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Development of control strategies for the spread of Styela clava in PEI; an epidemiologic study of processing plants



The clubbed tunicate, Styela clava, a recent invader to Prince Edward Island waters was first identified in 1998 on cultured mussels (Mytilus edulis). S. clava has had a devastating effect on mussel culture in Murray River and Brudenell River, attaching in high densities to mussel socks and equipment, competing for food resources and fouling equipment. Mussel productivity has been adversely affected by this infestation and it is posing challenges for farm husbandry due to the sheer weight of the tunicates. This pest was recently found in Malpeque Bay and Cardigan River and could have a devastating effect on the sustainability of the entire PEI mussel industry, if it establishes in these or other mussel growing areas with similar impacts. Heavy infestation has resulted in more equipment (i.e. buoys, etc) being needed for mussel production, increased labour costs due to injuries and a larger workforce required for the upkeep of farms, and processing costs due to the necessity for physical removal and prevention of spread to unaffected areas. In some cases, these mussel socks have less mussel biomass, usually caused by mussel falloff and/or mortalities due to the weight of tunicates. Since the introduction of S. clava, 3 more exotic species of tunicates have been identified on cultured mussels in Prince Edward Island waters.

Several preventative measures have been initiated to reduce further spread. These measures include restrictions on the movement of shellfish from tunicate infested areas which has resulted in additional economical stress to mussel growers from infested areas. Most of these restrictions are based on concrete scientific evidence, but some have been put into place as preventative measures without scientific confirmation. In these cases, growers are concerned that there is no evidence that the present restrictions will prevent the further spread of S. clava and that the related financial impacts are not justified.

One of the major concerns of the mussel industry is the spread of the tunicate from post harvesting activities, mainly associated to the processing infested socks in non infested areas. Restrictions placed on processing plants have been put in place to practically eliminate any possibility of introducing S. clava to non-infested areas, although the risk of introduction through processing activities has never been thoroughly evaluated. There is an urgent need to understand the risk of dispersal of aquatic invasive species (AIS) through processing facilities to avoid any further spread or undue restrictions leading to economical impacts. Processing plants are considered to be an important vector of AIS. They are essential components of the fishing and aquaculture industry. The main risk attributed to their activities is the importation of product (including hitchhikers) to the facility. Dispersal of established AIS can also be accelerated through processing activities. However, the levels of propagation pressure generated by processing plants have not been well documented. It is conceivable that the propagation pressure varies considerably in relation to environmental, husbandry and infestation characteristics, within these plants.

The first objective of this study will be to assess the risk of spread of S. clava associated to husbandry practices and environmental conditions within processing plants. This includes identifying processing stages that present a high risk of introducing S. clava into a new environment. Also the path and condition of S. clava within the plants will be evaluated and documented. Some of the processing practices have the potential to crush or tear the tunicates. Tearing of gonads would be sufficient to release gametes within the processing plant and potentially in receiving waters of the effluent. Release of early life stages (eggs and larvae) will be evaluated for each of the processing stages identified. Environmental parameters related to each processing stage will also be quantified. The main parameters that will be evaluated include temperature, salinity, photoperiod, oxygen, ammonia, pH, and turbidity.

The second objective is to develop control strategies to minimize the potential for gamete release. Laboratory trials will evaluate the impact of water parameters (e.g. temperature, salinity, etc.) found in the processing plants on the viability of the early life stages. This will be evaluated through laboratory simulations of processing plant conditions at various key stages. The effect of stressors and photoperiod (within plants) on the spawning of individuals will also be evaluated. Finally, laboratory findings will be evaluated in a processing situation to validate their effectiveness.

Program Name

Aquaculture Collaborative Research and Development Program (ACRDP)


2005 - 2008


Atlantic: Gulf of St. Lawrence, St. Lawrence Estuary

Principal Investigator(s)

Daniel Bourque

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