Assessment of mortality of Antarctic krill (Euphausia superba) escaping from a trawl

The overall purpose of this study was to estimate the mortality of Antarctic krill (Euphausia superba) that escape from the most common mesh size used for codends (16mm) in the current commercial fishery. The experiment was carried out off the South Orkney Islands (60°35′S, 45°30′W) using a covered codend sampling technique for retaining escaped krill, which thereafter were observed in holding tanks to monitor their mortality rate. Our results suggest that krill with smaller body lengths suffered higher mortality. However, sampling depth, haul duration and catch accumulation as well as handling effects onboard, such as exposure to temperature differences, likely increased the mortality rates in our experiment. The results indicates that mortality of krill which escape trawl nets is relatively small, suggesting that krill, in common with many other crustacean species, are fairly tolerant to a process of capture-and-escape

Antarctic krill (Euphausia superba) catch weight estimated with a trawl-mounted echosounder during fishing

Reporting reliable catch weight estimates is important for all fisheries management. This study explores the potential for precise and direct estimation of catch weight (green weight) for the Antarctic krill (Euphausia superba) fishery by employing a high frequency acoustic sensor in the trawl. Trials were performed off the coast of the South Orkney Islands during February 2020 using a scientific macroplankton trawl and echosounder providing a 18° beam pointing downwards across the fishing circle at the trawl mouth. The acoustically estimated catch weight and the observed catch weight had a linear relationship (R2 = 0.87, F(1,10) = 69.6, p < 0.000) where the acoustically estimated catch weight significantly predicted actual catch weight (β = 1.20, p = 0.000). The acoustic vertical densities of krill increased toward the center of the trawl opening suggesting that krill were herded during fishing. The current study demonstrates that acoustically based catch weight monitoring has the potential to be used for reporting total krill catch weight in each trawl, potentially in real-time, and that similar methods could also be employed in similar types of trawl fisheries.publishedVersio

Quantifying the escape mortality of trawl caught Antarctic krill (Euphausia superba)

<div><p>Antarctic krill (<i>Euphausia superba</i>) is an abundant fishery resource, the harvest levels of which are expected to increase. However, many of the length classes of krill can escape through commonly used commercial trawl mesh sizes. A vital component of the overall management of a fishery is to estimate the total fishing mortality and quantify the mortality rate of individuals that escape from fishing gear. The methods for determining fishing mortality in krill are still poorly developed. We used a covered codend sampling technique followed by onboard observations made in holding tanks to monitor mortality rates of escaped krill. Haul duration, hydrological conditions, maximum fishing depth and catch composition all had no significant effect on mortality of krill escaping 16 mm mesh size nets, nor was any further mortality associated with the holding tank conditions. A non- parametric Kaplan-Meier analysis was used to model the relationship between mortality rates of escapees and time. There was a weak tendency, though not significant, for smaller individuals to suffer higher mortality than larger individuals. The mortality of krill escaping the trawl nets in our study was 4.4 ± 4.4%, suggesting that krill are fairly tolerant of the capture-and-escape process in trawls.</p></div

Collecting size-selectivity data for Antarctic krill (Euphausia superba) with a trawl independent towing rig

For the development of efficient trawls to minimize catch loss, escape mortality and potential negative ecosystem impacts from the fishery, the understanding about trawl selectivity processes are crucial. Small crustaceans are regarded as being less motile than most fish species. Crustaceans also display low levels of active avoidance from trawl netting, which in turn may cause direct contact with netting on multiple occasions on their passage towards the codend increasing the probability for escapement. Full-scaled experiments to estimate gear selectivity are highly resource demanding and are highly technically challenging for several types of fisheries. In this study, we developed and tested a trawl-independent towed-rig construction designed to investigate size selectivity of Antarctic krill (Euphausia superba). The results indicate that valid selectivity estimates can be obtained using this method, but due to the small sample size, results are inconclusive. However, the findings of the current study show a potential for developing easier and more cost-effective ways of investigating and estimating size selectivity of Antarctic krill and other small crustacean species in trawls

Spatial structuring in early life stage fish diversity in the Scotia Sea region of the Southern Ocean

The fish community of the Scotia Sea is diverse and plays key roles in Antarctic food webs and biogeochemical cycling. However, knowledge of the spatial and community structure of their early life stages is limited, particularly in the region surrounding the South Orkney Islands. Here we examine the structure of the early life stage fish community in the epipelagic using data from a basin-scale survey conducted in early 2019, which sampled the top 200 m of the water column. 347 early life stage fish from 19 genera were caught in 58 hauls. A third of all specimens belonged to the genus Notolepis and the nine most common genera comprised over 90% of specimens. Cluster analysis revealed five distinct groupings, the most common were a group dominated by pelagic and shelf slope genera (Notolepis, Muraenolepis and Electrona) found mainly in oceanic waters (depth ≥ 1000 m), and a group dominated by species with demersal or benthopelagic adults (Chionodraco, Chaenocephalus and Nototheniops) found mainly in shelf waters. Bottom depth was the main environmental determinant of community structure, separating the diverse on-shelf assemblage at the South Orkneys from the less species-rich community of widespread oceanic taxa. Our results indicate the highest diversities of early life stages of endemic fish occur on the shelf and near-shelf areas. Dedicated monitoring is recommended to understand the seasonal differences in larval community assemblages and the implications of early life stages fish bycatch within the krill fishery