The life history and ecology of Bluefish, Girella cyanea, at Lord Howe Island
- Publication Type:
- Thesis
- Issue Date:
- 2012
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Girella cyanea is a conspicuous member of the reef-fish community in the Lord Howe Island Marine Park (LHIMP), but very little is known about its life history. Rareness of this species on mainland Australian coasts in recent years has initiated a fishing ban across the state of New South Wales, however recreational fishing is still permitted on LHI. Effective management and conservation of this population requires increased information on life history and demographics. Management currently in place for this species is limited. A bag limit of 5 fish person-1 day-1 is imposed in habitat protection zones across the Marine Park. It is difficult to measure the effectiveness of this strategy, however, without the knowledge of the resource requirements of the species and how these may change throughout the course of life. This study aimed to describe distribution, diet and growth in G. cyanea to provide important information for best-practice management of the LHI population.
An extensive literature search was conducted for published life history, ecology and management data on Girellidae, revealing relatively scarce information for the family. A pilot study assessed the utility of a roaming survey method towing a GPS-receiver behind an observer on snorkel/SCUBA. This new method proved effective and was used for size-based assessments of habitat-use at nearshore and offshore locations around the LHI archipelago. Densities of G. cyanea were highest in complex rocky intertidal and rocky-reef areas. Dietary analyses helped explain this distribution, with gut contents showing intertidal green algal species (i.e. Ulva and Enteromorpha) are important food resources for post-settlement fish.
An ontogenetic dietary shift was found, with fish < 40 mm standard length (Ls) found in intertidal habitat having a mainly carnivorous diet and a digestive system without pyloric caeca. In contrast, pyloric caeca were well-developed in fish> 40 mm Ls and diets exhibited increased ingestion of algae.
Age-at-size using otoliths and von Bertalanffy parameters revealed G. cyanea is fast-growing and long-lived (up to 41 yrs). It is likely the transition to sexual maturity occurs between 2 and 5 years of age or 200 mm Ls. Size-based observations place fish of this life stage within complex rock habitats at depths < 5 m. Future management policies should ensure adequate (representative) areas of rocky intertidal habitat are within sanctuary zoning to protect G. cyanea at this important life stage.
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