Being bigger is not always the best option to survive and mortality rates do not always decline with increasing larval size. Smaller size at a given age could under certain condition and at certain stages of development confer a survival advantage on individual members of a larval cohort.
Reference

Blanco, E. (Edurne)

de-la-Gándara, F. (Fernando)

Folkvord, Arild

Ortega-García, A. (Aurelio)

Reglero, P. (Patricia)

English

Folkvord, A.

Repositorio Institucional Digital del IEO

Otolith microstructure analyses in cultured Atlantic bluefin tuna larvae as a tool to provide accurate estimates of size selective growth and mortality

Blanco, Edurne

Reglero, Patricia

Ortega-García, Aurelio

de-la-Gándara, Fernando

Digital.CSIC

Folkvord, A. (Arild)

989OTOLITH MICROSTRUCTURE ANALYSES IN CULTURED ATLANTIC BLUEFIN TUNA LARVAE AS A TOOL TO PROVIDE ACCURATE ESTIMATES OF SIZE SELECTIVE GROWTH AND MORTALITY E. Blanco1*, P. Reglero1, A. Ortega2, F. de la Gándara2 & A. Folkvord3, 41Instituto Español de Oceanografía, Centre Oceanogràfic de les Balears, Moll de Ponent s/n, 07015 Palma de Mallorca, Spain* Corresponding author e-mail: edurne.blanco@ba.ieo.es2Instituto Español de Oceanografía, Planta de Cultivos Marinos. Carretera de la Azohía s/n, 30860 Puerto de Mazarrón, Spain3Department of Biology, University of Bergen, 5020 Bergen, Norway 4Institute of Marine Research, 5817 Bergen, NorwayIntroductionIn bluefin tuna aquaculture, mortality during the larval production represents one of the main problems. Transition from yolk sac utilization to exogenous planktivory feeding is the first bottleneck in larval survival (Hjort, 1914) and from planktivory to piscivory feeding another major one (Reglero et al., 2014). During the planktivorous feeding stages mortality is strongly size dependent with large size and fast growing larvae typically having increased survival probability (Takasuka et al., 2007). However, growth variations are apparent since the start of the piscivory (Tanaka et al., 2014) and how mortality affects the size spectrum of the larvae that manage to survive through piscivory has never been studied in Atlantic bluefin tuna (ABFT).The growth characteristics of the fraction of the initial population that survives can be determined using the information stored in the otoliths microstructure and estimated previous size-at-age. We conducted a laboratory experiment in which ABFT larvae were offered larval prey for first time at different times to simulate early onset of piscivory (YSL), delayed onset of piscivory (DYSL) or just a plantivorous diet (Rotifers). The otolith microstructure was then used to compare the size distribution at the onset of the experiment with the estimated previous size-at-age of the survivors at the end of the experiments.Materials and Methods In order to test the effect of different timings of onset of piscivory has on bluefin tuna larval size survival, we conducted a series of experiments with three different feeding treatments, with five 150 l tank replicates each. In the first treatment (Rotifers), only enriched rotifers were provided as food to the larvae until the end of the experiment. In the second treatment (DYSL), four days after the start of the experiment, sea bream yolk sac larvae were added ad libitum in the tanks along with enriched rotifers whereas in the third treatment (YSL), sea bream yolk sac larvae were added with enriched rotifers during the entire experiment. On the first day (Eday0, 19 dph), three larvae per tank were subsampled in darkness as starting point and the last day (Eday8, 27 dph) all the surviving larvae were counted, measured and sampled for accurate growth and survival estimates.Both sagittae otoliths from each sampled larva where removed, their increments and radii counted and measured. Increment widths were obtained by subtracting the distance from the core to the corresponding increment and the distance to the previously formed increment. Back calculation estimates to Eday0 were obtained from the Eday8 otoliths where 8 increments were counted backwards from the edge following the longest possible post-rostrum axis. These back-calculated otolith radii at Eday0 were then compared to the initially sampled otolith radii on Eday0 to obtain estimates of size-selective mortality within each group.  ResultsOur results, based on the analysis of the sampled survivors, show that the largest and fastest growth was obtained for the early piscivory feeding group (YSL) followed by the group in which piscivory was delayed (DYSL) and then the planktivorous group (Rotifers) (Fig.1). However, our otolith back-calculated results showed that the surviving larvae at the end of the experiment were the smallest ones at the beginning of the experiment in all treatments (Fig.1). The results corroborate the rapid response of ABFT to the piscivory in terms of growth reflected in the otolith increments width.(Continued on next page)990DiscussionThe growth differences between the three treatments may be caused by differences in the nutritional values and total energy content of the diets since rotifers are nutritiously inferior to yolk-sac larvae (Seoka et al., 2007). The surviving of the small larvae at the end of the rotifers and DYSL treatments can be explained as an agonistic behavior of the large larvae caused by the lack of suitable prey items at onset of piscivory. In the YSL treatment, it might be explained because larvae cannot express their behavior fully (Folkvord 2005), maintain the physiological machinery at maximum unlike the smallest ones and they might have been affected by a possible tank and transport effect. The mortality of the large larvae along with the rapid response of piscivory in terms of growth reflects that there is a cost within each treatment of growing fast. Being bigger is not always the best option to survive and mortality rates do not always decline with increasing larval size. Smaller size at a given age could under certain condition and at certain stages of development confer a survival advantage on individual members of a larval cohort. ReferencesHjort, J. (1914). Fluctuations in the great fisheries of northern Europe viewed in the light of biological research. Rapp PV Reun Cos Int Exp Mer, 20, 1-228.Seoka, M., Kurata, M., Hatanaka, Y., Biswas, A. K., Ji, S. C., Kumai, H. (2007). Possible nutrients in Artemia affecting the larval growth of Pacific bluefin tuna Thunnus orientalis. Aquaculture Science, 55(1), 55-64.Reglero, P., Ortega, A., Blanco, E., Fiksen, Ø., Viguri, F. J., de la Gándara, F., Seoka, M., Folkvord, A. (2014). Size-related differences in growth and survival in piscivorous fish larvae fed different prey types. Aquaculture, 433, 94-101.Takasuka, A., Aoki, I., Oozeki, Y. (2007). Predator-specific growth-selective predation on larval Japanese anchovy Engraulis japonicus. Marine Ecology Progress Series, 350, 99-107Tanaka, Y., Minami, H., Ishihi, Y., Kumon, K., Higuchi, K., Eba, T., Akefumi, N., Hideki, N., Shiozawa, S. (2014). Differential growth rates related to initiation of piscivory by hatchery-reared larval Pacific bluefin tuna Thunnus orientalis. Fisheries science, 80(6), 1205-1214.Acknowledges: E.Blanco was supported by a pre-doctoral FPI Fellowship from the Balearic regional government.

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Otolith microstructure analyses in cultured Atlantic bluefin tuna larvae as a tool to provide accurate estimates of size selective growth and mortality

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