Early life history phenology among Gulf of Alaska fish species: Strategies, synchronies, and sensitivities.

Synthesis of four decades of Gulf of Alaska ichthyoplankton data indicates that species diversity and total abundance peaks during spring, a common pattern in temperate and sub-arctic ocean regions due to synchrony with the spring peak in plankton production. Nevertheless, fish larvae occur in the plankton at all times of year and peak abundance periods vary significantly by species and habitat. Larval size at hatching and at transformation to the juvenile stage is also highly variable and associated with a variety of larval durations and temporal supply of larval cohorts to pelagic habitats. This phenological diversity represents variability in exposure and adaptation to seasonal cycles in the ocean. Water temperature, winds and currents, and availability of suitable zooplankton prey vary significantly on a seasonal scale affecting degrees of synchrony among larval species with optimal environmental conditions for growth, transport and survival. This synchrony is also affected by interannual shifts in the oceanographic environment, and different early life phenologies among species generate different sensitivities to such interannual variability. Early life history strategies and synchronies are evaluated here and environmental sensitivities are proposed for the numerically dominant species of fish larvae occurring in Gulf of Alaska plankton, including commercially and ecologically important species. For winter to early spring spawners, cold temperatures are an advantage in terms of slowing development so that larvae do not use up all their lipid reserves prior to optimal availability of suitable larval zooplankton prey. Interannual variability in winter temperature may therefore be a good indicator of survival outcomes, especially as influenced by the timing of the switch to exogenous feeding. Variability in temperature-influenced larval growth during late spring and summer months may be less consequential in maintaining synchrony with larval food availability for spring-summer spawners. Rapid growth in association with warm summer conditions facilitates access to a wide size range of prey organisms and minimizes critical periods of vulnerability to trophic mismatch. The Gulf of Alaska is a highly advective environment; storms and alongshore winds promote onshore advection of surface waters. This onshore Ekman transport is strongest during winter and spring when deep water spawned larvae are most abundant over the slope and require access to the shelf. Enhanced shoreward transport of larvae in the canyons intersecting the slope is also an important mechanism. Interannual variability in such transport mechanisms may be critical in determining early ontogeny survival for these species. During all seasons, but especially spring and summer, there are species of larvae for which retention nearshore is vital for survival and mesoscale oceanographic features as well as larval behavioral abilities may be crucial. Annual patterns in phytoplankton and zooplankton production and abundance indicate high-amplitude variation in the composition of prey fields available to larvae, including variability in abundance and the size spectrum of organisms that larvae might encounter and consume. Food limitation seems less likely for larval species that are most abundant in spring-summer than for species with peak abundance in winter-spring. It is probable that the more selective a species is in terms of zooplankton prey, the more susceptible that species is to a trophic mismatch. Species-specific intrinsic rates and morphological development during early ontogeny also influence the interaction of larvae with their environment, and larval growth trajectories can be quite different even among species with identical early life phenology. This insight clearly indicates that although phenology is critical, timing is not everything and all fish larvae are not equal. For the 23 species and two genera of fish in this study, a synoptic overview is provided of their early ontogeny environmental synchronies and proposed sensitivities. This ecological synthesis of phenologies helps us characterize vulnerability and resilience factors for intervals of the planktonic phase in the pelagic environment. It also identifies environmental signals that could be tested as species-specific ecosystem indicators of population trends for fish stocks in the Gulf of Alaska. Further, understanding seasonal dynamics in the ichthyoplankton is considered important for gauging food availability and energy flow more broadly in this and other pelagic ecosystems, as well as to understanding environmental forcing on the fish populations themselves. [ABSTRACT FROM AUTHOR]