Mesopelagic fishes dominate otolith record of past two millennia in the Santa Barbara Basin

Abstract

The mesopelagic (200–1000 m) separates the productive upper ocean from the deep ocean, yet little is known of its long-term dynamics despite recent research that suggests fishes of this zone likely dominate global fish biomass and contribute to the downward flux of carbon. Here we show that mesopelagic fishes dominate the otolith (ear bone) record in anoxic sediment layers of the Santa Barbara Basin over the past two millennia. Among these mesopelagic fishes, otoliths from families Bathylagidae (deep-sea smelts) and Myctophidae (lanternfish) are most abundant. Otolith deposition rate fluctuates at decadal to centennial time scales and covaries with proxies for upper ocean temperature, consistent with climate forcing. Moreover, otolith deposition rate and proxies for temperature and primary productivity show contemporaneous discontinuities during the Medieval Climate Anomaly and Little Ice Age. Mesopelagic fishes may serve as proxies for future climatic influence at those depths including effects on the carbon cycle.

Poleward distribution of mesopelagic fishes is constrained by seasonality in light

“…Mesopelagic fishes are found everywhere in the world’s oceans, yet their abundance and acoustic backscatter strongly decrease in polar waters, both in the Southern (Escobar‐Flores et al, 2018a, 2018b) and Northern Hemispheres (Gjøsæter et al., 2017; Knutsen et al., 2017; Kristoffersen & Salvanes, 1998; Norheim et al., 2016; Sameoto, 1989; Siegelman‐Charbit & Planque, 2016). In Arctic waters north of Svalbard (> 79°N), myctophids (or lanternfish) only represent 2% or less of the biomass (Geoffroy et al., 2019), while in most other areas they are a dominant taxon in the mesopelagic zone (Jones & Checkley, 2019). Given their abundance and near‐global distribution, the question is what limits their success at high latitudes?…”

Holocene fish assemblages provide baseline data for the rapidly changing eastern Mediterranean

“…Fishes are particularly important because they form the majority of organisms in the higher trophic levels of marine ecosystems, exercising control on the biomass of all other groups (Coll et al, 2010). Fish otoliths are one of the best tools for reconstructing past fish faunas; they are useful paleobathymetric and paleoclimatic indicators (Agiadi et al, 2017, 2018) and increasingly used in conservation studies (Jones and Checkley, 2019; Lin et al, 2019; Rowell et al, 2008; Schwarzhans, 2013a). Otoliths are aragonitic incremental stone-like structures in the inner ear of Teleost fish that facilitate sound and balance perception (Schulz-Mirbach et al, 2018).…”

Palaeontological evidence for community-level decrease in mesopelagic fish size during Pleistocene climate warming in the eastern Mediterranean

“…We reconstructed changes in body size and composition of Pleistocene mesopelagic fish assemblages using fossil otoliths. Fish otoliths are aragonitic incremental biomineralisates with species-specific morphology (43) that are commonly preserved as fossils in marine sediments, and whose assemblages faithfully record past fish faunas (14,46,47). Otolith size correlates with fish size through species-specific functions (48), although, in rare cases, otolith growth may become decoupled from fish growth (49).…”