Abstract
Myctophid fish are the most abundant and diverse mesopelagic fishes in the Southern Ocean. They are a conduit of energy between primary consumers and higher marine predators, and between the upper surface layers and the mesopelagic depths. However, there remain major uncertainties about their ecology, particularly regarding their role in Southern Ocean food webs, which are often regarded as dominated by Antarctic krill in waters south of the Antarctic Polar Front. Limited data on the feeding ecology of myctophids has made it difficult to assess the importance of myctophids as consumers of krill and how they fit in the traditional view of a krill-dominated system (diatom-krill-higher predator). We provide a new assessment of the role of myctophids in Southern Ocean food webs using information from recent trophodynamic studies of myctophids conducted in the Scotia Sea, one of the most productive regions of the Southern Ocean and a region that sustains both major populations of higher predators (sea birds, seals, whales) and important commercial fisheries (krill, toothfish and mackerel icefish). Collectively, these data show that myctophids have a central role in Southern Ocean food webs as both predators and prey. Large myctophid species are prevalent consumers of krill throughout their distributional range and in different seasons in the Scotia Sea. Moreover, best estimates of both myctophid and higher predator consumption of krill to date indicate that large myctophids are the greatest predators of krill in this region, consuming almost as much krill as all other vertebrate predators of krill. Nevertheless, there are several smaller myctophid species that do not eat krill, instead consuming copepods and other small euphausiids. Myctophids therefore link primary producers to higher predators through both krill-dependent and krill-independent trophic pathways, emphasizing their importance in regional food webs. Consequently, myctophid-based trophic pathways are unlikely to be exempt from the direct consequences of a redistribution and/or reduction in krill population biomass. The extent to which myctophids can maintain food web stability and sustain higher predator populations during periods of prolonged reductions in krill abundance is considered further.
Highlights
Myctophids (Family Myctophidae) are the most abundant and diverse mesopelagic fishes in most of the world’s oceans (Gjøsaeter and Kawaguchi, 1980), including the Southern Ocean, where they comprise around 35 species in 12 genera and an estimated biomass that may substantially exceed 70–200 million tonnes (Mt) (Hulley, 1981; Lubimova et al, 1987; Irigoien et al, 2014)
Other myctophid behavioral mechanisms that are important from a food web dynamics perspective include schooling, and many Southern Ocean myctophid species are generally thought to move through the ocean in dense aggregations that vary in size up to ∼42,000 m2 (Zasel’sliy et al, 1985; TABLE 1 | Summary of the predominant species of the Scotia Sea myctophid community (Hulley, 1981; Linkowski, 1985; Lubimova et al, 1987; Greely et al, 1999; Collins et al, 2008; Saunders et al, 2014, 2015b,c; Lourenço et al, 2017)
We provide new insights into the trophic role of myctophids in the Southern Ocean food web and update existing paradigms of the structure of this system
Summary
Myctophids (Family Myctophidae) are the most abundant and diverse mesopelagic fishes in most of the world’s oceans (Gjøsaeter and Kawaguchi, 1980), including the Southern Ocean, where they comprise around 35 species in 12 genera and an estimated biomass that may substantially exceed 70–200 million tonnes (Mt) (Hulley, 1981; Lubimova et al, 1987; Irigoien et al, 2014). Since myctophids are relatively abundant and thought to eat mostly primary consumers, such as copepods and amphipods (Pakhomov et al, 1996; Shreeve et al, 2009), as well as being a major food source for many higher marine predators, including penguins, seals, petrels, squid, and large predatory fish (Olsson and North, 1997; Cherel et al, 2002; Dickson et al, 2004; Reid et al, 2006; Collins et al, 2007; Connan et al, 2007), they have been identified as a major alternative trophic pathway in the Southern Ocean food web (Murphy et al, 2007b). It is an area where ecological change associated with rapid regional warming is marked (de la Mare, 1997; Curran et al, 2003; Atkinson et al, 2004, 2019; Murphy et al, 2007a; Whitehouse et al, 2008), which may be a useful model system with which to identify the changes in food web dynamics manifested by climate change to compare with elsewhere
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