We examine estimates of dispersal in a broad range of marine species through an analysis of published values, and evaluate how well these values represent global patterns through a comparison with correlates of dispersal. Our analysis indicates a historical focus in dispersal studies on low-dispersal/low-latitude species, and we hypothesize that these studies are not generally applicable and representative of global patterns. Large-scale patterns in dispersal were examined using a database of correlates of dispersal such as planktonic larval duration (PLD, 318 species) and genetic differentiation (F ST , 246 species). We observed significant differences in F ST ( p!0.001) and PLD ( p!0.001) between taxonomic groups (e.g. fishes, cnidarians, etc.). Within marine fishes (more than 50% of datasets), the prevalence of demersal eggs was negatively associated with PLD (R 2 Z0.80, p!0.001) and positively associated with genetic structure (R 2 Z0.74, p!0.001). Furthermore, dispersal within marine fishes (i.e. PLD and F ST ) increased with latitude, adult body size and water depth. Of these variables, multiple regression identified latitude and body size as persistent predictors across taxonomic levels. These global patterns of dispersal represent a first step towards understanding and predicting species-level and regional differences in dispersal, and will be improved as more comprehensive data become available.

Some of the longest and most comprehensive marine ecosystem monitoring programs were established in the Gulf of Alaska following the environmental disaster of the Exxon Valdez oil spill over 30 years ago. These monitoring programs have been successful in assessing recovery from oil spill impacts, and their continuation decades later has now provided an unparalleled assessment of ecosystem responses to another newly emerging global threat, marine heatwaves. The 2014–2016 northeast Pacific marine heatwave (PMH) in the Gulf of Alaska was the longest lasting heatwave globally over the past decade, with some cooling, but also continued warm conditions through 2019. Our analysis of 187 time series from primary production to commercial fisheries and nearshore intertidal to offshore oceanic domains demonstrate abrupt changes across trophic levels, with many responses persisting up to at least 5 years after the onset of the heatwave. Furthermore, our suite of metrics showed novel community-level groupings relative to at least a decade prior to the heatwave. Given anticipated increases in marine heatwaves under current climate projections, it remains uncertain when or if the Gulf of Alaska ecosystem will return to a pre-PMH state.

Eelgrass Zostera marina provides refuge to numerous fish species but is vulnerable to fragmentation through natural and anthropogenic disturbance. In Bonavista Bay, Newfoundland, eelgrass patch size was altered to measure changes in predation risk in Age-0 juvenile cod, Gadus morhua. Artificial eelgrass mats of 5 sizes (0.32, 1.1, 5.5, 11 and 22 m 2 ) were deployed in duplicate at each of 2 sites in Newman Sound in Terra Nova National Park during summer/autumn in 1999 and 2000. Predator distribution was determined using a combination of weekly underwater transect surveys and biweekly seining. Relative predation rates were measured by tethering Age-0 cod at the center of each patch and recording the incidence of predation (n = 1116 tether sets). Predation rates were negatively correlated with patch size during both years, suggesting that larger patches reduce predator foraging ability. However, high predator densities in the largest eelgrass patch resulted in higher than expected rates of predation. Therefore, habitat dimension affected predation risk in juvenile cod via 2 opposing mechanisms. Our results stress the importance of considering both habitat areal extent and predator distribution when estimating the effects of habitat fragmentation on predation rates.

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Pacific cod (Gadus macrocephalus) stocks in the Gulf of Alaska experienced steep, unexpected declines following an unprecedented 3-year marine heatwave (i.e., “warm blob”) from 2014 to 2016. We contend that stock reproductive potential was reduced during this period, evidenced by a combination of new laboratory data demonstrating narrow thermal hatch success (3–6 °C), mechanistic-based models of spawning habitat, and correlations with prerecruit time series. With the exception of single-year El Niño events (1998, 2003), the recent 3-year heatwave (2014–2016) and return to similar conditions in 2019 were potentially the most negative impacts on spawning habitat for Pacific cod in the available time series (1994–2019). Continued warming will likely reduce the duration and spatial extent of Pacific cod spawning in the Gulf of Alaska.

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In Bonavista Bay, Newfoundland, we monitored patterns of settlement and distribution of 2 species of gadids, Atlantic cod Gadus morhua and Greenland cod G. ogac, following a largescale alteration of nearshore eelgrass Zostera marina habitat. Comparisons between control and experimental sites, based on bi-weekly sampling from 1995 to 2001, indicated a significant increase in cod abundance at sites enhanced with simulated eelgrass and a corresponding decrease in cod numbers at sites where eelgrass had been removed. These data supported predictions, demonstrating that: (1) there was a sufficient supply of juvenile cod within the areas that have historically been unoccupied (i.e. sand) and (2) both species preferred to settle in complex habitats. However, G. ogac responded significantly to the removal of eelgrass in more comparisons than G. morhua (70 and 37% respectively), suggesting that G. ogac has a higher affinity for complex vegetative habitats than G. morhua at the scale of manipulation (ca. 800 m 2 ). Furthermore, despite an overall preference for eelgrass habitat, high within-site catch variation of post-settled juvenile cod indicated that both species were not restricted to a seine site. Such variation was occurring well after the settlement period, suggesting that juvenile cod were moving and occasionally aggregating (i.e. shoaling) throughout the study period. Our results support previously described associations between juvenile cod and eelgrass, but contradict other published accounts of high site-attachment and restricted movement in G. morhua following settlement.

Changes in Arctic fish assemblages resulting from climate change will likely be determined by the differential thermal response of key species during their early life history. In this study, we incubated multiple batches of eggs and larvae of two ecologically important gadids co-occurring at the Pacific–Arctic interface, Arctic cod (Boreogadus saida) and walleye pollock (Gadus chalcogrammus). Fertilized egg batches (n = 11 Arctic cod; n = 6 walleye pollock) were collected in the late winter/early spring from laboratory broodstock held under simulated seasonal environmental conditions. Image and lipid analyses indicated that Arctic cod eggs and larvae were ∼25–35% larger than walleye pollock and had nearly 3–6× more energetic reserves. Two batches of eggs from each species were incubated in replicated containers (n = 3/batch/temperature) at −0.4, 1.2, 2.5, 3.8, 5.0, 9.0, and 12.0°C for Arctic cod and −0.8, 0.3, 2.2, 4.5, 9.0, and 12.0°C for walleye pollock. Both species had very similar low thermal tolerance, but Arctic cod were much more sensitive to higher thermal stress in terms of hatch success and size-at-hatch. For example, Arctic cod hatch success declined precipitously at temperatures above 3.5°C yet remained above 50% in walleye pollock at 9°C. Arctic cod also had significantly longer development times, such that embryos could survive for ∼4 months at temperatures <0°C from the time of spawning to first-feeding. Collectively, these results indicate Arctic cod have a much smaller thermal window for survival, but can survive for longer periods in the absence of food than walleye pollock at cold temperatures. These temperature-dependent rates will be useful in the development of population forecasts and biophysical transport models for these species in the northern Bering, Chukchi, and Beaufort seas.