Understanding and predicting size selectivity and escape mortality in commercial zooplankton fisheries: case study on Antarctic krill

Antarctic krill is considered one of the least exploited fisheries resources worldwide. The size of annual quotas follows the precautionary principle and has great potential for increasing when more knowledge about population dynamics, biomass and ecosystem importance is better mapped. Norway is the largest krill fishing nation in the Southern Ocean. The Antarctic krill is an important food source for fish, sea mammals and seabirds. There have been expressed concern about the impact the fishery might have on the krill-dependent species. Concerns are further strengthened due to observed environmental changes, including temperature increase in some parts of the Southern Ocean, which may affect the production, distribution and life cycle of krill. From a previous project funded by the Norwegian Research Council, (NEAT Net Escapement of Antarctic Krill in Trawls No. 216574), we know that many different sizes of krill can escape even the smallest trawl masks used in commercial krill fishing. The purpose of this project was to investigate the faith the krill that are first captured in the trawl and thereby escapes trawl masks; if they become damaged and / or die due to the meeting with the fishing gear. We have also submitted to journals, 2 new manuscripts. One work describes an alternative method developed to collect selection data from the trawl body using a towing rig with different sampling compartments. This type of experiment design is also useful for studying effects from capture tools on other organisms, such as from lower trophic levels (Krag et al., Submitted to PlosOne 2017). The other manuscript is a mathematical modeling of the selection process that takes place in the codend and in the trawl side panels, so that the selection process for the entire trawlbody is now described. These calculations are based on results from a cover-codend experiment (Herrmann et al., Submitted to Fisheries Research 2017). We are in the process of completing a fourth manuscript describing a method for measuring physiological stress temporarily from krill exposed to stimuli similar to those they experience in a catch-escape situation. Such knowledge is essential in the process of understanding the vulnerability and potential degree of predation after trawl escape (Krag et al. In prep.). This new knowledge is essential to achieve sustainable exploitation of krill resources in Antarctica and may also contribute to the development of more efficient fishing gear. By combining the results from this study and the results from the previous project (NEAT) we can now with more precision calculate total mortality caused by the commercial fishery. We can also more reliably calculate the effects of the fishery on other species that depend on eating krill. CCAMLR regulates all fishing in Antarctica, and has long called for more knowledge about this field. References: Herrmann B, Krag LA, Krafft BA. Size selection of Antarctic krill (Euphausia superba) in a commercial codend and trawl body. Submitted to Fisheries Research 2017 Krafft BA, Krag LA. 2015. Assessment of mortality of Antarctic krill (Euphausia superba) escaping from a trawl. Fisheries Research 170:102-105 Krafft BA, Krag LA, Engås A, Nordrum S, Bruheim I, Herrmann B. 2016. Quantifying the escape mortality of trawl caught Antarctic krill (Euphausia superba). PlosONE 11(9):e0162311 doi:10.1371/journal.pone.0162311 Krag LA, Krafft BA, Engås A, Herrmann B. Collecting size-selectivity data for Antarctic krill (Euphausia superba) with a trawl independent towing rig. Submitted to PlosOne 2017 Krag LA, Krafft BA, Herrmann B, Skov PV. Using ISTAT point of care devices to estimate stress and post release vulnerability of Antarctic krill (Euphausia superba) escapees in the trawl fishery. In prep.

Antarctic krill is an important fisheries resource, regarded as one of the most under-exploited fisheries in the world. Concern is expressed regarding the future sustainability of harvesting and the impact this may have on dependant predators. This is associated with the cumulative pressure from ongoing environmental changes, which modify abundance, distribution and life cycle of krill. Due to large gaps in knowledge about this marine ecosystem and potential negative effects caused by fishery activities, both the Commission and Scientific Committee of CCAMLR strongly request knowledge about the effects of different fishing gear on krill escape and the indirect mortality on the krill stock. Indirect fishing mortalities include organisms that die after escaping from fishing gear due to injury. CCAMLR is currently improving their management system, by establishing feedback management procedures and Small Scale Management Units. They recommend members to have scientific observers on board to enhance control measures and it is initiated a process of establishing Marine Protected Areas around the continent with various degrees of allowing for exploratory fishing and rational use. To establish such a management regime without scientific knowledge about the impact of fishing on the ecosystem is unfavourable. A pilot study (NEAT) using both mathematical modeling techniques and practical experiments on size selection of krill shows that escape occurs even from some of the smallest commercial meshes used in the fishery. In this study, we assess the rate of escape mortality, establish selectivity predictions of krill for any trawl design, including trawlbody and codend. We will produce a set of design guides to allow evaluation and optimization of trawl designs used in the commercial krill fishery to reduce potential negative effects on the ecosystem. There is an urgent need to address these questions, also for the sake of the development of other new and exploratory fisheries.