Migration timing has evolved in many animals, allowing them to maximize breeding and feeding success by matching seasonal changes in abiotic conditions and resource pulses. These seasonal changes can shift with the climate, resulting in mismatches between migrations and resource availability unless the populations respond through phenotypic plasticity or evolutionary adaptation. It is common, however, for factors unrelated to climate to affect phenology. Salmon are an exceptionally well-studied group of fishes whose breeding migrations can serve as a template to consider the complex factors affecting migration phenology. In this paper, hypotheses for explaining changes in adult salmon migration phenology are reviewed. Pathways through which climate change may influence migration timing are first summarized, including shifting migration cues, limiting freshwater conditions, changes in distribution and conditions at sea, and alterations in embryo development. Alternative causes of phenological change in salmon are then explored including anthropogenic modifications of river habitat, demographic effects, hatcheries, and fisheries. The effects of these factors on phenology can mimic and mask climate effects, making it challenging to disentangle the causal basis of observed patterns. Instead of inferring shifts from trends in timing data (as is often done), it is suggested that specific mechanistic hypotheses be proposed and tested rigorously, and alternative causes systematically ruled out. Overall, it is challenging to attribute causation to phenological change, but salmon exemplify the many ways in which migration timing can change, including shifts due to climate and other processes.

The pelagic thresher shark Alopias pelagicus is an understudied elasmobranch harvested in commercial fisheries of the tropical Indo-Pacific. The species is endangered, overexploited throughout much of its range, and has a decreasing population trend. Relatively little is known about its movement ecology, precluding an informed recovery strategy. Here, we report the first results from an individual pelagic thresher shark outfitted with a pop-up satellite archival transmitting (PSAT) tag to assess its movement with respect to the species' physiology and trophic ecology. A 19 d deployment in the Red Sea revealed that the shark conducted normal diel vertical migration, spending the majority of the day at 200-300 m in the mesopelagic zone and the majority of the night at 50–150 m in the epipelagic zone, with the extent of these movements seemingly not constrained by temperature. In contrast, the depth distribution of the shark relative to the vertical distribution of oxygen suggested that it was avoiding hypoxic conditions below 300 m even though that is where the daytime peak of acoustic backscattering occurs in the Red Sea. Telemetry data also indicated crepuscular and daytime overlap of the shark’s vertical habitat use with distinct scattering layers of small mesopelagic fishes and nighttime overlap with nearly all mesopelagic organisms in the Red Sea as these similarly undergo nightly ascents into epipelagic waters. We identify potential depths and diel periods in which pelagic thresher sharks may be most susceptible to fishery interactions, but more expansive research efforts are needed to inform effective management.

Ocean sunfishes or molas (Molidae) are difficult to study as a result of their extensive movements and low densities in remote waters. In particular, little is known of the environmental niche separation and differences in the reproductive or movement ecology of molids in sympatry. We investigated spatiotemporal dynamics in the distribution of the common mola Mola mola, sharptail mola Masturus lanceolatus, and slender mola Ranzania laevis in the eastern North Pacific. We used observer data from a commercial fishery consisting of 85000+ longline sets spanning 24 yr, >50° in longitude, and >45° in latitude. Satellite altimetry analysis, species distribution modeling, and multivariate ordination revealed thermal niche separation, spatiotemporal segregation, and distinct community associations of the 3 molid species. Our quantitative findings suggest that the common mola is a more temperate species, while slender and sharptail mola are more (sub)tropical species, and that slender (and possibly also sharptail) mola undergo spawning migrations to the region around the Hawaiian Islands. In addition, we identified potential effects of fishing gear type on molid catch probability, an increasing trend in catch probability of a vulnerable species perhaps related to a shift in the distribution of fishing effort, and the possible presence in the fishery of a fourth molid species being misidentified as a congener, all of which are important conservation considerations for these enigmatic fishes.