A. Gimpel et al.
2014b) cod spend at least part of their life cycles closely associated with the
structures of the wind farm. The presence of larger individuals in the GB in
winter requires analyses with a higher temporal resolution. However, it
seems likely that persistent favorable feeding conditions inside the offshore
wind farms would support local reproductive success regardless of
spawning taking place inside or outside the offshore wind farm (Casini
et al., 2016; Kjesbu et al., 1991).
4.2. Limitations and opportunities of study design
For our study, we combined on-site samples collected by angling with
data from outside the OWF collected with bottom trawls. We assumed
‘hat the bottom trawl survey data were representative of the cod population
in the German Bight knowing that a comparison of the different sampling
strategies (angling vs. bottom trawling) has caveats. Angling, for example,
might be selective for larger, actively hunting individuals and does not nec-
essarily capture local size distributions. The fact that we did not catch cod
smaller than 15 cm by angling can be explained by the size of the bait we
used for fishing (Section 2.1). However, angling was shown to be a suitable
tool to investigate fish abundance, length frequencies, diet composition and
densities and even indicators for reproduction such as sex and maturity
"Haggarty and King, 2006). Winter catches inside the OWF should be
interpreted with caution, because a sample size of 12 individuals limits
he robustness of possible conclusions. A better monitoring of cod abun-
dance and spawning activity including the use of telemetry studies to inves-
:jgate distribution and activity patterns of cod inside wind farms (Dean
et al., 2014; Reubens et al., 2013a) seems inevitable to understand move-
nent patterns. However, even the sampling of ichthyoplankton with plank-
von nets inside German offshore wind farms is already often restricted. In
such situations, prior knowledge on the hydrodynamic conditions and
chus, anticipation of egg transport within the study site would allow the in-
vestigation of spawning activity inside an offshore wind farm. With hind-
sight, such information could have been exploited more efficiently with
regard to the adequate timing and orientation of sampling relative to tidal
oscillation and the direction of residual currents (Fig. 6). Our approach,
10wever, shows that already a simplified setup may provide valuable de-
:ails for the design of future monitoring concepts: The combination of sam-
ling and modelling enabled us to reconstruct individual drift trajectories,
:eveal the distance the eggs travelled, the direction of transport and even
che location and footprint of larger spawning events.
The variety of techniques applied, ranging from classical sampling
methodologies to laboratory/molecular analyses of fish eggs, stable isotope
analysis, and simulation modelling might be a way forward to address
emerging questions in fisheries science in relation to renewable energy de-
velopment with such enlarged geographical scope.
4.3. Implications of our findings for area-based management
A first evidence that wind farms can provide a suitable habitat and eco-
‘ogical refuge to Atlantic cod has been provided by studies in the Belgian
„art of the southern North Sea (Reubens et al., 2012; Reubens et al.,
2013a; Reubens et al., 2013b; Reubens et al., 2014a; Reubens et al.,
2013c). Our results add important indications that cod during summer ben-
efit from more diverse feeding conditions around wind turbines with scour
protection, and during winter spawns inside offshore wind farms.
This study includes a simple 2D modelling approach to demonstrate
how egg dispersal corridors change under the influence of variable ocean
currents and how this variability challenges the investigation of larval ex-
port and potential spillover effects of an offshore wind farm. Admittedly,
‘here are many physical and ecological pathways by which a single wind
urbine or entire clusters of offshore wind farms could possibly advance
or impede spawning, transport and larval survival due to changes in
wind-forcing, tidal dynamics, stratification strength and local water resi-
dence time or increased turbulence around the foundations, which in turn
can affect the resilience of fish populations and whole communities
(Christiansen et al., 2022: van Berkel et al.. 2020). For example, a recent
Science of the Total Environment 878 (2023) 162902
;tudy by Barbut et al. (2020) suggests a significant cumulative effect of
‘he envisaged offshore wind farm development in the North Sea on the con-
gectivity between spawning areas and nursery grounds of some commer-
cially important flatfish species. Any such statement is out of scope of our
study and is probably difficult to verify anyway considering superimposed
climate change effects and high natural variability. Here, we rather advo-
cate a stronger collaboration between observational and modelling efforts
:o substantiate the role of offshore wind farms as artificial habitats for en-
langered fish species as key for sustainable planning and especially for ad-
litional management measures (e.g. co-use with fisheries or conservation
neasures) to be defined for offshore wind areas.
The fishing sector still has strong concerns about the expansion of off-
;hore wind energy, which limits their abilities to fish. Whether or not com-
nercial fishing inside the wind farms should be legalized is difficult to
decide. There certainly is a chance for mismanagement and thus, negative
affects on fish stocks (Hammar et al., 2014; Leonhard et al., 2013; Russell
et al., 2014). Even a limited, passive (static) fishery targeting cod inside
of wind farm areas might have detrimental effects on the spawning stock
»iomass of the cod population and its genetic pool (Heath et al., 2008).
Jur results indicate that turbines with scour protection might function as
ırtificial reef and offer the last refuge for locally threatened species that
'avor such habitats. Thereby they could offer synergetic opportunities for
climate change mitigation and species conservation.
Taken together, if similar studies will confirm our observations, one
»ossibly has to rethink the actual concept of Marine Protected Areas
'MPA) and include wind farms as potential stepping-stone habitats for re-
storing and protecting local populations. This could have positive impacts
1ot only on the spawning stock biomass and resilience of cod at larger,
acosystem-wide, scales, but also safeguard the survival of other species.
dence, such observational evidence provides a strong basis for defining
wind farms with monopile foundations and scour protection overlapping
or instance with known spawning grounds of cod as OECMs which could
»rovide effective species protection and contribute to biodiversity conser-
vation. This would be a first important step towards an adequate integrated
area-based management and support the action plan towards the EU Biodi-
versity Strategy for 2030.
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.scitotenv.2023.162902.
CRediT authorship contribution statement
A. Gimpel: Conceptualization, Methodology, Resources, Formal analy-
sis, Data curation, Project administration, Writing — original draft. K.M.
Werner: Conceptualization, Methodology, Resources, Formal analysis,
Jata curation, Writing - review & editing. F.-D. Bockelmann: Conceptual-
zation, Methodology, Resources, Formal analysis, Data curation, Writing —
:‚eview & editing. H. Haslob: Conceptualization, Methodology, Resources,
Writing - review & editing. M. Kloppmann: Methodology, Resources, Data
curation, Writing — review & editing. M. Schaber: Methodology, Writing -
:eview & editing. V. Stelzenmüller: Conceptualization, Methodology, Re-
sources, Data curation, Funding acquisition, Writing - review & editing.
Data availability
Data will be made available on request.
Declaration of competing interest
The authors declare that they have no known competing financial inter-
ests or personal relationships that could have appeared to influence the
work reported in this paper.
Acknowledgements
We would like to thank Meerwind Süd/Ost for their collaboration and
support during the experimental fishing. Further, we would like to thank
