International audienceAntarctic krill Euphausia superba (hereafter `krill') occur in regions undergoing rapid environmental change, particularly loss of winter sea ice. During recent years, harvesting of krill has in creased, possibly enhancing stress on krill and Antarctic ecosystems. Here we review the overall impact of climate change on krill and Antarctic ecosystems, discuss implications for an ecosystem-based fisheries management approach and identify critical knowledge gaps. Sea ice decline, ocean warming and other environmental stressors act in concert to modify the abundance, distribution and life cycle of krill. Although some of these changes can have positive effects on krill, their cumulative impact is most likely negative. Recruitment, driven largely by the winter survival of larval krill, is probably the population parameter most susceptible to climate change. Predicting changes to krill populations is urgent, because they will seriously impact Antarctic ecosystems. Such predictions, however, are complicated by an intense inter-annual variability in recruitment success and krill abundance. To improve the responsiveness of the ecosystem-based management approach adopted by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR), critical knowledge gaps need to be filled. In addition to a better understanding of the factors influencing recruitment, management will require a better understanding of the resilience and the genetic plasticity of krill life stages, and a quantitative understanding of under-ice and benthic habitat use. Current precautionary management measures of CCAMLR should be maintained until a better understanding of these processes has been achieved. [GRAPHICS]

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Considering that swim-flume or chasing methods fail in the estimation of maximum metabolic rate and in the estimation of Aerobic Scope (AS) of sedentary or sluggish aquatic ectotherms, we propose a novel conceptual approach in which high metabolic rates can be obtained through stimulation of organism metabolic activity using high and low non-lethal temperatures that induce high (HMR) and low metabolic rates (LMR), This method was defined as TIMR: Temperature Induced Metabolic Rate, designed to obtain an aerobic power budget based on temperature-induced metabolic scope which may mirror thermal metabolic scope (TMS = HMR—LMR). Prior to use, the researcher should know the critical thermal maximum (CT max) and minimum (CT min) of animals, and calculate temperature TIMR max (at temperatures −5–10% below CT max) and TIMR min (at temperatures +5–10% above CT min), or choose a high and low non-lethal temperature that provoke a higher and lower metabolic rate than observed in routine conditions. Two sets of experiments were carried out. The first compared swim-flume open respirometry and the TIMR protocol using Centropomus undecimalis (snook), an endurance swimmer, acclimated at different temperatures. Results showed that independent of the method used and of the magnitude of the metabolic response, a similar relationship between maximum metabolic budget and acclimation temperature was observed, demonstrating that the TIMR method allows the identification of TMS. The second evaluated the effect of acclimation temperature in snook, semi-sedentary yellow tail (Ocyurus chrysurus), and sedentary clownfish (Amphiprion ocellaris), using TIMR and the chasing method. Both methods produced similar maximum metabolic rates in snook and yellowtail fish, but strong differences became visible in clownfish. In clownfish, the TIMR method led to a significantly higher TMS than the chasing method indicating that chasing may not fully exploit the aerobic power budget in sedentary species. Thus, the TIMR method provides an alternative way to estimate the difference between high and low metabolic activity under different acclimation conditions that, although not equivalent to AS may allow the standardized estimation of TMS that is relevant for sedentary species where measurement of AS via maximal swimming is inappropriate.

Vertical distribution and abundance of three numerically dominant krill species (Nyctiphanes simplex, Nematoscelis difficilis, and Euphausia eximia) were surveyed in the Gulf of California to understand the role of oxidative stress in their daily vertical migration (DVM) and zoogeographic patterns. Superoxide radical production, lipid peroxidation, and antioxidant enzyme activities were analyzed from krill collected with stratified nets from the surface down to 200 m during January, July, and October 2007. The upper boundary of the oxygen minimum zone (OMZ) was significantly shallower during October than during January. N. simplex was always distributed above the hypoxic layers, mostly in coastal upwelling areas. Ne. difficilis and E. eximia were relatively abundant during January, but detected mostly during their ascending migration. N. simplex was the most sensitive species to high temperatures and low oxygen concentrations, showing evidence of oxidative stress during summer (100 times more lipid peroxidation and 30 times more antioxidant enzyme activities than in winter). Ne. difficilis had higher glutathione peroxidase activity than N. simplex, which could facilitate its larger DVM. Low abundance of Ne. difficilis at 100 m during summer suggests that high temperature was also an environmental limiting factor. Oxidative stress indicators could explain the absence of N. simplex and Ne. difficilis in the eastern tropical Pacific and the ability of E. eximia to live in the OMZ and the eastern tropical Pacific. The latter had higher superoxide radical production and smaller lipid peroxidation during October. This suggests that E. eximia antioxidant enzyme activities are enough to avoid oxidative damage when exposed to hypoxic conditions during DVM.