CCAMLR

A stratified bootstrap of icefish catch density based on the UK 2019 demersal fish survey catch rate data indicated a biomass of 53,124 tonnes (lower one sided 5th percentile: 32,399 tonnes) within Statistical Subarea 48.3. Projections for the 2019/20 and 2020/21 seasons applying the CCAMLR harvest control rule lead to catch limits (TAC) of 3,225 tonnes for 2019/20 and 2,132 tonnes for 2020/21.

This paper describes an updated CASAL based assessment of Patagonian toothfish (D. eleginoides) in Subarea 48.4. The assessment data are updated with the observations for the 2017/18 season and the data weighting method revised to be consistent with those applied in other CCAMLR assessment model fits. Stock projections indicate that the stock was at 67% of B₀ in 2018/19 and that a yield of 27 tonnes in 2019/20 and 2020/21 is consistent with the application of the CCAMLR harvest control rule.

Assessment of the Patagonian toothfish (D. eleginoides) in Subarea 48.3 indicates that the current status of the stock is at 50% of B₀. Spawning biomass has been relatively constant in recent years. Projections indicate that a constant catch of 2,420 tonnes in the 2020  and 2021 seasons would be consistent with the CCAMLR decision rule.

The biomass of Antarctic toothfish (D. mawsoni) in CCAMLR subarea 48.4 is estimated from tagging returns to average 1109 tonnes since 2010, with a five-year (2015-2019) average biomass of 1187 tonnes. Applying the CCAMLR agreed precautionary assumption of average biomass across the time series and harvest rate of γ = 0.038, implies a 2019/20 yield of 45 tonnes using the 5-year average biomass.

Historically, a precautionary approach has been applied in treating the 48.4 Antarctic toothfish as a separate stock. Based on the biological characteristics of the catches in Subarea 48.4, and the surrounding regions, the Antarctic toothfish around the southern South Sandwich Islands are now hypothesised as being part of a much larger stock that extends south into Subarea 48.2, 48.6 and possibly 48.5. The current method of assessment, based on tag returns, consequently, is considered to provide an estimate of the local biomass.  

In order to investigate and monitor the ecological impact of longlines on the benthic environment and subsequently develop strategies for its management, the UK is conducting a series of research projects studying the potential impact of longline fishing gear on benthic habitats. The research has been structured around three approaches, each deploying a different format of benthic camera system and movement sensors. Data collected by the cameras and sensors will be input to models which predict the areas in which benthos is likely to occur and identify potential risks, in order to evaluate their validity and further develop their relevance to advice.

Biomarker analysis, especially fatty acids (FA) and stable isotopes (SI), has become a useful tool to elucidate the flow of energy and trophic interactions in an ecosystem and to analyse the diet of species that are hard to observe while feeding. Herein we compare FA profiles and SI composition (nitrogen, δ¹⁵N and carbon, δ¹³C) of muscle tissue from two sympatric Antarctic fish species - Notothenia rossii and N. coriiceps – that are key components in the inshore ecosystem of the South Shetland Islands. For both nototheniids, potential food sources from the benthic community (algae, amphipods, polychaetes and gastropods) were screened in order to re-evaluate their trophic position (TP) and the energy flow, taking as representative of the area the local food web of Potter Cove, in King George Island/Isla 25 de Mayo. Significant differences in FA and SI composition between the two fish species (δ¹⁵N / δ¹³C N. rossii: 9.94±0.80‰ / -23.64±1.03‰, N. coriiceps: 11.44±0.42‰ / -21.46±0.46‰) were found. Notothenia rossii showed a higher total FA concentration, with high levels of polyunsaturated FA (PUFA) such as EPA, DHA and ARA. Conversely, the potential food sources tested showed low concentrations of these FAs. This could indicate that both nototheniids are feeding mainly on another food source that was not included in the present study, or that FA bioconversion takes place in these species. While the FA results might suggest a possible trophic niche segregation between N. rossii and N. coriiceps, both species occupy a similar TP (N. rossii = 3.1, N. coriiceps = 3.5). Furthermore, we found a higher total concentration of monounsaturated FA (MUFA) in N. rossii (4348.4 μl/µg) than in N. coriiceps (85.6 μl/µg), which is about 50-fold greater, and is related with a higher buoyancy capacity in N. rossii. In the present study the use of trophic biomarkers did not elucidate which was the main prey item as lipid source for N. rossii and N. coriiceps, suggesting that other food sources need to be analysed and the species fatty acid bioconversion capacity should be further investigated.

An observation on the interaction between marine mammal and krill midwater trawl was carried out onboard the Chinese krill fishing vessel Fu Rong Hai during the 2018/19 fishing season. The underwater and on sea surface observations were both deployed. Although large numbers of Antarctic fur seal Arctocephalus gazella were observed around the vessel during the vessel fishing in the Subarea 48.3, no identified marine mammal was observed in the main body of trawl net. There was no marine mammal by-catch observed either. The observation found the Antarctic fur seals would be attracted by the codend of krill trawl net, especially during the periods difficult to get foods. The observation also indirectly implied that well designed seal escaping devices might reduce the risk of accidently by-catch of Antarctic fur seals. Future researches on the interaction between marine mammals and krill fishing gears would be helpful to understand the relationships between krill fishing and krill predators.