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Abstract

Antarctic krill is an important component of the zooplankton production in the Southern Ocean and is a major food source for baleen whales. The role of commercial fishing and predation by whales on Krill abundance has been investigated here using the innovative ecosystem-based fishery management, EBFM which maintains the krill to whale food web ecosystem stability. The literature indicates the Krill fishery may have been overfished, so it was reduced to the current annual upper limit of 0.62 million tonnes for support other predators of krill, such as seals, penguins and flying sea birds. However, recent literature suggests a moderate reduction in krill catch in the Antarctic Peninsula area due to its importance for whale migration to temperate areas. The Peninsula area catch was estimated to be reduced by about 10% due to additional concerns about climate change effects on krill abundance in the Southern Ocean, reducing overall catch to 0.556 million tonnes, moderately higher than the maximum taken in 2022. Hence, the krill biomass fishing was reduced to allow for predation by baleen whales and other predators, giving a full ecosystem-based fishing mortality similar to that previously estimated to maintain krill production in the Southern Ocean.

Introduction

The role of commercial fishing on krill abundance and predation by baleen whales in the Southern Ocean has been investigated 1, 2 but the literature is inconclusive, apparently due to lack of information on the amount of whale predation on krill. To support baleen whale krill consumption, 3 estimated a catch of 5.61 million tonne for the fishery area of 3.7 million Km² or 1.516 t/Km²/year. However, krill were expected to be overfished at that level, so to support other krill predators, such as Penguins, an upper catch limit was set at 0.62 million tonnes. That gave a krill catch an order of magnitude lower at 0.168 t/Km²/year, whereas the reported highest catch in 2022 was lower at about 0.45mt, or 0.122 t/Km²/year (see CCAMLR https://fishdocs.ccamlr.org/FishRep_48_KRI_2022.html) and Box 2 in 4. Although there appears to be no published method, it is understood CCAMLR reduced the total catch to allow for krill consumption by baleen whales and other predators. In this paper, the Ecopath Model by 5 for the Southern Ocean, with the Ecopath Model krill and predator characteristics obtained from their supplementary Table S1, was used to estimate the EBFM fishing mortalities, krill catches and krill production consumption by predators.

As the effects on krill due to fishing are considered important for support of baleen whales, a dominant consumer of krill, the aim of this paper is to examine the current krill fishery catch in terms of the innovative approach of using ecosystem-based fishery management, EBFM. The EBFM to support the whale krill predator’s production by 6 was modified for the amount of krill consumption by baleen whales and the other main predators of seals, penguins and flying seabirds. The EBFM was also reduced for the suggested reduction in krill fishing in the Antarctic Peninsula area by 8. The reduction was considered appropriate after review of the potential climate change effects on the Southern Ocean. Hence, the approach used here was to estimate a modified EBFM to support functioning of the Antarctic Krill in fishery in the Southern Ocean ecosystem. That was undertaken by estimating the krill EBFM fishing mortality from the Ecopath Model results in 5.

Results

Results Summary of krill results for krill EBFM FMSY, Full EBFM FMSY and catches, predator cropping, fishing mortality and krill fishery catch

The estimated Krill EBFM F_MSY and Full EBFM F_MSY for baleen whales and other predators, as well as the predator diet and krill cropping compared with the fishery catch is shown in Table 1.

Table 1. Estimated krill EBFM FMSY and Full EBFM FMSY and predator krill crop rate compared with the fishery catch rate. Units: biomass tww/Km2, FMSY/year, krill cropping and fishery catch tww/Km2/year.

Krill Predators	Predator^aTL	AverageTTE	Predator BiomassB	Predator BiomassP/B	Predator Q/B	Krill EBFMF_MSY	Krill Full EBFMF_MSY	Predator Diet	Predator Krill Cropping Crop_krill and Fishery Catch
Baleen Whales	3.54	0.110	2.16	0.03	3.75	0.224	0.094	0.80	1.620
Seals	4.33	0.085	0.25	0.40	15.0	0.237	0.099	0.35	1.148
Penguins	4.1	0.091	0.30	0.75	75.0	0.234	0.098	0.50	9.84
Flying Sea Birds	4.2	0.089	0.08	0.75	100.0	0.235	0.098	0.40	0.975
Krill Fishery^b	2.44	0.1755	25.0	2.5	33.0	0.195	0.114	0.46	0.168

) Data from 5, Table S4: Krill TL 2.44, TTE 0.1755 by equation 1, B 25.0 t/Km², P/B 2.5.(/year). Krill phytoplankton and zooplankton prey weighted average TL 1.43, dominant prey phytoplankton diet 0.5 and micro-zooplankton diet 0.35, weighted average 0.46,

) Krill fishery fishing average mortality is estimated by dividing catch by the krill biomass 0.0067/year (0.168 (t/Km²/year)/krill biomass 25 (tKm²).

Table 1 shows the estimated TTE for predators ranged from 0.085 for seals to 0.110 for baleen whales. The Krill EBFM F_MSY for ecosystem stability of whales and other predators averaged 0.233, and the Full Krill EBFM F_MSY averaged 0.097/year. The total predator cropping of krill 13.583 t/Km²/year, mostly by penguins, is equivalent to 54% of the krill biomass. That is similar to the average of 46% predation loss for marine fisheries estimated by 22, higher than the average of 30% for five small pelagic TL3 predation mortalities, M2, by 23 (see their Table 2) and similar to the 58% for Horse Mackerel, Mackerel and other small pelagic fish. Hence, the Krill predation was subtracted from the fishery biomass to estimate the ecosystem-based fishing mortality in the next Section 3.2. Note that the krill consumption is about 50 mt/year in the fishery area, 9-fold higher than the 5.61 mt/year catch estimated by 3, indicating the possible reason for reduction in the fishing limit by a similar proportion to 0.62 mt/year. Consequently, the commercial fishery catch at the upper limit of 0.168 t/Km²/year is equivalent to a fishing mortality of 0.0067/year (see note b in Table 1, 0.168/krill biomass 25), an order of magnitude lower than the average Full EBFM F_MSY. Given that finding, the expected fishing mortality by applying the Full Krill EBFM F_MSY to the krill biomass with reduction by predator consumption is examined below.

Equivalent krill fishery biomass after allowing for predator consumption

The Antarctic krill had a high biomass of 25 t/Km² at the time of Ecopath Modelling, giving a total biomass in the fishery area of 92.5 m. tonnes (25 x 3.7 mKm²) and high biological production of 62.5 t/Km²/year (P = B 25 x P/B 2.5). The findings by 17 Christensen and Pauly (1995) show predator predation reduces the fishery biological production because P equals biomass accumulation plus predation plus catch plus other mortality and losses, so the total predation was subtracted from the biomass to estimate the ecosystem-based catch. That gives a remaining biomass 11.417 (25.0 -13.583) t/Km²/year and subtracting the existing fishery catch of 0.168, gives an available biomass 11.249 t/Km²/year. Applying to the average krill Full EBFM F_MSY of 0.097/year in Table 1 gives an estimated catch 1.091 t/Km²/year, a total krill fishery catch of about 4.04 million tonne in the fishery area, which is similar to that estimated by 3 at 5.61 mt/year, but about 28% lower.

Discussion

The reduction in krill biomass by predation mortality for estimation of ecosystem-based fishery management is consistent with the investigation by 24 who proposed ecosystem-based fisheries management make adjustments for significant levels of predation mortality. They noted biological preference points, such as recruitment included in estimation of MSY and F_MSY, were to minimise effects of overfishing. The literature found similar high predation effects on the krill fishery 2325. A moderate reduction in catch as a precautionary measure to maintain krill and related predator biological production in the Antarctic fishery areas was prompted by near-term potential climate change effects on phytoplankton and krill production. Further monitoring and research could be undertaken to see if temperature related climate change effects could be offset to maintain production in the Southern Ocean.

The difference with krill catch by 2, or the above estimated Full EBFM F_MSY, and the current much lower catch limit could be considered to provide a buffer for near-term climate change, proposed as 25 years until net zero carbon emissions is reached 26. That is an important consideration due to the amount of literature that suggests climate change effects may cause reduction in Antarctic krill abundance, so a brief review of the fundamental processes that climate change may have on phytoplankton and krill (Euphausia superba) abundance in the Southern Ocean was undertaken. The review is for current and near future, not to the end of century effects because of proposed global action to reduce CO₂ emissions to carbon-neutral 26, meaning net removal by land and aquatic environments to equal annual emissions. For example, uptake by the oceans, particularly in the North Atlantic and the Southern Ocean, is about 25% per year by phytoplankton production 27. The relevant climate change literature findings are briefly summarised in the next section.

Summary of potential near-term Antarctic Ocean climate change effects

Information on climate change effects in the Arctic Ocean are used to provide some perspective on likely effects in the Antarctic Ocean.

Effects of increased water temperature on phytoplankton growth

Phytoplankton diatoms in the Southern Ocean are indicated as the main food for Antarctic krill 28, so significant changes could affect krill production. However, they found the net effect of temperature related climate change is uncertain, but suggested deep water circulation changes may eventually affect nutrient inputs and alter food web flows and biogeochemistry. The early study by 29 see Figure 2 on low temperature effects on diatom growth showed a mounded curvilinear relationship for the diatom Detonulaconfervacea. The curve is indicated as beginning at about 2.2^oC, peaking at about 11.9^oC doublings/day and decreased at higher temperatures. That species has been reported as occurring in the Arctic Ocean in Baffin Bay, further south in Davis Strait and in the Bay of Fundy (see World Register of Marine Species https://www.marinespecies.org/aphia.php?p=taxdetails&id=149286, so a similar response to water temperatures may occur for diatoms in the Southern Ocean. At the Antarctic Peninsula, 30 measured the grow rates in container bags of the diatoms Thalassiosirasp., Nitzschla sp., and Chaetocerossp. at the typical water temperature of 1.5^oC, having an average growth rate of 0.39/day, which was suggested by published models to be about 30% of the rate at 20°C. On the other hand, in situ measurements showed no net growth, apparently due to losses such as sinking and predation 31. However, 32 found climate change reduced cloud cover over the northern Antarctic Peninsula. The increased exposure to sunlight increased water temperature and stratification, and hence phytoplankton growth rates and abundance in the upper euphotic zone.

The current water temperatures in the Southern Ocean are reported by 33 as ranging from -2.62^oC to 5.2^oC. As the lower -2.62^oC temperature is about 4.8^oC lower than the minimum 2.2^oC in the diatom curve in the Arctic Ocean by 29, and assuming a similar response curve for diatoms in the Southern Ocean, a peak production at around 7.1^oC (Arctic peak 11.9 - 4.8) may occur. That suggests a further temperature increase by climate change to higher than only about 1.9^oC (7.1 - Southern Ocean upper temperature 5.2^oC) could cause a reduction in phytoplankton production. Although the recent water temperature increase in the Southern Ocean in 2023 has not been published, the sea ice area decreased by about 20% in 2023 since the area in 1979 34, their Figure 3b, indicating a significant temperature increase. Furthermore, 35 measured a 1.4^oC increase in the Arctic Ocean due to global temperature increases. By comparison, 36 reported the overall Southern Ocean temperature trend in the upper 800m as +0.29 ± 0.09 °C per decade from 1993 to 2017, giving a 0.7^oC increase in 2017, or 0.9^oC extrapolated to 2024. If the dominant diatoms of Rhizosoleniasp. and Thalassiothrixsp in the Southern Ocean 28 have a similar curvilinear relationship with temperature as shown by 29, the reduction of phytoplankton production, and associated krill production in the near future is a possibility. However, 32 found a recent increase in production with climate change. Hence, further research on the effects of water temperature on phytoplankton growth rates in the Southern Ocean is suggested.

In the summary of climate change effects on phytoplankton in the Southern Ocean, 28 noted the oceans have taken up 25 to 30% of annual atmospheric CO₂, with about 40% in the Southern Ocean and sea ice uptake about 58% of that uptake. However, sea ice extent around the West Antarctic Peninsula was indicated as declined by up to 40% over the past 26 years. Importantly, 37 noted ice sheet tipping points at about +1.5 to 2.0^oC, similar to the above suggested increase that may adversely affect phytoplankton production. Conversely, spring melt water in the Southern Ocean with released high iron, which was indicated by 28 to contribute 40-50% of the productivity in the entire ocean. Furthermore, 38, see their Figure 2 found increased phytoplankton production from 1998 to 2018 in the Arctic Ocean due to increased water temperature, reduced sea ice area causing greater exposure to light, and likely increase in nutrients from deep ocean waters. Therefore, the various interrelationships of climate change effects on phytoplankton indicate ongoing monitoring and assessments need to be undertaken.

Water temperature effects on krill growth rates

The natural temperature range of krill was suggested to lie between -1.8 and 5.5 ◦C by 39. They found smaller size and higher oxygen demand at > 3.5◦C. The findings where similar to those by 40 who noted from the literature that the krill growth optimum temperature was 0.5 ◦C to 1◦C and growth rates decreased between 3◦C and 4 ◦C and found effects on lengths at ≥ 3.5◦C, potentially affecting the South Georgia krill fishery. However, according to the rate of overall temperature increase by 36, a constant increase of 3.5^oC in the Southern Ocean may not occur in the near term, consistent with their rate of water temperature increase.

Suggested reduction in upper krill catch limit as precaution for potential climate change effects

A reduction in krill fishing intensity in the Antarctic Peninsula area was suggested by 8 to protect the krill egg and larvae recruitment to krill production because the area has a high proportion of the current krill catch. That was supported by 41 who suggested the area be made into a marine protected area. The study by 42 suggested krill fishing by new countries and climate change caused decreasing recruitment of krill near the Antarctic Peninsula by reduction in sea ice coverage, and a larger average body length being fished. It was also suggested the existing level of fishing is poorly quantified and controlled. Earlier, 43 suggested krill fishing should be stopped in existing protection zones of the South Georgia and Antarctic Peninsula fishery areas due to the high proportion of krill consumed by predators, particularly land based seabirds. For those reasons, a reduction of the current krill catch in the Peninsula area is suggested, which may also give some precaution for near-term climate change effects on the whole krill fishing area.

The Antarctic Peninsula fishing area 48.1 to the 1000m bathymetry is shown in 8 see their Figure 1. If fishing was stopped in the Peninsula area, the reduction in fishing for the area indicated in 8 was estimated to be about 10.3%. Assuming the Peninsula area has a similar catch per krill biomass as in the whole Antarctic fishing area, the 10.3% reduction is expected to be equivalent to reducing the fishery area upper limit of 0.62 mt/year to 0.556 mt/year (0.62 x (1 - 0.103)), which is still higher than the highest reported catch of 0.45 mt/year in 2021/22. The reduced catch is suggested because climate change effects, particularly for the Southern Ocean, indicated by 27 for the importance to carbon uptake, 44 for expected phytoplankton changes and 37 on marine ecosystem effects cannot be disregarded. Obviously, such a change requires further investigation and research on environmental change effects on larvae recruitment to krill production due to climate change, and the fishery manager and stakeholder approval for the final fishing level decision.

Conclusion

Although the suggested reduction in krill catch may address climate change effects in the near term, if carbon neutral is not achieved in about 25 years, or climate change effects accelerate, it is likely significant changes in the Southern Ocean ecosystem could occur. Hence, the suggested 10.3% reduction in krill catch is a first step in trying to address potential short-term climate change effects.

Journal of Plant and Animal Ecology