A. Gimpel et al.
1. Introduction
The implementation of marine renewable energy is worldwide key to
enable the transition to a carbon free energy production, hence leading to
a rapid increase of installed structures in coastal and offshore waters
(Gourvenec et al., 2022). In support of the EU Green Deal, the latest revision
of the EU Directive 2018/2001 (EU, 2018) proposed to increase the current
:arget to at least 40 % renewable energy sources in the EU's overall energy
nix by 2030. The rapid expansion of offshore wind development inflates
:he concern of adverse environmental effects due to the risk of cumulative
auman pressures, particularly in intensively used regions such as the North
Sea (Gimpel et al., 2013; Gusatu et al., 2021; Stelzenmüller et al., 2015). In
addition to these direct environmental perturbations, the increasing con-
Zlict potential with other wide-ranging human activities such as fisheries
challenge area-based management approaches such as Marine Spatial Plan-
aing (MSP) (Gimpel et al., 2013; Stelzenmüller et al., 2022).
The North Sea is one of the most intensively fished shelf seas in the
world (Guiet et al., 2019). Hence direct anthropogenic pressure on ecosyS-
;ems through for example bottom trawling is already higher than in many
other seas globally and threatens the sustainable exploitation of many fish
species (Amoroso et al., 2018; Zimmermann and Werner, 2019). Atlantic
cod (Gadus morhua) has been a crucial source of nutrition and income for
coastal communities around the North Atlantic for centuries (Kurlansky,
1997). Cod had been the quantitatively most important ground fishery re-
source in the North Sea for many decades, until the stock and the fishery
collapsed in the early 2000s. This collapse is especially pronounced for
(he southern stock component (ICES, 2022), representing the species’
southernmost area of occurrence, where trawling effort is highest through-
out the North Sea (Couce et al., 2020). Here, cod does not only face popu-
lation declines through fishing, but also habitat loss and physiological
stress through ocean warming (Nünez-Riboni et al., 2019; Pörtner et al.,
2014) and it appears that a strict management of the remaining spawning
areas is key to population recovery (Vitale et al., 2008).
Wind farms attract juvenile and adult life stages of cod in search of food
or shelter between gravel, stone fields and various other hard structures
‘hat secure the wind turbine piles' foundations (Reubens et al., 2012;
Zeubens et al., 2013a; Reubens et al., 2013b; Reubens et al., 2014a;
Reubens et al., 2013c). Monopile foundations with scour protections of
stones offer a different and potentially more diverse, hard-structure related
diet composition in comparison to the surrounding, muddy-sandy habitats
of the southern North Sea because they increase local habitat heterogeneity
‚Degraer et al., 2020; Langhamer, 2012).
It is the purpose of this study to investigate the effect of pile foundations
with scour protection, a deposit of rocks and stones to protect the founda-
ions against soil erosion, and the potential suitability of offshore wind
arms as feeding and spawning areas for cod, a threatened species in the
southern North Sea. Safety regulations of the wind farm operators, MSP
and shipping prohibited the use of telemetry or other tagging methods
within German wind farms as well as the deployment of bottom trawling.
Aence, we used rod and line fishing and plankton nets to collect biological
samples for early and adult life stages. These data were combined with sam-
les from bottom trawls and a physical drift simulation. As yet, such a com-
»ination of monitoring concepts has not been used to investigate the
ecological impacts of offshore wind farms. This study provides valuable de-
:ails for the strategic design of future wind farm monitoring and allowed us
also to reflect on the wider implications of the rapid expansion of offshore
cenewable energy.
Further, we elaborate on wind farms as existing management systems
chat can provide effective biodiversity conservation and cover thereby
‘Other Effective area-based Conservation Measures’ (OECMSs), a manage-
nent measure agreed at the 14th Conference of Parties of the Convention
on Biological Diversity (CBD).* This new conservation approach, where
conservation is achieved mainly as a by-product of other management,
1 https://biodiversity.europa.eu/protected-areas /other-effective-area-based-conservation-
measures.
Science of the Total Environment 878 (2023) 162902
night help and support EU Member States in their National Strategies
and Action Plans under the CBD and in implementation of the EU Biodiver-
sity Strategy for 2030.
2. Methods
2.1. Study area, sampling strategies and database
2.1.1. Study area
As case study area we chose the wider German Bight of the North Sea
with the Offshore Wind Farm Meerwind Süd/Ost (OWF) located 25 km
1orth-west of the Island of Helgoland, where the North Sea is on average
22-26 m deep. The OWF is at the southern edge of a larger wind energy
cluster (Fig. 1), which contains currently two other wind farms with vary-
ng foundation types. The OWF has been in operation since autumn 2014,
covers an area of approximately 8 x 4 km and comprises 80 turbines
with monopile foundations and scour protection consisting of rocks and
stones that surround the monopile foundation. The scour protection is ho-
nogeneously distributed around the turbine and has a two-layer structure:
The filter layer consists of smaller stones (D°° = 60 mm) and extends up to
13.5 m. The armor layer consists of large stones (D°° = 600 mm), which
completely cover the filter layer over —8.5 m and flatten out over the last
3 m. The height of the scour protection is about 1.2 m.
2.1.2. Sampling strategies
Adult cod were sampled during summer and winter 2019 and 2020 in-
side the OWF (Fig. 1, Table 1) using hook and line fishing (referred to as
‘angling”) up to a maximum distance of 10 m around the piles in June
2019 (OWF summer) and January 2020 (OWF winter). Angling has been
;hown to be a suitable method to investigate local abundance of fish
‘Haggarty and King, 2006). We fished from crew transfer vessels close to
‘he turbines using metal lures of 200 g weight and a method called “jigging”
during daytime. Samples were collected by two experienced anglers and
‘he duration of angling recorded to calculate catch rates in the number of
cod caught per angler and hour (Table 1) (Haggarty and King, 2006;
zeubens et al., 2013b). When catch rates around one turbine declined the
site was left and a new one approached.
To compare information from samples taken inside the OWF against ref-
erence data from the surrounding area, we collected samples from an area
we refer to as “German Bight (GB)” during summer (GB summer) and win-
;er (GB winter, Fig. 1 and Table 1). Those samples were taken with bottom
rawls i) during the annually conducted and standardized scientific ‘Inter-
ıational Bottom Trawl Survey (IBTS)’ and ii) the ‘German Small-scale Bot-
:om Trawl Survey (GSBTS)’ in summer (Fig. 1 and Table 1). The IBTS
»rovides annual abundance indices for a variety of commercially important
Vorth Sea fish species for stock assessment. At the IBTS the standard bottom
rawl GOV (Chalut au Grande Ouverture Verticale) is towed for 30 min at a
arget speed of 4 knots. Starting time is defined as the moment when full
jottom contact of the ground gear is reached at a predefined warp length.
Shooting positions and tow directions are randomly chosen within a re-
;tricted area at predefined clear-tow-positions. For each haul, speed and
;hooting and hauling positions are recorded by satellite navigator (GPS).
The catch is then sorted and weighed, counted and measured by species
‚Adlerstein and Ehrich, 2002). The German GSBTS provides complemen-
:ary investigations to the IBTS with the goal of providing highly resolved in-
ormation on the bottom fish fauna of the North Sea, using comparable
nethodology. At the GSBTS the standard gear is a cod hopper deployed
.n 12 selected survey areas (10 x 10 nm; “Boxes”), which are distributed
over the entire North Sea. Within each box, a high sampling frequency of
15 to 21 stations in two to three days is conducted. Compared to the two
BTS hauls per ICES rectangle (30 x 30 nautical miles), around 20-30
aauls are made in each Box, covering one ninth of an ICES rectangle
‘Ehrich et al., 2007).
In order to investigate if offshore wind farms offer suitable conditions or
even enhance reproductive efforts for cod, we collected further information
on early life stages during winter spawning season. We performed
