Experimental studies on growth and survival in Atlantic bluefin tuna (Thunnus thynnus) and Atlantic bonito (Sarda sarda) larvaeeffects of light, food availability and temperature on their physiology and behavior

Abstract

Understanding what determines the growth and survival of individuals among large pelagic fish populations, particularly during the larval stage, is crucial, because these factors will eventually determine recruitment variability and the number of survivors that reach the juvenile stage. Both Atlantic bluefin tuna and Atlantic bonito serve important ecological role as top predators, are targets of some of the most important fisheries in the world and have a fundamental influence on the structure and function of marine communities. Besides, they are the only two scombrid species in the Mediterranean Sea that have been successfully reared in captivity. However, there are still several unresolved problems in their rearing due to our current lack of knowledge regarding their development during these early life stages, knowledge that is difficult to obtain in the field. This thesis aims to improve current knowledge on the growth and survival of the larval stage of Atlantic bluefin tuna and Atlantic bonito under different light, food and temperature conditions. The results of this thesis are based on laboratory experiments (Chapters 3–6) and combined field and laboratory results (Chapter 7). Chapter 1 provides an introduction with a background to the topic of study. Chapter 2 discusses the materials and methods used for the studies upon which this thesis is based. In Chapter 3, growth differences during the first days of life of Atlantic bluefin tuna and Atlantic bonito are discussed. The nutritional condition of the larvae, measured by RNA:DNA ratio, did not explain the differences in developmental stage in larvae that had been cultured under the same conditions. Chapter 4 examines how larval size is related to vulnerability to manipulation in the timing of onset of piscivory. This is an important aspect to examine, since mortality during the larval stage is considered to be size-selective. The effect that changing the feeding regime, from continuous to intermittent piscivorous feeding, had on larval growth and survival is explored in Chapter 5. In Chapter 6, the metabolic cost of different light and darkness conditions was estimated. The effect that temperature had on fish larval vertical distribution was analysed in Chapter 7 by a comparative laboratory analysis between field-sampled individuals at different depths and experimental thermocline columns. The results suggest that the vertical distribution of Atlantic bluefin tuna and Atlantic bonito larvae is spatially constrained to the upper warmer layers due to larval thermal tolerance. The results of this thesis demonstrate the importance of studying the larval stages of these species, stages which are still not well understood. The characteristics of Atlantic bluefin tuna and bonito are very different from other commonly studied cold water species, such as herring, cod and salmon, for which many empirical studies have been conducted. The results presented in this thesis demonstrate the need to use experimentation in order to obtain knowledge on the processes driving growth and survival—knowledge that is difficult to obtain in the field. Culture of larvae could be improved using alternating light regimes instead of the common long light photoperiod regimes that are used in aquaculture to maximize larval ingestion rates. Field and experimental work can be combined in order to have a more complete view of the processes driving these species and their ecosystems.