Hypoxia effects on stock assessment: A case study of the South-Western Baltic (HypCatch) (39807)

Fisheries stock assessments depend on two key data sources, commercial catch data and fishery independent survey indices. Fishery-independent surveys sample regularly and uniformly across a stock area and are used as input to models of stock abundance and population dynamics. If the distribution of fish changes and the survey therefore cannot access the full stock, these data can decrease the estimated stock size in the subsequent stock assessment and hence the projected catch advice given to managers.

The basins of the Baltic Sea frequently experience extended periods of low oxygen. This is especially true in areas of the SW Baltic and Lillebælt, due to higher nutrient loading. Such conditions are physiologically stressful for many commercially important fish species, which are known to avoid these areas, hence potentially shifting the stock distribution. HypCatch will investigate changes in distribution, in particular looking at increased use of coastal areas relative to deep basins by using data from the Danish coastal fish monitoring programme and interviews of Danish Fishers from the SW Baltic.

Surveys of commercially important fish predominantly operate in and around the borders of these basins and so survey catches of these species depends both on the occurrence of hypoxia and the species’ response to these conditions. The hypoxia induced variability in survey catches is not accounted for in survey indices used to inform stock assessments, and subsequent TAC advice. HypCatch will investigate the extent to which varying oxygen conditions increase the variation of survey indices for cod and plaice of the SW Baltic. As part of this investigation, HypCatch will also develop a method to account for oxygen conditions in calculating survey indices and subsequently investigate the impact these new oxygen mediated indices have on stock assessments.

Attention was drawn to the issue of stock indices’ variability by the plaice and cod stocks inhabiting the SW Baltic (ple.27.21-23, cod.27.22-24) in the 2020 assessment. A marked decline in relative abundance was reported from the 2019 Q4 survey and these low stock indices were confirmed by survey operators as real observations. An investigation into potential causes identified that there was wide-spread hypoxia across the SW Baltic during the Q4 survey. This was identified as the most likely explanation for the very low catches reported from the survey. Retaining the Q4 survey in the assessments had different effects on the quality of the assessment for each species. Advice drafters were required to use expert judgement in deciding whether to include or discard the data from this survey, which led to a different approach being selected for each species. Both the Baltic Fisheries Assessment WG (ICES, 2020) and the Advice Drafting Group for the Baltic highlighted the need for a better understanding of the effects of hypoxic events on survey variability to increase the quality of the advice.

HypCatch is focussed on solving a clearly identified challenge for fisheries management. The results will be widely applicable and HypCatch will liaise closely with the relevant ICES assessment working group in both the early development stage and in encouraging operationalisation of the developed methods.

Funding
The project is funded by the European Marine Fisheries Fund and The Danish Fisheries Agency. 

Research area: Ecosystem based Marine Management