Optimizing cultured mussel yields: second-set dynamics and avoidance strategies

Macneill, Patrick Sean (2005) Optimizing cultured mussel yields: second-set dynamics and avoidance strategies. Masters thesis, Memorial University of Newfoundland.

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Abstract

A major production constraint for some mussel farms in Canada and elsewhere is related to 'second-set'- an accumulation of unwanted mussel seed on mussel socks. Accumulated seed may originate from primary settlement (annual settlement of mussel larvae) or secondary settlement (post-settled spat that drift) and may be severe enough to decrease growth of production mussels, reduce harvest yields, and increase production costs (extra flotation, transportation and processing costs). The objectives of the present study were to identify the biotic and abiotic factors involved in second-set dynamics through environmental and biological monitoring, as well as current husbandry observations. A multifactorial field experiment was undertaken to examine the temporal (monthly) and spatial patterns (2 sites; 3m, 6 m, 9 m depth) of larval and post-larval mussel settlement at two commercial mussel farms in Newfoundland in an attempt to understand second-set dynamics. Laboratory trials investigated mussel seed crawling behaviour under varying environmental conditions (food, temperature) with two seed sizes (5-10 mm and 15-20 mm) to explore a possible relationship with second-set accumulation. Finally, the influence of initial socking density (approximately 100, 200, 250, and 300+ mussels per 30 cm), sock deployment depth ( 4 m and 9 m), time of deployment (spring and autumn) and husbandry practices on the timing and intensity of second-set was examined. -- Results indicated that environmental conditions influenced mussel spawning times, with seed collection heaviest during August. The seasonal thermocline may have led to heavy seed collection at a depth of 9 m, however, growth of seed was less than at 3 m or 6 m. There was evidence of secondary settlement of post-settled spat (byssal drifting) which may be a source of second-set accumulation. Crawling behaviour of seed mussels was influenced by temperature and seed size, with implications for optimal socking strategies. Second-set accumulation was significantly reduced with higher initial socking densities and with depth of deployment. Fouling was heavier on low density socks. Spring deployments showed the highest sock yields and least amount of second-set accumulation after one year. Socks deployed at 9 m yielded less marketable product per 30 cm of socking than socks deployed at 4 m, yet respectable yields of 70% of gross were attained at 9 m after 1.5 years deployed. Observations of present culture practices indicated a lack of understanding of the impact of environmental conditions and seed handling practices on sock quality. Poorly formed mussel socks had high accumulations of second-set. It is recommended that careful consideration be given to site conditions and mussel seed handling practices when socking. To avoid second-set, it is concluded that high sock quality (fullness, uniformity) be obtained, with consideration of environmental influences on mussel seed quality. For the present study, densities of250+ per 30 cm of socking at 25-27 mm shell length socked in the spring, deployed in deeper water, at or below the seasonal thermocline, worked well in reducing unwanted accumulations of second-set mussels.

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