Meso-scale patterns in the distribution of larval fishes across the central Great Barrier Reef lagoon and relationships with environmental variability

Thorrold, Simon Robert (1993) Meso-scale patterns in the distribution of larval fishes across the central Great Barrier Reef lagoon and relationships with environmental variability. PhD thesis, James Cook University.

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Events occurring during the larval phase of marine fishes have important ramifications for the demographic structure of marine fish populations. Differential larval survival may drive recruitment variability and ultimately determine stock biomass. While considerable scientific attention has been devoted to larval fishes in temperate regions, studies of fish larvae in tropical environments have been rare and of uneven quality. This dissertation aimed to present detailed descriptions of the distribution patterns of small fishes, along with concurrent measurements of environmental variability, in the central Great Barrier Reef lagoon.

Larval fish were collected using both conventional plankton nets and light traps during the spring-summer periods of 1988-89, 1989-90 and 1990-91. Light traps have not been used to sample fish in open water before this study; they collected numerous fishes at sizes larger than those routinely captured in plankton nets. Data from both techniques were analysed to examine spatio-temporal variability in the distribution patterns of small fishes. Plankton nets revealed relatively stable cross-shelf patterns, with a distinctive nearshore component characterised by gobiids, callionymids, leiognathids and teraponids; a cross-shelf group including nemipterids, carangids, platycephalids and scorpaenids; and an offshore group dominated by clupeids, lutjanids, scombrids, and pomacentrids. Significant temporal coherence, across spatial scales up to 50 km, in abundance of a number of taxa with cross-shelf and offshore affinities was also found. This coherence could be generated by synchronous spawning, or by a hydrographic event acting over synoptic scales of at least 50 km in both cross-shelf and long shore directions. Light trap catches in both years were dominated by a catches at a single station in one month. In 1988, a multi-specific 'patch' of larvae were found 24 km off the coast on the CB transect, while in 1989 a similar 'patch' was located 16 km off the coast on the LR transect.

Remote sensing, utilising NOAA's AVHRR polar-orbiting satellite, was used to synoptically assess sea-surface temperatures and visible reflectance of water masses across the central Great Barrier Reef lagoon. While there was little evidence of significant thermal structure across the sampling transects, more structure was noted in false-water colour images. Turbid, coastal water was found along the coast, although the the offshore extent of this water mass varied through time. This appeared to be related to wind strength acting to determine the depth at which wave action led to resuspension of bottom sediments. The offshore extent of this nearshore water mass also appeared to delineate the approximate boundary between nearshore and offshore ichthyoplankton groupings from the net samples.

Zooplankton distributions across the inner shelf did not coincide with that of the larval fish community. Given that zooplankton are more accurate tracers of water movement than larval fish, hydrography alone could not explain the maintenance of larval fish assemblages through time. It is suggested that larval fish may be actively maintaining their positions within water masses. An alternative hypothesis that differential survival between masses was responsible for the patterns could not, however, be rejected.

Both zooplankton abundance and egg production rates of Acrocalanus gibber, a common copepod species, were significantly higher within a low-salinity plume derived from terrestrial runoff associated with cyclonic rains. Larval fish with cross-shelf affinities were also concentrated in the plume during the initial two days of a total of five sampling occasions. Offshore larvae became abundant at the plume front on the final two days of sampling. This accumulation may have been caused by the swathe effect of the plume front as it moved progressively offshore.

Episodic events such as cyclonic rainfall and resuspension of nutrients resulting from strong winds may be of critical importance to planktonic communities, and hence larval fish survival, in this region. Such events are impossible to predict, and therefore will only be elucidated by long-term, meso-scale monitoring of larval abundance within the pelagic environment and concurrent measurements of biological oceanography at similar scales. This study demonstrates that new developments in both biological sampling methods and satellite technology make programs eminently feasible. The renewed presence in space of a satellite (SeaWiFS) capable of determining ocean colour will further enhance the prospects of considerable advancements in determining the causes of variable survival and subsequent recruitment of tropical fish larvae.

Item ID: 27240
Item Type: Thesis (PhD)
Keywords: larval fishes; distribution patterns; Great Barrier Reef; GBR; environmental variability; recruitment patterns; recruitment variability
Date Deposited: 24 Jun 2013 23:42
FoR Codes: 06 BIOLOGICAL SCIENCES > 0608 Zoology > 060809 Vertebrate Biology @ 33%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 34%
05 ENVIRONMENTAL SCIENCES > 0501 Ecological Applications > 050102 Ecosystem Function @ 33%
SEO Codes: 96 ENVIRONMENT > 9606 Environmental and Natural Resource Evaluation > 960603 Environmental Lifecycle Assessment @ 25%
96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 25%
97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 50%
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