Demography and life history characteristics of the New Zealand geoduck, Panopea zelandica

Abstract

This thesis provides information on the demography, population dynamics and life history characteristics of the New Zealand geoduck, Panopea zelandica (Quoy and Gaimard, 1835), from Wellington Harbour and Kennedy Bay on the Coromandel Peninisula. The distribution, abundance and habitat characteristics were determined for both populations and estimates of age, growth and mortality were also investigated. Reproduction, sexual development, fertilization kinetics and larval development of P. zelandica are also described. These studies provide a preliminary assessment of the effects that harvesting may have on the sustainability of populations of P. zelandica, and the potential for developing aquaculture industries for this species. Geoducks in Kennedy Bay were found from 4-8 m water depth, and from 4-16 m in several beds in Wellington Harbour. In Wellington Harbour, there was a pattern of increasing numbers with depth up to~15 m (Fig. 2.2). Numbers decreased thereafter. Analysis of sediment samples indicated that P. zelandica was more prevalent in fine sand to fine silty sand substrata, with reduced numbers in siltier sediments. The water depths and sediment characteristics presented for the populations considered appear to be general habitat characteristics for this species. This will aid in finding further populations and in establishing ongrowing methods for this species. Experiments indicated that counts of siphon holes could be quickly and reliably used to provide estimates of abundance. Results also suggest that a show-factor of ~0.90 may be applicable to abundance estimates of other P. zelandica populations, and can be used year round. Estimates of density for P. zelandica in several bays in Wellington Harbour (0.14 to 0.49 geoducks.m-2) and Kennedy Bay (0.058 geoduck.m-2) were very low. P. zelandica is also very long lived (up to 86 years) and populations had low estimates of natural mortality (<0.14). These factors raise serious concerns about the sustainability of harvesting populations of P. zelandica and that harvest yields may be only a fraction of estimated biomass. Also, significant differences in the growth rates between Kennedy Bay and Shelly Bay with respect to shell length (p<0.001) and whole wet weight (p=0.001), and differences in estimates of natural mortality between sites suggest that stocks will have to be managed separately. Investigation of the reproductive cycle of P. zelandica over a 22 month period indicated a difference in the timing of spawning between Kennedy Bay (spring) and Shelly Bay (late summer/early autumn). However, both populations spawned when water temperatures reached ~l5˚C. Monthly mean oocyte values and Gonado-Somatic Indices (GSI's), such as gonad weight versus whole wet weight, wet shell weight, dry somatic tissue weight and wet somatic tissue weight closely followed the patterns evident in the histological analysis of the reproductive cycle suggesting that GSI's may be of use to marine farmers wanting a quick and easy method for assessing the reproductive state of potential broodstock. Patterns of sexual maturity were equivalent for both populations with 50% maturity calculated to be at ~55 mm and ~57 mm at Shelly Bay and Kennedy Bay, respectively. Samples of geoducks collected from June 1999 to March 2001 determined that reproductive development was protandric, as all immature geoducks from a single cohort collected over the sampling period matured into males in their third year of life. Females became more prevalent as size and age increased. Sex ratios with age were equal in the larger size classes. However, females dominated the larger size classes with respect to size. Thus harvesting may potentially target female geoducks. However, the relationship between siphon hole size and shell length was not strong (r=0.57). In terms of aquaculture, ripe broodstock were easily spawned, and resultant larvae grown through to settlement (which occurred spontaneously after l6 days at a size of ~270 μm) indicating that this species may be amenable to hatchery culture. Optimal fertilisation was achieved at concentrations around 2x102-103 sperm/μl over a range of contact times with the proportion of monospermic zygotes decreasing rapidly at higher sperm concentrations. An increase in gamete age from 0 to 30 mins adversely affected the fertilisation success of P. zelandica. Photomicrographs of developing larvae indicated an absence of fully developed ctendia and siphons in settling larvae, similar to the pattern described for the North American geoduck, Panope abrupta. Measurements of shell morphology were found to be adequate descriptors of larval development. SEM indicated that the provinculum of straight hinge and developing veliger larvae lacked any teeth, although there was some development of a small lamelliform tooth on the hinge structure of newly settled larvae. Prominent spines common on dissoconch shells of other hiatellid clams were absent in newly settled post-larvae of P. zelandica. These descriptions provide vital information for identifying larvae and newly settled recruits in plankton and sediment samples and will aid in determining patterns of recruitment, vital for assessing the sustainability of natural populations. The life-history characteristics and population characteristics described in this thesis for P. zelandica raise serious concerns regarding the sustainability of harvesting this species. However, the ease with which ripe P. zelandica could be spawned, and the resultant larvae grown through to settlement, suggests that the aquaculture of this species warrants further investigation.