Trawl records of krill (Meganyctiphanes norvegica) and glass shrimps (Pasiphaea sp.) in the inner Oslofjord between February 2020 and April 2023
2024
Christiansen, Svenja | Kaartvedt, Stein
An Isaacs-Kidd Midtwater Trawl (IKMT) with 2.5 m depressor and ca. 7 m2 opening and a 100 m2 pelagic trawl were used for sampling krill and glass shrimps in the inner Oslofjord between February 2020 and April 2023. Both gears had decreasing mesh sizes towards the cod end (IKMT: 3.5 cm to 1 cm, 4 mm cod end liner with 1 mm cod end bucket; pelagic trawl: from ca 20 cm to 1 cm, 3 mm cod end liner), and neither provides quantitative catches. The catch data was used for characterizing acoustic targets and relative comparisons between depths, seasons, and stations. Sampling depths were determined based on hull-mounted Simrad EK500 echosounder recordings to sample above, within and partly below krill scattering layers. The sampling depth was monitored by a ScanMar depth sensor. Towing speed was 2.8- 3 knots and tows lasted ~15 min. Trawl samples were sorted upon retrieval and the volume of the krill catch determined. Krill (Meganyctiphanes norvegica) and glass shrimps (Pasiphaea sp.) were frozen separately and, for subsamples of up to 50 specimens, length and partly wet weight determined (i.e. individual krill/glass shrimps were measured with a scale in the laboratory after thawing). Length was measured from the tip of the rostrum to the end of the telson. In some cases, the rostrum of the animals was damaged, and then the distance from the eye to the end of the telson was measured. Both methods were applied in one sample, and the resulting regression slope coefficient (slope = 1.07) can be used to correct the measurements from individuals with broken rostrums. The dataset includes in total 43 catches from 2 stations and 13 dates and measurements of 2253 krill and 425 glass shrimps.
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