Examining surface and subsurface drivers of variability in the distribution of mesopelagic prey through the diet of longnose lancetfish, Alepisaurus ferox

Our observations and understanding of the ways pelagic ecosystems respond to climatic variability are heavily skewed toward the responses of primary producers and top predators to environmental conditions at the ocean’s surface. This focus on the trophic “bookends” is largely driven by their accessibility from the ocean’s surface and the economic importance of many predators, allowing for the relatively high-resolution sampling required to understand their dynamics. Food-web models suggest that the effects of climatic variability on pelagic predators may be mitigated or intensified by the responses of organisms at the middle of the food web. However, the effects of climatic variability on midtrophic species, many of which live at mesopelagic depths, and the latency of their response to climatic variability in the surface ocean remain poorly understood. We generated a diet time series from longnose lancetfish (Alepisaurus ferox, n = 2600, 2009-2019) collected by federal observers in the Hawaii based, deep-set, longline fishery from the central North Pacific (10–40ºN, 135–180ºW), providing a window into the climatic responses of mesopelagic prey. Using Boosted Regression Trees, we examined changes in the distribution of dominant prey species in relation to variability in surface temperature and chlorophyll-a concentration, climate indices (ONI, PDO), and modeled temperature and oxygen at depth. Latency in responses to climatic variability between epipelagic and mesopelagic prey was examined with respect to the seasonal movement of the Transition Zone Chlorophyll Front. This work provides unique insights into the dynamics of a midtrophic, largely midwater prey community with respect to climatic variability in the North Pacific and the potential indirect effects of climatic variability on the many predators that forage at mesopelagic depths.