Although there is a fair amount of field and laboratory research showing that the mortality of fish larvae is growth-dependent (e.g. In the growth-dependent mortality hypothesis, faster-growing individuals within a population have lower risks of mortality due to predation since i) larger larvae have stronger swimming capabilities than smaller ones and ii) fast-growing larvae experience a shorter duration of the larval period, decreasing the period of time when they are most vulnerable to predators. The high mortality experienced during the larval phase of fishes is widely regarded to be size-dependent 5. Therefore, understanding the phenotypic variability in selected traits is fundamental to understanding the dynamics of fish populations and anticipating which species/populations may be most affected by environmental change. Moreover, selection greatly varies amongst cohorts, implying that selective mortality is an important source of variability in total mortality and thus in the variability in recruitment strength 4. Recent findings suggest that the great majority of the mortality occurring at these early phases is selective and that an individual’s phenotype influences its probability to survive (review in 4). It is commonly accepted that mortality occurring during the early stages of development of marine fish (eggs through to post-larvae) is a major cause of the natural variation in the abundance of marine fish populations 1, 2, 3. Our findings help to better understand the mass mortalities occurring at early stages of marine fish. Our study offers evidence indicating that a significant portion of fish mortality occurs during the endogenous (yolk) and mixed (yolk /prey) feeding period in the absence of predators, revealing that marine fish with high fecundity, such as small pelagics, can spawn a relatively large amount of eggs resulting in small larvae with no chances to survive. Larval mortality has frequently been related to adverse environmental conditions and intrinsic factors affecting feeding ability and vulnerability to predators. ![]() For sardine larvae collected in the Bay of Biscay during the spring of 2008, otolith radius-at-hatch was also significantly related to viability. Otolith diameter-at-hatch was also significantly correlated with survival-at-age in fed and unfed larvae in the laboratory. Larval size-at-hatch was not related to the egg size but was significantly, positively related to the diameter of the otolith-at-hatch. ![]() In this study, the relation between the size-at-hatch and early survival was assessed using laboratory experiments and on field-caught larvae of the European sardine ( Sardina pilchardus). Mortality during the early stages is a major cause of the natural variations in the size and recruitment strength of marine fish populations.
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