Consumption of Antarctic krill Euphausia superba by mackerel icefish, Champsocephalus gunnari off the South Orkney Islands: filling an information gap in the current ecosystem based management approach

Summary

This study calculated how much Antarctic krill is consumed by mackerel icefish around the South Orkney Islands under three different population scenarios. Current icefish populations (7,000 tonnes) consume only about 0.07% of available krill, while pre-fishing exploitation levels (140,000 tonnes) would consume approximately 1.4% of krill biomass. Individual fish consumption was estimated at 153.5 ± 77.9 grams per day. Both icefish species showed highest concentrations north of the islands with significant geographic overlap (Williamson overlap index = 7.22). Icefish distribution followed optimal foraging patterns in relation to krill presence, but population density patterns were not directly related to krill density. The study addresses an important knowledge gap in ecosystem-based fisheries management by measuring fish predation on krill, concluding that even fully recovered icefish populations would not significantly affect krill availability for other predators like Adélie penguins.

Map showing estimated probability of Antarctic krill presence (green shading), potential distribution area of mackerel icefish (depth range 0-1000 meters), and potential distribution of Adélie penguins (from Frolkina 2002) around the South Orkney Islands, Antarctica.

Key Findings

Individual icefish consume an average of 153.5 ± 77.9 grams of krill per day (ranging from 22.7-267.5 grams depending on fish size).

Current fish populations (7,000 tonnes) consume only 0.07% of available krill in penguin feeding areas.

Historical pre-fishing populations (140,000 tonnes) would consume 1.4% of available krill biomass.

The two icefish species show high geographic overlap (Williamson overlap index = 7.22) north of the South Orkney Islands.

Icefish distribution follows optimal foraging patterns, but population density patterns are not directly linked to krill density.

Total summer krill consumption ranges from 3,249 tonnes (current populations) to 64,978 tonnes (historical populations).

Even at maximum historical abundance levels, icefish impact on krill availability for other predators would be minimal.

Abstract

The Antarctic marine ecosystem is largely dominated by Euphausia superba, the most abundant krill species consumed by a large number of predators, including whales, penguins, flying birds, seals, fish and cephalopods. Current management of the krill fishery follows an ecosystem-based approach which takes into account the distribution, abundance of E. superba and its main predators plus their interactions. Mackerel icefish, Champsocephalus gunnari, was once considered the most abundant meso-pelagic fish species and a very important consumer of E. superba until its population was collapsed by overfishing in the early 90s. Currently, C. gunnari populations are slowly recovering which will increase E. superba predation. Therefore, in the present work, we analyze the distribution and density of E. superba and C. gunnari, gain some insights about the type of foraging strategy of C. gunnari and estimate its consumption on E. superba is consumed by C. gunnari under three different biomass scenarios: 7,000 (Bt), 70,000 (B50%) and 140,000 (B0) t. We put special focus on the potential area of overlapping between C. gunnari and Adélie penguin (Pygoscelis adeliae) off the South Orkney islands. Bayesian geostatistical models were used to test whether C. gunnari followed an ideal or generalized ideal free distribution. Spatially explicit consumption estimates were obtained under scenarios of 7,000 (Bt), 70,000 (B50%) and 140,000 (B0) t of C. gunnari biomass. We found that both species have a higher probability of presence and density north of the South Orkney Islands, where they present a high overlap. The probability of presence of Champsocephalus gunnari was best explained by the ideal free distribution whereas its acoustic density was best explained by an independent spatial model that lacked any relation with the distribution of E. superba. Individual consumption of E. supeba by C. gunnari was estimated to be 153.5 ± 77.9 g ind-1. Thus, current biomass (Bt) would only remove~ 0.07% of E. superba biomass in the focus area. Under a pre-exploitation biomass level (B0), C. gunnari would remove~ 1.4% of E. superba biomass. The present work present novel information on the relationship between a key species like E. superba and a highly specialized consumer such as C. gunnari around the South Orkney Islands and contributes to fill in an important gap regarding fish consumption of krill in the Antarctic ecosystem.

Published in

Research Square

2023

Authors

Canseco, J. A., Alegría, N., Niklitschek, E.

Institutions

Spanish Institute of OceanographyInstituto de Investigación PesqueraUniversidad de Los Lagos

Methods

DataAcousticFieldBiological sampling

Read the full paper

Go to doi.org