Eric Graynoth

Macroinvertebrate distribution in the littoral zone of Lake Coleridge, South Island, New Zealand—effects of habitat stability, wind exposure, and macrophytes

Abstract The macroinvertebrate community was surveyed at eight littoral zone sites in Lake Coleridge, New Zealand. Sites covered a range of conditions including exposure to prevailing winds, shore‐line topography, and proximity to river inputs—which are a major source of suspended sediment to the lake. The littoral zone contained an abundant and diverse macroinvertebrate fauna (47 taxa). Three communities were identified: a shallow‐water community inhabiting the wave‐break zone, dominated by Trichoptera and chironomid larvae; a middle‐zone community associated with macrophyte beds, with high macroinvertebrate abundance and dominated by gastropods and oligochaetes; and a community in fine sediments below the macrophytes dominated by oligochaetes. The depth distribution of these communities was related to substrate, macrophyte abundance, and shore‐line profile. Superimposed on this pattern were the effects of hillside slope and exposure to prevailing north‐west winds. Greatest abundance was associated with ...

Improved otolith preparation, ageing and back-calculation techniques for New Zealand freshwater eels

Abstract New Zealand freshwater eels are long-lived and their otoliths are difficult to age using conventional “burn-and-break” techniques. This study developed a number of new and improved techniques which increased the accuracy of age estimates and enabled growth rates to be back-calculated. The “burn-and-break” technique was significantly improved by sawing otoliths through the nucleus before burning. Criteria were developed to distinguish annuli from summer growth bands and supernumerary checks on the basis of colour, width, structure, continuity, and spacing. Annuli and supernumerary checks were most easily identified by examining thin sections stained in an acidic toluidine blue solution. Back-calculation techniques were also improved by using a curvilinear model, originating from a common biological origin, to describe the relationship between fish length and otolith radius. Annulus radii were measured accurately from the flat surface of sawn, burnt, half otoliths and back-calculated lengths were validated by comparison with the lengths of aged wild fish. Using the improved techniques developed in this study it may now be possible to determine factors influencing annual variations in growth and to study inter-relationships between sex and growth rates.

Influence of fluctuating lake levels and water clarity on trout populations in littoral zones of New Zealand alpine lakes

Abstract Brown trout (Salmo trutta Linnaeus 1758) and rainbow trout (Oncorhynchus mykiss Richardson 1836) populations in littoral zones of eight South Island, New Zealand alpine lakes were compared using gill and seine net sampling during summer. Lakes were selected to provide a matrix of lake level and water clarity variations and to assess how these variables influenced trout abundance (as reflected by catch rate), depth distribution, and size. Brown trout were small and in poor condition in three turbid lakes with shallow littoral zones and were scarce in one of these with a 14 m fluctuation. Although brown trout condition was generally higher in clear lakes, Lake Wanaka fish were an exception. Trout depth distribution was positively related to depth of the littoral zone. Brown trout were caught at most depths whereas rainbow trout showed variable depth preferences in different lakes. Rainbow trout catch rate, weight, and condition factor were unrelated to differences in depth of the littoral zone or lake level fluctuations, and it was concluded that spawning success and limnetic food supplies may be more important for this species. Spawning interactions may account for the different species composition between the stable clear Lake Wanaka, and the adjacent fluctuating clear Lake Hawea. Moderate and slow lake level fluctuations appear to have limited effects on trout in clear lakes, probably because trout are able to use deep littoral habitat and food supplies.

Recruitment and distribution of juvenile salmonids in Lake Coleridge, New Zealand

Abstract Rainbow trout (Oncorhynchus mykiss Richardson) and chinook salmon (O. tshawytscha Walbaum) fry and 0+ fingerlings entered Lake Coleridge, a deep, oligotrophic lake in the Southern Alps of New Zealand, in spring and early summer and dispersed throughout the shallow littoral. In contrast, most juvenile brown trout (Salmo trutta Linnaeus) remained in the tributaries for 1–2 years before migrating down stream to the lake. Juvenile salmonids ( 150 mm) and adult salmon and rainbow trout (>210 mm). The abundance of 0+ rainbow trout in the littoral zone declined rapidly from February to August and stocks were supplemented by a migration of yearling fish from the tributaries in spring. The shallow littoral zone and tributaries are both important for juvenile trout rearing in this lake.

Abundance and daytime vertical distribution of planktonic fish larvae in an oligotrophic South Island lake

The vertical distribution of common bully (Gobiomorphus cotidianus) and koaro (Galaxias brevipinnis) larvae in the limnetic zone of Lake Coleridge were determined using a high-frequency (200 KHz) echosounder. Planktonic bully larvae first appeared in appreciable numbers in January. By February, they formed a scattering layer between depths of 12 to 24 m during the day, where they achieved a maximum density of 0.59 fish m-3. Larger (> 18mm) fish migrated to the littoral zone and densities declined to < 0.01 fish m-3 by July, when remaining larval fish occupied greater daytime depths. Their vertical distribution during the day appeared to be influenced mainly by light levels and water temperatures. Larvae grew more slowly (0.12 mm d-1) than in more productive North Island lakes, and were also present in lower densities for a more restricted period of time. Koaro larvae first appeared in November and December and were found in low numbers (< 0.01 fish m-3) in summer at depths of 10 to 26 m. Salmonid production in the limnetic zone is probably limited by the small size and relative scarcity of forage fish present.

Growth of landlocked sockeye salmon (Oncorhynchus nerka) in New Zealand

Abstract The growth of landlocked sockeye salmon in the Waitaki lakes, New Zealand, was studied from 1975 to 1984. Age and growth were determined by length frequency analysis and by back calculation of growth from annual rings laid down on otoliths and scales. Growth was also determined for fish reared at three hatcheries. Salmon resident in Lake Ohau, a large deep oligotrophic lake, grew relatively slowly. Mature fish from two to five years of age averaged 182, 224, 263, and 329 mm in length, respectively. Growth was more rapid in Lakes Benmore, Aviemore, and Waitaki, which are relatively shallow and productive hydro‐electric storage reservoirs. In Lake Benmore, mature fish from two to five years of age averaged 243, 288, 352, and 450 mm in length, respectively. Fish reared in hatcheries grew faster than in the wild with some averaging 382 mm at two years of age. Food supply was considered to be the principal factor influencing growth rates. The fish originated from anadromous ancestors in the Frazer Riv...

Prey selection and dietary overlap of shortfinned (Anguilla australis) and longfinned (A. dieffenbachii) eels during summer in the Horokiwi Stream, New Zealand

Abstract Benthic macroinvertebrates were collected and the diets of shortfinned eels (Anguilla australis) and longfinned eels (Anguilla dieffenbachii) inhabiting a small coastal New Zealand stream were examined to investigate intraspecific and interspecific trophic partitioning and prey selection. Our results showed intraspecific segregation of preferred prey among three size classes of juvenile shortfinned eels, but there was significant overlap between different size classes of longfinned eels. All size classes of both species of eel fed nocturnally on similar prey, and so there was no indication of temporal or trophic segregation. Therefore, habitat separation is assumed to be the main mechanism to reduce interspecific competition in these two co‐occurring species of eels.

Growth, survival, and recruitment of juvenile shortfinned eels (Anguilla australis) in a large New Zealand coastal lake

Abstract Annual changes in the size composition and relative abundance of juvenile shortfinned eels (Anguilla australis Richardson) in Lake Ellesmere, New Zealand, were studied. Juveniles (<300 mm) were captured using a small beam trawl from 1994 to 1999. Age and growth rates were determined from otoliths and validated from trends in the length frequency of different year classes. Growth in length was linear from age 1 year onwards. Mean annual growth rates declined in recent years to exceptionally low values, averaging 13.2 mm year−1 (range 6.3–22.9 mm year−1), and are probably limited by the lack of food. Juvenile eels had high survival rates, exceeding 0.80 year−1 and were probably close to 1.0 year−1. Recruitment into this shallow, turbid, brackish lake was variable and was correlated with the duration and timing of the lakes opening to the sea. The decline in growth rates and restricted recruitment in recent years could reduce commercial catches of migrating silver eels for the next 10–15 years.

SPAWNING MIGRATIONS AND REPRODUCTION OF LANDLOCKED SOCKEYE SALMON (ONCORHYNCHUS NERKA) IN THE WAITAKI CATCHMENT, NEW ZEALAND

Abstract The spawning migrations of landlocked sockeye salmon (Oncorhynchus nerka) within the Waitaki catchment from 1975 to 1992 are described. In late February and early March an overlapping succession of spawning runs, consisting of fish of different origins and ages, spawned in cool (8–12°C) spring‐fed tributaries of lakes Ohau and Benmore. The size, age, and sex compositions of the runs were highly variable. Mature fish ranged in age from 2 to 5 years and in length from 138 to 462 mm. Redds were constructed in gravel (15–30 mm diam.) in water 0.15–0.50 m deep, flowing at 0.20–0.50 m s‐1. Runs increased during the 1970s, reaching a peak number of over 18 000 in 1979. Dam construction that was completed in late 1979 blocked spawning migrations from Lake Benmore to Lake Ohau; spawning runs are now fewer than 100 fish per annum and may be insufficient to maintain the wild stock. Attempts to increase the spawning runs by liberating hatchery‐reared juveniles and by the transfer of mature fish into warm (14...

Relative influence of local and landscape-scale features on the density and habitat preferences of longfin and shortfin eels

The relative structural influence of local-scale versus landscape-scale habitat features on size-class matrices of both longfin (Anguilla dieffenbachia) and shortfin (Anguilla australis) eels collected along six different New Zealand rivers (10 river-year combinations) are quantified using variance decomposition. The total explained variation of these species and size-class matrices was related to both local-scale and landscape-scale variables, and some proportion was shared between these two scales. When compared with landscape-scale variables, local-scale physical habitat variables such as water velocity and fish cover explained more about patterns in the size-distribution of longfin eels. The opposite situation was true for shortfin eels whose size-distribution was more strongly related to landscape-scale variables such as distance from sea and channel slope. These findings suggest that management of river flows is required to ensure flow regimes that maintain availability of suitable local-scale hydraulic conditions, and that biodiversity conservation efforts need to be targeted at protecting a gradient of rivers across New Zealands river landscape.