Finding the way to the top: how the composition of oceanic mid-trophic micronekton groups determines apex predator biomass in the central North Pacific

Abstract

We updated and expanded a model of the pelagic ecosystem for the area of the central North Pacific occupied by the Hawaii-based longline fishery. Specifically, results from the most recent diet studies were used to expand the representation of the lesser-known non-target fish species (e.g. lancetfish, opah, snake mackerel) and 9 mid-trophic micronekton functional groups. The model framework Ecopath with Ecosim was used to construct an ecosystem energy budget and to examine how changes in the various micronekton groups impact apex predator biomass. Model results indicate that while micronekton fishes represented approximately 54% of micronekton biomass, they accounted for only 28% of the micronekton production. By contrast, crustaceans represented 24% of the biomass and accounted for 44% of production. Simulated ecosystem changes resulting from changes to micronekton groups demonstrated that crustaceans and mollusks are the most important direct trophic pathways to the top of the food web. Other groups appear to comprise relatively inefficient pathways or 'trophic dead-ends' that are loosely coupled to the top of the food web (e.g. gelatinous animals), such that biomass declines in these functional groups resulted in increased biomass at the highest trophic levels by increasing energy flow through more efficient pathways. Overall, simulated declines in the micronekton groups resulted in small changes in biomass at the very top of the food web, suggesting that this ecosystem is relatively ecologically resilient with diverse food web pathways. However, further understanding of how sensitive micronekton are to changes in ocean chemistry and temperature resulting from climate change is needed to fully evaluate and predict potential ecosystem changes.

Diet and stable isotope analyses reveal the feeding ecology of the orangeback squid Sthenoteuthis pteropus (Steenstrup 1855) (Mollusca, Ommastrephidae) in the eastern tropical Atlantic

“…High abundances of gelatinous zooplankton [ 75 ] as well as cephalopods [ 76 ] have been found in the equatorial upwelling region and subtropical and tropical waters of the Atlantic, respectively, where our samples were taken. Gelatinous zooplankton play an important role in energy and matter transformation and its direct importance as prey may be largely underestimated [ 69 , 77 ]. Even though it is of low caloric value due to its high water content, a large predator may satisfy parts of its energy requirements by preying on large amounts of gelatinous zooplankton [ 78 ] and focusing on body parts of higher energetic value such as gonads or stomachs.…”

Persistent Enhancement of Micronekton Backscatter at the Summits of Seamounts in the Azores

“…Large pelagic predators, such as tuna, seabirds, and marine mammals, feed on various micronektonic organisms (Choy et al, 2016). The effect of seamounts at aggregating micronekton may play an important role in the feeding success of these top predators, especially in oligotrophic environments, where food is often scarce and too patchy to be efficiently exploited.…”

Dining in the Deep: The Feeding Ecology of Deep-Sea Fishes

“…Many demersal species are themselves commercially exploited (e.g., grenadiers, orange roughy, and Chilean sea bass; Koslow et al 2000, Norse et al 2012. Pelagic food-web models are now emphasizing the importance of mid-trophic-level species in overall oceanic ecology (Choy et al 2016, Ruzicka et al 2012. For example, in the California Current region, the biomass of mesopelagic fishes is an order of magnitude greater than the biomass of sardines and anchovies (Davison et al 2015); mesopelagic fishes therefore consume approximately the same amount of zooplankton as sardines and anchovies do despite their lower metabolism (Koslow et al 2014), and they can contribute significantly to vertical carbon flux through their vertical migrations (Davison et al 2013).…”