Effects of an Offshore Wind Farm (OWF) on the Common Shore Crab Carcinus maenas: Tagging Pilot Experiments in the Lillgrund Offshore Wind Farm (Sweden)

Abstract

Worldwide growth of offshore renewable energy production will provide marine organisms with new hard substrate for colonization in terms of artificial reefs. The artificial reef effect is important when planning offshore installations since it can create habitat enhancement. Wind power is the most advanced technology within offshore renewable energy sources and there is an urgent need to study its impacts on the marine environment. To test the hypothesis that offshore wind power increases the abundance of reef species relative to a reference area, we conduct an experiment on the model species common shore crab (Carcinus maenas).Overall, 3962 crabs were captured, observed, marked and released in 2011 and 1995 crabs in 2012. Additionally, carapace size, sex distribution, color morphs and body condition was recorded from captured crabs. We observed very low recapture rates at all sites during both years which made evaluating differences in population sizes very difficult. However, we were able to estimate population densities from the capture record for all three sites. There was no obvious artificial reef effect in the Lillgrund wind farm, but a spill-over effect to nearby habitats cannot be excluded. We could not find any effect of the wind farm on either, morphs, sex distribution or condition of the common shore crab. Our study found no evidence that Lillgrund wind farm has a negative effect on populations of the common shore crab. This study provides the first quantitative and experimental data on the common shore crab in relation to offshore wind farms.

Fig 1

Photographic illustration of the gravity based foundations in the Lillgrund offshore wind farm: A. Six-sided bottom slab of the foundations; the six pockets and circular section in the shaft are filled with ballast. B. Transportation of the gravity based foundations for deployment in Lillgrund. With kind support from RSB Formwork Technology.

Fig 2. Location of Lillgrund OWF and the two control sites, Sjollen in the North and Bredgrund in the South.

Fig 3. Calculated mean± SE in catch per unit effort (CPUE) of the common shore crab, Carcinus maenas in Lillgrund OWF and the two control sites Bredgrund and Sjollen in 2011 and 2012.

Letters above bars indicate significantly different means based on Tukey`s post-hoc comparisons.

Fig 4

Calculated mean± SE in carapace size of the common shore crab, Carcinus maenas in A. Lillgrund OWF and the two control sites Bredgrund and Sjollen in 2011 and 2012, and B. in the two morphs, red and green in Lillgrund OWF and the two control sites Bredgrund and Sjollen. Letters above bars indicate significantly different means based on Tukey`s post-hoc comparisons.

Fig 5. Size frequency distribution of CW in each site (Lillgrund OWF, Bredgrund and Sjollen) sampled in summer 2011 and 2012.

N, number of measured individuals.

Fig 6

Proportion estimates from A. model 1. Coloration (red and green) of shore crabs in Lillgrund OWF and the two control sites Sjollen and Bredgrund in the years 2011 and 2012 and, B. from model 2. Body condition (good or reduced) of shore crabs in Lillgrund OWF and the two control sites Sjollen and Bredgrund in the years 2011 and 2012.

Fig 7. Mean catch per unit effort in Lillgrund OWF, Sjollen and Bredgrund during the years of construction and during operation of the wind farm.

Catch per unit effort for all years from 2002 and 2010 are calculated from data from Swedish Agency for Marine and Water Management [24], data from 2011 and 2012 are added from our results.