Return of warm conditions in the southeastern Bering Sea: Phytoplankton - Fish

<div><p>In 2014, the Bering Sea shifted back to warmer ocean temperatures (+2 ^oC above average), bringing concern for the potential for a new warm stanza and broad biological and ecological cascading effects. In 2015 and 2016 dedicated surveys were executed to study the progression of ocean heating and ecosystem response. We describe ecosystem response to multiple, consecutive years of ocean warming and offer perspective on the broader impacts. Ecosystem changes observed include reduced spring phytoplankton biomass over the southeast Bering Sea shelf relative to the north, lower abundances of large-bodied crustacean zooplankton taxa, and degraded feeding and body condition of age-0 walleye pollock. This suggests poor ecosystem conditions for young pollock production and the risk of significant decline in the number of pollock available to the pollock fishery in 2–3 years. However, we also noted that high quality prey, large copepods and euphausiids, and lower temperatures in the north may have provided a refuge from poor conditions over the southern shelf, potentially buffering the impact of a sequential-year warm stanza on the Bering Sea pollock population. We offer the hypothesis that juvenile (age-0, age-1) pollock may buffer deleterious warm stanza effects by either utilizing high productivity waters associated with the strong, northerly Cold Pool, as a refuge from the warm, low production areas of the southern shelf, or by exploiting alternative prey over the southern shelf. We show that in 2015, the ocean waters influenced by spring sea ice (the Cold Pool) supported robust phytoplankton biomass (spring) comprised of centric diatom chains, a crustacean copepod community comprised of large-bodied taxa (spring, summer), and a large aggregation of midwater fishes, potentially young pollock. In this manner, the Cold Pool may have acted as a trophic refuge in that year. The few age-0 pollock occurring over the southeast shelf consumed high numbers of euphausiids which may have provided a high quality alternate prey. In 2016 a retracted Cold Pool precluded significant refuging in the north, though pollock foraging on available euphausiids over the southern shelf may have mitigated the effect of warm waters and reduced large availability of large copepods. This work presents the hypothesis that, in the short term, juvenile pollock can mitigate the drastic impacts of sustained warming. This short-term buffering, combined with recent observations (2017) of renewed sea ice presence over southeast Bering Sea shelf and a potential return to average or at least cooler ecosystem conditions, suggests that recent warm year stanza (2014–2016) effects to the pollock population and fishery may be mitigated.</p></div

Fish catch.

<p>Catch (circles) of age-0 pollock (number h^-1) as determined from oblique and surface trawling in 2015 (A) and 2016 (C). X indicates a trawl conducted but no catch. Heat map presents bottom temperatures (^oC) over the southeast and Northern Bering Sea shelves. Cold Pool denoted where bottom temperatures <2 ^oC (blue color ramp). Catches of age-0 pollock were low over the southern shelf in 2015, with higher catches that year in the vicinity of the Cold Pool (<2 ^oC). In 2016 trawl catches of age-0 pollock over the southern shelf were higher than in 2015 and reduced in the north relative to the southern shelf. Acoustic backscatter (NASC, m²/nmi²) estimates in 2015 (B) and 2016 (D) indicate higher backscatter in the Cold Pool relative to the shelf in both years.</p

Zooplankton.

<p>Mean abundance (estimated number m^-3) of small copepods < 2 mm (A), large copepods > 2 mm (B), and euphausiids < 5 mm (C) in the northern and southern shelf during spring and autumn of 2015–2016. Error bars represent ± standard error of the mean.</p

Thermal gradients.

<p>Vectors denoting strength of temperature gradients in 2015 (A) and 2016 (B). Note that vectors are adjusted to orient toward lower temperatures.</p

Diet and condition analyses.

<p>Stomach contents of age-0 pollock collected from trawl surveys over the southeast and Northern Bering Sea shelves (A: 2015, C: 2016). Diets over the southeastern shelf were comprised of euphausiids while large copepods made up the majority of the diets of fish collected in the Northern Bering Sea. Incidence of large copepods in the diet increased latitudinally, similar to observations field-collected zooplankton. Spatial heat map of age-0 pollock energy densities in 2015 (B) and 2016 (D). Heat map with color ramp denotes increasing energy densities. Energy densities were generally higher in the north relative to the south, possibly reflecting diet differences.</p

Walleye pollock abundance estimates.

<p>Walleye pollock abundance estimates.</p

Energy content.

<p>Average energy densities of age-0 pollock collected from the southeast Bering Sea shelf during Warm (red), Average (purple) and Cold (blue years). Circles indicate data collected from surface trawls, squares indicate data collected from oblique trawls. Estimates from 2015 indicate a return to Warm Year bioenergetic status for age-0 pollock, though values were higher than during a previous Warm Year stanza. Estimates from 2016 were even higher than 2015, though not as high as during Cold Years. Results imply that age-0 pollock were able to forage on prey of sufficient quantity and quality to partially mitigate Warm Year effects on provisioning.</p