Michael J. H. Hickford

Population sinks resulting from degraded habitats of an obligate life-history pathway

Many species traverse multiple habitats across ecosystems to complete their life histories. Degradation of critical, life stage-specific habitats can therefore lead to population bottlenecks and demographic deficits in sub-populations. The riparian zone of waterways is one of the most impacted areas of the coastal zone because of urbanisation, deforestation, farming and livestock grazing. We hypothesised that sink populations can result from alterations of habitats critical to the early life stages of diadromous fish that use this zone, and tested this with field-based sampling and experiments. We found that for Galaxias maculatus, one of the most widely distributed fishes of the southern hemisphere, obligate riparian spawning habitat was very limited and highly vulnerable to disturbance across 14 rivers in New Zealand. Eggs were laid only during spring tides, in the highest tidally influenced vegetation of waterways. Egg survival increased to >90% when laid in three riparian plant species and where stem densities were great enough to prevent desiccation, compared to no survival where vegetation was comprised of other species or was less dense. Experimental exclusion of livestock, one of the major sources of riparian degradation in rural waterways, resulted in quick regeneration, a tenfold increase in egg laying by fish and a threefold increase in survival, compared to adjacent controls. Overall, there was an inverse relationship between river size and egg production. Some of the largest rivers had little or no spawning habitat and very little egg production, effectively becoming sink populations despite supporting large adult populations, whereas some of the smallest pristine streams produced millions of eggs. We demonstrate that even a wide-ranging species with many robust adult populations can be compromised if a stage-specific habitat required to complete a life history is degraded by localised or more diffuse impacts.

Catch vs count: Effects of gill-netting on reef fish populations in southern New Zealand

Abstract This study investigated the relationships between visual counts of fishes and the catch from gill-nets on rocky reefs in southern New Zealand. Visual censuses were done and then gill-nets of three mesh sizes (2.5″, 3.5″ and 4.5″) were set in the surveyed areas. There were significant differences among habitats in the assemblages of reef fishes. The number and species of fishes caught by the gill-nets had little overlap with those recorded in visual surveys. The standard lengths of fishes caught in gill-nets were significantly larger than those recorded in the visual surveys, primarily because of differences in the species composition of fishes sampled by the two methods. Resident reef fishes, especially labrids, comprised most of the visual surveys, while transient pelagic species and wide-ranging reef fishes made up the largest proportion of the gill-net catch. The three mesh sizes caught different size fractions of fish populations, but all mesh sizes caught fishes larger than those seen in the visual surveys. There was a significant species × mesh size interaction in the number of fishes caught, indicating that some species were more vulnerable to particular mesh sizes. Both the number of fishes and number of species caught declined sequentially with increasing mesh size. This study shows that visual surveys and the more passive gill-netting sample different fractions of fish populations, and that gill-netting is ineffective at targeting individual species in complex reef habitats.

Transient effects of an invasive kelp on the community structure and primary productivity of an intertidal assemblage

Invasive species can have significant impacts on the diversity and productivity of recipient ecological communities. The kelp Undaria pinnatifida (Harvey) Suringar is one of the worlds most successful invasive species but, although itspurported impactsarestrong, thereis littleempirical evidence that itdisplacesnative species.Furthermore, as thisspeciesnaturalisesinlocalcommunities,itspotentialeffectsoncommunitydynamicshavenotbeenwelltested.Here, we test the ecological impacts of Undaria in intertidal communities in southern New Zealand using a combination of surveys, a 2.5-year press-removal experiment and in situ measures of net primary production to gauge its impact on community structure and productivity. Undaria had transient effects on the composition of communities, affecting two seasonally abundant species in 1 year, but these impacts did not persist into the following year. Overall, there were only small effects of Undaria removal on diversity and abundance of native algae and invertebrates at two sites. However, the presence of Undaria more than doubled net primary production of recipient communities during its annual peak abundance when it increased biomass by 606 g DW m � 2 . We conclude that the invasion of Undaria represents an additionalandsubstantialcarbonsubsidytocoastalecosystemswithpotentiallypositiveeffectsonnearshoreproductivity. Additional keywords: intertidal biodiversity, net primary production, NPP, Undaria pinnatifida.

Synergistic Interactions within Disturbed Habitats between Temperature, Relative Humidity and UVB Radiation on Egg Survival in a Diadromous Fish

Anthropogenic impacts, including urbanization, deforestation, farming, and livestock grazing have altered riparian margins worldwide. One effect of changes to riparian vegetation is that the ground-level light, temperature, and humidity environment has also been altered. Galaxias maculatus, one of the most widely distributed fishes of the southern hemisphere, lays eggs almost exclusively beneath riparian vegetation in tidally influenced reaches of rivers. We hypothesized that the survival of these eggs is greatly affected by the micro-environment afforded by vegetation, particularly relating to temperature, humidity and UVB radiation. We experimentally reduced riparian vegetation height and altered shading characteristics, tracked egg survival, and used small ground-level temperature, humidity and UVB sensors to relate survival to ground-level effects around egg masses. The ground-level physical environment was markedly different from the surrounding ambient conditions. Tall dense riparian vegetation modified ambient conditions to produce a buffered temperature regime with constant high relative humidity, generally above 90%, and negligible UVB radiation at ground-level. Where vegetation height was reduced, frequent high temperatures, low humidity, and high UVB irradiances reduced egg survival by up to 95%. Temperature effects on egg survival were probably indirect, through reduced humidity, because developing eggs are known to survive in a wide range of temperatures. In this study, it was remarkable how such small variations in relatively small sites could have such a large effect on egg survival. It appears that modifications to riparian vegetation and the associated changes in the physical conditions of egg laying sites are major mechanisms affecting egg survival. The impacts associated with vegetational changes through human-induced disturbances are complex yet potentially devastating. These effects are particularly important because they affect a very small portion of habitat that is required to complete the life history of a species, despite the wide distribution of adults and juveniles across aquatic and marine environments.

Evidence of iteroparity in the widely distributed diadromous fish inanga Galaxias maculatus and potential implications for reproductive output

Gaps in understanding variability among populations of inanga Galaxias maculatus in the timing of reproduction were addressed in southern New Zealand (NZ), where G. maculatus constitutes a declining fishery. Reproductive activity was delayed by 1 month on the west coast compared with the east coast and the west coast spawning season was prolonged into winter. The evidence for post-spawning survival of some fish was unequivocal from histological studies. These older and larger fish contributed disproportionately to egg production. Estimates of fecundity were considerably lower than those previously calculated for NZ populations. The importance of quality habitats being available during critical life history periods are highlighted. It was apparent that some streams supported fish that were larger and in better condition and that this translated into greatly increased fecundity. Future research should focus on whether this is a legacy of these fish experiencing better pre-settlement marine habitat as larvae, or higher quality instream habitat enhancing the growth and development of adults.

Census survey approach to quantifying īnanga spawning habitat for conservation and management

ABSTRACT Here, we describe a methodology for quantifying the spawning habitat of īnanga (Galaxias maculatus), a protected native fish species. Our approach is demonstrated with a survey of the Heathcote/Ōpāwaho following the Canterbury earthquakes that produced unexpected findings. Spawning habitat was detected over a 2.5 km reach and the area occupied by spawning sites (75m2) was much larger than in previous records (ca. 21m2). Sites dominated by the invasive Phalaris arundinaceae were found to support high egg numbers. Spawning has not previously been recorded on this species and it is identified in the literature as a threat to spawning habitat. Considerable spatio-temporal variation was also detected in the location of spawning sites and pattern of egg production. Together, these aspects illustrate the need for a comprehensive survey methodology to reliably quantify spawning habitat. The Heathcote/Ōpāwaho example shows the utility of our census approach for achieving this, and supporting habitat conservation objectives.

Protected area effectiveness for fish spawning habitat in relation to earthquake-induced landscape change

We studied the effectiveness of conservation planning methods for Galaxias maculatus, a riparian spawning fish, following earthquake-induced habitat shift in the Canterbury region of New Zealand. Mapping and GIS overlay techniques were used to evaluate three protection mechanisms in operative or proposed plans in two study catchments over two years. Method 1 utilised a network of small protected areas around known spawning sites. It was the least resilient to change with only 3.9% of post-quake habitat remaining protected in the worst performing scenario. Method 2, based on mapped reaches of potential habitat, remained effective in one catchment (98%) but not in the other (52.5%). Method 3, based on a habitat model, achieved near 100% protection in both catchments but used planning areas far larger than the area of habitat actually used. This example illustrates resilience considerations for protected area design. Redundancy can help maintain effectiveness in face of dynamics and may be a pragmatic choice if planning area boundaries lack in-built adaptive capacity or require lengthy processes for amendment. However, an adaptive planning area coupled with monitoring offers high effectiveness from a smaller protected area. Incorporating elements of both strategies provides a promising conceptual basis for adaptation to major perturbations or responding to slow change.