ICES answers to BSAC questions from June 2023

Following BSAC Joint Working Group of June 2023, members had tabled questions to ICES that was to come back later in writing with some answers.

On the 17th January 2024, BSAC received a first set of answers in blue below and BSAC questions to ICES June2023-answers.

Eastern cod

• Could ICES clarify why the results of TABACOD project have not been considered in the assessment?

A benchmark is needed to include the results and in the last benchmark they considered that the growth parameters derived from the TABACOD tagging program (TABACOD project) were considered to be an overestimate of the Eastern Baltic cod growth, although they were used to validate the change in growth.  This is because the tagged fish most likely include individuals of Western Baltic cod. The incorporation of the results will probably  be considered again in the next benchmark.

Western Baltic herring

• In the ICES advice on conservation aspects, why is the issue of spawning and nursery area pointed to and not other conservation aspects such as mismatch between larvae and food availability?

They focused on spawning and nursery ground management because they can be addressed in the short term(at least in Germany not part of any marine conservation programs). We see that coastal modification and eutrophication are affecting herring significantly by sometimes complex stressor cascades (e.g. Moll et al. 2018, von Nordheim et al. 2020). Probably a good summary of those is provided by Moyano et al. 2023 (attached). These subjects could potentially be more directly addressed by adapted (or at least informed) coastal zone management plans.

As this particular herring population relies to a big deal in transient waters, such as estuaries, bays and lagoons for spawning and nursery (Polte et al. 2017), the immediate impacts of coastal use are strong. A new study demonstrating determined homing of herring to particular areas (70% natal homing) even more underlines the need to identify and manage these areas. This study will be submitted this month.

What are the evidence used to choose these 2 aspects? Text and references below

a) Climate change related phenology shifts in herring nursery areas have already resulted in lower early life-stage survival and reduced productivity of the subpopulation (Polte et al 2021, Moyano et al 2022). As this development will likely continue in the following years, ICES advices to minimize other anthropogenic stressors for WBSSH.

b) Eutrophication and spawning habitat degradation have negative effects on the early life-stage survival and the productivity of WBSSH (Kanstinger et al 2016, Kotterba et al 2014, Moll et al 2017, von Nordheim et al 2020, Finke et al 2022, Moyano et al 2022). The homing behaviour of WBSSH and the corresponding strong spawning site fidelity (Moll et al. 2019, Moll et al in prep) implies a strong vulnerability of early life-stages to these threats. The implementation of environmental legislation (e.g. the EU Water Framework [WFD] and the Marine Strategy Framework directives [MSFD]) aims to improve the continental environment and could have a positive effect on the reproductive potential of WBSSH. ICES further advices to take action to protect known spawning habitats from further alteration and degradation.

At present, ICES is not able to exactly quantify the level and the relative impact of non-fisheries anthropogenic factors on the reproductive capacity of the stock. However, given the state of the stock, ICES advises that all non-fisheries anthropogenic impacts (e.g. those caused by eutrophication and spawning habitat degradation) that decrease the early life-stage survival of herring should be reduced.

 Sources and References

•  Finke, A., von Nordheim, L., Kotterb, P., Polte, P. 2022. Impact of spawn concentrations on Atlantic herring (Clupea harengus) egg survival in Baltic Sea inshore spawning areas. ECSS 275:107961. doi: 10.1016/j.ecss.2022.107961
• Kanstinger, P., Beher, J., Grenzdörffer, G., Hammer, C., Huebert, K. B., Stepputis, D., Peck, M. A. 2016. What is left? Macrophyte meadows and Atlantic herring (Clupea harengus) spawning sites in the Greifswalder Bodden, Baltic Sea. ECSS 201: 72-81. doi: 10.1016/j.ecss.2016.03.004
• Kotterba, P., Kühn, C., Hammer, C., Polte, P. 2014. Predation of threespine stickleback (Gasterosteus aculeatus) on the eggs of Atlantic herring (Clupea harengus) in a Baltic Sea lagoon. Limn. and Oceanogr. 59(2): 578-587. doi: 10.4319/lo.2014.59.2.0578
• Moll, D., Kotterba, P., von Nordheim, L., Polte, P. 2017. Storm-Induced Atlantic Herring (Clupea harengus) Egg Mortality in Baltic Sea Inshore Spawning Areas. Estuaries and Coasts. doi: 10.1007/s12237-017-0259-5
• Moll, D., Kotterba, P., Jochum, K.P., von Nordheim, L. and Polte, P. 2019. Elemental Inventory in Fish Otoliths Reflects Natal Origin of Atlantic Herring (Clupea harengus) From Baltic Sea Juvenile Areas. Front. Mar. Sci. 6:191. doi: 10.3389/fmars.2019.00191
• Moyano, M., Illing, B., Akimova, A., Alter, K., Bartolina, V., Börner, G., Clemmesen, C., Finke, A., Gröhsler, T., Kotterba, P., Livdane, L., Mittermayer, F., Moll, D., von Nordheim, L., Peck, M. A., Schaber, M., Polte, P. 2022. Caught in the middle: bottom‑up and top‑down processes impacting recruitment in a small pelagic fish. Rev Fish Biol Fisheries. doi: 10.1007/s11160-022-09739-2
• Polte, P., Gröhsler, T., Kotterba, P., von Nordheim, L., Moll, D., Santos, J., Rodriguez-Tress, P., Zablotski, Y., Zimmermann, C. 2021. Reduced Reproductive Success of Western Baltic Herring (Clupea harengus) as a Response to Warming Winters. Front. Mar. Sci. doi: 10.3389/fmars.2021.589242
• von Nordheim L., Kotterba, P., Moll, D., Polte, P.  2020.  Lethal effect of filamentous algal blooms on Atlantic herring (Clupea harengus) eggs in the Baltic Sea. Aquat Conserv Mar Freshw Ecosyst 30:1362–1372.  https:// doi. org/ 10.1002/ aqc. 3329

Pelagic stocks

Has ICES considered mortality of pelagic fish escaping through meshes? No.

Which indicator has ICES used for selectivity? Selectivity is only considered explicitly in Her.27.3031 and Her.27.25-2932 stocks. It is modelled in both cases as an age based process and most of the information to estimate the selectivity curves comes from the catch at age

Sea trout

• The headline advice mentions “Existing fishing restrictions for sea trout in subdivisions 21, 23, 25, 27-30 and 32 should be maintained”. Why are existing restriction applied in area SD31 not included here? No answer yet.

Date Posted: January 26, 2024