Patterns of movement of Plectropomus leopardus (Serranidae) in relation to spawning aggregations and marine protected areas, as determined by ultrasonic telemetry

Zeller, Dirk Christoph (1996) Patterns of movement of Plectropomus leopardus (Serranidae) in relation to spawning aggregations and marine protected areas, as determined by ultrasonic telemetry. PhD thesis, James Cook University.

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Abstract

The importance of patterns of movement and space use by fishes to the understanding of population dynamics, community structure and spatial population models, is being increasingly recognised. Despite this realisation, information regarding patterns of movement is rare for fishes. Two important aspects in coral reef fisheries which are affected greatly by the lack of knowledge about movements, are the uncertainties associated with high fishing pressures on spawning aggregations, and the potential use of marine protected areas as a fisheries management strategy.

The main aim of this research was to determine patterns of movement and space use of a species of major fishing importance (Plectropomus leopardus, Serranidae), in relation to annual spawning aggregation events, and with respect to existing marine protected area zoning. Given the known limitations of the conventional technique for assessments of movements, i.e. external mark-release-recapture techniques, an alternative methodology, ultrasonic telemetry, was adopted to address these aspects.

The first objective consisted of methodological evaluations of ultrasonic telemetry for use on P. leopardus and in coral reef fish and fisheries research in general. The second objective was to estimate home ranges and basic temporal patterns of space use by the study species. The third objective was to locate previously unknown spawning aggregation sites, estimate their minimal catchment areas, and determine patterns of participation and residence of individual fish at aggregation sites. The fourth objective was a comparison of data obtained through telemetry with comparable data collected independently using a mark-release-resighting study, and to evaluate the data obtained through both methods in relation to the existing marine protected area zoning at the study location, with considerations to the use of marine protected areas as a fisheries management tool.

Preliminary assessments of ultrasonic telemetry for use on P. leopardus included the evaluation of three ultrasonic transmitter placement methods (force feeding, external attachment, and surgical body cavity insertion) in conjunction with three different fish anaesthetics (Metomidate, Phenoxyethanol, and MS-222). The most suitable method of transmitter placement for long-term application in P. leopardus was surgical implantation into the body cavity. Attaching transmitters externally led to severe aggravation of the attachment wounds due to repeated attempts by the fish to dislodge the transmitter. Force feeding transmitters was unsuitable due to the short gastric retention times observed (18 to 216 hours). Tricaine methanesulfonate (MS-222) was the anaesthetic chosen due to the ease of induction and maintenance of deep anaesthesia. Post-surgery recovery periods in aquaria avoided field losses due to injury-induced predation, and permitted examination of each specimen prior to release for proper closure and healing of incisions.

The pilot evaluation of telemetry in the coral reef environment, and initial tracking trials indicated that manual ultrasonic tracking using visual triangulation should be conducted by taking bearings at approximately right angles (90°) to each other, with approximate distances of 50-75 m between tracking vessel and estimated location of transmitter. Bearings taken at angles considerably less than 90°, or taken at sharp angles to the prevailing wind, should be avoided. These considerations will result in minimal directional bias of bearings due to wind effects, while ensuring maximum accuracy and precision of position estimates. Observer training prior to tracking, and regular re-evaluation of bearing accuracy and precision during tracking sessions is recommended.

Thirty-nine individual P. leopardus (fork length: mean = 49.04 cm, range = 37.6 to 67.5 cm) were tracked successfully between 1993 and 1995. Eight of these were tracked during two subsequent field trips, resulting in 47 separate tracking sessions, comprising a total of 2,024 fish-tracking days. Average minimum area polygon home ranges of P. leopardus differed between fish from fringing reefs (10,458.4 m² ± 962.3 (SE)) and patch reefs (18,796.9 m² ± 3,188.8 (SE)). This difference was caused by differences in width of home ranges, with fringing reef home ranges being narrower than patch reef ones. Length of home ranges did not differ between reef types. Home ranges did not differ between male and female fish, and were stable within and between each tracking session (maximum 202 days between sessions). Plectropomus leopardus were day-active, predominantly using a small number of physical locations (3-4 positions) within their home ranges. Mean daily distance moved within home ranges was 192.2 m ± 5.09 (SE), with the maximum being 1121.8 m. Patterns of space use were relatively consistent throughout the day. Position fidelity was very high at night, with very limited movements.

A distinct pattern of home range use existed in relation to the prevalent current direction, with P. leopardus showing a strong preference for utilising positions located in the upcurrent portions of their home ranges. This study demonstrated, for the first time, distinct movements of P. leopardus in relation to changes in tidal currents. Thus, the observation commonly made by fishers of better catches on "run on" sides of reefs may be explained by the observed preference of upcurrent positions utilised by coral trout.

Using ultrasonic telemetry, four major spawning aggregation sites of P. leopardus were detected at Lizard Island. Spawning aggregation activities displayed a lunar pattern, with peak activities during new moon periods in the southern-hemisphere spring-early summer period. Of 35 fish tracked during the spawning periods, only 31% participated in spawning aggregations. Thus, this study demonstrated, at least for the periods studied, that a limited number of individuals in the population aggregated in large groups to spawn, despite all specimens being sexually mature. All specimens that aggregated displayed site fidelity with respect to their chosen aggregation site. Highest density estimates of fish at aggregations were 60 fish/1,000m² (1994) and 35 fish/1,000 m² (1995), based on visual census. The mean distance between home ranges and spawning aggregation sites was 911.95 m ± 223 (SE) (range: 223 to 5,213 m). Total spawning movement distances back and forth in the spawning season for individual fish ranged from 604 m to over 17 kilometres. One-way inter-reefal movements were recorded for three fish, moving 3, 7.5, and 11 kilometres between release and recapture locations. Total residence time at aggregations differed between males and females, with males spending on average 8 times more time at aggregations than females (males: 316:33 h:min ± 65:04 (SE), females: 36:42 h:min ± 17:42 (SE)). Females undertook day or overnight trips only, while males regularly did multi-day trips also.

The reliance on several aggregation sites per reef makes P. leopardus potentially less vulnerable to overexploitation of spawning aggregations, compared to species which utilise fewer sites but in larger numbers per site. However, the strong site fidelity observed for all individuals makes individual aggregations vulnerable to depletion. The low participation rate at major aggregation events, combined with the observation that all recovered tracking specimens showed histological patterns of reproductive activity for the current season, suggested that not all spawning activity took place at the known aggregation sites. This, together with the discovery of several smaller courtship sites, should be regarded as evidence for the possibility of additional, localised spawning events. The movement data obtained indicated within-reef catchment areas covering linear distances of over five kilometres, with some evidence of inter-reefal movements in relation to spawning. The observed differences between male and female fish in residence duration at aggregation sites indicated definite sex dependent variations in turnover rates at these aggregations, making males potentially more vulnerable to fishing pressures on aggregations. The observed sex dependent turnover rates, as well as the problem of visual identification of gender, need to be considered in the use of aggregation events for stock assessment purposes.

A mark-release-resighting study using hook and line as capture, and underwater visual census as resighting tool of fish marked with numerical freeze brands, indicated that catch per unit effort by hook and line was significantly higher in management zones closed to fishing than in zones open to fishing. However, the average density of fish (5.31 fish/1,000 m²) did not differ significantly between management zones. Thus, differences existed in catchability of fish between management zones, providing further evidence that concerns regarding reduced catches by fishers on the Great Barrier Reef may partly reflect behavioural changes in targeted species.

No freeze branded fish were recorded as having crossed the management zone boundaries. However, fish carrying ultrasonic transmitters, and having home ranges straddling management zone boundaries, spent, on average, 27.49% of their time in the 31.23% of their home ranges located in zones open to fishing. These fish crossed zone boundaries at an average rate of 15.27 times/month (range: 3.62 - 29.09 times/month), indicating that these individuals moved regularly across the zone boundaries. Any of the monitored fish had the chance of being caught outside the closed zone in proportion to the area of home range located in the open zone.

This study successfully demonstrated the viability and suitability of ultrasonic telemetry to the study of fishes on coral reefs. Application of ultrasonic telemetry provided unique information about movement and space use patterns of P. leopardus that could not be obtained in any other manner. The information obtained provided not only the basis for future ecological and behavioural investigations of P. leopardus and other coral reef fishes, but may serve as the foundation for the development of improved management strategies for long-term sustainable fisheries and marine protected area management on coral reefs.

Item ID: 27498
Item Type: Thesis (PhD)
Keywords: Plectropomus leopardus; spawning aggregations; ultrasonic telemetry; transmitter placement method; site fidelity; fisheries management strategies
Date Deposited: 25 Jun 2013 04:46
FoR Codes: 05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050202 Conservation and Biodiversity @ 49%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 51%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 51%
97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 49%
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