Early life history dynamics of the New Zealand whitebait species.
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
In New Zealand, ‘whitebaiting’ describes a nationwide recreational and commercial fishery that is of high cultural significance. The New Zealand whitebait fishery is comprised predominantly of five diadromous galaxiid species, īnanga (Galaxias maculatus), kōaro (G. brevipinnis), banded kōkopu (G. brevipinnis), giant kōkopu (G. argenteus) and shortjaw kōkopu (G. postvectis). In recent years, four out of the five whitebait species have been listed as ‘declining’ or ‘threatened’ in large part due to increased stressors such as habitat degradation. The whitebait fishery is governed by regulations that were set 20 years ago and management is separated into two spatial units: the West Coast of the South Island and the rest of New Zealand.
With the increasing recognition that restricted connectivity is driving local adaptation in a large number of marine species, this study investigated possible local adaptations in early life history traits of the five whitebait species. To provide insight into what is occurring at sea, the age at migration and daily pelagic growth rate of post-larvae were compared across broad spatial and temporal scales. Whitebait were sampled from 26 rivers within nine regions of New Zealand during September-December 2015. From these samples, a total of 1567 whitebait were aged by counting daily rings in their otoliths: īnanga (n=794), kōaro (n=296), banded kōkopu (n=446), giant kōkopu (n=30) and shortjaw kōkopu (n=2).
Strong spatial trends in age at migration and daily pelagic growth were seen in four whitebait species (īnanga, kōaro, banded kōkopu and giant kōkopu). At lower latitudes of New Zealand (North Island) whitebait entering rivers were on average younger with higher levels of average daily pelagic growth. At higher latitudes (South Island) whitebait were older with lower growth rates. Additionally, whitebait on the east coast of the South Island tended to be older with lower growth rates than those on the west coast at similar latitudes.
Age at migration decreased and average daily pelagic growth increased between months for īnanga (September- November) and banded kōkopu whitebait (September- December) in most regions. However, in other regions the age of whitebait increased during the settlement with concomitant reductions in growth or did not vary significantly.
Back-calculated hatch dates of īnanga and kōaro whitebait had broad distributions from May until August. Banded kōkopu and giant kōkopu whitebait hatch dates had a narrower, unimodal distribution and the majority of banded kōkopu hatched during June and July. Giant kōkopu whitebait hatched later, predominately during July and August.
This study is the first to make broad-scale spatial comparisons of age and growth of the five whitebait species. From the findings it is apparent that the New Zealand whitebait fishery is not comprised of a single meta-population and that spatial differences in oceanic conditions drive local adaptation and variability in early life history traits.