Vertical Distribution of Age‐0 Walleye Pollock during Late Summer: Environment or Ontogeny?

Abstract

Variability in the late‐summer vertical distribution of age‐0 Walleye Pollock Gadus chalcogrammus in the southeastern Bering Sea has been attributed to a range of physical and biological factors. Using acoustic data (38 and 120 kHz) collected during the 2010 Bering Aleutian Salmon International Survey (BASIS) and dedicated high‐resolution surveys (HR1 and HR2), we evaluated whether late‐summer distributions could be explained by water column properties (environment) or whether sampling was likely occurring during the ontogenetic shift of age‐0 Walleye Pollock from near‐surface habitat to demersal habitat (ontogeny). Neither water column attributes (temperature, relative temperature, salinity, dissolved oxygen, and density gradient) nor the acoustic density of zooplankton prey strongly predicted the acoustic estimates of age‐0 Walleye Pollock vertical presence or density. At 6 of 10 paired BASIS–HR1 stations, age‐0 Walleye Pollock shifted deeper in the water column between BASIS sampling and the HR1 sampling conducted 8–34 d later. There were no consistent differences in FL (P > 0.05 for 2 of 4 station pairs) or energy density (P > 0.05 for 3 station pairs) between age‐0 Walleye Pollock caught in near‐surface trawls and those caught in midwater trawls. Our data suggest that the observation of both near‐surface and midwater age‐0 Walleye Pollock during late summer is likely due to an ontogenetic habitat shift; however, the causative factor was not clear given the limited sample sizes and explanatory variables. The timing of the ontogenetic shift, which appears to have begun before August 18, 2010, can ultimately affect survey strategies, and knowledge of this timing can provide additional insight into factors affecting the overwinter survival of age‐0 Walleye Pollock. Received December 5, 2014; accepted May 10, 2015

Timing of ice retreat alters seabird abundances and distributions in the southeast Bering Sea

“…12: 20160276 age-0 pollock only in the upper water layers, we do not know whether these fish had a larger population size in early-iceretreat-years or if their vertical distribution in the water column changed. Low densities of large, lipid-rich crustacean zooplankton may be responsible for age-0 pollock foraging longer near the surface to accumulate lipids needed for winter survival [9], thereby delaying their ontogenetic vertical migration during early-ice-retreat-years [16].…”

Changes in the vertical distribution of age‐0 walleye pollock (Gadus chalcogrammus) during warm and cold years in the southeastern Bering Sea

“…Age‐0 pollock are also associated with movements of the jellyfish, Chrysaora melanaster , which is a potential sheltering strategy to avoid predators such as seabirds, seals, and adult pollock (Brodeur, 1998a; Livingston, 1993). Parker‐Stetter et al (2015) found that aggregating behavior, which aids in predator refuge, played a larger role than environmental variables in the vertical distribution of age‐0 pollock; however, these findings were based on a single cold year (2010). Our findings support the importance of aggregating behavior in cold years but less so in warm years.…”

“…In addition, age‐0 pollock are also known to change their vertical distribution by altering their migration behavior in response to light, thermal gradients, prey, and predators (Bailey, 1989; Olla & Davis, 1990; Sogard & Olla, 1993, 1996a, 1996b; Schabetsberger et al, 2000). It is known that age‐0 pollock perform ontogenetic migration into deep layers of the water column within the first year of life (Parker‐Stetter et al, 2015), but there is only anecdotal evidence that age‐0 pollock adjust their vertical distribution in response to ocean temperatures. We analyzed the vertical distribution of age‐0 pollock in the EBS with respect to warm and cold stanzas and examined factors that may have influenced distributional shifts.…”

Jellyfish and forage fish spatial overlap on the eastern Bering Sea shelf during periods of high and low jellyfish biomass

“…They also found substantial inter-annual variability depending on oceanographic changes in water column structure and mixed layer depth. The vertical distribution of age-0 walleye pollock and presumably Pacific cod may change seasonally due to the autumn breakdown of the thermocline, when pollock undergo an ontogenetic movement to deeper layers (Parker-Stetter et al 2015). Given this infor mation and also the differential catchability of jellyfish and forage species with the trawl used (e.g.…”