Physical oceanographic influences on Queensland reef fish and scallops
Department of Agriculture and Fisheries (QLD)
Tony J. Courtney
There is a strong need for Queensland fishery managers to obtain a better understanding of key physical oceanographic influences on target species of commercial and recreational fisheries. Tropical cyclones have been associated with reef fish catch rates. Coral trout (Plectropomus leopardus) catch rates typically fall after a major cyclone, while those of red throat emperor (Lethrinus miniatus) rise (see “Background” above). The effects on catchability can last several years. While the exact causal mechanism is not known, it is thought to be related to water temperature. Nutrient-rich cold water eddies, which break from the East Australian Current and move westward onto the Queensland continental shelf are likely to affect the spat settlement, growth, abundance and catch rates of saucer scallop (Amusium balloti). Understanding these relationships may lead to improved management, assessment and forecasting of catch in these fisheries, and it may also lead to improved acceptance of quantitative stock assessment results by industry. This proposal differs from previous abiotic studies because it focuses more on offshore, oceanic influences, rather than coastal rainfall and flow data.
1. Review recent advances in the study of physical oceanographic influences on fisheries catch data, and describe the major physical oceanographic features that are likely to influence Queensland reef fish and saucer scallops.
2. Collate Queensland’s physical oceanographic data and fisheries (i.e., reef fish and saucer scallops) data.
3. Develop stochastic population models for reef fish and saucer scallops, which can link physical oceanographic features (e.g., sea surface temperature anomalies ) to catch rates, biological parameters (e.g., growth, reproduction, natural mortality) and ecological aspects (e.g. spatial distribution).
Principal Investigator: Tony J. Courtney
Key Words: coral trout, Plectropomus leopardus, saucer scallop, Amusium balloti, ocean data
reanalysis, BRAN, tropical cyclones, wave height, Chlorophyll-a, eddy kinetic energy, Capricorn Eddy
Cyclones and reef fish
Cyclones and other low pressure systems that create large swells and wave heights offshore do not
necessarily result in reduced coral trout catch rates. This is because the outer barrier reef efficiently
dissipates offshore swells and protects coral reef habitats. However, systems that create large
southeasterly swells in waters between the GBR and the coast can cause significant habitat damage.
GBR waters between Rockhampton and Townsville act as a long southeast–northwest fetch over
which such a swell can build. Also Townsville has its own fetch which runs nearly east–west. These
swells bypass the GBR’s natural protection from its barrier reefs. It is the inshore fetches that are most
important for explaining the decline in coral trout catch rates after a cyclone. Declines in coral trout
catch rates are believed to stem mainly from reduced catchability rather than reduced abundance.
From Cairns north, due to the lack of suitable fetches, cyclones have almost no discernible effect on
• We recommend recognition of the importance of inshore wave-height measurements, and the
collation of available historical measurements. These data could then be used in new fishery
catch rate standardisations that can properly take account of the effects of tropical cyclones.
• We also suggest that new oceanographic models for the GBR be developed to take account of
the prime importance of the inshore-fetch mechanism in generating damaging swells.
New catch rate standardisations should prove very valuable in fishery management as they may finally
separate the effects of tropical cyclones from the effects of fishing, which to date has been impossible.
The process of setting quota in the fishery could then be based on the effects of fishing alone and made
largely independent of cyclones.
Saucer scallops and coastal and oceanographic influences
The project examined larval advection patterns for Queensland saucer scallops and relationships
between commercial catch rates of scallops and coastal and oceanographic variables.
Current strength and direction vectors from a 19-year hindcast dataset (i.e. BRAN 3.5) were used to
model scallop larvae advection trajectories in the scallop fishing grounds. Larvae were generally
found to move in a northwest direction towards the coast. Although larval connectivity between areas
was highly variable, the three scallop replenishment areas (SRAs, Yeppoon, Bustard Head and Hervey
Bay), which are opened and closed to trawling on a rotational basis, each showed relatively high rates
of self-seeding. This may explain why adult scallop densities in the areas are relatively high, and why
the areas receive high levels of fishing effort. No one single area is responsible for supplying larvae in
the fishery and larval advection patterns differed significantly between years, over the 19-year time
• Future larval advection modelling of the fishery should use oceanographic models that include
tidal vectors. This was not captured in the BRAN 3.5 hindcast dataset.
• Studies that provide some understanding of scallop larval vertical migration behaviour,
especially diel and tidal variation, would also improve the modelling.
The study examined a large number of correlations between commercial catch rates in November
(when the fishery has traditionally opened each year) and coastal and physical oceanographic
variables, over 26 years (1988-2013). The strongest correlation found was 0.85 for Chl-a
concentration five months earlier in June. Chl-a appears to have a significant influence on scallop
catch rates, probably as an important food source to scallops and other fauna, although the relationship
may not be a simple one.
Physical oceanographic properties of the area where the Capricorn Eddy forms, adjacent to and east of
the fishery, produced twice as many strong, statistically significant correlations with scallop catch rates
than any other area, suggesting that the eddy has a significant effect on the fishery. November catch
rates are negatively correlated with bottom water temperature anomalies (correlation = -0.74) and sea
level (-0.60) at the eddy area three months earlier in August.
November catch rates declined when surface water temperature anomalies were elevated 16-18 months
prior (May-July, winter previous year) when the scallops were spawned. Saucer scallop larvae have a
relatively narrow range of water temperature tolerance and we speculate that elevated SST during the
winter spawning reduces reproductive output and/or lowers survival of larvae. This negative
correlation was common to all five areas examined and is consistent with research from Western
Australia’s Shark Bay and Abrolhos Is. saucer scallop fisheries where elevated SST during spawning
results in reduced recruitment.
Chl-a, freshwater flow from rivers adjacent to the scallop fishery, and temperature anomalies had the
most statistically significant correlations with scallop catch rates. Sea level and eddy kinetic energy
generally had relatively few significant correlations with catch rates, while the SOI had no significant
correlations. This ranking helps identify the most influential environmental drivers in the scallop
fishery and future areas for research.
• We recommend that key environmental variables be monitored in conjunction with scallop catch
rates, including Chl-a, SST and freshwater flows. Key correlations suggest catch rates in
November (when the fishery commences each year) may be predicted using environmental data.
• Key relationships should be incorporated in the quantitative models used to assess the scallop
fishery. We examined how three such relationships, for water temperature anomalies, eddy
kinetic energy and Chl-a, affected performance of the age-structured model used to assess the
fishery. Two of the relationships improved the model fit and the estimate of maximum
sustainable yield (MSY). Further work is required to determine which relationships result in the
most improvement to the models.
• Catch rates and annual landings in the Queensland saucer scallop fishery are highly variable. In
1996 the fishery experienced recruitment failure, resulting in the lowest catch rates on record,
severe hardship to industry and emergency management measures which included spatial
closures (SRAs). These rotational closures remain today. Given that the project identified a
number of strong correlations with environmental variables (although no causal mechanisms
were demonstrated), particularly relationships with Chl-a and water temperature, we recommend
reviewing the need and function of the SRAs.