The Bay of Biscay, a good feeding environment for the larvae of anchovy, sardine and horse-mackerel

The anchovy, sardine and horse-mackerel are three species of great commercial interest to the Basque fishing fleet. However, all three have suffered considerable fluctuations in their catches over the last few decades, reaching all time lows at times. This variability in the catches, due in part to the high mortality rate of their larvae, with lower numbers reaching adulthood, generates great uncertainty when trying to plan good fisheries management.

Predation and food availability are the most important factors in the mortality of larvae and this is why they were the target of the PhD thesis that Ms Estibaliz Díaz Silvestre has defended at the University of the Basque Country (UPV/EHU): Condition and biochemical composition of three pelagic larvae in the Bay of Biscay. The research work was led by doctors Fernando Villate Guinea and Jesus Mari Txurruka Argarate of the Department of Plant Biology and Ecology at the University’s Faculty of Science and Technology. Dr. Díaz is a Biology graduate and currently working as a researcher at the AZTI-Tecnalia Foundation. Apart from this foundation, participating in the project within which the PhD thesis was based, was the CSIC of Barcelona, the Instituto Español de Oceanografía and the University of Vigo.

This research was based on the general hypothesis that better-fed larvae grow better and, thereby, undergo less risk of depredation and lack of nutrition. However, the metabolism of the larvae has to reach a balance between the energy devoted to growth and fleeing predators, on the one hand and, on the other, creating reserves of energy to withstand periods when they are needed, such as winter or during metamorphosis. Thus, Dr. Díaz studied, on the one hand, the nutritional environment and state of the larvae during the period of reproduction of the anchovy, sardine and horse-mackerel and, on the other, different strategies of growth adopted by each species in order to achieve this balance.

Well fed larvae

Dr. Díaz’s research was undertaken using six sample surveys (May, June and July 2000, and April, May and June 2001) and carried out off the coast of the City of Donostia-San Sebastian. Her first objective was to describe the nutritional environment inhabited by the larvae of the fishes during the period of maximum reproduction and, to this end, based on the biochemical composition of the seston (the particles on which the larvae feed). The initial hypothesis was that the nutritional environment along the Basque coast was sufficiently rich to enable the survival of the larvae of the anchovy, sardine and horse-mackerel, and the results from the period studied confirmed this.

Apart from the quantity and quality of food available to the fish larvae, Dr. Díaz has analysed their nutritional state. To this end, the analysis was based on the RNA/DNA index, taking into consideration the quantity of RNA present in cells in comparison to DNA. Her conclusion was that the nutritional condition of the three species is good, coinciding with previous studies, observing scant presence of larvae with signs of starvation.

Two different strategies for growth

From the start of the development stage of the anchovy, sardine and horse-mackerel, proteins are their major food component, followed by lipids and carbohydrates. Nevertheless, the temporary patterns of accumulation of these substances are different in the case common to the first two species, both belonging to clupeids, from the horse-mackerel: with the first two, the percentages of proteins and RNA growth reaching a threshold, while the percentage of lipids and carbohydrates drop reaching a specific minimum for each species (anchovy or sardine). On the other hand, the horse-mackerel shows highly variable percentages during the first stages of growths, that stabilise at later stages, and reach values similar to those of other species.

According to Dr. Díaz, these differences in the biochemical composition of the species could be due to the different growth strategies adopted by each. The anchovy and the sardine, being larvae with anguiliform morphology (elongated), need to grow more rapidly at early stages than the horse-mackerel and, thus, their metabolism is given over to accumulating proteins to form muscle. The larva of the horse-mackerel, on the other hand, is like a tadpole; it swims better than a clupeid larva of the same size — thus enabling it to flee predators more easily — and it has a bigger mouth to capture more and bigger prey.

The conclusions of this PhD defended at the UPV/EHU state that, although initially both groups accumulate nutrients in different proportions, both anchovy and sardine and the horse-mackerel finally coincide with the same the biochemical composition. Dr. Díaz believes that if other species of larvae are studies, these proportions will not differ much from those found in the study, as the patterns of growth and development follow the rigorous rules dictated by natural selection and the biochemical composition of the larvae only means a variation of these patterns.

Dr. Díaz also states that it would be advisable to measure the changes in the biochemical composition of the anchovy, sardine and horse-mackerel in other locations and under different environmental conditions, in order to confirm or otherwise the existence of two different strategies of growth for the two morphological groups studied.