The 2014-2016 Northeast Pacific marine heatwave (MHW) induced the warmest 3year period on record in the California Current Ecosystem. We tested whether larval fish assemblage structure, phenology, and diversity dynamics were comparable to past warming events from 1951 to 2013. First, we hypothesized, based on past observations of biological effect of warming, that mesopelagic species with southern distributions relative to southern California and Pacific sardine Sardinops sagax (a coastal pelagic species) would increase during the MHW while northern mesopelagics and northern anchovy Engraulis mordax (coastal pelagic) abundances would decline.Similar to past warming, southern mesopelagics increased and northern mesopelagics decreased. Unexpectedly, however, a common southern mesopelagic, Mexican lampfish Triphoturus mexicanus, was approximately three times more abundant than the previous annual high. Furthermore, whereas sardine abundance did not increase, larval anchovy abundance rose to near-record highs in summer 2016. Second, we hypothesized that fishes would spawn earlier during the MHW. Fishes did not spawn in an earlier season within a year, but five of six southern mesopelagic taxa spawned earlier than typical within winter and spring. Third, we predicted that species richness would increase moderately due to an influx of southern and exodus of northern species. Richness, however, was very high in all seasons and the highest ever during the summer as multiple species with primarily southern distributions were recorded spawning for the first time in southern California. The richness of northern species was also unexpectedly high during the MHW. Northern species likely persisted in the study area because in addition to the warm water, pockets of cold water were consistently present. If, as predicted, conditions similar to the MHW become more common as oceans warm, this unique and largely unexpected combination of fishes may reflect future biological conditions.

We investigated the potential use of open coastal habitat over the continental shelf as a nursery area for the common thresher shark Alopias vulpinus. Seven juvenile threshers were tracked using acoustic telemetry to determine their movement patterns and nursery habitat in the Southern California Bight (SCB). Tracked sharks occupied waters over the continental shelf 87% of the time. These waters had an average (± SD) sea surface temperature of 18.8 ± 1.6°C and chlorophyll concentrations that were an order of magnitude higher than in adjacent waters offshore of the continental shelf. Tracked sharks had a mean rate of movement of 1.63 ± 0.56 km h-1 , and some sharks exhibited high site fidelity. The vertical distribution of juvenile threshers was generally limited to the upper 20 m of the water, and most sharks showed diel depth distribution patterns, with daytime depths significantly greater than nighttime depths. An analysis of SCB commercial fishery observer data confirms that juvenile common threshers are most frequently captured over the continental shelf. This region appears to provide juvenile threshers with ample food resources and reduced predation risk relative to adult habitat, and partially satisfies more quantitative nursery area criteria recently established in the literature.

1. Globally, one quarter of shark and ray species are threatened with extinction due to overfishing. Effective conservation and management can facilitate population recoveries. However, these efforts depend on robust data on movement patterns and stock structure, which are lacking for many threatened species, including the Critically Endangered soupfin shark Galeorhinus galeus, a circumglobal coastalpelagic species. 2. Using passive acoustic telemetry, we continuously tracked 34 mature female soupfin sharks, surgically implanted with coded acoustic transmitters, for 7 years via 337 underwater acoustic receivers stationed along the west coast of North America. These sharks and an additional six were also externally fitted with spaghetti identification tags. Our tagging site was a shallow rocky reef off La Jolla (San Diego County), California, USA, where adult females were observed to aggregate every summer. 3. Tagged soupfin sharks were highly migratory along the west coast of North America, between Washington, USA and Baja California Sur, Mexico. However, every 3 years, they returned to waters off La Jolla, California, where they underwent gestation. This is the first conclusive evidence of triennial migration and philopatry ('homeloving') in any animal, which is apparently driven by this species' unusual triennial reproductive cycle. Females of other shark and ray species with triennial reproductive cycles are also likely to exhibit triennial cycles of migration and philopatry. 4. At least six (15%) of our tagged soupfin sharks were killed in commercial gillnets in Mexico. 2 | Journal of Applied Ecology NOSAL et AL. 1 | INTRODUC TI ON Migration, the long-distance movement between distinct habitats for distinct purposes, is widespread among animal taxa (Dingle & Drake, 2007). In long-lived, iteroparous species (i.e. most vertebrates), loop and to-and-fro migrations are most common, involving recurring round-trip journeys between breeding and nonbreeding habitats in response to seasonal changes in the environment (Ramenofsky & Wingfield, 2007). Along these migratory circuits, many animals are philopatric ('home-loving'), returning to previously occupied 'bottleneck sites' for feeding, mating, parturition, molting or staging (Mayr, 1963). Such predictable site fidelity can be used to monitor individual animals via automated tracking and markrecapture methods, to study the long-term patterns of migration within an individual's lifetime. Understanding where, when and why animals move can improve management and conservation, by identifying essential habitat, migratory corridors and bottleneck sites, and enabling more targeted management actions that are flexible in space and time (Allen & Singh, 2016). Migration and other phenological events, such as reproduction, molting and hibernation, usually cycle with a period of 1 year, regulated by endogenous circannual rhythms that are entrained to seasonally varying environmental cues such as photoperiod, temperature and rainfall (Helm et al. 2013; Visser et al. 2010). Circa...