Age, growth, and mortality of the New Zealand geoduck clam, Panopea zelandica (Bivalvia: Hiatellidae) in two North Island populations

There is increasing interest in developing fisheries and aquaculture industries for the New Zealand geoduck clam, Panopea zelandica (Quoy and Gaimard, 1835). However, little is known about the age structure, growth rates, and mortality of different populations of P. zelandica. Annual bands in polished shell sections were used to obtain estimates of age, growth rates, and mortality of P. zelandica at two sites in northern New Zealand. Panopea zelandica in Kennedy Bay ranged in age from 2 to 34 yrs whilst those in Shelly Bay ranged from 3 to 85 yrs. There was a significant difference (P < 0.001) in growth rates (shell length-at-age) between the populations (estimated asymptotes were 111.5 mm and 103.6 mm for Kennedy Bay and Shelly Bay, respectively). However, the growth characteristics (i.e., rapid growth for the first 10-12 yrs and minimal thereafter) were similar for both populations. Drained wet weight-at-age followed a similar pattern to shell length-at-age, although growth in terms of weight was rapid until 12-13 yrs of age. The estimated maximum drained wet weight of P. zelandica was higher in Kennedy Bay (275.5 g) than in Shelly Bay (223.1 g). There was also a significant difference (P = 0.02) in the relationship between total shell length and drained wet weight for the two populations. The width between the siphons visible at the sediment surface was a reasonable predictor of the shell length (r² = 0.57) of P. zelandica in Kennedy Bay. Estimates of natural mortality using catch curve analysis, estimates of maximum age, and the Chapman-Robson estimator were very low (0.02-0.12 proportion yr^-1). This study is the first to confirm that the shell bands in P. zelandica are deposited annually and can be successfully used to age this species. Given the low estimated rates of mortality and longevity of P. zelandica, fisheries managers will need to carefully consider the feasibility of commercially harvesting this species.