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Estuarine Use by Spotted Grunter <I>Pomadasys Commersonnii</I> In African Journal of Marine Science ISSN: 1814-232X (Print) 1814-2338 (Online) Journal homepage: https://www.tandfonline.com/loi/tams20 Estuarine use by spotted grunter Pomadasys commersonnii in a South African estuary, as determined by acoustic telemetry A-R Childs , PD Cowley , TF Næsje , AJ Booth , WM Potts , EB Thorstad & F Økland To cite this article: A-R Childs , PD Cowley , TF Næsje , AJ Booth , WM Potts , EB Thorstad & F Økland (2008) Estuarine use by spotted grunter Pomadasyscommersonnii in a South African estuary, as determined by acoustic telemetry, African Journal of Marine Science, 30:1, 123-132, DOI: 10.2989/AJMS.2008.30.1.12.462 To link to this article: https://doi.org/10.2989/AJMS.2008.30.1.12.462 Published online: 08 Jan 2010. Submit your article to this journal Article views: 108 Citing articles: 18 View citing articles Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=tams20 African Journal of Marine Science 2008, 30(1): 123–132 Copyright © NISC Pty Ltd Printed in South Africa — All rights reserved AFRICAN JOURNAL OF MARINE SCIENCE EISSN 1814–2338 doi: 10.2989/AJMS.2008.30.1.12.462 Estuarine use by spotted grunter Pomadasys commersonnii in a South African estuary, as determined by acoustic telemetry A-R Childs1*, PD Cowley2, TF Næsje3, AJ Booth1, WM Potts2, EB Thorstad3 and F Økland3 1 Department of Ichthyology and Fisheries Science, Rhodes University, PO Box 94, Grahamstown 6140, South Africa 2 South African Institute for Aquatic Biodiversity, Private Bag 1015, Grahamstown 6140, South Africa 3 Norwegian Institute for Nature Research, Tungasletta 2, NO-7485, Trondheim, Norway * Corresponding author, e-mail: [email protected] Manuscript received September 2007; accepted January 2008 Factors influencing estuarine use and marine excur- environmental conditions. On average, fish departed sions by spotted grunter Pomadasys commersonnii the estuary on the outgoing tide at night and returned in the Great Fish Estuary, South Africa, were studied to the estuary on the incoming tide during the day. using manual and automated telemetry methods. In all, Spotted grunter <400 mm TL spent more time in the 20 individuals, ranging from 362 mm to 698 mm total estuary than larger fish (403–698 mm TL). The number length (TL), were caught and tagged with acoustic- of fish recorded in the estuary was significantly corre- coded transmitters in the estuary. Tagged individuals lated with barometric pressure, wind direction and sea spent most time in the estuary (67%), interspersed with temperature. The spotted grunter under study exhibited a number of marine excursions — 15 in all. The number a high degree of estuarine use, highlighting the impor- (6–53) and mean duration (0.8–9.5 days) of sea excur- tance of this habitat to this overexploited, estuarine- sions varied among individuals, and were correlated dependent fishery species. significantly with tide, time of day, fish size and various Keywords: environmental variables, estuarine-dependence, estuarine conservation, fi sh movement, Great Fish River Introduction Estuaries are important in the life history and the main- 1994). The return of post-spawning fish coincides with tenance of the diversity of coastal fish species because increased catches by fishers in estuaries between July and of their function as nursery areas for juveniles as well as January. These events are known as ‘grunter runs’ (Wallace feeding grounds for adults (Cyrus 1991). The dependence 1975a, Marais and Baird 1980, Marais 1988, Pradervand of many fish species on estuaries is well documented (e.g. and Baird 2002). It is suggested that adults spend up to Wallace et al. 1984, Lenanton and Potter 1987, Blaber et several months in estuaries, before moving back to sea al. 1989, Whitfield 1990, Hoss and Thayer 1993). Spotted where they undergo gonadal development and ultimately grunter Pomadasys commersonnii (Haemulidae) (Lacepède spawn (Wallace 1975b, Wallace and van der Elst 1975). 1801) is an estuarine-dependent species which spawns in It is believed that adult fish also enter estuaries in a pre- the KwaZulu-Natal inshore coastal waters, between August spawning state to gain condition en route to their spawning and December (Wallace 1975b, Wallace and van der Elst grounds in KwaZulu-Natal (Webb 2002). 1975, Harris and Cyrus 1997, 1999). The eggs and larvae Spotted grunter is one of the most important linefishery are transported southwards by the Agulhas Current, and species along the South African coastline and, although it juveniles between 20 mm and 50 mm TL recruit into the dominates the catch of recreational and subsistence fishers KwaZulu-Natal and south-eastern Cape estuaries (Wallace in most estuaries in the Eastern Cape and KwaZulu-Natal and van der Elst 1975, Whitfield 1990). Juvenile spotted (James et al. 2001, Mann et al. 2002, Pradervand and Baird grunter make use of the abundant food resources in estuar- 2002), it is far less prevalent in the marine shore-based ies, where they grow rapidly and remain for a period of 1–3 fishery (Coetzee et al. 1989, Bennett et al. 1994, Brouwer years (Wallace and Schleyer 1979, Day et al. 1981). Upon et al. 1997). Increased fishing pressure placed on estuarine attaining sexual maturity (at between 300 mm and 400 mm systems (Lamberth and Turpie 2003) and the estuarine- TL), they return to the marine environment (Wallace 1975b). dependent nature of spotted grunter render them vulnerable Some adults, however, return to estuaries to feed and to to exploitation during both their juvenile and post-spawning regain condition after spawning (Wallace 1975b, Whitfield adult phases. Consequently, an understanding of the degree Published online 08 Jan 2010 124124 Childs, Cowley, Næsje, Booth, Potts, Thorstad and Økland of estuarine use and the factors influencing estuarine use is accounts for continuous nutrient inputs and elevated phyto- fundamental to the design of management strategies. plankton production. Consequently, the Great Fish Estuary Conventional tag-and-recapture techniques are considered is a highly productive and turbid system. Perennial river flow to be cost-effective to monitor fish movements. However, (mean annual river discharge of 250–650 × 106 m3) and tidal the animal has to be physically recaptured to obtain the data exchange ensures a permanently open connection to the (Heupel et al. 2006) and, more importantly, a description of sea (Grange and Allanson 1995, Grange et al. 2000). The small-scale localised movements is not pos sible owing to the estuary channel is narrow (30–100 m wide) and its depth lack of information on movements between the fish’s release (0.5–3.5 m) is dependent on flooding events (Whitfield et al. and recapture (Egli and Babcock 2004, Matthews 1990). 1994). The spring tidal prism is 1.6 × 106 m3 and the spring Although newer, alternative techniques (e.g. otolith micro- tidal range is between 1.0 m and 1.5 m in the lower reaches chemistry) are also available to quantify patterns of migra- and decreases towards the head (Whitfield et al. 1994). tion between estuarine and coastal habitats (Gillanders 2005, Secor and Piccoli 2007), particularly at a life-history Research approach level, telemetry is the only method that can establish the precise real-time movements of individual fish and provide Acoustic telemetry methods were used to track the move- fine-scale temporal and spatial data that are essential for ments of spotted grunter in the Great Fish Estuary between behavioural ecology (Baldwin et al. 2002). The use of 29 September 2003 and 12 February 2004. This 137-day acoustic monitoring systems is expanding worldwide and is study period coincided with the known time spotted grunter likely to continue to increase in the field of aquatic sciences enter estuaries along the South African coastline (Webb (Heupel et al. 2006). However, only a few studies have used 2002). A total of 20 fish of between 362 mm and 698 mm telemetry to determine the extent of estuarine use by a fish TL (mean 478 mm TL) was tagged with VEMCO V13SC- species (Able and Grothues 2007, Hartill et al. 2003, Miller 1L-R256 coded transmitters. The transmitters (69 kHz) and Sadro 2003). In South Africa, Kerwath et al. (2005) and had an expected battery life of 130 days, were 13 mm in Næsje et al. (2007) have used acoustic telemetry methods diameter, 36 mm in length and weighted approximately to examine the temporal and spatial use of estuaries by 6 g in water — on average 0.5% of the fish’s body mass spotted grunter, but only Cowley et al. (in press) have used (min: 0.2%, max: 1.2%). Each trans mitter emitted a unique a fixed array of automated data-logging listening stations acoustic pulse train randomly every 5–15 s. According to to accurately determine estuarine residence of a estuarine- Wallace (1975b), the length-at-50% maturity of spotted dependent species, the dusky kob Argyrosomus japonicus. grunter is 300 mm TL for males and 360 mm TL for Although Kerwath et al. (2005) conducted a pilot study females, so those under study were likely a mix of both on spotted grunter in a small temporarily open estuary, juveniles and adults. and Næsje et al. (2007) provided the first comprehensive attempt of describing the movement patterns of undersized Tagging of fish spotted grunter in a permanently open estuary, neither study provided detailed information on the factors influencing Fish were caught from a boat using rod-and-line with baited estuarine use. barbless hooks. After capture, each fish was immediately Environmental factors play an elemental role in fish distri- placed in a plastic container with about 100 l of estuary bution and recruitment patterns in estuaries (e.g. Abookire water containing 2-phenoxyethanol (c.
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