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.