Hypoxia (low dissolved oxygen) is a mounting concern for aquatic ecosystems as its prevalence increases with rising anthropogenic nutrient inputs. Hypoxia is most commonly defined as 2.0 mg l–1 of dissolved oxygen, although this level varies widely across studies and agency regulations. Such definitions may be too conservative, as ecologically‐relevant non‐lethal effects (e.g. consumption and growth) of hypoxia on important aquatic species, such as fish, often occur at oxygen levels much higher than 2.0 mg l–1. In addition, many mechanisms that regulate hypoxia tolerance in fish have been proposed, including temperature, habitat, location in the water column, and body size, but there is ongoing debate over which mechanisms are most important. Using a structured meta‐analysis of published studies, we showed consistent, significant negative effects on fish growth and consumption below 4.5 mg l–1. While the total amount of variation explained was generally low, below 4.5 mg l–1 of dissolved oxygen, phylogenetic relationships accounted for most of the explained variation in fish growth. Ecological factors including body size, location in the water column (pelagic, demersal, or benthopelagic), habitat (freshwater, marine, or diadromous), and temperature explained very little of the effect of hypoxia on fish growth and explained only a moderate level of variation in consumption. Our results suggest a dramatically higher threshold for sub‐lethal effects of hypoxia on fish than oxygen levels generally set for regulation purposes, and provide little support for accepted ecological mechanisms thought to influence hypoxia tolerance.

Despite the high rate of egg mortality due to predation, few teleost fishes utilize external casings for protecting their eggs. The gelatinous egg matrix, or skein, produced by Yellow Perch Perca flavescens may provide a variety of benefits including deterring egg predators. This study explored the chemical components of the skein in addition to testing the preferences of two common egg predators, Round Goby Neogobius melanostomus and calico crayfish Orconectes immunis, when presented with three potential egg prey: Yellow Perch eggs in the skein, Yellow Perch eggs without the skein, and Fathead Minnow Pimephales promelas eggs. Preliminary analyses showed that the skein may contain a variety of potentially noxious components, including piperideine and the galactose-specific lectin, nattectin. In preference trials, Yellow Perch eggs in the skein were often approached first; however, both predators preferred Yellow Perch eggs with the skein removed and Fathead Minnow eggs rather than Yellow Perch eggs in the skein. Further experiments demonstrated that crayfish spent less time attempting to consume Yellow Perch eggs in the skein after prior exposure to the skein (day 1: 11.0 min ± 1.3 [mean ± SE]; day 2: 7.6 min ± 1.4), reducing time spent damaging the eggs. These results indicate that the skein may help protect eggs from predation.

While severe hypoxia can be lethal and is usually avoided by mobile aquatic organisms, moderate hypoxic conditions are likely more prevalent and may affect organisms, such as fishes, in a variety of systems. However, fishes have the potential to adjust physiologically and behaviorally and thus reduce the negative effects of hypoxia. Quantifying such physiological responses may shed light on the ability of fishes to tolerate reduced oxygen concentrations. This study assessed how two different hatchery populations of yellow perch Perca flavescens, a fish that is likely to encounter moderate hypoxic conditions in a variety of systems, responded to moderate hypoxic exposure through three experiments: 1) a behavioral foraging experiment, 2) an acute exposure experiment, and 3) a chronic exposure experiment. No marked behavioral or physiological adjustments were observed in response to hypoxia (e.g., hemoglobin, feeding rate, movement frequency, gene expression did not change to a significant degree), possibly indicating a high tolerance level in this species. This may allow yellow perch to utilize areas of moderate hypoxia to continue foraging while avoiding predators that may be more sensitive to moderately low oxygen.

Understanding and predicting recruitment, longstanding goals in fisheries science and ecology, are complicated by variation in the importance of environmental drivers coupled with the dynamic nature of individual ecosystems. Developing an understanding of recruitment from well‐monitored stocks offers an opportunity to overcome these complexities. We used a systematic literature review, a survey, and a workshop attended by professionals with expertise in recruitment of Walleye Sander vitreus to identify common environmental drivers of Walleye recruitment and additional sources of variation (i.e., context dependencies) among populations. The importance of individual environmental drivers, as well as the direction of their influence, differed as a function of geographic region, lake surface area, and Walleye life stage. The literature suggested abiotic conditions (e.g., temperature) during the first year of life were influential in determining recruitment. Professional opinion noted the importance of biotic factors, with prey availability and predation risk having the most consistent relationships with recruitment. We synthesized this information to propose a conceptual model that illustrates the suite of characteristics that shape Walleye recruitment over large spatial and temporal scales. Our findings emphasize the importance of first‐year growth and system‐specific contextual factors, which can alter the relative importance of the environmental drivers of recruitment.

1. Across diverse systems, nutrient loading from anthropogenic sources into aquatic systems has increased over the past century. Such nutrient inputs may

Early-life conditions can have long-lasting effects (experiential legacies) on an individual's performance. Experiential legacies are an important source of variation among mature individuals because responses to early-life environments vary widely. Yet, the factors influencing the magnitudes and directions of phenotypic responses to experiential legacies are poorly understood, hindering our ability to predict adult phenotypes and population-level consequences of environmental stressors. To better understand these issues, we examined how experiential legacies varied with the type of phenotypic response (e.g., reproduction, longevity), characteristics of the individual, and characteristics of the stressful conditions imposed. We conducted a meta-analytic review (n species = 65, n studies = 81), examining experiential legacies of early-life nutritional restriction. We found generally consistent negative or neutral impacts of early nutritional stress on later-life phenotypes, indicating that positive responses to early nutritional restriction may be rare among organisms. Our results also demonstrated differences in how experiential legacies were expressed in specific dimensions of an individual's phenotype; for example, magnitude and direction differed among responses in development rate (weak negative response), offspring quality and quantity (strong negative), and longevity (neutral response). We also found that the harsher the early-life nutritional stress, the stronger the negative nutritional legacy. Our results emphasize the complicated interactions among a suite of phenotypic responses in determining individual performance. Given the potential for individual performance to inform the demography and dynamics of populations, we offer avenues for future research that can improve our understanding of how experiential legacies of nutrition or other early-life conditions affect populations.

The rapid decline in Pacific cod (Gadus macrocephalus, Gadidae) biomass following multiple Gulf of Alaska marine heatwaves (2014–2016 and 2019) may be one of the most dramatic documented changes in a sustainably managed marine fishery. As such, fisheries managers are exploring new recruitment paradigms for Pacific cod under novel environmental conditions. In this review, we address the challenges of managing and forecasting Pacific cod populations in the Eastern Pacific where thermal habitats for early life stages are undergoing varying rates of change across space and time. We use observational data to examine changes in distribution, abundance and demographics of the population from 1993 to 2020, and model contemporary and future changes of thermal habitat for both spawning success and age‐0 juvenile growth potential. Results indicate that reduced spawning habitat and early life stage abundance may be a precursor to regional population decline, but the recent apparent increases in size‐at‐age of pre‐recruits will have unknown impacts on future recruitment in these regions. We contend that continued monitoring of early life stages will be necessary to track changes in phenology and growth that likely determine size‐at‐age and the survival trajectories of year classes into the adult population. These include complex size‐ and temperature‐dependent energetics spanning seasonal habitats through the first winter. Climate‐ready management of Pacific cod will, therefore, require new process investigations beyond single‐season surveys focused on one‐life stage.

Climate‐induced shifts in plankton blooms may alter fish recruitment by affecting the fatty acid composition of early‐life diets and corresponding performance. Early‐life nutrition may immediately affect survival but may also have a lingering influence on size and growth via experiential legacies. We explored the short‐ and longer‐term performance consequences of different concentrations of polyunsaturated fatty acids (PUFA) for juvenile Walleye (Sander vitreus, Mitchill 1818). For the first 10 days of feeding, juveniles were provided Artemia enriched with: oleic acid (low PUFA), high docosahexaenoic acid and high eicosapentaenoic acid (high PUFA), or high PUFA and a form of vitamin E (high PUFA + E). After 10 days, all fish were fed a high‐quality diet and reared for an additional 27 days. Juveniles fed either high PUFA diet were 1.15‐fold larger (PUFA mean ± SD = 20.0 ± 3.3 mg; PUFA + E = 19.8 ± 3.3 mg) than those fed the low PUFA (17.3 ± 2.8 mg) diet after 10 days of feeding. After 27 days, juveniles initially fed the high PUFA diet were still 1.10‐ to 1.20‐fold larger (PUFA = 407.0 ± 61.6 mg; PUFA + E = 422.7 ± 58.7 mg) than those initially fed the low PUFA diet (356.5.0 ± 39.5 mg). Our findings demonstrate that fatty acid composition of juvenile Walleye diets has immediate and lingering size effects. As changes in climate continue to alter lower trophic levels, fish management and conservation may need to consider short‐ and long‐term effects of temporal or spatial differences in early‐life diet quality.