Is starvation a cause of overmortality of the Mediterranean sardine?

Abstract

Animal mortality is difficult to observe in marine systems, preventing a mechanistic understanding of major drivers of fish population dynamics. In particular, starvation is known to be a major cause of mortality at larval stages, but adult mortality is often unknown. In this study, we used a laboratory food-deprivation experiment, on wild caught sardine Sardina pilchardus from the Gulf of Lions. This population is interesting because mean individual phenotype shifted around 2008, becoming dominated by small, young individuals in poor body condition, a phenomenon that may result from declines in energy availability. Continuous monitoring of body mass loss and metabolic rate in 78 captive food-deprived individuals revealed that sardines could survive for up to 57 days on body reserves. Sardines submitted to long-term caloric restriction prior to food-deprivation displayed adaptive phenotypic plasticity, reducing metabolic energy expenditure and enduring starvation for longer than sardines that had not been calorie-restricted. Overall, entry into critical fasting phase 3 occurred at a body condition of 0.72. Such a degree of leanness has rarely been observed over 34 years of wild population monitoring. Still, the proportion of sardines below this threshold has doubled since 2008 and is maximal in January and February (the peak of the reproductive season), now reaching almost 10% of the population at that time. These results indicate that the demographic changes observed in the wild may result in part from starvation-related adult mortality at the end of the winter reproductive period, despite adaptive plastic responses.

Temperature-dependent metabolic consequences of food deprivation in the European sardine

“…In our study, despite a decrease in respiration after 60 days of food deprivation, the metabolic rate of food deprived fish at 20°C remained higher than that measured at 12°C in fed or food deprived fish. Moreover, as observed in Queiros et al, 2021, the respiration of sardines food deprived at 20°C actually increased after 50 days of food deprivation, which may indicate an entry into the critical fasting phase (Bar, 2014;Queiros et al, 2021). At a cellular level, the combination of food deprivation and warmth for 60 days elicited energy-saving mechanisms characterized by a decrease in mitochondrial maintenance costs (i.e.…”

“…In vivo group metabolism was measured as oxygen consumption by the sardines in their custom-designed holding tanks, using automated stop-flow respirometry (Steffensen, 1989) modified for open tanks (McKenzie et al, 2007;McKenzie et al, 2012), exactly as described in Queiros et al (2021). Briefly, this system alternated periods in a cycle comprising 30 min when the tanks were aerated and received a water supply versus 30 min when water supply and aeration were stopped, and the fish consumed the oxygen in the water.…”

Temporal changes in zooplankton indicators highlight a bottom-up process in the Bay of Marseille (NW Mediterranean Sea)

“…An increased mismatch between the spring peak of zooplankton biomass (mainly calanoids) and the growth phase of small pelagic fish [84][85][86] could certainly explain their body condition. In their experimental study, Queiros et al [87], showed that sardine can display adaptative phenotypic plasticity to food condition changes. Under lower food quantity and quality conditions, the smallest phenotypes experienced lower mortality by starvation than the larger ones during the critical post-reproductive period (i.e.…”