Effects of warm and cold climate conditions on capelin (Mallotus villosus) and Pacific herring (Clupea pallasii) in the eastern Bering Sea

Abstract

Climate warming has impacted the southern extent of sea ice in the eastern Bering Sea (EBS) ecosystem, leading to many changes in ocean conditions and food webs there. We explore how these changes have affected two key forage fish species, capelin (Mallotus villosus) and Pacific herring (Clupea pallasii), examining the effects of climate change on this commercially important ecosystem in the EBS. Catch per unit effort (CPUE) data from surface trawls, size, and diet of capelin and Pacific herring were collected during a series of warm and cold years by fisheries oceanographic surveys conducted from mid-August to early October 2003 through 2011. Overall, mean CPUE for both species was higher in the northeastern Bering Sea [NEBS; capelin=1.2kg/km2 (warm) and 40.0kg/km2 (cold); herring=141.1kg/km2 (warm) and 132.4kg/km2 (cold)] relative to the southeastern Bering Sea [SEBS; capelin=0.2kg/km2 (warm) and 5.8kg/km2 (cold); herring=15.8kg/km2 (warm) and 24.5kg/km2 (cold)], irrespective of temperature conditions. Capelin mean CPUE was significantly lower during warm years than during cold years [p<0.001; 0.6kg/km2 (warm), 19.0kg/km2 (cold)]. Pacific herring mean CPUE was less variable between warm and cold years [p<0.001; 63.8kg/km2 (warm), 66.2kg/km2 (cold)], but was still significantly less during warm years than cold. Capelin and herring lengths remained relatively constant between climate periods. Capelin lengths were similar among oceanographic domains [104mm (South Inner domain), 112mm (South Middle domain), 107mm (North Inner domain), and 104mm (North Middle domain)], while herring were larger in domains further offshore [123mm (South Inner domain), 232mm (South Middle domain), 260mm (South Outer domain), 129mm (North Inner domain), and 198mm (North Middle domain)]. Diets for both species were significantly different between climate periods. Large crustacean prey comprised a higher proportion of the diets in most regions during cold years. Age-0 walleye pollock (Gadus chalcogrammus) contributed >60% to the diets of Pacific herring in southern Middle Domain and >30% in the northern Middle domain during warm years. A switch to less energetic prey for these forage fishes during warm years may have implications for fitness and future recruitment. The shifts in the distribution and lower biomass of capelin in the EBS during warm years could lead to disruptions in energy pathways in this complex marine ecosystem.