Shannan K. Crow

Does one size fit all? An evaluation of length–weight relationships for New Zealand's freshwater fish species

Length–weight relationships are a fundamental tool for assessing populations and communities in fisheries science. Many researchers have collected length–weight data throughout New Zealand, yet parameters describing these relationships remain unpublished for many species of freshwater fish. We compiled 285,124 fish records from researchers and institutions across New Zealand to parameterise length–weight equations, using both power and quadratic models, for 53 freshwater species belonging to 13 families. The influence of location and sex on length–weight relationships was also assessed. Location, in particular, generated different length–weight relationships for 65% of the species examined. Length–weight equations were validated by comparing predicted weights against independently measured weights from 25 electrofished sites across New Zealand and the equations were highly accurate (R2>0.99). Recommendations are made about how to robustly apply this new resource which should assist freshwater fisheries researchers throughout New Zealand.

The effects of suspended sediment on brown trout (Salmo trutta) feeding and respiration after macrophyte control

Sediment resuspension during and after mechanical excavation of macrophytes may have a significant impact on resident fish populations. Unfortunately, little is known about the influence of this sediment on the respiratory performance and feeding abilities of fishes in New Zealand waterways. We examined the effects of suspended sediment (SS) concentrations previously observed after a large-scale macrophyte removal operation on oxygen consumption (MO2) and feeding rates of brown trout (Salmo trutta). MO2 at 0 mg L−1, 150 mg L−1, 300 mg L−1, 450 mg L−1 and 600 mg L−1 of SS was measured using semi-closed respirometry. Feeding rates at the same SS concentrations were also measured using laboratory tank experiments. Results suggest that SS concentrations up to 600 mg L−1 have no effect on MO2. Conversely, feeding rates were significantly reduced at 450 mg L−1 (22% reduction) and 600 mg L−1 (31% reduction), indicating that sediment concentrations above 450 mg L−1 may negatively affect brown trout populations.

Ontogenetic changes in morphology of flathead galaxiid fishes (Osmeriformes: Galaxiidae) in South Island, New Zealand

Abstract We examined ontogenetic changes in morphology of four non-migratory ‘flathead’ galaxiid fishes and interspecific differences. Geometric analyses were used to determine where ontogenetic shifts occur, examine growth trajectories between taxa and compare interspecific differences with taxa standardised to each of three sizes. Ontogenetic changes in all taxa were mostly associated with the pelvic and anal fins developing slower than overall length. Growth trajectories differed between all taxa except Galaxias ‘southern’ and G. ‘northern’, with the largest interspecific differences occurring in the overlap between the anal fin and the caudal flange. This overlap discriminated taxa at each of the three size classes despite ontogenetic changes. Morphological shifts suggest that habitat use changes with size. Interspecific differences shown in geometric shape require structural linkages before suggestions can be made on identification or systematics.

Relationships between Maori values and streamflow: tools for incorporating cultural values into freshwater management decisions

ABSTRACT This study aimed to develop quantitative relationships for Maori cultural values supported by streamflow. We examined the results of a Cultural Flow Preference Study (CFPS) for five streams in Canterbury, New Zealand, and used mixed effects models to quantify how observed streamflow supported scores for Overall Health (OH). Relationships between streamflow and OH differed between sites and assessors within each site. Ignoring differences between sites and assessors, there was a positive relationship that explained 5% of the variation between streamflow and OH. After accounting for differences between sites and assessors, there was a positive relationship that explained 53% and 62% of the variation respectively. Flow attributes sustaining cultural values are based on the characteristics of the site and relationships change between rivers. Our study provides a transferable framework for developing cultural health–flow relationships that will enable Maori to be involved from the outset of any environmental flow assessment.

Effects of mechanical macrophyte control on suspended sediment concentrations in streams

ABSTRACT Suspended sediment (SS) is an important pollutant in freshwater ecosystems and can be detrimental to fish communities. Although macrophytes mediate sediment deposition, little effort has been put into determining how their removal affects sediment resuspension. The present study examined the immediate and long-term impacts of mechanical macrophyte removal on SS concentrations in streams. The results of this study suggest that bed disturbance during mechanical excavation of macrophytes significantly increases SS in the short term, and concentrations were found to increase by as much as 15,687 mg L–1 immediately after macrophyte removal. Significant long-term (77 day) increases in SS were also observed, indicating that without macrophytes, disturbed material is continually resuspended after excavation by fluvial processes. These results demonstrate that macrophyte removal can result in SS levels that have previously been shown to harm fish, and indicate that this activity may be more detrimental to fish than previously thought.

The effect of ramp slope and surface type on the climbing success of shortfin eel (Anguilla australis) elvers

Instream structures such as dams, weirs and culverts reduce diadromous fish (e.g. freshwater eels) biodiversity and abundance by restricting upstream migrations. Fish passes are used to facilitate fish passage around instream structures, but limited information is available about design aspects that influence the ability of fish to negotiate the structures. We used experimental ramps to evaluate the effects of surface type and slope on the climbing success of shortfin eel (Anguilla australis) elvers (total length <155mm). Climbing success was examined for three surface types (smooth plastic, sand and gravel, Miradrain) at slopes of 30, 50 and 70°. Elver climbing ability differed among substrates, with the highest success observed on the Miradrain surface (a moulded plastic drainage product). Climbing success decreased with increasing ramp slope. The mean length of elvers successfully negotiating the ramps was longer than that of elvers that failed to climb because only larger elvers could climb the Miradrain surface as ramp slope increased. Results indicated that the optimal ramp-designed passage for climbing species would be lined with Miradrain and set at an angle close to 30° and not exceeding 50°. Elver passage will be detrimentally affected by steep surfaces, which will be exacerbated if suboptimal ramp-surface types are used.