Effects of anthropogenic stressors on Helgoland’s lobsters (Homarus gammarus)

Abstract

As meroplankton, lobsters make up a great portion of both benthic communities and planktonic fauna in the water column. Furthermore, they represent a mayor food source across the marine food web and a vital source of protein for humans. As an economically important species, lobsters are highly susceptible to anthropogenic stressors (e.g habitat destruction, over-fishing, noise pollution). Moreover, climate change may magnify the impact of human activities on lobsters’ fitness. The collapse of the population of European lobster (Homarus gammarus) around Helgoland constitutes a good example and prompted a large-scale restocking program. Yet, the question arises if recruitment of remaining natural individuals and program released specimens could be stunted by ongoing climate change and human activities.

In my thesis I investigate the effect of several anthropogenic stressors that could potentially be affecting the route to recovery of Helgoland’s lobsters. Since monitoring of lobster larvae has not been implemented on the island yet, I performed laboratory and field experiments to evaluate the potential of using light-traps on the island of Helgoland to catch lobster larvae. Developing adequate traps is useful to support research on the European lobster in the wild and conservation efforts. The light traps used white LED lights and successfully trapped lobster larvae in laboratory experiments at different densities of 100 and 10 ind/m3 , and in the field experiment, six traps were deployed weekly throughout lobster larvae hatching season in the months of May to August. Traps were deployed in two different rocky seabed sites, that are adequate habitats for larvae, and at different depths (1, 2, 4, and 6 m). However, no lobster larvae were caught in the field. This may be due to several reasons: (1) low population numbers of lobster larvae, (2) a rapid loss of positive phototactic response of larvae to light or (3) the chosen deployment sites were not appropriate. Future research is needed to construct a specialized trap to sample Helgoland’s lobster larvae and provide information on the current larval fitness and population numbers. Nevertheless, the light traps used in my thesis successfully caught a variety of small crustaceans and can be a useful tool to sample small benthic organisms for experimental purposes.

Owing to the difficulties in catching lobster larvae in the field, I used larvae from lobster-rearing facilities to study the effects of anthropogenic stress on larval development and physiology. Studies on the effects of climate change on European lobster larvae have mostly focused on the isolated effect of ocean acidification or warming. Acidification treatments were based on two shared socio-economic pathways emitted by the Intergovernmental Panel on Climate Change (IPCC) regarding the amount of atmospheric CO2 for the end of the century. This study is the first to provide a more complete picture of the thermal limits at different levels of biological organization of lobster larvae under acidification by including a ten-level temperature gradient setup (13-24°C) The results show temperature was positively correlated with growth and energy metabolism; while, pCO2 had a negative impact on survival and morphology. Thus, climate change could potentially stunt the European lobster restocking efforts taking place on the island.

Lastly, I investigated the impact noise pollution may have on juvenile lobster’s behavior and substrate choice. Around the island of Helgoland there are numerous offshore windfarms (OWF) which are a source of constant low-frequency noise. As OWF foundations also have the potential to act as a hard substrate habitat for lobsters, I assessed the separate and joint effect of predator presence and a constant low-frequency noise on young-of-year European lobsters’ behavior and substrate choice. The results suggest that the single and joint effect of added noise and predator presence influenced their initial substrate choice. Furthermore, young-of-year lobsters under the added noise and predator presence treatment spent more time outside their substrate (shelter), exploring, and less time hiding. The latter is ecologically detrimental as animals may expose themselves more to predator in natural conditions. These behavioral responses raise concerns about the effects constant low-frequency noise may have on young-of-year and adult European lobster living among artificial reef structures such as foundations of operational offshore wind farm naturally, or in the frame of restocking, multi-use and temporary no-take zones programs.

This thesis documents a field sampling trial with light traps that, while failing its main aim for lobster larvae, it could be followed up to obtain more abundance and biomass data on epibenthic specimens. Nevertheless, with specimens obtained from rearing-facilities, the thesis provides a comprehensive and contemporary picture of the effects of anthropogenic stressors on the early-life stages of the European lobster larvae, physiologically and behaviorally. Ocean warming and acidification can lead to an increase in mortality and lower rostrum size in lobster larvae, while noise pollution can disturb the behavior of juvenile lobsters, thus affecting their attention and decision-making processes. Helgoland’s lobsters are a vulnerable population that will continue to be affected by offshore human activity and climate change, therefore restocking programs, scientific support and fisheries management should also be a permanent action.