Geographic origin and culture method influence the overwinter mortality of juvenile hard clams, Mercenaria mercenaria (L.)

• An overwintering method for M. mercenaria was tested using local and Maine stocks.
• Maine stocks had highest survivorship through the winter months at each study site.
• All mortality coincided with a decline in carbohydrate energy reserves.
• Local stocks survived better in bottom planting vs. new overwintering method.
• Overwintering methods developed in Maine are not advantageous in mid-Atlantic.

Overwinter mortality of juvenile northern quahogs, Mercenaria mercenaria, is a major impediment to the aquaculture industry from the mid-Atlantic US through Atlantic Canada. Mortality occurs during prolonged exposures to low temperatures and/or during the winter–spring transition when water temperatures are rising. Recent efforts to overwinter juveniles of M. mercenaria produced from native Maine (ME, USA) broodstock in eastern ME have found high survival while using a newly adopted methodology (i.e. cages in the water column rather than the standard practice of bottom planting) for overwintering. The current study aimed to determine if the high survivorship was a result of the culture methodology or if the ME genetic stock is better adapted for overwinter survival. Field studies were performed across two consecutive winters in ME, New York (NY) and New Jersey (NJ, USA) to compare survivorship of two size classes of juvenile M. mercenaria using the new methodology to the standard practice of bottom planting, and to compare the survival and physiological condition of the ME stock vs. the NY and NJ stocks across a latitudinal gradient. The ME stock had greater survival than the NY and NJ stock at all sites and in both study years. Both of the ME size classes exhibited lower mortality (< 12%) through the onset of winter and when temperatures were < 5 °C suggesting greater tolerance to low temperatures, while the NY and NJ stocks experienced very high mortality (up to 87%). However, the ME stock did experience significant mortality in the spring when water temperatures were rising in NY and NJ in both years and in ME during the second study year which was abnormally warm. Mortality of all stocks coincided with a decline in carbohydrate reserves. Survival was greater in field plots as compared to the overwintering cages in NY and NJ but no significant difference was observed in ME. The results demonstrate that alternate methodologies used to successfully overwinter juvenile M. mercenaria in eastern Maine cannot be applied effectively to more southern locations. However, the results suggest that selective breeding programs may be able to develop aquaculture stocks that are better adapted to overwintering conditions.