Spread and status of seven submerged pest plants in New Zealand lakes

The distribution of seven submerged aquatic pest plants is reported. Lake vegetation surveys recorded pest plants in 27.9% of 344 lakes, with two species co‐occurring in 5.8%, and three species in 2.6% of lakes. Egeria densa was most frequent (15.4% of lakes), followed by Ceratophyllum demersum (9.0%), Lagarosiphon major (7.3%), and Utricularia gibba (5.5%). Spread since 2000 has continued for five pest plants, with 34 lakes invaded by U. gibba over 2004–08 alone. Early regional sites in proximity to human population centres were likely plant liberations and numerous potential founder colonies remain in garden ponds. Human activities were important for inter‐lake dispersal, with the exception of bird‐dispersed U. gibba. Significant lake associations between pest plants, and with presence of six exotic fish species, suggest common dispersal pathways and similar introduction risks. Therefore, predictions of future spread should be possible based on sources, dispersal pathways, and identifying key risk factors for lakes.     

Nursery grounds of Tarakihi (Teleostei: Cheilodactylidae) around New Zealand

Nursery grounds of the tarakihi Cheilodactylus macropterus (Bloch & Schneider) were discovered in 1963–72 mainly off the south‐western coast of the North Island, in Tasman Bay, along the entire eastern coast of the South Island, and around the Chatham Islands. They occurred at depths of 20–100 m, and mostly between 10 km and 30 km from shore. The tarakihi nurseries had a dense and varied invertebrate bemthic epifauna dominated by sponges and small corals. Few signs of nurseries were found in. other New Zealand waters, in spite of the existence of major spawning grounds there. It is not known with which spawning grounds the various nursery grounds are linked.

The Tasman Bay nursery ground is 18–33 km wide and about 75 km long, with a surface area of about 2000 km². There is a rich demersal fish fauna (37 species were recorded) dominated by tarakihi, red gurnard Chelidonichthys kumu (Lesson & Garnot), snapper Chrysophrys auratus (Bloch & Schneider), and leathei'iacket Novodon convexirostris (Guenther). The young tarakihi occur at depths of 20–45 m and concentrate during winter in the warmer deeper water. They first appear in trawl catches in summer, towards the end of their first year (assumed birthdate is 1 March), and stay on the nursery until the age of 3 y. They occur in the deeper water off the nursery during their fourth and fifth year and depart during their sixth year, possibly for the spawning grounds off the west coast of the South Island. The mean lengths at the ages of 15, 27, and 39 months were 12.6, 18.0, and 24.0 cm respectively in 1970. There was some variation in growth rate within the nursery ground, but the age groups could nevertheless be recognised easily from, length frequency modes. Monthly modal lengths showed that growth slows down in winter. Evidence for fluctuations in year class strength was found; year classes 1966 and 1969 were weak and year class 1968 was strong.

The vulnerability of tarakihi less than 4y old to commercial New Zealand trawl gear is low, and the local fishing fleets do no serious damage to the habitat on the nursery grounds. However, a large part of the tarakihi nursery grounds off the east coast of the South Island lies outside the territorial fishing zone. Intensive fishing on these grounds by large foreign vessels using heavy trawl gear could have an adverse effect on the habitat of the young tarakihi in this region.     

Reconnaissance survey of the mineralogy and geochemistry of some New Zealand lake and nearshore sediments

131 sediment samples representing 4 different environments (lakes, sounds, harbours, and fiords) in New Zealand have been studied and characteristic differences in chemical composition and mineralogy noted. These differences are related principally to the lithology of the sediments and the productivity of the lake waters. This study should provide a baseline for future surveys of lake sediment geochemistry in New Zealand.     

Some effects of eutrophication and the removal of aquatic plants by grass carp (Ctenopharyngodon idella) on rainbow trout (Salmo gairdnerii) in Lake Parkinson,New Zealand

Temporary removal of aquatic plants in Lake Parkinson, a small, eutrophic dune lake, resulted in a number of changes to the population of stocked rainbow trout (Salmo gairdnerii). During each summer the lake stratified and low oxygen levels limited the distribution of trout to shallow (0–4 m) surface waters. In the first summer following weed removal the numbers of black shags (Phallacrocorax carbo) counted at the lake increased, and their predation resulted in a decline in trout density. However, the growth rate and condition of the trout population then exceeded that of trout present before weed removal. During the second summer after weed removal a cladoceran bloom was followed by low phytoplankton levels and high ammonia concentrations. A prolonged calm compounded this situation with the result that oxygen levels in bottom and surface waters dropped below 2 ppm. These low oxygen levels eliminated the trout population, but other fish species present survived. Elimination of aquatic plants affected the population dynamics of other fish species in the lake with potential implications for the trout. The experiment demonstrated the importance of weed beds in maintaining a stable fish community in lakes such as Lake Parkinson.     

Effects of fluctuations in water level and growth of Lagarosiphon major on the aquatic vascular plants in Lake Rotoma, 1973–80

Fluctuations in water level and the growth of the introduced exotic Lagarosiphon major (Ridley) Moss have significantly influenced the submerged vegetation of Lake Rotoma over the period 1973–80. Low lake levels temporarily reduced the proportion of native vascular plants by removing available shallow‐water habitats through erosion, siltation, or desiccation. High lake levels have allowed native vascular plants to re‐establish from seed and rhizomes. Fluctuations in water level appear to have reduced the long‐term replacement of native species by L. major, which has none the less spread progressively around the lake. Water‐level fluctuations enhanced its rate of fragmentation, and thus its dispersal and establishment. The annual increase in the proportion of L. major has been primarily at the expense of the shallow‐water characean algae, but also partly by competitive displacement of native vascular plants. The southwest inlet of Lake Rotoma had an exceptionally high plant density, with up to 3518 g/m² dry weight of L. major (believed to be a world record for submerged plant biomass); this is attributed to local enrichment and protection from wave exposure.     

Life history of Galaxias divergens (Salmonoidea: Galaxiidae)

Galaxias divergent Stokell is a small, bottom‐dwelling fish inhabiting streams in the North Island and the northern part of the South Island. Its ecology is similar in many respects to that of G. vulgaris. The fry develop in the same environment as the adults, unlike the sea‐going young of several other Galaxias spec'es. Spawning occurs in spring and summer, and at least some of the fish mature at the beginning of their second year. The maximum age attained seems to be about 3 years, and the fish can reach a length of about 80 mm in this time. The food consists mostly of small benthic invertebrates taken from both the stream bed and the drift fauna.     

Early oligocene squalodont cetacean from Oamaru,New Zealand

Records of Tertiary fossil cetacea from New Zealand are revised. Probable proto‐squalodontid teeth from the McDonald Limestone Formation (Whaingaroan Stage, Lower Oligocene), Oamaru, are described, and represent the oldest cetacean remains found in New Zealand. This record demonstrates the probable extension of the Squalodontidae into the Lower Oligocene.     

Glauconitic coating on a gastropod,Fusitriton laudanum,from the Chatham rise,New Zealand (note)

Superficial coatings of glauconite grains have been observed on the gastropod mollusc, Fusitriton laudanum, from the Reserve Bank, Chatham Rise. The glauconite predates the shell on which it accretes and is incorporated on to the shell surface by agglutination of sand‐size material by mucus secreted by epizoic anemones.     

New Zealand lakes research, 1967–91

Research on New Zealand lakes over the past 25 years has focused mainly on eutrophication, measures of water quality, patterns, processes and production in plankton communities, and the spread and growth of submerged adventive macrophytes. Compared with most northern temperate lakes many New Zealand lakes show: low levels of inorganic nitrogen (N) and frequent N‐limitation of phyto‐plankton growth; diffuse, rather than point source, nutrient inflows; a great diversity of optical properties; unusual and often variable timing of plankton growth (attributed to the pervasive influence of New Zealand's oceanic climate); low productivity and biomass of zooplankton, which appear to be limited by food rather than predators; and sensitivity to invasion by adventive macrophytes. Notable research develop‐ments include improved methods of measuring nutrients, empirical predictive models that relate indices of eutrophication to nutrient loading, and significant advancements in methods of quantifying and measuring water colour and clarity.     

Recent occurrence of Cylindrospermopsis raciborskii in Waikato lakes of New Zealand

Cylindrospermopsis raciborskii is a toxin‐producing species of cyanobacteria that in autumn 2003 was recorded for the first time in three shallow (max. depth ≤5 m) Waikato lakes and a hydro‐electric dam on the Waikato River, New Zealand. It formed water blooms at densities >100 000 cells/ml in Lakes Waahi and Whangape. Net rates of population growth >0.2 day_–1 were recorded for C. raciborskii in Lakes Ngaroto, Waahi, and Karapiro, based on comparisons of low numbers (detection of <10 cells/ml) from initial samples and its presence at bloom densities (>15 000 cells/ml) in the subsequent sample “x"‐"y” days later. C. raciborskii may be well adapted to rapid proliferation in the Waikato lakes, which are eutrophic to hypertrophic, with high light attenuation, and where nitrogen (N) fixation may provide it with a competitive advantage over non‐nitrogen fixing algae under N‐limited conditions.