For eight or 10 months, there is indeed a turnover of foreign seasonal workers. They mainly come to work in the agricultural sector. You're right. However, and this is also true for old stock Quebec workers, regional agreements are reached with processing businesses which must process their food products during the rest of the year. There is often a labour shortage in this sector in the regions.
Temporary seasonal agricultural workers could very well stay in the country as permanent residents because, during the rest of the year, they could meet labour shortages in the processing industry. I know because I've seen that commonly happens, particularly in Montérégie. The argument concerning harvest and planting times and so on should be qualified. Once again, it's not certain that this labour force couldn't have work throughout the year.