- Title
- Movement behaviour and reproductive biology of adult spotted grunter (Pomadasys commersonnii) in the Breede Estuary
- Creator
- Ziko, Bantony Alford
- Subject
- Fishery management
- Subject
- Freshwater fishes
- Subject
- Fishes--Nutrition
- Date
- 2021-06
- Type
- Master's theses
- Type
- text
- Identifier
- http://hdl.handle.net/10353/20970
- Identifier
- vital:46870
- Description
- Movements of the adult spotted grunter (Pomadasys commersonnii), an overexploited estuary-dependant fishery species, were investigated in relation to its reproduction behaviour and selected environmental influences. The South African P. commersonnii population is known to spawn at sea along the KwaZulu-Natal (KZN) coastline, whereas there no published records on spawning in the Eastern and Western Cape Provinces exist. However, due to the occurrence of ripe running fish that were reported in the Breede Estuary (Western Cape - WC), an investigation into its reproductive behaviour was necessary, as was gaining a greater understanding of its movements between the estuary, the sea, and other connected habitats. Seven adult P. commersonnii (610 – 690 mm fork length, FL) were captured and implanted with long-life acoustic transmitters and their movements were logged on a linear array of 16 acoustic receivers in the Breede Estuary (WC) and six acoustic receivers in the adjacent marine environment. Movements of the tagged fish to other marine and estuary habitats were also determined using data collected by the Acoustic Tracking Array Platform (ATAP) which is an extended network of acoustic receivers along the South African coastline with receivers at sea and in selected estuaries. Furthermore, adult P. commersonnii tagged in the Goukou (n = 6) (WC) and Sundays (n = 8) (Eastern Cape - EC) estuaries, approximately 50 km and 470 km away, respectively, were also monitored to facilitate an improved understanding of inter-habitat connectivity. Fish tagged in Sundays and Goukou estuaries were monitored by the Acoustic Telemetry Array Platform (ATAP). Each individual fish tagged in the Breede Estuary was monitored between 356 and 1166 days and selected environmental parameters, including water temperature, river flow rate and rainfall, were measured during the study period (November 2016 – March 2020) to assess their influence on the movements of the tagged fish. Furthermore, to determine time of spawning and duration, gonads of 112 adult fish (383 – 700 mm FL) were opportunistically collected from recreational anglers in the Breede Estuary to determine the gonadal maturation cycles by using a combination of Gonado-somatic index (GSI) and histology. The tagged adult P. commersonnii spent significantly more time in the estuary (83.5 percent) than the marine (16.5 percent) environment, with all of the tagged P. commersonnii making sea trips of varying frequency and duration. While in the estuary, fish showed a strong preference for the lower reaches of the estuary compared to the middle and upper reaches. Area use varied seasonally, with the mouth area being heavily utilized during winter, then followed by summer. Sea trips in all three tagging estuaries also fluctuated seasonally, with more frequent movements during summer which lasted for longer periods. This coincided with the peak of the spawning period inferred from the gonad developmental cycle of fish captured in the Breede Estuary. Results from a generalized linear mixed model showed that the presence of Breede tagged P. commersonnii in the sea, where the species is known to spawn, was significantly influenced by water temperatures in the estuary and river flow rate with fish more likely to occur at sea during warmer months (summer) and periods of high river flow (winter). Furthermore, circular analysis showed that season (month) had a significant influence on the departure from and arrival in the Breede Estuary, with most departures and arrivals occurring during January. Moreover, P. commersonnii tagged in the Sundays Estuary also showed similar results where both departures and arrivals were influenced by season, mostly taking place in December and January, respectively. The increased frequency of sea trips by adult P. commersonnii in summer suggests regional spawning occurring in the WC. Based on the similar movement behaviour displayed by tagged fish in the Sundays Estuary, it is possible that spawning also occurs in the EC. Most of the P. commersonnii tagged in all the three estuaries never left their tagging estuaries (57 percent Breede, 66 percent Goukou, 38 percent Sundays), only making short sea trips, while others displayed use of multiple habitats. On average, the tagged P. commersonnii that left their tagging estuaries travelled a maximum distance of 377 km (Breede), 63 km (Goukou) and 300 km (Sundays). Tagged fish were more likely to visit nearby estuaries and offshore marine habitats (< 130 km) compared to distant ones (> 130 km). These results suggest that there are high levels of localised connectivity between P. commersonnii populations in different habitats although connectivity between distant habitats appears to be rare. Connectivity will likely occur on habitats located close to the boundaries of the WC, EC and KZN. The findings from this long-term monitoring study have shown that adult P. commersonnii rely heavily on the estuarine environment, undertaking short sea trips, thus demonstrating the importance of estuaries to the adult population. Due to various threats faced by estuaries such as habitat degradation and overexploitation, management of these systems is paramount to conserve such fishery resources. Due to the resident behaviour of the P. commersonnii within estuaries, Marine Protected Areas (MPAs) and area closures, in conjunction with Estuarine Protected Areas (EPAs) will be significant in the recovery and management of P. commersonnii populations. Since P. commersonnii appears to reach peak spawning activity during summer when fishing pressure is usually highest, improved enforcement during this period is necessary to conserve the spawning stock. As connectivity is an important aspect in the life of the P. commersonnii, ensuring such connectivity is maintained by protecting all these habitats is important.
- Description
- Thesis (MSc) (Zoology) -- University of Fort Hare, 2021
- Format
- computer
- Format
- online resource
- Format
- application/pdf
- Format
- 1 online resource (115 pages)
- Format
- Publisher
- University of Fort Hare
- Publisher
- Faculty of Science and Agriculture
- Language
- English
- Rights
- University of Fort Hare
- Rights
- All Rights Reserved
- Rights
- Open Access
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UFH Department of Zoology & Entomology
| Thumbnail | File | Description | Size | Format | |||
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| View Details | SOURCE1 | ZIKO_BA_FINAL_THESIS.pdf | 2 MB | Adobe Acrobat PDF | View Details |