A warming climate poses a fundamental problem for embryos that develop within eggs because their demand for oxygen (O 2 ) increases much more rapidly with temperature than their capacity for supply, which is constrained by diffusion across the egg surface. Thus, as temperatures rise, eggs may experience O 2 limitation due to an imbalance between O 2 supply and demand. Here, we formulate a mathematical model of O 2 limitation and experimentally test whether this mechanism underlies the upper thermal tolerance in large aquatic eggs. Using Chinook salmon ( Oncorhynchus tshawytscha ) as a model system, we show that the thermal tolerance of eggs varies systematically with features of the organism and environment. Importantly, this variation can be precisely predicted by the degree to which these features shift the balance between O 2 supply and demand. Equipped with this mechanistic understanding, we predict and experimentally confirm that the thermal tolerance of these embryos in their natural habitat is substantially lower than expected from laboratory experiments performed under normoxia. More broadly, our biophysical model of O 2 limitation provides a mechanistic explanation for the elevated thermal sensitivity of fish embryos relative to other life stages, global patterns in egg size and the extreme fecundity of large teleosts.

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Understanding the mechanisms that affect larval dispersal is critical to management of marine populations. Rockfishes Sebastes spp. do not settle to benthic habitats immediately after metamorphosis, but instead remain in the water column for weeks to months. Movements of larvae and pelagic juveniles during their months at sea are largely unknown. It is traditionally thought that young rockfishes are planktonic, moving at the mercy of ocean currents, but this assumption is unverified. In this study, swimming capabilities (critical speed) of larval and pelagic juvenile stages of 6 rockfish species (blue [S. mystinus], yellowtail [S. flavidus], brown [S. auriculatus], kelp [S. atrovirens], gopher [S. carnatus], and splitnose [S. diploproa]) were evaluated to determine their ability to behaviorally influence dispersal. Rockfish larvae have critical speeds of 0.5 to 1.8 cm s −1 (1 to 3 body lengths per second [bl s −1 ]) at parturition, whereas newly settled juveniles are capable of swimming 8.6 to 53.5 cm s −1 (5 to 9 bl s −1 ). Swimming ability increases throughout ontogeny and postflexion rockfishes can swim faster than typical water motions in their natural habitat (i.e. mean ocean currents off central California). Critical speeds for Sebastes spp. are substantially lower than those for larvae and juveniles of tropical species at similar body sizes. Rockfishes, however, have swimming speeds at settlement comparable to some tropical species, as rockfishes settle at larger sizes. The increasing ability of rockfishes to outswim currents during their pelagic phase (acting as nekton rather than plankton) may promote individual survival as well as enhance retention and/or long-distance dispersal -thus swimming has important implications for population connectivity and sustainability.

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Forecasts from climate models and oceanographic observations indicate increasing deoxygenation in the global oceans and an elevated frequency and intensity of hypoxic events in the coastal zone, which have the potential to affect marine biodiversity and fisheries. Exposure to low dissolved oxygen (DO) conditions may have deleterious effects on early life stages in fishes. This study aims to identify thresholds to hypoxia while testing behavioral and physiological responses of two congeneric species of kelp forest fish to four DO levels, ranging from normoxic to hypoxic (8.7, 6.0, 4.1, and 2.2 mg O 2 /L). Behavioral tests identified changes in exploratory behavior and turning bias (lateralization), whereas physiological tests focused on determining changes in hypoxia tolerance (pCrit), ventilation rates, and metabolic rates, with impacts on the resulting capacity for aerobic activity. Our findings indicated that copper rockfish (Sebastes caurinus) and blue rockfish (Sebastes mystinus) express sensitivity to hypoxia; however, the strength of the response differed between species. Copper rockfish exhibited reduced absolute lateralization and increased escape time at the lowest DO levels, whereas behavioral metrics for blue rockfish did not vary with oxygen level. Both species exhibited decreases in aerobic scope (as a function of reduced maximum metabolic rate) and increases in ventilation rates to compensate for decreasing oxygen levels. Blue rockfish had a lower pCrit and stronger acclimation response compared to copper rockfish. The differences expressed by each species suggest that acclimatization to changing ocean conditions may vary, even among related species that recruit to the same kelp forest habitat, leading to winners and losers under future ocean conditions. Exposure to hypoxia can decrease individual physiological fitness through metabolic and aerobic depression and changes to anti-predator behavior, with implications for the outcome of ecological interactions and the management of fish stocks in the face of climate change. K E Y W O R D S aerobic scope, climate change, lateralization, low dissolved oxygen, respirometry, Sebastes MATTIASEN ET Al. | 3499

Viviparous rockfishes (Sebastes spp., family Scorpaenidae) mate and store sperm in the ovaries for several months prior to fertilization, as oocytes develop for the parturition season. Although multiple paternity has been documented in single‐brooding rockfishes, paternity in consecutive broods of multiple‐brooding species has not been studied. Analyses of multilocus microsatellite genotypes in both residual larvae left in the ovary from a previous parturition and upcoming fertilized broods in the same ovary demonstrated evidence of the same sires in consecutive broods in chilipepper (Sebastes goodei) and speckled (Sebastes ovalis) rockfishes. One S. goodei mother showed evidence of multiple paternity from the same two sires in both consecutive broods. The ability to retain sperm, even after a parturition event, for use in subsequent broods, confers an advantage to ensure fertilization and allows for extension of the parturition season. This life‐history strategy provides a bet‐hedging advantage in the California Current system, an environmentally dynamic ecosystem where larval survivorship and subsequent recruitment to adult populations can vary temporally by orders of magnitude.