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Shift of young anchovy from the estuary to the sea may be important for success of recruitment
25 March 2011
The more anchovies grow, the greater the probability of their survival, and it is precisely those born at the peak of the season of egg-laying and promptly moving out to the ocean area which enjoy optimum growth. These are the key factors to good recruitment, according to the conclusions of biologist Naroa Aldanondo in a thesis drawn up at Azti-Tecnalia and defended at the University of the Basque Country (UPV/EHU).
Dr Aldanondo investigated patterns of growth and movement of young anchovies, in order to elucidate their connection with survival. The thesis is entitled, Young-of-the-year European anchovy in the Bay of Biscay: study of recruitment-determining processes based on otolith microstructure analysis. Furthermore, she has just received the CAF-Elhuyar Prize for the best article on science and technology for the general public based on the author’s PhD thesis. The text she wrote regarding her research is entitled Antxoaren gorabeherak ulertu nahian (Wanting to understand the fluctuations of the anchovy) and, according to the panel, it is a “well-rounded” article, “well told and accompanied by fine images”.
Otoliths as a base
Otoliths were the main work tool for undertaking this PhD research thesis, forming part, as they do, of the balance system of the fish, and used to determine their age and how they have grown, given that the growth process is accompanied by rings accumulating on the otolith. Many fish accumulate these rings with a daily frequency and, in order to verify that this is in fact what happens in the case of anchovies, Dr Aldanondo cultivated larvae in a laboratory under various thermal conditions. With the results, she was able to show that, effectively, the accumulation of these rings also occurs on a daily basis in the case of anchovies. Thus, the accumulated rings in the otolith can help in making a precise estimation of age.
It was also confirmed that there is a close link between the width of these rings (the radius of the otoliths) and the length of the fish, although with certain reservations. For example, cold temperatures can deform this proportionality. Thus, despite being a useful tool for tracking the growth of the anchovy, certain factors have to be considered.
Better growth in external waters
As regards the research undertaken out at sea, over a period of three years (2004-06), Dr Aldanondo caught anchovy larvae and young anchovies, in order to analyse the data on young anchovy growth, on the measures of their otoliths and their rings and on their patterns of movement.
Based on these in-situ data and on those obtained in the laboratory, the researcher classified young anchovies in the Bay of Biscay into two groups: those which, being young, moved out to the sea to later return to the Gironde estuary, and those who settle along the coast from the beginning. The chemical analysis of the otoliths indicated that the anchovies belonging to the first group grow much more rapidly (0.88 mm a day) than the second group (0.32 mm a day). According to the researcher in her thesis, the great difference in terms of growth could be due to the fact that the external waters are warmer in spring and in summer, and that there is greater visibility in these waters, despite having lower concentrations of food.
Moreover, the results show that individuals that swim out to sea and then return make up 99 % of the population of young anchovies. Thus, it is suggested that this movement of young anchovies to the exterior zone of the estuary is important for successful recruitment.
Date of birth, key
As regards other factors affecting recruitment, date of birth is also crucial, according to the thesis. In the years in which individuals born at the peak of the season of egg-laying (May-June) survive, the probabilities of obtaining successful recruitment increase. Moreover, it has been observed that there are different models of growth, depending on the date of birth of the anchovy. For example, despite the fact that individuals born at the peak of the season of egg-laying grow more slowly at their larval stage, their growth is more rapid in the juvenile phase. As a consequence, their size is greater in autumn and, thereby, their probability of survival too.