Hi
Collecting mussels from the shore for a feed is not something I would advise unless you really know what your doing or really know the area. I am not trying to scare here but Shellfish even when cooked can cause serious human health issues.
There are two elements to how shellfish can make you sick.
1. Eating shellfish from where there is human or agricultural sewage. There is a classification of shellfish waters in Ireland where if the water has been assessed over time and is monitored for what they call coli-forms (bacteria from a mammal gut). Shellfish from Category A waters can be eaten with no issues. Category B the shellfish need to be allowed to clean out over a period of time, usually about 24-36 hours, in clean water usually in a closed system called depuration. Category C is basically a no for human consumption.
http://www.sfpa.ie/SeafoodSafety/Shellf ... Areas.aspx2. The second and most important factor to take into account are biotoxins. These are microscopic plants that are eaten by the shellfish and contain something that we are severely allergic to. These biotoxins can make you very ill and can occur at different times of the year. In Ireland we have one of the safest early warning system in the world for making sure that no mussels of oysters can be sold on the market without first having the all clear on biotoxins. Weekly water samples are taken by shellfish farmers and the SFPA take samples of the shellfish, for testing, before the shellfish can be released to market. Only accredited labs can test and in Ireland this is done by the Marine Institute.Shellfish contaminated with Biotoxin cannot be depurated or cleaned. Only time will clear them and this can take months.
http://www.marine.ie/Home/site-area/dat ... afety-dataSo in a nut shell. Know your water classification for the area your taking the mussels from and check the situation with the biotoxins for the area as well using the websites attached.
Hope this helps.
CAZ