Pacific sand lance Ammodytes personatus are a key forage fish in the North Pacific for many species of salmon, groundfish, seabirds, and marine mammals and have historically been important to predators in relatively warm years. However, extreme declines in the nutritional value of sand lance in Prince William Sound, Alaska, USA, during 2012−2016 indicate that energy transfer from lower trophic levels to predators via sand lance may have been disrupted during the North Pacific marine heatwave in 2015 and 2016. Nutritional value (length, energy density, and whole-body energy) was measured in age-0 and age-1 sand lance collected during July in cool (2012−2013) and increasingly warm (2014−2016) years. The value of age-0 fish was relatively stable, with only minor differences among years for length and whole-body energy. By contrast, the value of age-1 fish significantly declined in 2015, and by 2016 they were 38% shorter and 13% lower in energy density compared to cooler years. This contributed to significant declines in whole-body energy of 44% in 2015 and 89% in 2016 compared to cooler years (2012−2014). The 2015 sand lance cohort experienced little growth or lipid accumulation from July 2015 at age-0 to July 2016 at age-1. This effective disruption of energy flow through pelagic food webs probably contributed to population declines and/or breeding failures observed among several predators in the Gulf of Alaska and suggests that tipping points were reached during the heatwave.

Acute anomalous ocean warming events, including marine heatwaves (MHWs), have significant effects on reproduction and survival of piscivorous seabirds. Additionally, MHWs have negative effects on seabird fish prey, exacerbating these consequences and resulting in population implications for seabirds. We evaluated the relative body condition of Pacific sand lance Ammodytes personatus, an important seabird forage species, in Haro Strait, a highly productive region of southern British Columbia, Canada. We compared body condition and length of fish cohorts that experienced the 2016 MHW year (MHW cohorts) with fish hatched during 3 subsequent post MHW years (2017-2019). Age-0 MHW cohorts had a seasonal decline in body condition in age-0 fish from 100% in the summer to 81% in the winter, while age-1 fish showed a decline from summer-fall highs of 93.5% to wintertime low of 79.5%. In comparison, post MHW cohorts had a winter body condition that was 2-4 times higher than their MHW cohorts. Similar to previous studies in Alaska during the MHW, age-1 fish failed to grow and reach the typical size that distinguishes them from age-0 fish. Poor sand lance condition and growth in winter may explain the ramifications of a warming ocean for top predators, including seabirds and Pacific salmon, which depend on these prey fish in Haro Strait. Our results support the idea that Haro Strait, which is influenced by estuarine circulation resulting in cooler temperatures than surrounding areas, serves as a climate refugium for sand lance populations in summer and provides buffering capacity to ocean climate warming events.