Butler, GL 2009, 'Biology and conservation of the endangered eastern freshwater cod Maccullochella ikei Rowland, PhD thesis, Southern Cross University, Lismore, NSW.
Copyright GL Butler 2009
The eastern freshwater cod, Maccullochella ikei Rowland 1985, is an endangered species native to the Clarence and Richmond rivers of northern New South Wales, Australia. A combination of factors resulted in a significant decline in the abundance and range of the species in the early 1900s. By the 1970s, M. ikei was considered extirpated from the Richmond River system, and there remained only one self-sustaining population in an isolated sub-catchment of the Clarence River system. A Recovery Plan was implemented in 2004 to assist in the conservation of M. ikei, in accordance with the threatened species provisions (Part 7A) of the Fisheries Management Act 1994. A major issue highlighted within the plan was that the lack of published information on the biology of wild M. ikei might be a limiting factor in the recovery of the species. Therefore, to ensure the effective management and ongoing conservation of M. ikei, the aim of this study was to undertake research into a number of key areas of the species’ biology.
Underwater cameras were used to observe the breeding behaviour of wild M. ikei, over three years and across three areas in the Mann and Nymboida rivers, tributaries of the Clarence River system. Each area was 5-8 km in length and included a mix of pools, glides, runs, riffles and rapids. The annual breeding season for M. ikei was short and succinct, lasting only 8-10 weeks. Spawning commenced each year during the first week of spring in the area of lowest altitude, and approximately one week later in the closest upstream area. Day-length is the primary spawning cue for M. ikei, but increasing water temperature may also be of importance. Nesting sites were located in slow-flowing pools, under cover such as large boulders and bedrock shelves, at depths of 0.9 to 4.0 m, and had one or two distinct entrances only. The nesting site was vigorously cleaned by the male up to one week prior to spawning and was only entered by the female for spawning. Paternal care of eggs and larvae was undertaken for up to 24 days, after which larvae dispersed. Greater protection of breeding M. ikei must be a management priority to ensure long-term conservation of the species.
Aspects of the ontogenetic behaviour and, growth and development of hatchling to day-30 M. ikei were studied under controlled, experimental conditions. Experiments included: stamina and burst swimming ability using a speed controlled flume; depth preference using a water-filled vertical tube; habitat preference using an artificial environment containing rocks and sand, and neutral areas constructed from white polyurethane; and photo-sensitivity using an enclosed light-box. Maccullochella ikei larvae grew constantly, but not consistently over the 30 days of the experiment. Burst swimming ability improved daily through until day-30, but swimming stamina declined significantly beyond day-13. Larvae were initially photonegative but switched to be positively phototactic beyond day-10. Depth preference was for the benthos until day-8, beyond which larvae dispersed to all depths when released. Habitat preference was for sand in young larvae but changed to the upstream neutral zone as the experiment progressed. The results suggest the period between day-10 and day-20 is the most critical in the early ontogeny of M. ikei, when larvae switched phototrophic behaviour, transitioned from endogenous to exogenous feeding, and exhibited a decline in sustained swimming ability.
The weekly and diel movements, and habitat selection of radio-tagged M. ikei were studied over a 12-month period in the Mann River. Inter-pool movement was significantly related to increases in flow, but was largely independent of changes in water temperature. Males but not females increased activity during the breeding season, moving distances of up to 30 km to locate suitable spawning sites. Diel movements were typified by increased crepuscular activity, with significantly less movement occurring during the night compared to the night. River channel preference was for deep pools, with large rocks and woody debris preferred as cover. The maintenance of flows, particularly to facilitate movement during the breeding season, is essential for the ongoing conservation of M. ikei.
The stomach contents of juvenile and adult M. ikei were sampled over two consecutive years in the Mann and Nymboida rivers to determine summer and winter feeding preferences. Food items were non-destructively collected using a modified gut-flushing technique. The stomachs contents of 268 M. ikei were evacuated, of which 191 contained at least one food item. A large variety and size range of items were recovered, encompassing 39 different taxa. There were significant differences between the food items recovered between seasons and an interaction between season and site. Seasonal differences related to a greater consumption of crustaceans, small fish, and terrestrial animals such as birds and a range of small mammals in winter, and more aquatic insects and molluscs in summer. There were also significant differences in food preference among size classes, with larger M. ikei (>550 mm) consuming fewer crustaceans, and more large fish and terrestrial animals. Maccullochella ikei has a diet different to other Maccullochella taxa, displays high plasticity in seasonal dietary choice, changes diet and foraging tactics as it grows, and does not always feed in accordance with optimal foraging theory.
Otoliths, opercular bones and dorsal spines were gathered from archival collections, hatcheries, and opportunistically from the wild to determine age and growth of M. ikei. Examination of opercular bones and dorsal spine sections revealed no consistent annuli. Sectioned otoliths exhibited consistent bipartite rings throughout the structures, and 106 otoliths were used to estimate age of M. ikei from 0+ to 15+ years. Edge-increment analysis and known-age M. ikei were used to validate age estimates. The von Bertalanffy growth equation for M. ikei is Lt = 704.9 (1-exp (-0.20 (t + 0.14))). Length-weight relationship of W = 2.80 x 10-6 x L3.2467 was established from 372 M. ikei collected using non-destructive techniques. Significant differences were found in relative condition of M. ikei in summer (Kn=0.999) and winter (Kn=1.026). Maccullochella ikei was found to be relatively long-lived, has a strong length-weight relationship, but growth and longevity differed from the closely related Murray cod, Maccullochella peelii peelii.
This study has added significantly to the knowledge of one of Australia’s most endangered fish species. I successfully quantified and qualified a number of previously undescribed aspects of M. ikei’s biology using innovative sampling methods that minimised the impact on the remnant population. I described; the reproductive behaviour; larval ontogeny; movement patterns and habitat choice; dietary choice and; the age and growth of M. ikei in the wild and where applicable, using lab-based experiments. The results of my research have already brought about significant changes in the management of M. ikei. The information presented will not only assist in the conservation of M. ikei, but will also provide a guide for future management and studies of other threatened fish species.