David K. Rowe

Effects of turbidity on the feeding ability of the juvenile migrant stage of six New Zealand freshwater fish species

Abstract Laboratory tank experiments were used to determine the effect of turbidity levels ranging from 0–640 Nephelometric Turbidity Units (NTU) on mean feeding rates for each species. Mean feeding rates for banded kokopu (Galaxias fasciatus), smelt (Retropinna retropinna), inanga (G. maculatus), and common bullies (Gobiomorphus cotidianus) were all reduced by increased turbidity, but significantly so only for banded kokopu and inanga. Banded kokopu was the most sensitive species as feeding rates at 20 NTU were significantly lower than in clear water, compared with levels over 160 NTU for the other species. Feeding rates for redfinned bullies (Gobiomorphus huttoni) increased with turbidity up to 40 NTU then decreased. Reductions in feeding rate at elevated turbidity levels were the result of a reduced inability to feed, rather than to stress, as exposure to the highest turbidity level did not reduce feeding motivation or appetite. Feeding tests in the dark indicated that, whereas vision was important for...

EFFECTS OF LAND USE ON NATIVE FISH COMMUNITIES IN EAST COAST STREAMS OF THE NORTH ISLAND OF NEW ZEALAND

The community structure, density, and biomass of native fish species was determined for first‐ to second‐order streams draining mature pine, native forest, and pasture catchments in seven, North Is...

Exotic fish introductions and the decline of water clarity in small North Island, New Zealand lakes: a multi-species problem

Changes in water clarity (secchi disc transparency) in relation to the presence/absence of introduced, exotic fish, including rudd (Scardinius erythrophthalmus), tench (Tinca tinca), perch (Perca fluviatilis), brown bullhead catfish (Ameiurus nebulosus), goldfish (Carassius auratus), and koi carp (Cyprinus carpio) were determined for 49 small, North Island, New Zealand lakes. There was a negative association between water clarity and the presence of exotic fish independent of lake depth. Moreover, a ‘before-and-after’ comparison and examination of case-studies indicated that introductions of exotic fish reduce water clarity. The number of species introduced affected the relationship between lake depth and water clarity but the specific role of each species could not be distinguished because most of the lakes (83%) contained more than one exotic fish species. A model incorporating the known mechanisms by which planktivorous, benthivorous and herbivorous fish can influence water clarity in lakes showed that control over just one species or feeding guild may not result in an improvement in water clarity because of the additive and synergistic effects of different species on lake trophic processes.

Effects of turbidity on the ability of juvenile rainbow trout, Oncorhynchus mykiss, to feed on limnetic and benthic prey in laboratory tanks

Abstract The feeding rate of juvenile rainbow trout, Oncorhynchus mykiss Richardson, 1836, on small (1–2 mm diam.) Daphnia spp. in laboratory tanks was not reduced by turbidity levels up to 160 Nephelometric Turbidity Units (NTU). Furthermore, the high feeding rates on larger, benthic prey (viz. Deleatidium spp. and chironomid larvae) in clear water (0 NTU) were maintained at turbidities up to 160 NTU. Therefore, any impairment of vision by increased turbidity did not affect the ability of trout to feed on these prey, and non‐visual senses may be used for the capture of such prey in turbid waters. Although trout were strongly size‐selective for both large chironomid and Deleatidium larvae in clear water, turbidities over 20 and 160 NTU, respectively, reduced size‐selection. Vision is therefore needed by trout to select larger prey. However, chironomid larvae were consumed by trout in the complete absence of light, so their ability to capture these prey in tanks was not dependent on visual cues. It is apparent that trout do use other senses such as the lateral line system to detect and capture such prey when turbidity levels are high and when light levels or water clarity are low. This ability is expected to offset any reduction in visual feeding caused by increased turbidity, and it helps explain the increased emphasis on epibenthic feeding by trout in turbid waters.

Reduced abundance of banded kokopu (Galaxias fasciatus) and other native fish in turbid rivers of the North Island of New Zealand

Abstract Laboratory experiments demonstrated that migrant juvenile banded kokopu (Galaxias fasciatus Gray) were more sensitive to suspended sediment (SS) than other native fish species. If juvenile migrants avoid waters made turbid by SS and their recruitment to adult habitats up stream is reduced, then adult abundance may decline in turbid rivers. To test this, we compared the abundance of diadromous native fish between turbid and clear rivers. The duration (% time) for which SS concentrations exceeded 120 mg litre‐1 (a critical level from laboratory experiments) during the migration season (August‐December) was estimated for over 150 New Zealand river sites. Turbid rivers were defined as those where SS concentrations exceeded 120 mg litre‐1 for over 20% of the time and clear rivers as those where SS concentrations exceeded 120 mg litre‐1 for less than 10% of the time. Eight turbid rivers and seven clear ones were identified where sufficient data on SS and native fish populations existed to permit a comparison. The mean occurrence of banded kokopu was reduced by 89.5% in turbid rivers and, although other diadromous fish species were also less common, banded kokopu was most affected. Densities of adult banded kokopu were also significantly lower in optimal stream habitats in three turbid compared with three matched clear rivers. We therefore concluded that the abundance of adult banded kokopu was reduced in turbid rivers and propose that this is because of reduced recruitment of juveniles in turbid rivers.

Effects of logging with and without riparian strips on fish species abundance, mean size, and the structure of native fish assemblages in Coromandel, New Zealand, streams

Abstract We determined the effects of logging, both with and without a riparian buffer strip, on the native fish fauna at 27 stream sites in an exotic pine (mainly Pinus radiata) forest on the Coromandel Peninsula, New Zealand. Fish abundance at the logged sites was compared with reference sites in both unlogged pine and native forest. The abundance of Anguilla dieffenbachii (Gray) and Anguilla australis (Richardson) was not significantly affected by logging. However, the abundance of Galaxias fasciatus (Gray) and Gobiomorphus huttoni (Ogilby) was. There were fewer Ga. fasciatus at the logged sites without buffers than at the reference sites, but more at the logged sites with buffers. The abundance of Go. huttoni was higher at the logged sites than at the reference sites, and was highest at the logged sites with riparian buffers. Overall, the different, species‐specific responses to logging maximised total fish numbers at the logged sites. As total fish numbers, the abundance of Ga. fasciatus, and species equitability, a measure of fish assemblage structure, were all highest at the logged sites with riparian buffer strips, we concluded that riparian strips enhanced the native fish community of streams within these logged catchments.

Effects of oxygen, temperature and light gradients on the vertical distribution of rainbow trout, Oncorhynchus mykiss, in two North Island, New Zealand, lakes differing in trophic status

Abstract Vertical distributions of adult rainbow trout (> 25 cm fork length, FL) were determined with a SIMRAD ES470 split‐beam echosounder in two 80–90 m deep lakes differing in water quality. Between November 1993 and February 1994, most trout (> 80%) were between 10 and 40 m, within or close to the thermocline. However, a small group of fish occupied colder waters, deeper than 50 m. In February, surface water temperatures > 21.0°C and hypolimnetic oxygen levels < 2.5 g m−3 compressed the habitable depth range for trout in Lake Rotoiti to 12–35 m compared with 12–80 m in Lake Rotoma. Deeper‐dwelling trout inhabiting waters over 50 m in Lake Rotoiti would have been forced into shallower waters at this time. However, the vertical distribution of the remaining trout in Lake Rotoiti was not compressed. In March 1994, adult trout were still present in waters 10–40 m deep in both lakes, but many of the smaller fish had moved into shallower waters (< 10 m deep), probably because of declining water temperatures...

Effects of turbidity on the migration of juvenile banded kokopu (Galaxias fasciatus) in a natural stream

Abstract Laboratory experiments have shown that the juvenile migratory stage of banded kokopu (Galaxias fasciatus (Gray 1842)) is more sensitive to turbidity than other native fish species and avoids turbidity levels of >25 nephelometric turbidity units (NTU). Field trials using juvenile fish collected from the Tarawera River and Hays Stream, New Zealand, were used to test the results from these laboratory experiments by measuring the effects of turbidity on the migration direction and rate for banded kokopu in a natural stream setting. In the stream setting, neither the migration rate nor the migration direction were affected at turbidity <25 NTU. At higher turbidity levels, significantly fewer fish migrated up stream within a given time period. Because there was rarely any downstream movement, this suggests the fish either halted or slowed their upstream movement. A slower rate of migration could result in fewer juveniles reaching adult habitat, and would account for the reduced abundance of adult banded kokopu in rivers that are turbid during the migration season. Achieving turbidity levels of <25 NTU in rivers and streams during the migration season would therefore help maintain upstream migrations and populations of banded kokopu, and hence other native fish species.

Disappearance of koaro, Galaxias brevipinnis, from Lake Rotopounamu, New Zealand, following the introduction of smelt, Retropinna retropinna

SynopsisKoaro, Galaxias brevipinnis, were once the only fish present in Lake Rotopounamu but, after a comprehensive survey in 1990, none were found in the lake or its tributary streams. Introduced native fish, specifically smelt, Retropinna retropinna, and the common bully, Gobiomorphus cotidianus, now occur in this lake. As koaro co-exist with bullies in other lakes, but have declined in landlocked lakes containing smelt, the disappearance of koaro in Lake Rotopounamu is attributed to the introduction of smelt alone. Interspecific competition for food between 0 + year old koaro and smelt, combined with predation by 2 + year old smelt on koaro larvae, are thought to be responsible. Such a mechanism would be consistent with theoretical predictions of predator-prey regulation systems within same chain food webs. Rainbow trout, Oncorhynchus mykiss, which were introduced into a number of local lakes before smelt, and which preyed on the koaro, have been blamed for the decline of the koaro populations. However, the disappearance of koaro in Lake Rotopounamu shows that smelt can reduce koaro populations independently of trout predation.

A rapid method to score stream reaches based on the overall performance of their main ecological functions.

A method was developed to score the ecological condition of first- to third-order stream reaches in the Auckland region of New Zealand based on the performance of their key ecological functions. Such a method is required by consultants and resource managers to quantify the reduction in ecological condition of a modified stream reach relative to its unmodified state. This is a fundamental precursor for the determination of fair environmental compensation for achieving no-net-loss in overall stream ecological value. Field testing and subsequent use of the method indicated that it provides a useful measure of ecological condition related to the performance of stream ecological functions. It is relatively simple to apply compared to a full ecological study, is quick to use, and allows identification of the degree of impairment of each of the key ecological functions. The scoring system was designed so that future improvements in the measurement of stream functions can be incorporated into it. Although the methodology was specifically designed for Auckland streams, the principles can be readily adapted to other regions and stream types.