36
J. Haure et al.
Discussion
To ensure a high quality seawater supply for land-based systems designed for
shellfish safe storage or detoxification, a primary goal was to remove any toxic
cells from the seawater supply. It was shown that micro-filtration membranes were
more efficient than sand filters for removing micro-algae. Both membrane and sand
filter units were supplied with seawater containing toxic micro-algae at a flow rate
of 2 m
3 /h. An economic evaluation showed that for a ban on shellfish harvesting
lasting 21 days, the annual running costs would be approximately equal to 1,470 or
6,100 euros year
1 for the sand filter and the membrane process, respectively.
Another objective of this work was to define industrial processes that would
enable the storage of market-sized shellfish in good condition, to ensure high
survival rates and maintain the initial flesh quality. It appeared that oysters could
be stored dry (Buzin et al. 2011) or wet for 3 weeks, before being sold, regardless
of the process used.
As observed in other studies (Chen and Chou 2001; Lassus et al. 1999,
2005), the detoxification trials performed here showed that, with active feeding of
contaminated shellfish with non-toxic micro-algae, a reduction in the time required
to complete detoxification to a level below the safety threshold can be achieved.
With this simple process, a 60 % drop in PSP toxin content can easily be obtained
within 5 days in oysters. However, as PSP toxic events have become rare in France
in the last 10 years, there is less need to research this type of toxin today. Likewise,
a 90 % drop in DSP toxin content can be obtained by feeding non-toxic microalgae to mussels within 3 weeks, but this result is hardly better than that observed
under natural conditions. Thus in this case, the detoxification treatment is not cost
effective for the producer. Finally, when using oxidizing agents, such as H 2 O 2 , for
phycotoxin removal, it gave encouraging results for DSP toxins, provided that initial
toxin concentrations were not too high. A future challenge will be to understand how
lipophilic toxins are metabolised in mussels.
Preventative shellfish safe storage must include adaptation of existing equipment to reduce investment costs and future solutions must overcome the spatial
constraints often faced by producers. The main limiting factor is the low capital
investment in the industry, while energy costs represent much of the operating costs.
A simple model working in Excel software was developed and successfully used to
simulate the economic impact of 113 days of harvesting closure on cash flows for
three different groups of producers (which were defined in a survey of Bourgneuf
Bay farmers). For some oyster farmers, pooling filtration equipment would offer a
real advantage by sharing some of the financial losses caused by market closures.
The loss assessment is a short term and simplified one: it does not take into account
various factors such as shellfish mortality, price variations or wage savings due to
changes in work organisation during a market closure. In order to produce a more
generalised economic model it could be improved by integrating these additional
factors, as well as estimates of the closure frequency. This study provided the
opportunity to test various mitigating measures, and apply cost benefit analyses in a
case-study of the French shellfish industry.
J. Haure et al.
Discussion
To ensure a high quality seawater supply for land-based systems designed for
shellfish safe storage or detoxification, a primary goal was to remove any toxic
cells from the seawater supply. It was shown that micro-filtration membranes were
more efficient than sand filters for removing micro-algae. Both membrane and sand
filter units were supplied with seawater containing toxic micro-algae at a flow rate
of 2 m
3 /h. An economic evaluation showed that for a ban on shellfish harvesting
lasting 21 days, the annual running costs would be approximately equal to 1,470 or
6,100 euros year
1 for the sand filter and the membrane process, respectively.
Another objective of this work was to define industrial processes that would
enable the storage of market-sized shellfish in good condition, to ensure high
survival rates and maintain the initial flesh quality. It appeared that oysters could
be stored dry (Buzin et al. 2011) or wet for 3 weeks, before being sold, regardless
of the process used.
As observed in other studies (Chen and Chou 2001; Lassus et al. 1999,
2005), the detoxification trials performed here showed that, with active feeding of
contaminated shellfish with non-toxic micro-algae, a reduction in the time required
to complete detoxification to a level below the safety threshold can be achieved.
With this simple process, a 60 % drop in PSP toxin content can easily be obtained
within 5 days in oysters. However, as PSP toxic events have become rare in France
in the last 10 years, there is less need to research this type of toxin today. Likewise,
a 90 % drop in DSP toxin content can be obtained by feeding non-toxic microalgae to mussels within 3 weeks, but this result is hardly better than that observed
under natural conditions. Thus in this case, the detoxification treatment is not cost
effective for the producer. Finally, when using oxidizing agents, such as H 2 O 2 , for
phycotoxin removal, it gave encouraging results for DSP toxins, provided that initial
toxin concentrations were not too high. A future challenge will be to understand how
lipophilic toxins are metabolised in mussels.
Preventative shellfish safe storage must include adaptation of existing equipment to reduce investment costs and future solutions must overcome the spatial
constraints often faced by producers. The main limiting factor is the low capital
investment in the industry, while energy costs represent much of the operating costs.
A simple model working in Excel software was developed and successfully used to
simulate the economic impact of 113 days of harvesting closure on cash flows for
three different groups of producers (which were defined in a survey of Bourgneuf
Bay farmers). For some oyster farmers, pooling filtration equipment would offer a
real advantage by sharing some of the financial losses caused by market closures.
The loss assessment is a short term and simplified one: it does not take into account
various factors such as shellfish mortality, price variations or wage savings due to
changes in work organisation during a market closure. In order to produce a more
generalised economic model it could be improved by integrating these additional
factors, as well as estimates of the closure frequency. This study provided the
opportunity to test various mitigating measures, and apply cost benefit analyses in a
case-study of the French shellfish industry.