are essentially zooplantonophages. they mainly consume copepods (59.4% of identified prey for sardines and over 78% for anchovies). Comparing diets suggests little overlap between the two species, particularly in areas of high biological productivity. However, in the Mediterranean, where the waters are known to be oligotrophic and thus unproductive, the trophic niches of the two species may overlap.

En esta Tesis Doctoral se ha analizado las relación de la comunidad ictioplanctonica con su ambiente físico y trófico en dos condiciones hidrográficas contrastadas (estratificación estival y mezcla otoñal) y se ha caracterizado la dieta larvaria de cinco especies de teleósteos presentes en el Noroeste del Mediterráneo: los tres clupeiformes de la región [anchoa (Engraulis encrasicolus), sardina (Sardina pilchardus) y alacha (Sardinella aurita)], un pequeño túnido [la melva (Auxis rochei)] y la especie de pez demersal más importante de la región [la merluza (Merluccius merluccius)]. Estas especies han sido elegidas porque representan dos morfotipos distintos, 1) morfotipo estilizado representado por los clupeiformes, con boca pequeña, cuerpo alargado y tracto digestivo ligado a la cola, y 2) morfotipo robusto representado por la melva y la merluza, con boca grande, cuerpo robusto y digestivo compacto e independiente de la cola. Además se han estudiado las variaciones de la dieta i) a lo largo del desarrollo, en relación ii) con la disponibilidad de presas y iii) con la morfología de las estructuras implicadas en la alimentación como son el sistema visual, el aparato bucal y el sistema digestivo. La alimentación es un aspecto de la ecología larvaria de los peces que en último termino afecta a la dinámica de sus poblaciones (a través de la supervivencia larvaria y las consiguientes fluctuaciones en el reclutamiento). Los factores que influyen en la preferencia de las larvas de peces por sus presas y las estrategias de alimentación han sido poco o nada estudiados para estas especies escogidas en el mar Catalán. Estos factores pueden diferir entre especies y cambiar a lo largo de la ontogenia. Esta tesis doctoral ha tenido como principal finalidad comprender las estrategias tróficas específicas de las larvas de estas cinco especies importantes en las pesquerías mediterráneas. A grandes rasgos se puede observar tres tipos de estrategias: 1) La que sigue la merluza, que desde el comienzo de la alimentación exógena se especializa en un tipo de presa (Clausocalanus spp.) y solo aumenta el número de copépodos que ingiere para responder al aumento de los requerimientos nutricionales de crecimiento. 2) La que siguen la anchoa y sardina, que muestran una baja intensidad alimenticia, y que varían el tamaño y número de presas poco a poco a lo largo del desarrollo sin grandes cambios en la dieta. 3) La descrita para la melva y la alacha, que contrariamente la las tres especies anteriores, son oportunistas y en su etapa larvaria ajustan los tipos de presa (y tamaños) conforme mejoran sus habilidades de una forma mucho más marcada que el resto de especies. El trabajo presentado en esta tesis ha estado dirigido a obtener información básica y necesaria para interpretar los patrones de distribución y dilucidar el efecto del tipo de ecología trófica de diversas especies de peces en la supervivencia larvaria. Ha sido la primera vez que se estudian las larvas de estas especies en el mar Catalán en relación a la distribución de sus presas potenciales del plancton para evaluar el papel que juega el acoplamiento espacio-temporal de larvas y presas en la estrategia alimentaria. También es la primera vez que se estudia la dieta de estas especies en relación a la morfología de estructuras relacionadas con la alimentación como son el sistema visual, la boca y el tracto digestivo. Los resultados de este trabajo muestran como la morfología opuesta del tipo de digestivo, junto con el tamaño de boca y ojos de las larvas explica las diferencias entre clupeiformes y melva y merluza en lo que se refiere a incidencia alimentaria de la población, tamaños extremos de presas grandes y agudeza visual, pero esto no es suficiente para explicar las estrategias de la alacha que se asemeja mas en sus hábitos alimentarios a la melva que a las especies morfológicamente similares (la anchoa y sardina), así como tampoco sirve para explicar las diferencias de dieta entre merluza y melva pese a que morfológicamente comparten características. Pese a que tanto la melva como la merluza parten de unas características morfológicas mas ventajosas como son el cuerpo robusto, tamaño de boca grande, mayor agudeza visual y digestivos con mayor capacidad de almacenaje, la melva sí refleja estas ventajas en unos hábitos predadores intensos y con incorporación de presas mas grandes y móviles conforme se desarrolla mientras que la merluza no cambia su dieta pese a que estaría capacitada para detectar y capturar presas más nutritivas. La merluza parte de un tamaño de boca relativamente grande al eclosionar que le permite ingerir presas grandes desde el comienzo de la alimentación exógena, patrón similar a las especies que nacen con desarrollo directo o con tamaños de larvas grandes, y no varía su dieta a lo largo del desarrollo larvario / We have analysed the relationship of the ictioplankton community with its fisical and trophic environment in two hydrographical contrasting conditions (summer stratification versus autumn mixed column), and we have characterized the larval diet of five teleost species in the NW Mediterranean: the three clupeiform of the region [anchovy (Engraulis encrasicolus), sardine (Sardina pilchardus) and round sardinella (Sardinella aurita)], a small tuna [bullet tuna (Auxis rochei)] and the most important demersal species of the region [hake (Merluccius merluccius)]. These species have been chosen because they represent two different morphotypes, 1) that of the clupeiforms, which correspond to a stylized larvae with small mouth, long body and digestive tract linked to the tail, and 2) that of bullet tuna and hake, which correspond to a robust larvae with big mouth, robust body and compact digestive tract independent of the tail. Moreover, we have studied the diet variations i) along development, in relation to ii) prey availability and iii) morphology of the feeding-related structures like visual system, mouth and digestive tract. Feeding is an important aspect of the larval fish ecology that affects the population dynamics (through larval survival and subsequence recruitment fluctuations). The factors that affect the larvae in their prey selection and the feeding strategies have been scarcely studied for these species in the Catalan Sea. These factors can be different depending on the species and can change with ontogeny. The main objective of this Ph.D has been to understand the specific trophic strategies of the larvae of these five important species in Mediterranean fisheries. We can point out three types of strategies: 1) The hake’s one, which from the onset specializes feeding in a prey type (Clausocalanus spp.) and only increases the number of copepods to respond to the nutritional requirements of growth. 2) Anchovy and sardine’s type, which show low feeding incidence at the beginning, and they change the size and number of preys step by step along development without showing abrupt changes in the diet. 3) Bullet tuna and hake’s type, opportunist species which deeply adjust their prey type (and size) in their larval period as they enhance their abilities. This research has aimed to obtain the basic and necessary information to explain the fish larval distribution and to elucidate the effect of the trophic ecology of several species of fishes in the larval survival. It has been the first time to study these species in the Catalan Sea in relation to the potential prey distribution to evaluate the rol of spatial-time match of larvae and preys in the feeding strategies. Moreover, it is the first time that the diet composition of the five species has been studied in relation to the morphology of the feeding-related structures such as the visual system, the mouth and the digestive tract. The results of this research highlights that the contrasting morphology of the digestive tract together with the mouth and eyes size can explain the differences in the feeding habits (feeding incidence, size range and visual acuity) among clupeiforms, bullet tuna and hake, but this is not enough to explain the strategies of round sardinella. The feeding habits of this clupeiform is closer to the bullet tuna’s than to the morphogically similar species (anchovy and sardine), nor serves to explain the differences between hake and bullet tuna diet despite sharing morphological characteristics. Although both bullet tuna and hake have a more favourable morphological features such as robust body, large mouth size, higher visual acuity and greater storage capacity, bullet tuna does reflect these advantages in an intense predatory habits with the incorporation of larger and more mobile prey as it develops, but hake diet does not change even though it would be able to detect and capture more nutritious prey. Hake has a relatively large mouth size at hatching therefore can eat larger prey since the beginning of exogenous feeding, similar to species that are born with direct development of larvae or large sizes, and does not change its diet during the course of larval development.

The high variability in the recruitment of marine fishes is probably explained by small fluctuations in the mortality rates of early developmental stages. A popular proxy for the potential mortality of fish larvae is the study of nutritional condition (or simply "condition"). According to the growth-mortality hypothesis, a lowered condition has a high associated probability of total mortality.This thesis offers new information on several aspects of nutritional condition and growth indices, by working on two main lines: 1) Laboratory Studies: Three main types of condition indices were investigated in larvae of a well-known laboratory-reared species (sea bass Dicentrarchus labrax). Acknowledging for the species-specificity of condition indices, this experimental Chapter was devoted to the study of various properties of morphometric, histological and biochemical indices, in response to several feeding schemes. Also, effort was placed in the improvement of some of the indices and the study of their relationship with survival in the laboratory.In the last Section of this Chapter (comparative analysis), a model to relate potential survival with the different indices is proposed. 2) Field Studies: the relationships between environmental variables, nutritional status and growth of Sardina pilchardus (Walbaum 1972) larvae in an area of the Catalan Sea (NW Mediterranean) were studied. Pilchard is the most abundant commercial pelagic fish species in the NW Med. Despite its economic importance and the known decline of the stock since 1993, the studies on possible links between environmental conditions and pilchard early life history are hardly developed in this area.The field studies were based on three oceanographic cruises conducted in November 1998, February 1999 and November 1999. Firstly, biotic and abiotic environmental variables were related to larval distribution, abundance and size-structure. Secondly, the nutritional condition and growth was studied in two of the cruises. Environmental variables considered at each station included potential food, fluorescence, temperature, salinity and the maximum Brünt-Väisälä frequency. Long-term growth was analysed through otolith analyses and muscle fibre growth patterns. Condition was studied through hystological an biochemical (protein ad RNA/DNA ratios) indices. The possible effect of environmental variables on potential survival is discussed.The main conclusions of the thesis were: LABORATORY STUDIES1)Morphometric indices proved useful in distinguishing the shape of larvae subjected to differing feeding conditions. This was done through multivariate analysis on variables from which the effect of size had been completely removed, whilst incorporating the information on allometry. 2)The quantitative determination of muscle fibre separation (MFS) showed to be the best histological method (within those tested) to detect a suboptimal nutritional state. 3)The RNA/DNA ratio is more appropriate in detecting a nutritional deficiency in post-flexion than in pre-flexion larvae. 4)Within the biochemical indices studied in post-flexion larvae subjected to a short-term food deprivation and re-feeding, the RNA/DNA and the LDH/DNA ratios are considered particularly useful.5)In post-flexion larvae subjected to 2 days of fast, the cell proliferation rate measured in the muscle was significantly lower than in Fed larvae. This was not observed in the brain, which is more conservative to the food withdrawal. 6)The comparison of morphometric, histological and biochemical indices shows that, in pre-flexion larvae, the quantitative determination of muscle fibre separation is the index that soonest detects the effect of food deprivation. For the same age-range, the RNA/DNA yields the maximum correct discrimination percentage (100%). In this study, the MFS was the condition index that held the best correspondence with larval mortality. 7)The larval susceptibility to mortality at sea may be more related to those deficiencies that involve organs or tissues that are of crucial importance for feeding behaviour, escape response or maintenance of floatability, like the muscle. It is here proposed an scheme of how indices could be weighted in order to relate them with survival potential. FIELD STUDIES8)In November 1998, certain environmental characteristics were found to be positively associated with nutritional condition of S.pilchardus, measured through histological and RNA/DNA indices. These areas can be characterised by an optimum "environmental window" defined by temperature values under 19ºC, values of the Brunt-Väissälä (B-V) water stability index under 0.8 cycles h-1 and values of potential food abundance over 4.5 nauplii l-1 and 5.5 individuals l-1 of the rest of the microzooplankton. The variables that best related to larval condition were the B-V index and the amount of potential food. Moreover, the larval abundance was higher in those areas. Therefore, it is considered that larval survival would be enhanced in these areas. 9)In February 1999 the eggs and larval abundance was lower than in November 1998. 10)The long-term growth studied through otolith analyses suggests that larval growth was enhanced in November 1998 with respect to February 1999. This result was confirmed by the analysis of the muscle fibre growth patterns. Condition was also better in November 1998, although the February data were less clear. 11)The measures of growth and condition showed a general coincident pattern. The analysis of muscle growth patterns suggests that in pilchard larvae growth is hypertrophic until 6-7 mm SL, becoming hyperplastic at least until 13.5 mm SL. Within the histological measures studied, the MFS was the most informative. The RNA/DNA appeared more useful in larvae over 8-10 mm SL.12)The amount of larvae in the November 1998 and February 1999 cruises was anomalously low for the time of the year. Hydrographic conditions during both cruises were highly unusual, exhibiting high stratification in November and an inverted salinity pattern in February. The presence of a mesoscale anticyclonic eddy of new AW, that remained in the area from October 1998 to February 1999, is thought to account for the unusual hydrographic conditions and the low numbers of eggs and larvae in some areas. 13)Overall, the data on larval abundance, nutritional condition and environmental characteristics suggest that the spawning season autumn 1998-winter 1999 was characterised by a low larval production of this species. It is concluded, from the joint interpretation of all data, that putative larval survival would be the result of a reduced spawning and nutritional condition in some areas and periods, and a reduced growth in the winter cruise.