Restoring soft-sediment mussel beds using transplanted adults

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Chapter 2 Restoring mussel beds onto soft-sediment using transplanted adults

Introduction

Bed-forming marine bivalves, such as mussels and oysters, are an integral component of many coastal ecosystems, altering nutrient and energy dynamics and also providing three dimensional structure which supports abundant and diverse communities of associated organisms (Hall-Spencer & Moore, 2000; Meyer & Townsend, 2000; Coen et al., 2007; Commito et al., 2008; Trigg et al., 2011; McLeod et al., 2014). These habitats provide increased foraging potential for both resident and transient species and provide refuge from predation (Lee & Kneib, 1994; Jiang & Carbines, 2002; Grabowski & Powers, 2004). The bivalves that constitute these beds can reduce the phytoplankton biomass in the water column by as much as 74% and effectively control the phytoplankton community in shallow waters, providing a potential buffer for eutrophication (Officer et al., 1982; Alpine & Cloern, 1992; Norén et al., 1999; Dolmer, 2000). The result of this enormous filtering potential is a transfer of pelagic productivity to the benthos, known as benthic-pelagic coupling (Dame et al., 1991; Loo & Rosenberg, 1996; Norkko et al., 2001). Along with providing three-dimensional structure, it is this exchange of energy from the pelagic to the benthic environment that makes these biogenic habitats such important sources of diversity and productivity in the marine environment.
Despite the importance of these bivalve habitats, anthropogenic disturbance such as decreased water quality and overharvesting have led to the degradation and loss of many bivalve habitats around the world (de Jonge et al., 1993; Rothschild et al., 1994; Service & Magorrian, 1997; Cranfield et al., 1999). The increasing knowledge and awareness of the numerous benefits provided by bivalve beds and the consequences of their loss have spurred recent efforts to restore degraded bivalve habitats. These restoration initiatives aim to establish a persistent population such that there is sufficient recruitment to offset the many sources of mortality to the adult population. Despite the critical importance of assessing the population dynamics in bivalve restoration, the monitoring of recruitment and mortality of restored populations has received relatively little attention (Mann & Powell, 2007). This lack of monitoring makes it difficult to determine the ultimate success of these initiatives or to identify the ecological processes, such as recruitment limitation, which may be impinging on the persistence of the restored population.
The green-lipped mussel, Perna canaliculus, is endemic to New Zealand where it is found throughout the country, often forming extensive beds in shallow coastal waters (Jeffs et al., 1999; Morrison et al., 2010). These mussels once covered more than 1300 km2 of soft-sediment sea floor in the Hauraki Gulf, a large coastal embayment in the northern North Island, (Greenway, 1969; Reid, 1969) before being nearly extirpated by intensive commercial dredge fishing from 1910 – 1969 and subsequent poaching until 1978 (Paul, 2012). Only a few small remnant mussel beds remain totalling around 0.64 km2 (McLeod, 2009).
Experiments into the survival of small quantities of caged adult mussels transplanted into these soft-sediment environments revealed that the lack of population recovery was not due to adverse environmental conditions inhibiting the survival of adults that were experimentally placed on the seabed in cages to protect them from predators (McLeod et al., 2012). However, the survival of these caged mussels does not reflect the potential survival in the presence of predation or incorporate the effects of density-dependant factors that mussels would experience as part of a larger mussel bed. It is unclear whether beds of adult mussels are capable of persisting in the current natural environment of the Hauraki Gulf which has changed markedly over the last 50 years (HGF (Hauraki Gulf Forum), 2014) and what potential factors might inhibit or help to promote the long term persistence of restored mussel beds. Therefore, this study aims to determine if mussel beds can be re-established on the soft-sediment environment in the Hauraki Gulf and to identify any potential limitations to their subsequent persistence. Seven experimental mussel beds were established by transferring large quantities of adult mussels from aquaculture onto soft-sediments at a restoration site and four of these beds were subsequently assessed regularly over two years for changes in population size and the size structure of mussels. The results of this study will help contribute to the development of best practice methods for future mussel restoration initiatives .

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Materials and methods

Site and mussel bed deployment

Cable Bay (S 36º 48′ 32″, E 175º 11′ 37″), off the northern tip of Rotoroa Island, in the Hauraki Gulf was selected as a suitable site for deploying adult mussels (Figure 2.1). This bay has a large expanse of soft-sediment in shallow water (3-12 m below chart datum) with low tidal currents and reasonable water clarity (diver visibility of 1.5 – 3 m). The seabed substrate in the area of deployment of the mussels consisted of a layer of about 5 cm of fine mud overlyinge a more stable mix of fine mud, sand, and shell hash. Preliminary surveys showed no signs of mussels within the study site, and the rocky foreshore was dominated by oysters, with no existing mussel beds present within the surrounding Cable Bay. On 28 November 2013 seven mussel beds were created using adult mussels (70 – 100 mm shell length, SL) from mussel aquaculture (North Island Mussels Ltd) and deployed from the company’s barge. Each mussel bed was formed from approximately 1 t of freshly-harvested adult mussels that were released from the barge at the water’s surface and allowed to sink onto the sea floor. Each mussel bed was defined by a distinct margin between the predominantly contiguous bed and the surrounding benthic environment. Where mussels aggregated into clumps, these were included as part of the bed when clumps comprised five or more mussels. These clumps of mussels were never greater than 0.25 m from one another and no other clumps of mussels were observed to be closer than 3 m from the defined bed margin. The distance between the seven adjacent mussel beds ranged from 9 – 35 m at depths of 3.9 – 5.1 m below chart datum. Due to logistic constraints of deploying SCUBA divers, only the four westernmost mussel beds, hereafter referred to as the experimental mussel beds, were monitored for changes in their population. These beds were labelled I-IV running west to east (Figure 2.1).

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Chapter 1 General Introduction
1.1 Ecosystem services and functions of bivalve mollusc beds
1.2 Loss of bivalve habitat
1.3 Bivalve restoration
1.4 Green-lipped mussels
1.5 Study aims
Chapter 2 Restoring soft-sediment mussel beds using transplanted adults
2.1 Introduction
2.2 Materials and methods
2.3 Results
2.4 Discussion
2.5 Conclusion
Chapter 3 Impacts of sea star predation on restored mussel beds
3.1 Introduction
3.2 Materials and methods
3.3 Results
3.4 Discussion
3.5 Conclusion
Chapter 4 Does the provision of attachment substrate in restoration initiatives enhance persistence of mussels on soft-sediment substrate?
4.1 Introduction
4.2 Materials and methods
4.3 Results
4.4 Discussion
4.5 Conclusion
Chapter 5 Larval settlement within a restored mussel bed site: enhanced settlement in the presence of adult conspecifics
5.1 Introduction
5.2 Materials and methods
5.3 Results
5.4 Discussion
5.5 Conclusion
Chapter 6 General Discussion
6.1 Recruitment pathways in mussels
6.2 Lack of recovery in green-lipped mussels in the Hauraki Gulf
6.3 Best practice methods for restoration of green-lipped mussel beds
6.4 Conclusion
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Population dynamics of restored green-lipped mussel (Perna canaliculus) beds in the Hauraki Gulf, New Zealand

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