Condition and biochemical composition of three pelagic larvae in te bay of biscayengraulis encrasicolus, sardina pilchardus and trachurus trachurus

Resumen

The aims of this thesis were to study: 1. The nutritional environment for fish larvae in the Basque continental shelf (Bay of Biscay), during the main spawning periods of anchovy (Engraulis encrasicolus), sardine (Sardina pilchardus )and horse mackerel (Trachurus trachurus), on the basis of abundance and biochemical composition of size fractioned seston particles. 2. The condition and growth strategy in anchovy and sardine larvae from the Basque continental shelf (Bay of Biscay) by obtaining information on larval protein, lipid, carbohydrate, RNA and DNA contents. 3. The variations in the dynamics of the accretion of the various biochemical constituents, because they are key factors in the understanding of the different larval growth strategies of anchovy, sardine and horse mackerel, linked to body shape: the eel like group (anchovy and sardine) and the tadpole-like group (horse mackerel) The study was carried out in the inner Bay of Biscay, on the Basque continental shelf area off San Sebastian, at 1°55'W and 43°20' to 34'N. To obtain the seston, microplankton, and mesoplankton samples to study the nutritional environment, and the larvae, to study the condition and biochemical composition, six surveys were undertaken during the spring to early summer period of 2 consecutive years: 3 in 2000 (6 May, 13 June and 5 July) and 3 in 2001 (10 April, 9 May and 14 June). Sampling was carried out at 3 stations (D1, D2 and D3) on a cross-shelf transect. The analysis of the nutritional environment as well as the rarity of the starved larvae, according to the RNA/DNA ratio, during the studied period lead to the conclusion that nutritional environment for fish larvae was rich over the Basque continental shelf during spring and early summer of 2000 and 2001. The RNA/DNA ratio was size-related and increased during larval growth up to an asymptotic optimal value The 'actual' RNA/DNA ratios found in a given larval population when compared with the 'starving' RNA/DNA ratio, and using the 'absolutely optimal' value, determined in the laboratory using optimal combinations of food and temperature, as complementary information, could be used to determine used to determine the overall status of the population. Larval condition and survival, seemed to be affected by both factors: the biochemical starting point of the larval development determined by the parents through the initial biochemical composition of the egg, and the growth trajectory made possible by the environmental conditions. Regarding the biochemical composition, from the very beginning of larval development, proteins are the main organic component of anchovy, sardine and horse mackerel larvae, followed by lipids and, finally, carbohydrates. The trends in biomolecules accretion with larval growth in anchovy and sardine larvae differed from those observed in horse mackerel. In anchovy and sardine: protein and RNA percentage increased during growth up to reaching a ceiling, whereas percentages of carbohydrates and lipids decreased until they stabilized in a minimun value specific for each biomolecule. However, biochemical composition did not show any trend during growth in horse mackerel; percentages in biomolecules showed a considerable variability in the smaller sizes, but they stabilized around a value very close to the average percentage. The different trends in biochemical composition during growth of the clupeoids anchovy and sardine, and horse mackerel could be explained by a different growth strategy, since clupeoids need to grow more rapidly than horse mackerel to reach the phases of better swimming ability and bigger mouth. However, the two larval morphological groups, the eel like and the tadpole-like groups, show different trends in the accretion patterns of biomolecules during growth, they come together in the same 'optimal biochemical composition', indicating convergences in life-history patterns of different species. However, it would be convenient to further measure the changes in biochemical composition during growth of anchovy, sardine and horse mackerel in other environmental circumstances and places to confirm the eventual existence, as biochemical composition is concerned, of two different growth strategies.