Protandry, maturation, and spawning in cultured intertidal geoduck(Panopea abrupta) clams

摘要

Among the challenges facing aquaculture of endemic species are potential negative effects of gene flow from cultured to wild populations. Solutions include maturation control, monosex out-plants, spatial or temporal separation, and harvest before sexual maturation. To assess whether cultured intertidal geoduck clams mature and spawn before harvest, we sampled 12 geoduck ten times over the course of one year (2004-2005) from each of five year classes (1999--2003) planted conterminously in south Puget Sound, Washington. Additional sampling occurred through a second spawning season from the 2001, 2002, and 2003 year classes. Individuals were sexed, weighed, and measured; histological sections of gonad were classified by developmental stage (early active, late active, ripe, partially spawned, spent). Using image analysis, we calculated a gonadal-somatic index for each individual. Of geoducks age 1.5-2.2 years, approximately 23% exhibited evidence of spawning and 71% were mature, compared to 68% and 89% respectively in geoducks age 4.5-5.0 years. We estimate that 50% maturation occurs at 64 mm shell length at this site. Male:female ratios in two to five year old geoducks were male-biased relative to the predominate 1:1 sex ratios observed in wild populations (p < < 0.05), providing evidence of facultative protandric dieocy. Maturation stage in cultured geoducks was temporally correlated with maturation stage in wild populations, indicating no temporal separation. Taken together, these data suggest that (1) pre-maturation harvest to prevent gene flow may be untenable and (2) future efforts be directed toward exploring the viability of maturation control in this species.