Ecology of Biofouling and Impacts on Mussel Aquaculture: A Case Study with Didemnum Vexillum

Over the past decade, several species of non-indigenous ascidian have had adverse effects in the marine environment and on associated industries. The colonial ascidian Didemnum vexillum is a recent successful invader in temperate marine communities worldwide, proving problematic to mussel aquaculture in New Zealand. At the inception of this thesis, control strategies to manage the threat from Didemnum to mussel aquaculture were implemented in the absence of information on the biological processes underpinning the species’ invasion success. Background information on Didemnum presented in Chapter 2 recognises this paucity of information on several key biological attributes as well as negative impacts of this species. The ability to obtain larvae and culture colonies in the laboratory was a crucial first step. Thus, Chapter 3 presents laboratory experiments that describe the first successful methods to induce spawning in adult Didemnum colonies, as well as techniques for the successful settlement and metamorphosis of the larvae produced, and for laboratory culture of juveniles. Chapters 4 to 6 address key aspects of the biological characteristics of Didemnum that relate to its invasiveness and spread. The recruitment and reproductive development of Didemnum were assessed over a 20-month period at two locations in central New Zealand. Results indicated that the reproductive season for Didemnum in New Zealand (at least 9 months) is considerably longer than previously believed, with recruitment patterns strongly correlated with seasonal water temperature fluctuations at each location. Secondly, the natural dispersal ability of Didemnum was assessed using a weight-of-evidence approach that combined laboratory and field studies. Larval competency trials revealed that > 70 % of larvae were able to settle and undergo metamorphosis following an artificial settlement delay of 2 hours. Larval viability decreased with increasing delay duration; however 10 % of larvae remained viable following a 36 hour delay. These findings were supported by a field-based study documenting larval recruitment at distances up to 250 m from source populations. Exponential decay models indicated that, given favourable conditions, larval dispersal distances greater than 1 km were theoretically possible, which is a much greater distance than previously assumed. Lastly, the impacts of Didemnum on cultured New Zealand green-lipped mussels (Perna canaliculus) were investigated. At the level of invasiveness experienced in a field experiment, only small mussel size classes were vulnerable to direct Didemnum impacts, with negative effects restricted to fouling-related displacement of mussels as opposed to reduced size or condition. However, at the greater levels of invasiveness evident at other places and times, Didemnum impacts have the potential to be considerably larger. As such, the ability to predict invasiveness, and hence impacts, is critical for stakeholders. However, for reasons discussed in the thesis, making reliable specific predictions of invasiveness is difficult. Despite such limitations, it is clear that an understanding of a species’ basic biological attributes can still greatly assist with management decisions, as highlighted throughout the chapters in this thesis. My research findings have led to a better awareness of commercial impacts and potential spread of this species. Simultaneously, my research also highlights the limitations inherent in inferring invasiveness from other situations (e.g. places, times, and related species); there is a need to specifically evaluate a species’ biological attributes and invasive behaviour when introduced into a novel environment.